Animal Proteins
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Title: Animal Proteins
Author: Hugh Garner Bennett
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Transcriber's Note:
Minor typographical errors and inconsistencies have been corrected. Some
words had inconsistent hyphenation throughout the book; these have been
made consistent.
In the mathematical and chemical expressions, the caret character
represents "to the power of", e.g. e^2 means e-squared. Curly braces are
used to represent underscores, e.g. n{1} means n with a subscript of 1.
Equal signs are used to represent =bold words=, and underscores are used
to represent italics.
On page 152 NaCO{23} has been corrected to Na{2}CO{3}. On page 212,
the variable n has been replaced with the correctly subscripted forms
n{1} and n{2}.
The index entry for Hemlock bark had no page number in the original
text, so the correct page number, 34, has been supplied.
Ions are shown as Fe+++, instead of using superscripts.
There is some inconsistency in the notation used in the original text
for chemical formulae such as Na{2}Cr{2}O{7}(2H{2}O). These have
been regularized to use the modern mid-dot, for example,
Na{2}Cr{2}O{7}·2H{2}O.
Greek letters in the original have been represented in this version by
the name of the letter enclosed in square brackets, e.g. [alpha].
ANIMAL PROTEINS
BY
HUGH GARNER BENNETT, M.Sc. (LEEDS)
MEMBER OF THE SOCIETY OF LEATHER TRADES' CHEMISTS; FORMERLY ASSISTANT
LECTURER AND DEMONSTRATOR AT THE LEATHER INDUSTRIES DEPARTMENT OF THE
UNIVERSITY OF LEEDS
AUTHOR OF "THE MANUFACTURE OF LEATHER"
LONDON
BAILLIÈRE, TINDALL AND COX 8, HENRIETTA STREET, COVENT GARDEN
1921
(_All rights reserved_)
GENERAL PREFACE
The rapid development of Applied Chemistry in recent years has brought
about a revolution in all branches of technology. This growth has been
accelerated during the war, and the British Empire has now an
opportunity of increasing its industrial output by the application of
this knowledge to the raw materials available in the different parts of
the world. The subject in this series of handbooks will be treated from
the chemical rather than the engineering standpoint. The industrial
aspect will also be more prominent than that of the laboratory. Each
volume will be complete in itself, and will give a general survey of the
industry, showing how chemical principles have been applied and have
affected manufacture. The influence of new inventions on the development
of the industry will be shown, as also the effect of industrial
requirements in stimulating invention. Historical notes will be a
feature in dealing with the different branches of the subject, but they
will be kept within moderate limits. Present tendencies and possible
future developments will have attention, and some space will be devoted
to a comparison of industrial methods and progress in the chief
producing countries. There will be a general bibliography, and also a
select bibliography to follow each section. Statistical information will
only be introduced in so far as it serves to illustrate the line of
argument.
Each book will be divided into sections instead of chapters, and the
sections will deal with separate branches of the subject in the manner
of a special article or monograph. An attempt will, in fact, be made to
get away from the orthodox textbook manner, not only to make the
treatment original, but also to appeal to the very large class of
readers already possessing good textbooks, of which there are quite
sufficient. The books should also be found useful by men of affairs
having no special technical knowledge, but who may require from time
to time to refer to technical matters in a book of moderate compass,
with references to the large standard works for fuller details on
special points if required.
To the advanced student the books should be especially valuable. His
mind is often crammed with the hard facts and details of his subject
which crowd out the power of realizing the industry as a whole. These
books are intended to remedy such a state of affairs. While
recapitulating the essential basic facts, they will aim at presenting
the reality of the living industry. It has long been a drawback of our
technical education that the college graduate, on commencing his
industrial career, is positively handicapped by his academic knowledge
because of his lack of information on current industrial conditions. A
book giving a comprehensive survey of the industry can be of very
material assistance to the student as an adjunct to his ordinary
textbooks, and this is one of the chief objects of the present series.
Those actually engaged in the industry who have specialized in rather
narrow limits will probably find these books more readable than the
larger textbooks when they wish to refresh their memories in regard to
branches of the subject with which they are not immediately concerned.
The volume will also serve as a guide to the standard literature of the
subject, and prove of value to the consultant, so that, having obtained
a comprehensive view of the whole industry, he can go at once to the
proper authorities for more elaborate information on special points, and
thus save a couple of days spent in hunting through the libraries of
scientific societies.
As far as this country is concerned, it is believed that the general
scheme of this series of handbooks is unique, and it is confidently
hoped that it will supply mental munitions for the coming industrial
war. I have been fortunate in securing writers for the different
volumes who are specially connected with the several departments of
Industrial Chemistry, and trust that the whole series will contribute
to the further development of applied chemistry throughout the Empire.
SAMUEL RIDEAL.
AUTHOR'S PREFACE
It has been the author's chief concern that this volume should fulfil
its own part in the programme set forth in Dr. Rideal's General Preface.
The leather, glue, and kindred trades have been for many years
recognized as chemical industries, but the great development of colloid
chemistry in the last few years has given these trades a more definite
status as such, and they can now be placed in the category of applied
physical chemistry. The time is probably not far distant when some
knowledge of pure physical chemistry will be a first essential to
students, chemists, chemical engineers, and to all engaged in these
industries in supervision, administration, or control. It is hoped that
this volume will stimulate the study of these industries from that
standpoint.
As the author has previously written upon one of the industries involved
herein ("The Manufacture of Leather": Constable & Co.), he has, rather
inevitably, found it difficult to avoid altogether his own phraseology.
The changes of a decade, however, together with the wider field and
newer view-point, have made possible a radical difference of treatment.
The author desires to acknowledge the help he has received from the many
books, essays, and researches which are mentioned in the references at
the end of each section, especially to Procter's "Principles of Leather
Manufacture," and also to thank Dr. Rideal for many useful suggestions.
The author would like also to acknowledge here his indebtedness (as well
as that of the trade generally) to the work of Dr. J. Gordon Parker, who,
through his researches, lectures, and teaching work, has done more than
any other man to disseminate a knowledge of practical methods of
tanning.
The author's thanks are also due to his brother, Mr. W. Gordon Bennett,
M.Sc., A.I.C., M.C., for assistance in proof revision, and to his
father, Rev. John Bennett, for some literary criticism.
H. GARNER BENNETT.
BEVERLY, _June_, 1921.
CONTENTS
GENERAL PREFACE
AUTHOR'S PREFACE
CONTENTS
INTRODUCTION
* * * * *
PART I.
HIDES FOR HEAVY LEATHERS.
SECTION
1. THE RAW MATERIAL OF HEAVY LEATHERS
2. THE PREPARATION OF PELT
3. VEGETABLE TANNAGE
4. FINISHING PROCESS
5. SOLE LEATHER
6. BELTING LEATHER
7. HARNESS LEATHER
8. UPPER LEATHERS
9. BAG LEATHER
10. PICKING BAND BUTTS
* * * * *
PART II.
SKINS FOR LIGHT LEATHERS.
1. PRINCIPLES AND GENERAL METHODS OF
LIGHT LEATHER MANUFACTURE
2. GOATSKINS
3. SEALSKINS
4. SHEEPSKINS
5. CALFSKINS
6. JAPANNED AND ENAMELLED LEATHERS
* * * * *
PART III.
CHROME LEATHERS.
1. THE NATURE OF CHROME LEATHERS
2. GENERAL METHODS OF CHROME LEATHER
MANUFACTURE
3. CHROME CALF
4. CHROME GOAT AND SHEEP
5. HEAVY CHROME LEATHERS
* * * * *
PART IV.
MISCELLANEOUS TANNAGES.
1. ALUM TANNAGES
2. FAT TANNAGES
3. OIL TANNAGES
4. FORMALDEHYDE TANNAGE
5. SYNTHETIC TANNING MATERIALS
6. COMBINATION TANNAGES
7. THE EVOLUTION OF THE LEATHER INDUSTRY
* * * * *
PART V.
GELATINE AND GLUE.
1. PROPERTIES OF GELATINE AND GLUE
2. RAW MATERIALS AND PRELIMINARY
TREATMENT
3. EXTRACTION
4. CLARIFICATION AND DECOLORIZATION
5. BLEACHING
6. EVAPORATION
7. COOLING AND DRYING
8. USES OF GELATINE AND GLUE
9. THE EVOLUTION OF THE GELATINE AND
GLUE INDUSTRY
* * * * *
PART VI.
MISCELLANEOUS PROTEINS AND BYE-PRODUCTS.
1. BYE-PRODUCTS OF THE LEATHER TRADES
2. BYE-PRODUCTS OF THE GELATINE AND
GLUE TRADES
3. FOOD PROTEINS
4. MISCELLANEOUS ANIMAL PROTEINS
INDEX
ANIMAL PROTEINS
INTRODUCTION
Proteins are organic compounds of natural origin, being found in plants
and in animals, though much more plentifully in the latter. They are
compounds of great complexity of composition, and of very high molecular
weight. The constitution of none of them is fully understood, but
although there are a great number of different individual proteids, they
present typical resemblances and divergences which serve to
differentiate them from other groups of organic bodies, and also from
one another.
Proteins resemble one another in both proximate and ultimate analysis.
They contain the usual elements in organic compounds, but in proportions
which do not vary over very wide limits. This range of variation is
given approximately below:--
Element. Per cent.
Carbon 49 to 55
Hydrogen 6.4 to 7.3
Oxygen 17 to 26
Nitrogen 13 to 19
Sulphur 0.3 to 3.0
The most characteristic feature of the protein group is the amount of
nitrogen usually present. This is generally nearer the higher limit,
seldom falling below 15 per cent. This range for the nitrogen content is
determined largely by the nature of constituent groups which go to form
the proteid molecule. Roughly speaking, proteins consist of chains of
amido-acids and acid amides with smaller proportions of aromatic groups,
carbohydrate groups and thio compounds attached. In these chains an acid
radical may combine with the amido group of another amido acid, the acid
group of the latter combining with an amido group of another amido acid,
and so on. Hydrogen may be substituted in these chains by alkyl or
aromatic groups. There is obviously infinite possibility of variation in
constitution for compounds of this character, the general nature of
which varies very little. Practically all of the proteins are found in
the colloid state, and this makes them very difficult to purify and
renders the ultimate analysis in many cases doubtful. It is, for
example, often difficult to ascertain their moisture content, for many
are easily hydrolyzed with water only, and many part easily with the
elements of water, whilst on the other hand many are lyophile colloids
and practically cannot be dehydrated or dried. A few, such as gelatin
and some albumins, have been crystallized.
The constituent groups have been investigated chiefly by hydrolytic
methods. The chains of amido acids are split up during hydrolysis, and
individual amido acids may thus be separated. The hydrolysis may be
assisted either by acids, alkalies or ferments, but follows a different
course according to the nature of the assistant. Under approximately
constant conditions of hydrolysis, the products obtained are in
approximately constant proportions, and this fact has been utilized by
Van Slyke in devising a method of proximate analysis. It is not possible
in this volume to enter deeply into the constitution of the different
proteids. Reference must be made to works on pure chemistry, especially
to those on advanced organic chemistry. It will be interesting, however,
to mention some of the amido acids and groups commonly occurring in
proteids. These comprise ornithine (1:4 diamido valeric acid), lysine
(1:5 diamido-caproic acid), arginine (1 amido, 4 guanidine valeric
acid), histidine, glycine (amidoacetic acid), alanine (amido propionic
acid), amido-valeric acid (amido-iso-caproic acid), liacine, pyrollidine
carboxylic acid, aspartic acid, glutamic acid (amido-glutaric acid),
phenyl-alanine, serine (hydroxy-amido propionic acid), purine
derivatives (_e.g._ guanine), indol derivatives (_e.g._ tryptophane
and skatol acetic acid), cystine (a thioserine anhydride),
glucosamine, and urea.
There are a few general reactions which are typical of all proteins, and
which can usually be traced to definite groupings in the molecule.
Amongst these is the biuret reaction: a pink colour obtained by adding a
trace of copper sulphate and an excess of caustic soda. This is caused
by the biuret, NH(CONH{2}){2} radical or by similar diacidamide
groups, _e.g._ malonamide, oxamide, glycine amide. Another general
reaction is with "Millon's reagent," a solution of mercuric nitrate
containing nitrous fumes. On warming the proteid with this reagent, a
curdy pink precipitate or a red colour is obtained. This reaction is
caused by the tyrosine group (p. oxy [alpha] amido phenyl-propionic
acid). Another general reaction is to boil the protein with 1:2 nitric
acid for some days. A yellow flocculent precipitate of "xanthoproteic
acid" is obtained, and this dissolves in ammonia and caustic alkalies
with a brown or orange-red colour. Another characteristic of proteins is
that on dry distillation they yield mixtures of pyridine C{5}H{5}N,
pyrrol C{4}H{5}N, and their derivatives.
On the subdivision, classification and nomenclature of the proteins much
ink has been spilled, and it is impossible in this volume to go into the
various systems which have been suggested. It should be noted, however,
that some writers habitually use the terms "proteid" or "albuminoid" as
synonyms for protein. The classification of proteins adopted in this
work is used because it is the most suitable for a volume on industrial
chemistry and has the additional merits that it is simple and is already
used in several standard works on industrial chemistry. It is based upon
the behaviour of the proteins towards water, a matter of obvious moment
in manufacturing processes. On this basis proteins may be divided into
albumins, keratins and gelatins.
Cold water dissolves the albumins, does not affect the keratins, and
only swells the gelatins. The behaviour in hot water confirms and
elaborates the classification. When heated in water, the albumins
coagulate at temperatures of 70°-75° C., the gelatins (if swollen)
dissolve readily, whilst the keratins only dissolve at temperatures
above 100° C. Albumins and keratins may be distinguished also from
gelatins by adding acetic acid and potassium ferrocyanide to their
aqueous solutions. Albumins and keratins give a precipitate, gelatins do
not. Another distinguishing reaction is to boil with alcohol, wash with
ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a
violet colour, keratins and gelatins do not.
=Albumins= may be first discussed. They are typified by the casein of
milk and by white of egg. Their solutions in water are faintly
alkaline, optically active, and lævorotatory. They are coagulated by
heat and also by mineral acids, alcohol, and by many poisons. The
temperature of coagulation (usually about 72° C.) is affected by
mineral salts, the effect being in lyotrope order (see Part V.,
Section I.). The coagulated albumin behaves in most respects like a
keratin. Some of the albumins (globulins) are, strictly speaking, not
soluble in cold water, but readily dissolve in weak solutions of salt.
The albumins are coagulated from these solutions, as usual, when
heated. Into this special class fall myosin (of the muscles),
fibrinogen (of the blood) and vitellin (of egg yolk). By a gentle or
limited hydrolysis of the albumins with dilute acids in the cold, a
group of compounds called albuminates are obtained. They dissolve in
either acids or alkalies, and are precipitated by exact
neutralization. They may also be "salted" out by adding sodium
chloride or magnesium sulphate. They are not coagulated by heat. After
further hydrolysis with either acids, alkalies or ferments, very
soluble compounds are obtained called albumin peptones or albumoses.
These are soluble in alkalies, acids and water, and are readily
hydrolyzed further into amido acids and acid amides. They are very
similar to the peptones obtained from keratins and gelatins. They are
not coagulated by heat.
=Keratins= are typified by the hair of animals. They soften somewhat
in cold water and even more in hot water, but are not dissolved until
digested for some time at temperatures exceeding 100° C. With some
keratins, however, the cystine group is to some extent easily split
off by warm water, and on boiling with water hydrogen sulphide is
evolved. The sulphur content of keratins is often greater than the
average for proteids. All keratins are dissolved with great readiness
by solutions containing sulphydrates and hydrates, _e.g._ a solution
of sodium sulphide. In solutions of the hydrates of the alkali and
alkaline earth metals, keratins behave differently. Some dissolve with
great ease, some with difficulty, some only on heating and some not
even if digested with hot caustic soda. They are dissolved (with
hydrolysis) by heating with mineral acids, yielding peptones and
eventually amido acids, acid amides, etc. Many keratins have a
comparatively low content of nitrogen.
=Gelatins= are very difficult to distinguish from one another, their
behaviour being closely similar to reagents. They are also very
readily hydrolyzed even with water, and the products of hydrolysis are
even more similar. The gelatins are known together, commercially,
under the general name of gelatine. Gelatins of different origin,
however, have undoubtedly a different composition, the nitrogen
content being variable. If the gelatins are not bleached whilst they
are being manufactured into commercial gelatine, they are called
"glue." Gelatine is colourless, transparent, devoid of taste and
smell. It is usually brittle. Its S.G. is about 1.42, and it melts at
140° C. and decomposes. It is insoluble in organic solvents. When
swelling in cold water it may absorb up to 12 times its own weight of
water. The swollen product is called a "jelly." Jellies easily melt on
heating and a colloidal solution of gelatine is obtained. This "sets"
again to a jelly on cooling, even if only 1 per cent. gelatin (or
less) be present. The solution is optically active and lævorotatory,
but with very variable specific rotation. Some observers have thought
that the different gelatins have different specific rotations and may
so be distinguished. Gelatins are precipitated from solutions by many
reagents, such as alcohol, formalin, quinone, metaphosphoric acid,
tannins, and many salt solutions, _e.g._ those of aluminium, chromium
and iron, and of mercuric chloride, zinc sulphate, ammonium sulphate,
potassium carbonate, acidified brine. Many of these precipitations
have analogies in leather manufacture (see Parts I. to IV.). The
gelatin peptones or gelatoses are formed by hydrolysis with acids,
alkalies, ferment or even by digestion with hot water only. A more
detailed description of the properties of gelatine is given in Part
V., Section I. Gelatine is sometimes called "glutin" and "ossein."
Animals are much the most important source of proteins, especially of
those which are of importance in industrial chemistry. Proteins occur
in nearly every part of all animals, and the "protoplasm" of the
living cell is itself a protein. The keratins include the horny
tissues of animals: the epidermis proper, the hair, horns, hoofs,
nails, claws, the sebaceous and sudoriferous glands and ducts, and
also the elastic fibres. The gelatins are obtained from the collagen
of the skin fibres, the bones, tendons, ligaments, cartilages, etc.
Fish bladders yield a strong gelatin. The albumins are obtained from
the ova, blood, lymph, muscles and other internal organs of animals.
The classification of proteins herein adopted fits in well with the
scope and purpose of this volume. The keratins are of little
importance in chemical industry, but are of immense importance in
mechanical industry, _e.g._ the woollen trade, which is based upon the
keratin comprised by sheep wool. The collagen of the hide and skin
fibres is of vast importance to chemical industry, and is the basis of
the extensive leather trades discussed in Parts I. to IV. The waste
pieces of these trades, together with bones, form the raw material of
the manufacture of gelatin and glue, as discussed in Part V. The
proteids of animals' flesh and blood, milk and eggs form the source of
the food proteins discussed in Part VI. The food proteins embrace
chiefly albumins, but gelatins and even keratins are involved to some
extent.
PART I.--HIDES FOR HEAVY LEATHERS
Section I.--THE RAW MATERIAL OF HEAVY LEATHERS
The term "hide" possesses several shades of meaning. In its widest sense
it applies to the external covering of all animals, and is sometimes
used derogatively for human skin. In this wide sense, it is almost
synonymous with the term "skin." The term "hide," however, has a
narrower meaning, in which it applies only to the outer covering of the
larger animals, and in this sense is used rather in contrast with the
term "skin." Thus we speak of horse hides, cow hides, camel hides, and
buffalo hides. It is used in this sense in the title of Part I. of this
volume. As such hides are from large animals, the leather which is
manufactured therefrom is thick and in large pieces, and is therefore
commercially designated as "heavy leather." From the standpoint of
chemical industry hides are amongst the most important of animal
proteins, and their transformation into leather for boots, shoes,
belting, straps, harness, and bags comprises the "heavy leather trade,"
which is one of the largest and most vital industries of the country.
The heavy leather trade predominates over other branches of leather
manufacture, not only because of the comparatively large weight and
value of the material handled, but also because the resulting products
have a more essential utility. There is also a still narrower use of the
term "hide," in which it applies only to the domesticated cattle--the
ox, heifer, bull and cow--which use arises from the fact that the hides
of these are both the largest and most valuable portion of the raw
material of the heavy leather industries. In a very narrow sense the
term is also sometimes applied only to ox hides, which for most heavy
leathers are the ideal raw material.
=The Home Supply= of hides forms a large important proportion of
the total raw material. Its importance, moreover, is rapidly increasing,
for the excellence and abundance of the home supply determines the
extent to which it is necessary for the industry to purchase its raw
material abroad. The position of our national finances makes this an
increasingly serious matter, for hides are comparatively a very
expensive material.
The quality of our home supply of hides is very valuable, being
determined by the conditions of the animal's life, its precise breed,
and by other factors such as age and sex. The best hides are usually
obtained from animals which have been most exposed to extremes of wind
and cold, as such conditions tend naturally to develop a thicker and
more compact covering. Broadly speaking, these include the hides from
cattle of the northern and hilly districts. The age of the animal when
killed is also a dominating factor. Calf skins are very soft, fine
grained and compact, the state of rapid growth favouring the existence
of much interfibrillar substance. The youngest animals supply suitable
raw material for various light leathers (see Part II., Section V.), and
are also very suitable for chrome work (see Part III., Section III.).
Bull and cow hides, on the other hand, are from animals whose growth is
complete, and show in consequence a lack of interfibrillar substance,
coarse fibres and a rough and often wrinkled grain. The resulting
leather tends consequently to be spongy, thin, empty and non-waterproof.
Intermediate between these extremes are the hides of the ox and heifer,
large, yet of good texture, and well supplied with interfibrillar
substance. These hides are much the best for sole leather, a firm,
smooth-grained and well-filled leather being needed. The term "kip" is
often applied to small hides and to hides from large calves. In the
trade, however, "kip" is sometimes used also for larger hides, as a
verbal enhancement of value; just as a man with a few old fowls is said
to keep "chickens." Cow hides tend to be "spready," _i.e._ to have a
large area per unit weight, and are therefore more suitable for dressing
leather. Bull hides are thicker in the neck and belly, and thinner in
the back, which characteristics reduce their commercial value.
Market hides are sold by weight, and are therefore classified chiefly by
their weight, which is marked on near the tail by a system of
knife-cuts. The animals are flayed after cutting the hide down the belly
and on the inside of the legs.
Of the various breeds, "Shorthorns" yield a large supply of useful
hides. The name, however, covers a variety of similar breeds, and the
hides therefrom are rather variable in texture and quality. They tend to
be greasy owing to high feeding. The "Herefords," obtained from Midland
markets, are generally excellent hides for sole and harness leathers.
They give a good yield of butt pelt, a stout and smooth shoulder, and
are not often greasy. "Devons" yield a good-textured and well-grown
hide, but are often badly warbled. The "Sussex" cross breeds yield
somewhat larger hides. "Suffolk Red Polls," common in East Anglia, yield
a good butt, and the cow hides make good dressing leather. "Channel
Island" cattle yield very thin hides, but with a fine undamaged grain.
Scotch hides possess deservedly the very highest reputation. The
climatic conditions favour the production of a hardy race of cattle with
thick well-grown hides, yielding a large proportion of butt. These hides
are amongst the best obtainable for heavy leather, and particularly for
sole leather. "Highlanders," "Aberdeen Angus," "Galloways" are typical
breeds, with short neck, legs and straight backs. Cross breeds are also
excellent (_e.g._ "Scotch Shorthorns"). The natural value of these hides
is further enhanced by the usual care in flaying. "Ayrshires" yield good
milch cows and consequently yield often a more spready hide. The Welsh
breeds for rather similar reasons also yield valuable hides. The Irish
"Kerrys" are small but stout, and yield hides suitable for light sole
leather. Irish cross-breeds, Shorthorns, have a rather bad reputation,
and are often ill flayed.
All the varieties of the home supply are subject to various defects,
which influence seriously their commercial value. One of these defects
is warble holes or marks, caused by the Ox Warble fly (_Hypoderma
bovis_). This is a two-winged fly about half an inch long. The larva of
this fly, the "Warble maggot," lives and thrives in the skin of cattle,
and causes a sore and swelling. The life-history of this insect is still
in dispute, but it is generally thought that the eggs are laid in the
hair on the animal's back, and the young larva eats its way through the
hide until just below the dermis, and there feeds until mature. It then
creeps out of this "warble hole," falls to the ground, pupates for a
month, after which the imago or perfect insect emerges from the
chrysalis. Hides which have been thus infected have, in consequence,
often quite a number of holes through the most valuable part of the
hide, thereby rendering it unsuitable for many kinds of leather. Even
old "warbles" which have more or less healed up are a weakness, and
warbled hides and leather fetch a decidedly lower price than undamaged.
Another of these defects is bad flaying. Clearly the hide should be as
little cut as possible, but many of our market hides are abominably
gashed and often cut right through. This, of course, often reduces
seriously the commercial value of the hide. Careless treatment after
flaying also results in another common defect, viz. taint. As the term
implies, the hide is partly putrefied, sometimes only in patches, but
sometimes also so extensively as to render the hide quite rotten and
quite incapable of being made into leather at all. Hides are of course
putrescible, and dirt, blood, dung and warm weather encourage rapid
putrefaction. As market hides are usually uncured, this defect is
constantly appearing, and is a cause of considerable loss. Other defects
are due to injuries to the animal before it is killed, _e.g._ brands,
scratches due to hedges and barbed wire, old scabs, goad and tar marks.
All these reduce the value of the hide.
All the defects in hides involve a very serious loss to the community,
and the time is rapidly approaching when their continuance is
insufferable. The loss is not usually very considerable to any
individual, though very large in the aggregate. The hide is a minor part
of the beast's value, and a somewhat damaged hide does not involve a
very serious loss to the farmer. Some with typical stupidity regard a
few warbles as "the sign of a healthy beast." These defects involve
practically no loss to the hide merchant, tanner or currier, as each
pays less for damaged material. The loss falls upon the community, and
the time is ripe for the community to insist upon the elimination of
these defects. The national resources will be for some years strained to
their uttermost, and preventable damage must be considered intolerable.
The principal defects in hides are preventable, and ought to be
prevented. The warble fly could, by a united effort, be rendered before
long practically extinct, a task which is facilitated by the fact that
it is not migrative. Bad flaying and careless treatment of hides
resulting in putrefaction are still more easily remedied. The communal
slaughter-house is long overdue from the standpoint of public health,
and would, under conditions of cleanliness and skilled workmanship and
oversight, also solve the problem of ill-flayed and tainted hides.
The question of the raw material is of first importance to the leather
trades. There was, before the commencement of the European War, a
steadily increasing scarcity of hides, causing a constant increase in
their price. This was due partly to the fact that cattle were increasing
at a less rate than the population, partly to the growth of
civilization, and more extensive use of leather in proportion to the
world's population, and partly to the constant discovery of new uses for
leather, _e.g._ for motor cars, aeronautics, etc. The question of raw
material was under these conditions serious enough. The terrific
slaughter, necessary at the same time to provide the belligerents with
food and the army with leather, is bound to result in a serious crisis
for the leather industries; and in conjunction with the country's
financial condition, will make it absolutely necessary that all care
should be taken with the raw material of one of our most important
industries. The farmer who pays no heed to the warble fly, the man who
gashes the hide in flaying and who allows the hide to putrefy, are
equally criminal with the man who throws bread crusts into the dustbin.
It is impossible to foresee, as yet, anything in the nature of a
satisfactory solution to the problem of raw material, especially in
respect to heavy leather production, for the food question will rank
first in the popular mind, and the earlier slaughter enjoined for the
more economical production of meat will scarcely tend to increase the
proportion of heavy hides.
=The Foreign Supply= of hides is also of great importance and value. In
the case of imported hides precautions to prevent putrefaction are
essential, and some method of "curing" is always used.
=Salting= the hides is one of the most satisfactory methods for
temporary preservation. The action of salt is hygroscopic, and mildly
antiseptic. Moisture is withdrawn from the hides, which are then under
conditions no longer favouring the growth of bacteria. Well-salted
hides will keep for years, especially if quite clean. A light salting
is also useful for a short preservation, and is becoming common in
hide markets and tanneries during the summer and autumn months.
Salting is a method used extensively in the United States. The "packer
hides" of the stockyards are carefully and systematically salted with
about 25 per cent. of salt and stored in cool cellars. The hides are
so piled up in heaps, that brine easily drains away. The great
disadvantage of salting is the so-called "salt stains." These stains
have been ascribed to the iron in the salt, to the iron in the blood,
to calcium sulphate in the salt, and also to chromogenic bacteria,
whose development is favoured by salting. The relative importance of
these factors is not yet satisfactorily determined, but cleanliness
and pure salt tend to eliminate the trouble.
=Drying= the hides is a less satisfactory cure. The principle is
similar, viz. removal of moisture. Dried hides are, however, much
drier than salted, and are quite hard and horny, hence the name "flint
hides." The hides also lose much weight, a considerable advantage in
reducing freight. Tropical hides are often flint-dry, and where
preservatives are expensive or unprocurable, it is often the only
practicable method of cure. Nevertheless, the method has many serious
disadvantages, and is difficult to execute. If dried too slowly the
hides putrefy partially; if too quickly they dry on the outside, and
the interior is left to putrefy. The fact that hides are of uneven
thickness, and the climate often hot, increases the difficulty, and
often results in partial destruction of the fibrous structure of the
hide. When dried, moreover, the hides are still subject to the attacks
of insect larvæ, for the prevention of which the usual sprinkling of
naphthalene or arsenic is only an imperfect remedy. This method of
cure is also a nuisance to the tanner, who has to employ labour, pits
and time in attempting to restore the hides to their original
condition, and often loses up to ten per cent. of the goods in so
doing. Dried hides are also subject to the presence of anthrax.
=Dry Salting= the hides is an excellent method of curing. As the name
implies, it combines methods of drying and salting which are used
alternatively. The method is used extensively in South America. A
modified form of it is also used for preserving the "E.I. kips," which
are cured, however, not with common salt, but with earth containing up
to 70 per cent. of sodium sulphate. Dry-salted hides are largely free
from the defects of dried hides, but of course are more trouble to the
tanner in the process of soaking (see Section II.) than the wet-salted
goods.
=Freezing= the hides is now a commercial process. On the whole the
process is satisfactory, but the expansion of water after freezing may
tend to damage the hide fibres.
Sterilizing the hides has been frequently suggested, but no method has
yet been advocated which does not interfere either with the tanning
processes or with the quality of the finished leather.
Hides from the European Continent, usually wet salted and well flayed,
exhibit much the same variable quality as the home supply, those from
highland districts tending to be thick, yet even, well grown, tight
textured and smooth grained, whilst those from lowland regions are
less satisfactory. Thus hides from the Swiss Alps and Scandinavia have
ranked high, whilst the spready Dutch cows are typical of a lowland
hide. In the hides which once came from Germany the same features
appear. Bavarian highland hides had an excellent reputation, whilst
those from Berlin, Cologne, etc., tended to be long in shank and not
well grown. French hides are often ill flayed, and Spanish and
Portuguese are often subject to scratches. Italian hides have a very
good name, being small but stout in butt.
The American supply is important. South America yields an excellent
class of hide, salted or dry-salted. They are from an excellent breed
of animals, slaughtered and flayed with every care, and efficiently
cured. A most serious defect in this class of hide is the "brand,"
which is both deep and large and in the most valuable part of the
hide. One side, however, is usually unbranded, so that each hide
yields one good "bend." These hides, _e.g._ "Frigorifics," have
recently been much more extensively tanned in Britain because of the
shortage in the home supply of market hides caused by the European
War. South America also yields good horse hides. North American hides
are usually wet-salted (_e.g._ packer hides). They are usually good.
Central America yields mostly dried hides exhibiting usual defects.
The Asiatic supply comprises the frozen China hides, which are clean
but small, with flaying of uncertain quality. There are the buffalo
hides from Asia and East Europe, which are suitable for cheap and sole
and strap leather, and also the dry-salted "E.I. kips," obtained from
a small breed of Indian cattle, and extensively made into upper
leather. The Asiatic humped cattle also provide a limited supply. The
African supply is of increasing importance. The tropical parts yield
dried hides of uncertain quality, but the more temperate parts of
South Africa yield a growing supply of good quality.
REFERENCES.
"The Manufacture of Leather" (Bennett), pp. 27-37.
"Principles of Leather Manufacture" (Procter), pp. 33-56.
"The Ox Warble or Bot Fly" (E. Ormerod).
"The Making of Leather" (Procter), pp. 2-22.
Section II.--THE PREPARATION OF PELT
Before hides are tanned it is necessary for them to pass through a
series of preparatory processes. The object of these processes is to
obtain from the hide the true hide substance in a pure and suitable
condition. Each class of leather has its own appropriate processes, the
adjustment of which largely determines the quality of the finished
article. So prominent is the influence of these preparatory methods that
the paradox "good leather is made before tanning" is in trade circles
almost a platitude. These processes, sometimes lumped together under the
general name of "Wetwork," comprise soaking, liming, beam house work and
deliming. These will be discussed in turn.
The term applied to the hide after these processes, but before tannage,
is "pelt."
=Soaking= has for its object the cleansing and softening of the hides,
chiefly by means of water. It aims at the removal of dirt, blood, dung,
and curing materials by washing. The process is usually simple, and is
much the same for all classes of leather. The ideal to be aimed at is to
restore the hide to its condition when it left the animal's back.
Cleanliness in leather manufacture is as essential at the commencement
as anywhere, for the hide is in its most putrescible state. The soluble
proteids (blood, lymph, part of dung, etc.) which always adhere to hides
encourage the rapid growth of putrefactive bacteria, and cannot be
washed away too soon. Dung is often difficult to remove, being caked on
the butt end amongst the hair. Soaking only softens it, and mechanical
removal is usually necessary. If such substances are not removed, they
go forward with the goods into the lime liquors, causing stains,
loss of hide substance, and counteracting plumping.
The detailed method and time of soaking are determined mainly by the
nature of the cure. One of the purposes of the soak liquors is to
dissolve the salt used in curing hides and to rehydrate the hide and
make it again soft and pliable. As a 10-per-cent. salt solution exerts a
solvent effect on hide substance, it is necessary soon to change the
first soak liquor of salted goods.
Market hides, which are uncured, require the least soaking, the
cleansing effect being most needed. The hides are inserted into pits
("water dykes") of water for a few hours, and the water changed once or
twice. The soaking should not be prolonged as the hides are so
putrescible, and where it is customary to leave the goods in a soak
liquor overnight, it is advantageous to add a little slaked lime to the
water before inserting the goods. This not only softens hard water, but
is mildly antiseptic and plumping, and forms a suitable introduction to
the liming proper. Each pit contains a "pack" of 30-50 hides, according
to its capacity, which varies in different tanneries from 1000 to 2000
gallons. Tainted goods, which are indicated by a characteristic white
colour on the flesh side and by loose hair, need a preliminary washing
either in a "drum," "tumbler" or in a "paddle." This ensures a rapid
change of liquor and the removal of most of the putrefactive agencies.
Bad cases may need the application of antiseptics, such as immersion in
0.1 per cent. carbolic acid; but if possible these should be avoided, as
they lengthen the time required for liming. After drumming or paddling,
tainted goods should be placed directly into a lime liquor.
Salted hides need very similar treatment to uncured hides, but the
soaking is longer, because of the dehydration caused by salting. Hence
they receive also a greater number of changes of water, three or four
usually, but often more. As much loose salt as possible should be shaken
from the hides before insertion into any liquor. The employment of drum
or paddle before pit soaking is extremely useful to effect the rapid
removal of superficial salt, and is also useful after pit soaking to
remove the last traces.
Dried and dry-salted goods need a soaking still more prolonged, up to
one week if water alone be used. With the assistance of caustic soda,
however, the process can be shortened to about two days. The first soak
liquor should consist of a 0.1 per cent. solution of caustic soda, and
after the goods have been inserted twenty-four hours, they will be
materially improved by a few hours' drumming or paddling. Another
caustic soda soak will complete the process. Sodium sulphide crystals
may replace caustic soda, but about three times the weight will be
needed. Carbonate of soda and caustic lime also are a convenient
commercial substitute for caustic soda. For 10 lbs. caustic soda, use 36
lbs. carbonate and 7 lbs. lime. Extra lime should be added in all cases
when the water is hard. Acid liquors will also soften dried and
dry-salted goods, but such processes do not fit in so well with the
subsequent liming. The use of putrid soaks and stocks may be now
considered out of date.
=Liming= follows soaking, and consists essentially in immersing the
hides for 7-10 days in milk of lime. The chief object in view is to
loosen the hair and prepare for its mechanical removal. Liming takes
place in pits, the tops of which are level with the limeyard floor.
The lime is slaked completely and mixed well with water in the pit,
being particularly well plunged just before the insertion of a pack of
goods. Saturated limewater is only a 0.13-per-cent. solution. The
goods are occasionally "handled" _i.e._ hauled out of the pit and
reinserted after plunging ("hauling" and "setting"). This is necessary
to keep the liquor saturated with lime. The hides are inserted one by
one, each being "poked down" to ensure its contact with the liquor.
The goods are invariably immersed first in a previously used lime
liquor. Most tanneries now carry this out in a systematic way, so as
to ensure regularity in the process. As the goods are large and heavy
it is less laborious to carry out the whole process in one pit. In
this "one-pit system" the goods are inserted for (say) four days in an
old used lime liquor, with occasional handling; this liquor is then
run to the drain and a new liquor made up in the same pit, into which
the goods are inserted for (say) five days. They are then hauled and
sent to the unhairers. Each pack thus gets two liquors, old and new.
A better method is the "three-pit system." In this case each pack
receives three liquors and has (say) three days in each, first an "old
lime," then a "medium lime," and finally a "new lime." This system
ensures a greater regularity of treatment, and is deservedly the most
popular method for liming hides for sole leather. After being used once
as a "new lime," a liquor then becomes a "medium lime," and after being
thus used becomes the "old lime" which receives the green hides from the
soaks. The system involves the goods being shifted twice to another pit,
which is more laborious than reinsertion into the old pit, but if the
limeyard be arranged in "sets" or "rounds" of three pits, the shift is
usually only to the adjacent pit. One special advantage of this system
is that the top hides in one pit become the bottom hides in the next
pit, and _vice versâ_. Rounds of more than three pits are sometimes
used.
Many factories have now adopted systems in which there is no handling at
all. The hides are suspended in lime liquors which are agitated by
mechanical contrivances (_e.g._ Tilston-Melbourne process), or by jets
of compressed air (_e.g._ Forsare process). The goods are soaked and
limed "mellow to fresh" by changing the liquors by means of pumps, air
ejectors, etc. Thus the hides need no labour from first being inserted
until drawn for depilation.
In liming, the whole of the epidermis as well as the hair is loosened,
and is subsequently removed in depilation. The corium or true hide
substance becomes much more swollen by imbibation of water, and when
taken out of the new lime is "plumped" to very firm jelly. This plumping
is a matter of prime importance to the tanner. The coarser fibres are
thereby split up into the finer constituent fibrils, which fact assists
very materially in obtaining a quick and complete tannage, good weight,
and a firm leather. During the liming, the natural grease of the hide is
saponified or emulsified, which prepares for its removal in scudding.
Liming is thus a complex process: the hair is loosened, the hide is
plumped, and the grease is "killed." All these results may be hastened
by the use of other alkalies in addition, and most heavy leather yards
assist the liming by adding also sodium sulphide or caustic soda or
both. Sodium sulphide is a powerful depilatant, and will alone unhair
hides easily in strong solutions even in a few hours. As in solution it
forms caustic soda by hydrolysis, it possesses also the powerful
plumping and saponifying powers characteristic of the latter. The
addition of arsenic sulphide (As{2}S{2}) (realgar) to the lime when
slaking causes the presence of calcium sulphydrate in the lime liquors
thus made. This is also a powerful depilatant, but not much used for
heavy leather.
The function of the lime in depilating is complex and has occasioned
much discussion. Its main purpose, however, is that of a partial
antiseptic. When hides putrefy, one of the first results is that the
hair is loosened. In America depilation by "sweating" is carried out
commercially by such a mild putrefaction, the lime liquor permits a
similar fermentation at a slower rate, and all tannery lime liquors are
swarming with putrefactive bacteria. Liming is thus a safer method than
sweating, which may be easily carried too far. Various workers have
isolated specific organisms--Wood a _bacillus_, Schmitz-Dumont a
_streptococcus_--but it seems highly probable that the limeyard bacteria
are just the common organisms of putrefaction sorted out or selected by
the exact nature of the liquor and the method of working the limes. Many
putrefactive bacteria are very adaptable and could easily accommodate
themselves in this way. It is known that the exact nature of the
culture medium has a great influence on the rate of development of such
organisms, and which particular species thrive and obtain predominance
in any limeyard will depend upon the amount and nature of the dissolved
organic matter available as food, and upon the exact alkalinity and the
concentration of other apparently inert substances, such as common salt
and sodium, calcium and arsenic salts. Hence no two lime liquors operate
alike, and approximate regularity is only assured by systematic method.
In handling and shifting, the organisms are subjected to further
selection, and the most adaptable survive. It is probable that different
species may act symbiotically. The depilating organisms of lime liquors
are probably mostly anærobes, but some may be anærobic by adaptation. It
is probable that ærobic ferments commence the depilation, but this will
be done before the goods are put into work, or at any rate before they
reach the limes. More strictly, it is the enzymes secreted by bacteria
which are directly responsible for the hydrolytic work; these enzymes
are chiefly proteolytic (proteid splitting), but the lipolytic (fat
splitting) enzymes have also a place.
The lime, however, not only limits and selects the course of the
putrefaction, but also affords more positive assistance. Lime plays its
own hydrolytic part and assists the depilation by purely chemical
action. Lime will unhair without the assistance of bacteria, but its
action is slow and forms a minor part of the operation in the average
limeyard. This action is due chiefly to its progressive formation of
calcium sulphydrate from the cystine group of the softer keratins. Lime
also plays an essential part in assisting the putrefactive fermentation.
It softens the keratins and thus assists the bacterial attack, it
hydrolyzes other proteids and provides the bacteria with food in
solution, the calcium ion increases the proteolytic action of certain
enzymes, and finally the apparently inert excess of undissolved lime has
an accelerating effect on the bacterial activity.
In the average limeyard these various functions are inextricably mixed
up, and it is impossible to assign any definite proportion of the total
depilatory effect to any of the factors at work. Lime alone will unhair,
bacteria alone will unhair, and sulphides will also unhair without lime
or bacteria, but in the limeyard all three agencies are at work.
Putrefactive fermentation, however, obtains a good start. Ærobic
fermentation commences with the slaughter of the animal, and the
anærobic organisms soon commence their part, and are at work in the hide
house and soaks. On entering the limes, the purely chemical hydrolytic
action of lime is added to that of the bacterial enzymes as well as the
action of lime as bacterial assistant, and the three continue to operate
side by side. Each gives rise to the formation of calcium sulphydrate,
whose own special solvent effect is superadded. If sulphydrates be
deliberately added to the liquors there is yet another factor assisting.
Speaking broadly, the bacterial enzymes have their maximum activity in
the old limes, and the chemical action of sulphydrate formed from the
keratin cystine is also at a maximum in these liquors. The chemical
action of added sulphide, and the simple hydrolytic action of calcium
hydrate have their maximum activity in the new limes. Most observers
would agree that in practice the bacteria shoulder the greater part of
the work.
From the limeyard is taken about the only waste bye-products of the
tannery, viz. the residues from the soak and lime pits. These consist
mainly of lime and chalk, with some hair and dung, and possibly a little
sulphide. The sludge possesses some value as a manure, especially if
from the soak pits on account of the greater nitrogen content. (Part
VI., Section I.)
=The Beam House Work= consists in the mechanical removal of those
parts of the hide not wanted for leather manufacture. _Unhairing_
removes the hair and the epidermis made loose in liming. The hides are
placed over a sloping "beam" with a convex surface, and the hair scraped
off with a blunt concave and double-handled knife. The hides are then
thrown into a pit of water. The hair is carefully collected, washed well
with water, preferably centrifuged, and then dried out by a current of
warm air. It forms a valuable bye-product. White hair is usually kept
separate and fetches a higher price. _Fleshing_ is the next process. The
hides are again placed over a beam, with the flesh side (_i.e._ the side
nearest the flesh) uppermost. Skilled workmen then cut off, with a sharp
convex knife, the fat, flesh and connective tissue left in flaying.
_Rounding_ is usually the next process. The unhaired and fleshed hide is
spread out flat and cut up into butt, shoulder and a pair of bellies.
These parts have different commercial values, and may afterwards be
tanned by different methods for very different purposes--for dressing
leather, and sometimes even for sole leather. _Scudding_ is the last
piece of beam work. The fleshed hides (whether rounded or not) are
washed, or at least rinsed, with water, and again placed on the beam
grain side up. They are then scraped with a rather sharp concave knife,
to remove "scud," which consists of hair roots and sheaths, lime soaps,
fat, pigment and other dirt. Short hair is shaved off by a very sharp
hand knife.
The beam work demands a certain amount of skill from the workmen,
especially from the flesher, whose sharp knife may prove very wasteful
in incompetent hands. Hand labour was slowly but surely being replaced
by machinery before the war, and war-time conditions have greatly
accelerated the rate of transition. Beam house machinery is rapidly
becoming universal. The machines are cumbrous and expensive in cost and
in power, but machine work is quicker, less laborious, and needs much
fewer workmen. Many types of machine have been suggested, but the most
useful are those in which the hides pass over rollers and are
simultaneously acted upon by a rapidly revolving cylindrical knife with
spiral blades, one half being a left-handed and the other a right-handed
spiral, so that the hide is scraped outwards as well as in the direction
of motion. The part of the hide being acted upon rests on a pneumatic
roller. By changing the type of spiral knife cylinder the machine will
unhair, flesh or scud.
=Deliming= is a general name covering a number of similar operations
whose primary object is the neutralization and removal of the caustic
lime and soda in the plumped pelt, or at any rate on the surface of the
hide. This is a preparation for the tan liquors. All the tannins and
many associated substances darken rapidly with oxidation when in
alkaline solution, so that to place the fully limed hide in a tan liquor
would give a dark-coloured leather. A short insertion in a bath of weak
acid would secure the elimination of surface lime and the disappearance
of this difficulty, but there are other purposes in deliming. The more
completely lime is removed the more the plumped pelt "falls" into a
soft, pliable, unswollen and relaxed condition, and this change assists
very materially in the production of a soft dressing leather, suitable
for boot uppers, bags, etc. For such leathers, therefore, the deliming
must be much more complete than for sole leather, in which the object is
to obtain a firm and plump leather.
In the case of the softer dressing leathers, experience indicates the
advisability of allowing some further bacterial action on the
interfibrillar substance in order to produce the requisite pliability
and softness. This is secured by "bating" the hides. This process
consists in immersing the goods into a cold fermenting infusion of hen
or pigeon dung. The infusion is made in a special tub or pit with warm
water and allowed to stand for a day or two until the fermentation has
commenced, and then run into the bating pit through a coarse filter such
as sacking. The hides are immersed for some days, but are handled
frequently to ensure an even effect. The bate is always slightly
alkaline. The caustic alkalinity increases rapidly at first owing to the
diffusion of caustic lime, then at a slower rate, afterwards slowly
declining. This is explained by the production of organic acids, and
their salts with weak bases from the dung infusion by the action of
bacteria. The total alkalinity of the bate liquor increases rapidly at
first owing to the diffusion of lime and its liberation of organic
bases, then very slowly, but towards the end of the operation the total
alkalinity increases very rapidly indeed, owing probably to the
commencement of a violent anærobic fermentation which produces ammonia
and other organic bases, and which heralds the approach of a
putrefactive action, which if allowed to continue for even a short time
will ruin the hides. Bating is consequently a risky process, and needs
experienced oversight. For goods which need only a mild bating, there is
the alternative of giving a longer liming in older limes. This of course
involves more bacterial hydrolysis, and perhaps does it in a safer, more
economical and certainly in a less offensive manner. Bating is often
followed by a further deliming by acids. Boric, lactic, acetic, formic
and butyric acids are all used, and with care even hydrochloric and
sulphuric acids may be employed. Innumerable "artificial" bates have
been put on the market, but most are merely weak acids, acid salts or
salts of strong acids with weak bases. An American "bacterial bate"
consists of a lactic fermentation of glucose in the presence of glue.
Closely similar to bating is "puering," investigated by Wood.
Drenching is another fermentive deliming process. In this the goods are
inserted into an infusion of bran. This is made by scalding the bran
with hot water, and allowing it to stand until it is about 70°-90° F.
The infusion is then "inoculated" with a few gallons of old drench
liquor, and the goods are immersed. This fermentation has been examined
carefully by J. T. Wood. First the enzyme cerealin converts bran starch
into glucose, which is then fermented by the drench bacteria with the
production of lactic acid, some acetic acid and small amounts of formic
and butyric acids. The butyric fermentation is liable to become too
violent. These acids, as they are formed, neutralize the lime in the
hides and plump the pelt slightly.
Various gases (carbon dioxide, hydrogen, nitrogen, methane and
sulphuretted hydrogen) are involved, and the proportion produced in the
pelt itself has a peculiar opening effect on the hide fibres. The
activity of the drench can be decreased by dilution and by using a less
starchy bran, and can be increased by adding pea meal or rye meal.
Drenching usually follows bating. Scudding sometimes follows deliming.
The theory of the volume and elasticity changes of pelt during
preparation will be better understood after considering the behaviour of
gelatine gels. The determining factors are the _nett_
charge of hydroxyl ions on the disperse phase, resulting from ionic
adsorptions, and the lyotrope influence of dissolved substances on the
continuous phase.
In softening dried hides the swelling may be due to either influence,
but the latter tends to loss of hide substance and the production of
soft leather.
In liming, the nett adsorption of hydroxyl ions is the principal factor,
but the lyotrope influence of the alkali cations and of the impurities
is important. Plump pelts are those in which the contained water is in a
relatively greater average state of compression. Few substances can
assist plumping, but many can hinder it. In plumping all lyotrope
influence is objectionable, and "sharp" (pure) alkali solutions are
required. Mellow limes reduce elasticity and plumpness by lyotrope
influence.
In bating and puering the essential change is that before the process
the swelling is due chiefly to adsorption of hydroxyl ions, whereas
afterwards it is due chiefly to a composite lyotrope influence.
REFERENCES.
"Principles of Leather Manufacture," Procter, pp. 108-184.
"The Manufacture of Leather," Bennett, pp. 49-113.
"Lyotrope Influence and Adsorption in the Theory of Wetwork,"
Bennett, _J.S.L.T.C._, 1920, pp. 75-86.
"Analytical Examination of Bating," Bennett, _Leather Trades
Review_, 1911, p. 972, and 1912, p. 28.
"The Bating, Puering and Drenching of Skins," by J. T. Wood.
SECTION III.--VEGETABLE TANNAGE
All tannages have for their object the conversion of the readily
putrescible hide tissue of the corium (the pelt) into an
imputrescible, insoluble and permanent material called "leather"
which, possessing considerable strength and pliability, is capable of
application to a variety of useful purposes. The conditions necessary
for this transformation have been clearly stated by Procter.[1] For
the production of leather from pelt "it is not only necessary to dry
the fibres in a separate and non-adherent condition, but so to coat
them or alter their chemical character that they are no longer capable
of being swelled or rendered sticky by water." Whatever substance will
secure this permanent dehydration of the hide fibres in a separate
condition is called a "tanning material." The change from pelt to
leather is known as "tannage," the process is termed "tanning," and
those who undertake it are "tanners."
[Footnote 1: "Principles of Leather Manufacture," p. 184.]
In "vegetable tannage" the tanning materials are of vegetable origin,
and contain a group of organic compounds called "tannins" which are
extracted by the infusion of these materials with water. Pelt, when
immersed in these infusions, is converted into leather, rather slowly;
but a gelatin solution gives an immediate precipitate of "amorphous
leather," even if the tannin infusion be exceedingly dilute. The
tannins are aromatic compounds of phenolic character, and contain
carbon, hydrogen and oxygen only, but our knowledge of their chemical
constitution is exceedingly small owing to their instability and
colloid nature, which make impossible their preparation in a pure
state. They are all, however, derived from either catechol or
pyrogallol, and yield these substances if carefully heated to about
200° C. The tannins are soluble in water, alcohol, acetone, ethyl
acetate and acetic acid, but insoluble in benzene, chloroform, carbon
disulphide, petroleum ether,
dilute sulphuric acid and _pure_ ethyl ether. The aqueous infusions of
the tannins are in reality colloidal solutions; _i.e._ heterogeneous
systems of two phases. The systems are lyophile, or, more particularly,
hydrophile, _i.e._ there is an affinity between the two phases. As usual
with lyophile systems the two phases may be considered as both liquid,
and an aqueous infusion of tannin forms an emulsoid sol, which therefore
is subject to the phenomenon of adsorption. The tannins are all
precipitated by solutions of basic lead acetate and copper acetate, and
many of them with varying completeness by solutions of many other
metallic salts and hydroxides, of basic dyestuffs and of alkaloids. They
give dark colorations with ferric salts.
The tannins are widely distributed in plant-life, but only in a limited
number of cases do the plants contain sufficient tannin to render them
of commercial importance. Tannin is found in all parts of plants, but
usually in greatest amount in the bark or fruit. The tannins are
classified into "pyrogallol tans" and "catechol tans," according to the
parent phenol. This classification is confirmed by their chemical,
analytical and practical behaviour, and the vegetable tanning materials
may be classified into the same two groups, for, although even the same
plant contains both pyrogallol and catechol tans, it is usual to find in
any one part of the plant that one group is predominant.
=Pyrogallol tans=, which are oftenest obtained from fruit or leaves,
contain usually about 52 per cent. of carbon. Used alone they produce a
rather soft and porous leather. Associated with them--in many cases
probably as decomposition products--are certain other substances of
well-known properties and constitution. These substances are not only
typical of the group, but also form the most valuable clue to the
chemical constitution of the group and the key to their chemical
behaviour. One of these substances is gallic acid (3:4:5
trihydroxy-benzoic acid C{6}H{2}(OH){3}COOH), which possesses
properties very similar to the tannins, but does not precipitate gelatin
and will not itself make leather. Another of these substances is ellagic
acid C{14}H{6}O{8}, a double lactone of a
hexa-hydroxy-diphenyldicarboxylic acid. This is deposited as an
insoluble yellow powder from infusions of many pyrogallol tans, by
boiling with dilute acids only, allowing them to stand for a few days.
In practice the deposit is found as mud at the bottom of the tan pits,
and also upon the leather, to which it strongly adheres. It is
technically known as "bloom." It is insoluble in acids and cold alcohol,
but soluble in alkalies. It is a feeble dye-stuff. The pyrogallol tans
yield very different amounts of bloom. Other associated substances are
the sugars. In practice these sugars ferment to lactic, acetic, and
other acids which cause "sour" liquors. Such liquors plump the hides and
tend to give firm, thick leather. These acids also probably cause
increase of adsorption of tannin by the hide and therefore assist in
giving "good weight." Solutions of pyrogallol tans all give a blue-black
colour with a dilute solution of ferric alum. If a solution of sodium
arsenate be added to an infusion of pyrogallol tan diluted until no
longer distinctly coloured, and the mixture allowed to stand for about
two hours, a green colour develops at the surface of the liquid. The
reaction is due to gallic acid or a similar grouping, and is, in the
author's experience, the most satisfactory qualitative test for the
group. Another test is to mix equal volumes of a 0.4 per cent. infusion
of tan and a 10 per cent. solution of sodium bisulphite; a few drops of
10 per cent. potassium chromate are added, and either a transient
blood-red colour or a more permanent deep purple is obtained. The former
colour is due to gallic acid. If a tannin infusion be largely diluted
with hard water and a little iodine solution added, the pyrogallol tans
yield either a purple-red or a dark blue colour, the former being a
reaction of gallic acid. Pyrogallol tans yield no precipitate with
bromine water. They yield a yellow or brown colour when one drop of
infusion is added to concentrated sulphuric acid.
Myrabolans is one of the most important of the pyrogallol tanning
materials. It is a name given to the dried fruit of _Terminalia chebula_
and other species of Indian trees. The nuts resemble an elongated
walnut. They are dried and exported from many parts of India to all
parts of the world, but largely to this country. The varieties of
commerce are named according to origin and quality: thus we have "J1's,"
_i.e._ Jubbelpore, No. 1 quality, "R1's" (Rajpore, No. 1),
"B1's" (Bhimley, No. 1), etc. The little difference in tannin strength
(about 32 per cent.) in these varieties is usually compensated by
corresponding differences in price. The quality of myrabolans cannot be
safely judged by appearance. Much bloom is deposited by myrabolans
liquors, especially by "J's." Myrabolans are amongst the most sugary of
tanning materials, containing up to 5-1/2 per cent. It is therefore one
of the best materials for giving a plump leather. Broadly speaking,
those varieties which yield most sugar yield least bloom, and _vice
versâ_. Myrabolans tannin has a small affinity for hide substance and
penetrates the hide very slowly. It gives a "mellow" tannage, but a
bright, good colour, which characteristics are imparted to the leather
when the material is blended with other materials containing dark or
astringent tannins. When used alone it yields a rather spongy leather,
and it is not considered a good weight-giving material, though its
acid-producing powers are very helpful to other more astringent tannins.
Valonia has been the other staple tanning material of the heavy leather
trade. It is the acorn cup of oaks common in Asia Minor and Greece,
chiefly the Turkish oak (_Quercus ægilops_). The fruit is gathered when
ripe and dried in layers of about one foot deep until the acorn drops
out, Smyrna is the great export centre. Greek valonia is obtained from
many parts of the Archipelago and mainland. It is gathered in a more
immature condition and includes the acorn. It is considered slightly
inferior in strength and colour to the Smyrna valonia. The exterior of
the acorn cup is covered with rather scaly protuberances known as
"beard," which contains usually about 40 per cent. of tannin. The cup
alone contains usually about 25 per cent. tannin, and the whole about 30
per cent. The valonia tannin has been thought to contain two chemical
individuals, only one of which produces bloom. Parker and Leach[2] found
that the tannin of the cup produces more bloom than that of the beard,
and that Smyrna valonia yields more bloom than Greek. The more bloom is
deposited, the less acid will be produced. Under all conditions the
yield of bloom is large, and its deposition in and on the leather
assists materially in giving the weight and water-resisting powers
associated with sole leather which has been largely tanned with valonia.
The valonia tannins have only a moderate affinity for hide, which, like
myrabolans, they penetrate very slowly. When used alone the leather is
less yellow than that from myrabolans, and is also duller. After most of
its bloom has been deposited valonia makes a very suitable tannage for
dressing leather, and in conjunction with gambier has been largely thus
used. Since the outbreak of war the Turkish product has, of course, not
been available for importation.
[Footnote 2: _J.S.S.I._, 1903, 1184.]
Sumach[3] is the other pyrogallol tan of commercial importance. It
consists of the leaves and small twigs of the Sicilian sumach (_Rhus
coriaria_) cultivated in Italy extensively for export. The leaves are
hand picked, dried and often ground to powder. It contains 26-28 per
cent. of a tannin which yields little or no bloom, but much gallic acid.
It is an unstable tannin, and its infusion rapidly ferments. Sumach is a
very valuable tanning material, and when used alone gives an exceedingly
durable leather of excellent light colour. It gives a soft mellow
tannage, and is therefore most suitable for light leather tanning, and
is extensively used for this purpose. It is used, nevertheless, in large
quantities by the heavy leather tanners for finishing purposes, for it
contains some organic reducing agent which exerts a powerful bleaching
action on other tannages, and which assists to brighten as well as
lighten the rather dull appearance of leathers largely tanned with
valonia. It is rather an expensive tannin, but most manufacturers find
that its results are worth its cost.
[Footnote 3: Also spelt Sumac and Shumac, and always pronounced like the
latter.]
Other pyrogallol tans are also used to a limited extent. Algarobilla and
divi-divi are the fruit pods of several species of American _cæsalpina_.
They are strong in tan (45 per cent.) and yield a light-coloured and
bright leather, but are unstable tans, yielding much bloom. Babla is a
small pod yielding a mellow tannage and much gallic acid. Celavinia is
another pod containing no colouring matter and giving an almost white
leather. The tannin is closely similar to that of oak galls. These last
were once extensively used for tanning in Austria. Willow bark is used
for tanning in Russia and Denmark. Valuable pyrogallol tannins are
obtained from oak wood and chestnut wood, but the woods are not used in
tanning as the percentage of tan is so small.
=Catechol tans=, often obtained from barks, contain usually about 60 per
cent. of carbon. They are seldom used alone, for they usually have
little or no sugar associated, and hence their liquors do not either
"sour" or "plump." They can be used alone if artificially acidified, but
without acidifying or blending would give a rather flat leather, though
possibly firm. They yield no bloom or gallic acid, but have associated
with this other characteristic substances. Of these the catechins are
the most typical, and have been considered as the parent substances of
the catechol tans.
The catechins are white crystalline substances, apparently isomers with
the general formula C{15}H{14}O{6}. They have different
melting-points, and varying amounts of water of crystallization, but are
otherwise exceedingly similar in properties. They are sparingly soluble
in cold water, but freely in hot, and in alcohol and ether. They are
precipitated by lead acetate, mercuric chloride and albumin, but not by
gelatin, tartar emetic or alkaloids. In gambier liquors they are
especially strong, and sometimes crystallize on the side of the pits,
being thus known as "whites." The phlobaphenes or "reds" are also
typical of catechol tans from which grow catechins; they can be formed
by boiling with dilute mineral acids. They are considered to be
anhydrides of the catechol tans. They are difficultly soluble in cold
water, but freely in hot, and in cold alcohol and dilute alkalies. They
are true tannins and alone are capable of making a red leather, but in
practice are often found as mud in the tan liquors owing to their
limited solubility. They naturally influence the colour of leather made
with catechol tans, which is usually distinctly redder than the leather
made from pyrogallol tans. Infusions of catechol (_cp._ catechin) give a
green-black colour with iron alum. The sodium arsenate test gives a red
colour due to catechin. The chromate and iodine tests mentioned for
pyrogallol tans give negative results with the catechol tans, but
bromine water gives a precipitate, and sulphuric acid a crimson colour.
Mimosa bark is one of the most important catechol tans. It is usually
obtained in this country from Natal ("Natal bark"); but the tree (Sydney
green wattle, _Acacia mollissima_) is a native of Australia. It is being
cultivated now extensively in South Africa, and forms a most valuable
portion of the Empire's stock of tanning material. Its more extensive
use has been long recommended by the author,[4] but its gradually
increasing employment in British tanneries has been greatly accelerated
by the war, which has prevented its delivery in Germany and has cut off
Turkish valonia from Britain. It yields about 30 per cent. of a stable
and excellent tannin, and will produce a firm, durable leather, with a
colour much less red than that obtained from many other catechol tans.
It is an astringent tan, and if carelessly used yields a harsh or even
"drawn" grain. Most of the tannin is easily extracted, yielding a clear
infusion which penetrates fairly quickly and gives good weight. It
contains less than 1 per cent. of sugar, which unfortunately rapidly
ferments to carbonic acid, so that it is not a good plumping material.
It makes in all respects an excellent blend with myrabolans. Like all
catechol tans, the resulting leather darkens on exposure to sunlight.
[Footnote 4: J.S.C.I., 1908, 1193.]
Oak bark, from _Quercus robur_, is the ancient tanning material of
Britain, and is still used to a limited extent. It contains about 13 per
cent. of tannin and is mainly a catechol tan, but also contains a
pyrogallol derivative. It yields catechin, and gives a red colour with
the sodium arsenate test, but also will yield some bloom and gallic
acid, and gives a blue-black with ferric salts. The tannin itself is
exceedingly similar to that of mimosa bark, but the material contains
about 2-1/2 per cent. of sugar, which makes it possible to employ oak
bark alone for making sole leather. It is noted for yielding a sound,
durable leather of good typical tan colour. Its tannin combines well
with hide and penetrates quickly. The fatal disadvantage of oak bark is
its weakness in tannin strength compared with other materials. This
results in heavy freight and heavy cost per unit tannin, bulky storage,
expensive handling in the factory, comparatively large bulk of spent
tan, after relatively greater trouble in extracting, and the
impossibility of making the strong liquors so necessary in these days to
produce good weight in a short time. No satisfactory extract has yet
been made from it.
Pine bark, from _Pinas abies_, is one of the staple materials of the
Continent. It contains up to 14 per cent. of a catechol tan, and, unlike
most of this group, contains a high proportion of sugar and will give
good results alone. Hemlock bark has been the staple tanning material of
North America. It is obtained from the hemlock, or _Pinus canadensis_.
It contains up to 11 per cent. of tan and much phlobaphene, and yields a
characteristic red leather of good quality, but which rapidly darkens
with sunlight. It contains some sugar, but is usually employed in
conjunction with sulphuric acid or with sugary materials. Mallet bark
yields another catechol tan similar to that of mimosa, but somewhat less
astringent and more yellow in colour. Quebracho wood and mangrove bark
have been used, but are now made into extracts.
=Leaching.=--Whatever class of leather is being made, and whatever blend
of tanning materials is being employed, the tannins must be efficiently
extracted by water in order to make the tanning liquors. This process is
called "leaching." The tanning materials, after being ground, crushed or
shredded, are placed in large pits arranged in "rounds," "sets," or
"batteries" of 6, 8 or 10 units, through which water is percolated
systematically, so as to secure a continuous extraction. Water itself is
added to only one of the pits of material. The liquor produced is passed
on to the next pit, and then to the next, and is continually gathering
strength. After passing thus through the series, the liquor becomes the
source of the strong extracted tan liquors which are used in the tannery
proper. With this system the stronger leach liquors are being acted upon
by fresh material, and the nearly "spent" material is being acted on by
the weakest liquors, and finally by water, thus ensuring a complete
extraction. In the press leach system, which is now practically
universal, the bottom of one pit communicates with the top of the next,
and the liquor presses round by gravity flow caused by a few inches
"fall." Liquor is thus constantly percolating downward through the
material in each pit. The "head leach" and "tail leach" are always
adjacent in a double row of pits, and when the material in the latter is
quite spent, it is "cast," and the pit is filled with fresh material.
The liquor is then pressed round into this pit by adding water to the
tail leach. Hot water is used to secure better diffusion. At least two
such sets of leaches ("taps" and "spenders") are necessary to spend the
material of the average tannery and to obtain liquors of the necessary
strength.
=The Manufacture of Extracts.=--In addition to the use of the natural
tanning materials described above, modern leather manufacturers employ
also a variety of "tanning extracts," _i.e._ vegetable tanning materials
in which the tannin has been already extracted, and which are supplied
in form of a solid or concentrated liquid. Such extracts only need to be
dissolved in warm water in order to make a tan liquor, and the cost and
trouble of leaching is avoided. They are a great convenience as making
strong liquors of definite strength. Many vegetable tanning materials
are too weak in tan for the tanner to leach, and indeed to justify the
cost of importation have been made available by manufacturing an extract
at the source of the material. With such weak materials the extract
manufacturer has had to secure a much more complete extraction than in
ordinary leaching, and to concentrate his infusions by means of
steam-heated vacuum pans. With such experience he has naturally begun to
make extracts also from the stronger materials, such as myrabolans and
mimosa bark, and it is now possible to have a tannery without any
leaches at all. Tanners also have begun to realize the advantages not
only of more rapid and complete extraction, but also of doing the work
for themselves, and extract factories are beginning to appear as an
adjunct to the larger tanneries. The more complete extraction of tan
also involves a greater extraction of unwanted colouring matters, hence
decolorization is a feature of extract manufacture.
=Chestnut Extract= is from the wood of the Spanish chestnut (_Castanea
vesca_), which contains 3-6 per cent. of a valuable pyrogallol tan very
similar to that of valonia. Its weight-giving and water-resisting powers
are as good as valonia, and its penetrating power is even better, so
that it forms an exceedingly suitable material for the modern short
tannage, and also for drum tannages. The extract is manufactured
extensively in France. The wood is stripped of bark and usually piled
for some months to dry and to allow the resins to become insoluble. Some
factories, however, use the green wood direct. There are two methods of
extraction, viz. in open vats and in closed vats under pressure. The two
methods yield extracts which differ in composition and properties. In
either case the vats have a capacity of up to 3,000 gallons, and hold up
to 6-1/2 tons of wood. They are arranged in series, as in leaching, and
the liquor passes in succession through all the vats over wood less and
less spent. The temperature is highest in the vat containing the fresh
water and nearly spent wood. In open vats of wood or copper the
temperature is near boiling-point, whilst in the closed autoclaves
(copper or bronze) the pressure reaches about two atmospheres and the
temperature about 130° C. (266° F.). The series may contain 5, 7, 9 or
even 12 vats, and the liquor obtained has a strength of 3° to 4-1/2°
Beaumé (22° to 33° Bkr.).
After extraction the liquor is allowed to stand, and much insoluble
matter settles out--resins, wood, fibre, etc. The clarified and settled
liquor is then passed through a cooler up to about 55° C., and then run
into the decolorizing plant, a deep vat fitted with a copper steam coil
and mechanical stirrer attached to power. The best decolorizer is
bullock's blood, which is run into the vat and well mixed. The
temperature is next raised to about 70° C., causing the blood albumin to
coagulate. It carries down with it a little tannin, but much colouring
matter. After standing a few hours the settled liquor is run off direct
to the evaporator. A multiple-effect evaporator is usually employed, and
the concentrated liquor, which has a strength of about 25° Beaumé, is
run into suitable oak casks. The extracts contain 27-32 per cent.
tannin. An extract made with open vats has about 7 per cent. soluble
non-tanning matters, whilst a "pressure extract" may contain up to 12
per cent. of these "non-tans." Pressure extracts obtain also a better
yield of tannin, which more than compensates for the slightly lower
price. Open extraction yields, however, the purer product and an extract
with better penetrating powers, and is consequently the more suitable
for drum tannages. Chestnut extract is extensively used by the heavy
leather tanners.
=Oakwood Extract= is manufactured from the wood of the common oak
(_Quercus robur_). The centre of the industry has been the oak forest of
Slavonia. The wood contains 2-4 per cent. of a tannin very similar to
that of chestnut wood, but somewhat more astringent.
The manufacture is also similar to that of chestnut extract, but
decolorization is often omitted, and greater care has to be taken and in
other ways to keep the colour within limits. One of these is to strip
the wood more completely of bark. Another is to operate at as low a
temperature as possible, about 110° C. The extraction is made in large
circular vats about 14 feet high and holding about two tons material. A
battery is composed of about eight vats or extractors. Open extraction
is used, and the liquor is passed forward after 2-3 hours' boiling, so
that the material is spent in about 24 hours. A liquor of about 5° Bé
(36 Bkr.) is obtained, and the strength of the material reduced from 4
to 1/4 per cent. of tannin. Getting rid of insoluble matter is a
difficulty, and is attained by settling, by rapidly cooling, and then
passing through a filter press of wood. For evaporation a double-effect
vacuum pan is preferred, which operates first at about 113° F., and
afterwards at 140° F. with a higher vacuum. The liquor is concentrated
from 5° to 25° Beaumé (s.g. 1.036 and 1.210 respectively).
The extract has a much higher colour than chestnut, and is not used now
as much as some years ago. As the principal supply was German, it has
been unavailable.
=Quebracho Extract= is made from the wood of the South American tree
_Loxopteryngium Lorenzii_, which contains about 20 per cent. of a
typical catechol tan. It is associated with a little catechin, much
phlobaphene, but practically no sugar. The tannin is very astringent,
penetrates quickly and gives a firm red leather which darkens on
exposure to light. It is not noted for weight-giving powers. The wood
itself, as chips or shavings, has been used in British tanneries, to a
limited extent, but the great bulk of the material is made into extract
chiefly in South America. The crude "extract," made by evaporating
aqueous infusions of the wood, is largely exported for refinement in
Europe. It is also refined on the spot to a large extent and converted
into solid extract containing 60 per cent. of tannin.
The great difficulty with quebracho has been the disposal of the
phlobaphenes, and a great variety of quebracho extracts are now
available which deal with this problem in different ways. In some the
more soluble reds are simply left in the extract under the idea that
they are really tannins and may be of some use in some part of the
tanning process; in others they have been removed by settling and
filtration at appropriate temperatures and concentrations; in most,
however, they have been solubilized by treatment with alkalies, in the
presence of reducing agents, notably by heating with sodium bisulphite
in closed vats. The base combines with the phlobaphenes, which are made
completely soluble and available for tanning. Sulphurous acid is
evolved, and its reducing powers assist materially in retaining and
promoting a good colour in the product. Such "sulphited extracts" are
now extensively manufactured in this country from the imported "crude"
extract, and sold as liquid extracts containing 30, 35 or 40 per cent.
of tan according to the requirements of the buyer; "mixed extracts"
which are solubilized quebracho blended with about 15 per cent. of
myrabolans, are also used.
By solubilizing quebracho with excess of bisulphite an extract is
obtained which possesses considerable bleaching powers, and such
extracts are also extensively manufactured for the "vatting" or
bleaching of heavy leather after tannage. The excess of sulphurous acid
not only bleaches the leather, but also swells it up and thus permits a
further absorption of strong tan liquor, which is conducive to good
weight. These bleaching extracts are usually of 36-38 per cent. strength
in tan.
=Gambier= is an extract of the leaves and twigs of the eastern shrub
_Nauclea gambir_. It is a catechol tan of peculiarly mellow quality and
great practical value. It contains much catechin, but little
phlobaphene, and yields a beautifully soft leather, but without weight.
It is an exceedingly suitable material for the early stages of tanning,
and is much liked for tanning leathers that have to be curried, and is
widely used in the manufacture of upper leather. It is, however, an
exceedingly expensive tannin, and the extract is made in a very crude
way by Chinese and Malays without much supervision. Hence its strength
in tan and general quality is extremely variable. The plant is
cultivated for the purpose of extract manufacture, and prunings are
taken in the plant's third year. They are bruised and boiled with water
in the open. The infusion is strained, concentrated, and poured into
cooling vessels in which it sets to a paste. Two varieties of gambier
are well known, "cube gambier" and "block gambier." In the latter the
extract remains as a paste containing 25 to 40 per cent. of tannin. It
is sold in oblong blocks of 1 or 2 cwt., either wrapped in cocoanut
matting or in wooden boxes. Cube gambier is made by running the
concentrated syrup into trays 2 inches deep and drying in the sun. When
partly dry, it is cut up into 1-1/2-in. cubes and dried further on
cocoanut matting. The rough "cubes" as imported contain 40-50 per cent.
of tannin.
=Myrabolans Extract= is now largely manufactured in this country. A
liquid extract of 25, 30 or even 35 per cent. strength is made for
home consumption, and a solid extract for export. The light colour,
high strength and easy extraction of the natural material have all
facilitated the task of the manufacturer.
The material is extracted in open vats or stills of copper, which take
one ton or more of nuts. A battery of 4, 6 or 8 of such stills is
usually employed, and the temperature is kept well below boiling-point
except in the vats containing the nearly spent material. The liquors
move forward quickly, and the material is quite spent in 24 hours. The
material when cast contains less than 1/2 per cent. of tannin. The
liquor obtained is 40°-50° Bkr. (6 - 7-1/2 per cent. tan), and after
settling is concentrated at 40°-50° F. in a single effect vacuum pan,
which though more costly in steam is quicker than the multiple effects,
and gives the low temperature required. For solid extract the more
concentrated liquor is run direct into tarred bags, in which it soon
solidifies.
=Hemlock Extract= is manufactured from the North American pines and
imported into this country to some extent. It gives a very red colour.
=Mangrove Extract= is made from the bark of _Rhizophona Mangle_ and
other species of mangrove which grow freely in the tropical swamps of
West Africa, Borneo, etc. Much solid and liquid extract has been made
from this material, but is not very popular on account of its harsh
tannage and dark red colour.
=Pine Bark Extract= (_Larch extract_) is made in Sweden from the Norway
spruce (_Pinus abies_). It is slightly sulphited and gives a good
colour. It is a liquid extract of about 30 per cent. strength, and is
sometimes used as a chestnut substitute. It should not be confused with
the so-called "spruce" or "pine wood" extract, which is a paper trade
bye-product and contains ligneous matters rather than tannin.
=American Chestnut Extract=, made from the chestnut oak, is either a
liquid or a solid extract in powder form. It gives a wretched
brown-black colour, which is quite unsuited to the usual British needs.
THEORY OF VEGETABLE TANNAGE.
Vegetable tannage is a phenomenon of colloid chemistry. The old
arguments as to whether tanning was a chemical or a physical process
have been rendered obsolete by the advent of a new set of explanations,
which, though shedding light on many obscure points, have enormously
increased the complexity of the problem. In vegetable tannage an
emulsoid gel (pelt) is immersed in a complex emulsoid sol (tan liquor),
which immersion results, not in simple reaction or change, but in a
series of changes.
One of these changes is _adsorption_. Pelt is a gel which possesses a
great development of surface. It not only exhibits like gelatine the
phenomenon of imbibition and dehydration to a very marked extent, but
also possesses a very fine fibrous structure due to its organic origin;
thus pelt possesses an enormous specific surface, further intensified by
the preparation processes previously discussed, which split up the hide
fibres into smaller bundles and into much finer constituent fibrils.
Tannins, on the other hand, are hydrophile colloids which in water form
emulsoid sols, and which may thus be expected to exhibit the phenomenon
of adsorption. A tan liquor usually contains several tannins in addition
to other closely similar substances, also in colloidal solution, and is
therefore a sol of considerable complexity. The immersion of pelt into a
tan liquor results in an adsorption, which consists essentially in an
inequality of concentration in the sol, the greater concentration being
at the interface. This inequality between the surface concentration and
the volume concentration of the sol, is due primarily to considerations
of surface tension and surface energy, and exists before the immersion
of the pelt. The surface layer having excess over the volume
concentration, any considerable extension of surface in a fixed volume
of sol must produce a very considerable decrease in the volume
concentration. This is what occurs when pelt is immersed in a tan
liquor, the immersion being the considerable extension of surface. It
should be especially remembered that the inequality of concentration is
in the sol, on the liquid side of the interface. In adsorption, the
substance adsorbed, _i.e._ the excess at the surface, is too frequently
regarded as bound to the solid immersed. This is because the excess is
in the layer which wets the solid and remains wetting it when the solid
is removed. Thus the immersion of pelt produces primarily only a change
in the distribution of the tannins in the liquor. It follows from this
that the adsorption is an equilibrium, and that if the sol be diluted,
the equilibrium will become the same as it would have been by immersing
the pelt directly into the dilute solution. Thus, if pelt be first
immersed in one tan liquor and then into a weaker one it will yield tan
to the latter solution.
The chief object in heavy leather tanning is to obtain the maximum
possible adsorption in the minimum possible time, or in other words, to
obtain good weight quickly. The amount adsorbed is proportional to the
actual extension of surface, _i.e._ the adsorption is a function of the
specific surface of the adsorbent. Hence, to obtain good weight it is
necessary to develop in the pelt its maximum possible specific surface.
This is one of the objects of "plumping," which splits up the fibres. It
is attained also by the solution of interfibrillar substance in limes
and bates.
The amount adsorbed is also a function of the volume concentration in
the sol after equilibrium is reached. Hence the better weights are
obtained with stronger liquors.
The adsorption law is
y/m = ac^(1/n)
where y is weight adsorbed by the weight m of adsorbent, and c the
volume concentration after adsorption; a and n are numeral constants.
Hence weight is determined by the strength of the liquor which the goods
finally leave. The commencement of tannage is necessarily in weak
infusions, in order to secure the maximum diffusion into interior of the
fibres before they become heavily coated on the exterior. As the
equilibrium is being established in such liquors the volume
concentration diminishes, and thus makes it less likely that good weight
will be attained; hence it is necessary in practice to move the goods
constantly into fresh liquors of gradually increasing strength, and so
maintain the rate of adsorption and save time. A further consequence of
the adsorption isotherm is that as y varies as c^(1/n) and n is > 1, y
is increased appreciably only by a relatively large increase in c.
Hence, though stronger liquors give better weight, there is a limit
beyond which any further gain in weight is not justified by the enormous
increase in the concentration necessary to attain it. Such great
increase in c is impracticable not only on the ground of expense, but
also on account of the great viscosity of the sol.
The amount of adsorption depends also upon the exact nature of the sol.
It has been previously pointed out that the tannins differ largely in
their penetrating and weight-giving powers. Some are readily adsorbable
and are deposited in great concentration at the surface of the fibre,
but for good weight it is necessary to use also the less adsorbable and
more diffusible tans, which penetrate the fibre itself. Hence it is
necessary for good weight to use a blend of materials, and so supply
many grades of liability to adsorption. It is particularly advantageous
to blend judiciously the two main types of material, the pyrogallol and
catechol tans. It is also necessary for good weight to present to the
pelt the more diffusible and less adsorbable tannins first, in order to
secure the maximum diffusion into the interior of the fibre before the
exterior of the fibre is heavily coated with the heavily adsorbable and
astringent tans. The least adsorbable materials are therefore used in
the early stages of tanning, and the most adsorbable materials at the
end of the tanning process. Thus gambier is added to the early liquors
(suspenders), solubilized quebracho to the later liquors (handlers), and
mimosa bark extract to the final liquors (layers). There is also another
excellent way of ensuring this progressive astringency of the liquors;
this consists in leaching the required blend of materials together (or
mixing them in the case of extracts) and presenting the mixed infusion
to the nearly tanned goods, which adsorb chiefly the more astringent
tannins. The liquor is then used for goods at a less advanced stage of
tanning, which again take the most adsorbable constituents. This is
repeated until the stage is reached when the fresh pelt is inserted into
the nearly exhausted liquor, which naturally contains only the least
adsorbable substances. This system is almost universal, and in practice
is known as "working the liquors down the yard." It has the additional
advantage of being a systematic method of economically exhausting
("spending") the tan liquors. When free acid is present in the tan
liquors, it tends to distend the fibres composing the pelt by a strong
and rapid adsorption. Thus distended or plumped the fibres present a
still greater surface for adsorptive operation, but the distension
naturally leaves less space between the fibres for the diffusion of the
sol. Hence acid or "sour" tan liquors give in the long run more weight,
but tan more slowly. Pelt tanned whilst thus plumped forms naturally a
thicker and less pliable leather. This occurs in tanning sole leather,
to a less extent with heavy dressing leather, and to a very small extent
in the case of softer dressing leathers.
In addition to adsorption, there is another phenomenon of colloid
chemistry in operation, viz. the _mutual precipitation_ of the sols in
the liquid by the gels in the hide. In most sols the disperse phase is
electrically charged. The sol therefore possesses electric conductivity,
and migration occurs in the electric field to the cathode or anode
according to the nature of the charge. Oppositely charged sols
precipitate one another, the precipitate containing both colloids. The
maximum precipitation occurs when the + charge of one sol exactly equals
and neutralizes the - charge of the other. There is thus an electrical
equivalence; an amount of sol which is equivalent to a given amount of
the other. This is not a chemical equivalence, however, and the
precipitate is not a chemical compound in spite of its fairly constant
composition. The composition of the precipitate, indeed, is not quite
constant, for the optimum precipitation may not correspond exactly with
the electrical equivalence, being influenced by the number of particles
required, their size (dispersity), the rate of mixture, and the relative
concentrations of the sols. This mutual precipitation is exhibited by
emulsoids as well as suspensoids, but the charge (+ or -) on an emulsoid
is in many instances largely an accidental matter, being determined by
the medium in which it happens to be, its normal condition being
electrical neutrality. Gelatin and pelt are such emulsoids, and a
positively charged gelatin sol has been observed to precipitate a
negatively charged gelatin sol. It is thought, however, that gelatin is
primarily a positive sol. Pelt (whether delimed or not) is rapidly
acidified by the quickly penetrating and strongly adsorbed organic acids
of the old tan liquors and becomes positively charged before the tannins
are adsorbed. The positive charge increases with the acidity of the
liquor. Other emulsoids are not electrically neutral, but are
electrically charged and exhibit considerable conductivity. Into this
class fall the tannins, and in tanning it is thought that there is a
mutual precipitation of the negative tannin sol with the positive hide
gel, the precipitation of the negative sol being favoured by the acid
condition of the liquor. The effect of increasing acidity soon falls
off, however, as a saturation limit is soon reached. This mutual
precipitation of colloids in tanning is in reality but an extension of
the adsorption theory, which explains the predominant effect of H+,
and OH- on the electric charge by stating that these ions are more
readily adsorbed than other ions, and that as OH- is more readily
adsorbed than H+ most sols are negative to water.
In addition to the adsorption phenomena described, there are in
vegetable tannage _secondary changes_ which are slow and "irreversible."
These changes are obscure and are difficult to investigate. Oxidation,
dehydration and polymerization have all been suggested, but there is
little direct evidence. Certain it is, however, that time renders the
tannage more permanent. It perhaps should be pointed out that in the
very strongest tan liquors the viscosity of the tannin sol is so great
that adhesion would be a better term than adsorption. There is no abrupt
division between the two phenomena.
In the theory of vegetable tannage there is another factor the
importance of which has been strongly emphasized by the author, viz.,
_lyotrope influence_. This has been most conveniently discussed in
connection with gelatin gels, but its effect on hide gels is analogous.
It has also an effect upon the diffusion and gelation of the tannin and
non-tannin sols.
=Mechanical Operations.=--In the tanyard the liquors are almost
invariably divided up into sections, called "rounds" or "sets," in
which the mechanical operations are different in aim and method. In
the first pits entered by the goods there is rapid adsorption in spite
of the low concentration and small astringency, and the great aim is
to obtain evenness of action and a good level colour. It is also
necessary to maintain the rate of adsorption. All the aims are
attained by frequently moving the goods. Heavy leather is suspended
vertically in the pits of tan liquor and handled up and down as well
as forward from pit to pit. Such pits are termed "suspenders." In the
earliest suspenders it is indeed advantageous to have the goods in
constant motion. This is done by suspending on wooden frames which are
rocked gently by mechanical power; such pits are termed "rockers." For
dressing leather in which firmness and smooth grain are not so
essential, the goods may be paddled in the first liquors. This is
occasionally done with stronger liquors for the express purpose of
working up the "grain" pattern. The goods after passing through the
suspenders are usually passed to "handler" rounds, in which they are
moved less frequently. In these pits the goods are laid horizontally
one above the other. One advantage of handlers is that the goods
flatten thoroughly and straighten one another by their own weight;
another is that more goods can be placed in one pit than in
suspenders. They are not so convenient to work, however, as
suspenders, and the goods do not feed so rapidly. Hence the tendency
is now to tan more in suspension, and to economize labour by an
extension of the rockers. The handling of the goods is also saved by
pumping the liquors and by working rounds of suspenders or rockers
like the press leach system, with the difference that the stronger
liquor is pumped in to the head pit, and the liquor passes upwards
through the goods.
Finally the goods are placed in "layers" or "layaways," in which they
remain undisturbed for a decidedly longer time. These pits contain the
strongest liquors of the yard, and their principal function is to
complete the tannage and give weight and firmness by the adsorption of
bloom, reds, etc., in the interior of the hide. The goods are placed in
horizontally, and are dusted in between with fresh tanning material
which maintains the local strength of the liquor and keeps the goods
somewhat apart. Drum tanning attains a more rapid penetration of the
pelt by giving constant motion in stronger infusions. It is of course
liable to result in an under-tannage of the interior of the fibre. After
the goods have been "struck through" in the ordinary way, however,
drumming in extract is increasingly used as a substitute for much labour
in handling, and also to save the time spent in the early layers.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 220-350.
Bennett, "Manufacture of Leather," pp. 113-179.
Bennett, "Celavima and Babla," L.T.R., 1914, 122.
Dumesny and Noyer, "Manufacture of Tanning Extracts."
_Theory_:--
Meunier and Seyewetz, Collegium, 1908, 195.
Stiasny, Collegium, 1908, 117-159, 289, 294, 337.
Procter and Wilson, Collegium (London), 1917, 3.
Wilson, Collegium (London), 1917, 97, 100, 105.
Moeller, Collegium (London), 1917, 13, 38, 46, 103; and
_J.S.L.T.C._, 1917, 22, 56, 92.
Bennett, _J.S.L.T.C._, 1917, 130-133, 169-182; 1918, 40; 1920,
75-86; _S.L.R._, 1916, March.
SECTION IV.--FINISHING PROCESSES
After the tannage is complete, leather is hung up to dry. In the case of
heavy leather this drying must be very carefully carried out in order to
obtain a product of satisfactory appearance and saleable qualities.
Associated with the drying are many mechanical operations (scouring and
rolling) which assist very materially in imparting the desired
qualities. After tanning, however, the quality of the final product is
most strongly influenced by the amount of grease added in finishing.
Some grease is always used in finishing, partly because even sole
leather requires some measure of pliability and partly because a coating
of oil over the leather during drying prevents the loose tannin from
being drawn to the surface of the leather by capillarity, thereby
causing dark and uneven patches and a "cracky" grain. The added grease
is also a contribution to the "weight" of the finished article--a
primary consideration for heavy leather, which is usually sold by
weight. The finishing processes, indeed, tend to be dominated by this
consideration, and become a series of efforts to retain as much tannin
and add as much grease as are consistent with the requirements of the
class of leather being manufactured. Sole leather does not contain more
than about 2 per cent. grease, or its firmness is impaired. Belting
leather, in which considerable pliability is needed, may contain about 9
per cent., whilst harness leather, which must be exceedingly tough and
durable, may contain up to 13 per cent. of fatty matters. Upper
leathers, which need to be soft and pliable as well as waterproof and
durable, are very heavily "stuffed" and often contain up to 30-40 per
cent. of grease. Sole leather is thus rather distinct from the rest,
which are called "curried," "stuffed," or "dressed" leathers. The actual
drying out before, after and between the various mechanical operations,
each have an appropriate degree of wetness. In this country the drying
is usually under the prevailing atmospheric conditions and is known as
"weather drying." The goods are suspended by hooks or strings or by
laying over poles in special sheds fitted with louvre boards by which
the rate of drying can be roughly controlled. Weather drying is cheap,
but exceedingly slow, and in unfavourable weather is very unreliable.
The goods, moreover, need constant attention to obtain an even result.
Steam pipes are usually laid along the shed floors, and are used in
winter and damp weather to accelerate the drying, and also in the final
shed stove to remove the last traces of moisture. Wet weather, however,
will not stand a high temperature, and steam drying is better avoided
when possible. Air-dried leather still contains about 14 per cent. of
moisture. Many systems of shed ventilation have been suggested to hasten
the drying and to secure a better control of the process. In one system
a screw fan is fitted at one end of a shed (without louvre boards) and
sucks air through the goods from an inlet at the other end. The air can
be heated by a steam coil near the inlet. In another system a
centrifugal fan blows air through an arrangement of pipes which
distributes it to the drying sheds, and discharges it close to the floor
by various branch pipes. The outlets are near the roof. A system of
dampers permits hot air, warm air and the used wet air to be blended in
the desired proportions. In America turret drying has been used. The
sheds are vertically above one another and have latticed floors. Heated
air is admitted at the bottom and rises through the goods up the
building just as in a chimney. For many of the finishing operations it
is important to obtain the leather in a uniformly half dry or "sammed"
condition. This may be done by careful drying, and wetting back the
parts that have become too dry with tepid water or weak sumac liquor,
and then leaving the goods "in pile" until of uniform humidity. It may
also be done by "wetting back" leather which has been completely dried
out. There are also "samming machines" which by means of rollers squeeze
out the excess liquor. Sole leather is dried out and finished
immediately after tanning, but dressing leather is often "rough dried"
out of tan liquors and wet back for finishing when required. Dressing
leather is often treated in different factories; tanners selling it as
rough leather and "curriers" finishing it.
Scouring is one of the first operations in finishing leather. The grain
side is wet and worked with brushes and stones until the bloom and loose
tannin are removed. This process aims at producing a good even colour
and level surface, but is liable to cause a loss of weight. Dressing
leather is often scoured on both grain and flesh, and weak soap or borax
solutions are used to assist the process. In this operation hand labour
has been now quite superseded by machine work. A great variety of
machines have been devised. The mechanical working of leather takes
place in various parts of finishing. These operations, known as
"striking," "setting," "pinning," "jacking," may be carried out often by
the same machine as used for scouring, but with a change of tool. The
object of these operations is to get rid of wrinkles and creases, to
produce softness, pliability and area, and to remove superfluous
moisture, grease, dirt. The tools are of steel, brass, slate or
vulcanite. Scouring is often effected by putting the goods into rotating
drums together with extract and sumach. The bloom is removed by
friction, the colour is improved by the sumach, whilst the extract keeps
up the weight.
In finishing sole leather firmness is enhanced by "rolling." A brass
roller passes to and fro over the goods with the exertion of
considerable pressure. The operation is carried out by machinery.
Shaving is an important operation in the case of many dressing leathers.
Its object is to produce a uniform thickness of the leather and an even
surface on the flesh side. The sammed goods are laid over suitable beams
and shaved with special sharp knives which possess a turned edge. This
hand process, which demanded considerable skill, is fast becoming
extinct, and machine shaving is already almost universal on account of
its greater speed. The machines consist essentially of two rollers, one
of which is smooth, whilst the other is a spiral knife-blade cylinder
(cp. Section II.). The sammed goods are held in the hands and placed
over the smooth roller, which is raised to the cutting roller by a foot
treadle. A number of similar operations ("flatting," "whitening,"
"buffing") are carried out by a suitable change of tool. In all these
operations good samming is important.
Splitting is another important operation on tanned leather. In this
process the leather is cut parallel to its grain surface, thus yielding
two pieces with the same area as the original, the "grain" and the
"flesh split." It is essentially a machine operation, and is carried out
by presenting the carefully sammed leather to a sharp knife-edge,
towards which it must be constantly pressed. The "band knife" machine is
the most popular arrangement. The knife is an endless belt, which
continually revolves round two pulley wheels of equal size. In between
these the knife is horizontal, and is then used for splitting. The
sammed leather is pushed towards the blade by two feed rollers, and the
grain passes above the knife on to a small platform, whilst the flesh or
"split" passes below and falls to the ground. Emery grinders and thick
felt cleaners in the lower part of the machine keep the knife in good
condition. The adjustment of the machine is delicate and requires
considerable experience. With care splits may be obtained down to 1/16"
thick, and sometimes as many as 6 or 7 splits are obtained from one
hide.
Oiling is still usually done by hand, and cod oil is still preferred for
many classes of goods. Of recent years there has been a great extension
of the use of sulphonated oils, which have the valuable property of
forming an emulsion with water or tan liquor. With these materials it is
easier to ensure the goods being completely covered with oil. The
penetration of the oil into the leather is also quicker and more
complete. These oils have often the disadvantage of leaving solid fats
on the exterior of the leather, which gives it an ugly smeared
appearance.
Stuffing the dressing leathers is carried out in a variety of ways and
with a variety of materials. The old process of hand stuffing employs a
mixture of tallow and cod oil called "dubbin." This is made by melting
the ingredients together and allowing them to cool with constant
stirring to a nearly homogeneous salve. The dubbin is brushed thickly on
to the flesh side of the sammed leather, which is then hung up to dry.
As the moisture dries out the oils and soft fats penetrate the leather
and leave the more solid fats on the outside. The proportions of tallow
and oil are varied with the time of year and with the method of drying,
for if the dubbin be too soft it will run off the leather, and if too
hard will not penetrate it so well.
Drum stuffing is a more modern development in which a higher temperature
is employed, about 140° F. The drum is heated up by steam or by hot air,
and the sammed goods are then inserted and drummed for a few minutes
until they are warmed. The drum is fitted with a heated funnel
containing the melted grease, which is run in through the hollow axle.
After a half to three-quarters of an hour's drumming the grease is
completely absorbed by the leather. The drumming is continued for a
while until the goods have cooled. Whilst still warm they are "set out"
to remove creases and superfluous grease. Drum stuffing is not only
quicker than hand stuffing, but also makes it possible to use the hard
fats, and so make a leather which carries more grease without appearing
greasy. Thus in drum stuffing, paraffin wax and wool fat are used, and
their penetration assisted by small proportions of cod oil or dégras. If
the leather be too wet the grease is not absorbed, whilst if it be drier
than usual the leather will take more grease, but the resulting colour
is not so good. There is also another method of stuffing which
originates from the Continent. It is known as "burning in" and involves
the use of still higher temperatures (195° to 212° F.). Wet leather
will, of course, not stand this temperature, so that it is first
necessary to make the leather absolutely dry. This is effected by drying
in stoves at temperatures up to 110°-115° F. There are two ways in which
the grease is applied. In one method the melted grease is poured by a
ladle on to the flesh side and brushed over until evenly distributed. A
second application of grease is made to the thicker parts. The hides are
then put into warm water (120° F.) for about a quarter of an hour, and
then drummed for half an hour. In the other method the goods are
completely immersed in the melted fats for a few minutes in a
steam-jacketer tank at a temperature of 195° F. After softening in water
at 120° F. the goods are drummed. "Burning in" is used for the heavier
dressing leathers such as belting and harness. It does not give good
colour, but permits the employment of still more hard fats.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 223, 378.
Bennett, "Manufacture of Leather," pp. 251-312.
Bennett, "Principles of Leather Stuffing," _Leather Trades Review_,
1911, 186.
SECTION V.--SOLE LEATHER
Leather for the soles of boots and shoes is a matter of essential
interest to all, and forms one of the best appreciated applications of
animal proteids to useful purposes. Methods for its manufacture are as
numerous as the factories producing it, hence all that can be done is to
describe broadly the general method which is typical of our time, to
classify the many varieties into types, and to indicate the recent
changes and present tendencies.
Sole leather is mainly manufactured from butt pelt, and the great aim is
to produce a firm, thick, waterproof and smooth grained leather which
will bend without cracking. It must have a light tan colour to be
saleable, and contain as much weight as possible to be profitable.
The modern mixed tannage of "sole butts" or "scoured bends" generally
utilizes ox-hides of the Scotch and English markets, though salted
Continentals and South Americans are also employed. After the usual
soaking a short and sharp liming is given. The special aim in liming
sole hides is to obtain the maximum plumping effect with the minimum
loss of hide substance. Both these achievements are necessary to obtain
good weight. The limes should be kept as clean as possible, which is
best obtained by putting clean hides into work. This reduces bacterial
activity and loss of hide substance. The "shortness" of the process is
attained by the use of sodium sulphide (from 2 to 16 ozs. per hide of
sulphide crystals), by which depilation may be accomplished easily in
about nine days. The amount of sulphide should be increased somewhat in
the short-hair season and in cold weather. Some factories take up to
about 12 days using less sulphide, whilst others will lime in about a
week by using the larger quantities. The amount of lime used varies
enormously, and is invariably in great excess of the actual
requirements. "Probably 2-3 per cent. on the green weight of the hides
is all that can be really utilized, the remainder being wasted."[5] This
amounts to about 2-1/2 lbs. lime per hide, but in practice it is more
frequent to find 7, 8, 9 or even 10 lbs. per hide being used. The excess
is innocuous, owing to the limited solubility of lime. Some excess is
desirable, to replace in the liquor the lime adsorbed by the goods in
plumping, to assist bacterial activity, and also because in
sharp lime liquors the undissolved portions do not remain so long in
suspension. The use of sulphide and other alkalies does not "make it
possible" to reduce the amount of lime used, it merely renders the
excess more superfluous. The use of sulphide not only shortens the
process, but also sharpens it, on account of the caustic soda produced
by hydrolysis. Usually for sole leather, however, it hardly sharpens it
sufficiently, and it is very common to add also caustic soda (or
carbonate of soda) to the limes. About 2 ozs. caustic soda (or its
equivalent in carbonate) is used per hide. The hides are limed generally
by the three-pit system, giving about three days in each pit. They
should be handled each day in the first pit (old lime) and once in the
other pits.
[Footnote 5: Procter, "Principles of Leather Manufacture," p. 129.]
Unhairing and fleshing by hand labour is still common, in order to avoid
great pressure on the plumped hide. Scudding should be very light, and
in some yards is entirely omitted. Only the lime on the surface of the
hide should be removed by deliming, and this immediately prior to the
insertion of the butts into the tan liquor. This is to ensure good
colour and yet keep the butts plump. Boric acid is the best for this
purpose, using 10-15 lbs. per 100 butts. The goods are inserted (and
preferably rocked) in a dilute solution for a few hours only. About the
same quantity of commercial lactic acid may be substituted for the
boracic. This deliming can also be accomplished by adding the acid to
the worst suspender in the tanyard.
To obtain firmness and plumping it is necessary that the early liquors
in tanning should be more acid than for other leathers. With old methods
of tanning one could trust to the natural sourness of the liquors to
complete the deliming and replump the goods with acid. In such cases any
deliming was also unnecessary. In the modern yard, however, we get
"sweet" liquors coming down the yard, partly on account of the greater
proportion of extract used and partly because the liquors themselves are
not so old. Hence it is now practically always necessary to acidify
artificially the tan liquors. This may be done by adding a few gallons
of lactic, acetic, formic, or butyric acid to the handlers and
suspenders, especially in the winter and spring. It is now increasingly
common to place sole butts in a special acid bath after they have been
in tan liquor for about a week. This bath is often made from sulphuric
acid, and may be 1 or 2 or even 4 per cent. in strength.
The actual tanning of sole butts lasts three to four months, and just
prior to the war the tannage consisted often of about one-third myrabs,
one-third valonia, and one-third extract. The myrabs and valonia were
leached together, and the extract added to the best leach to make layer
liquors of the required strength. Some mimosa bark was generally used
also, and now it is extensively employed to replace the valonia. The
most widely preferred extract is chestnut, but quebracho, myrabs extract
and mixtures have also a prominent place, and mimosa bark extract an
increasing importance. It is recognized that this tannage is if anything
too mellow, and that if only a smooth grain and plump butt can be
ensured in the first weeks of tanning, it is much better for sole
leather to employ the most astringent tans possible and the sharpest
liquors (_i.e._ liquors with a small relative proportion of soluble
non-tannin matters). Hence there is the tendency in sole-leather tanning
to employ fresh clear liquors for the butts and use up the more mellow
liquors on the "offal" (shoulders and bellies).
Four types of sole butt tannage will now be described, all of which
illustrate the methods employed in a modern mixed tannage.
=1.= The first type consists in a four-months tannage, in which the
liquors are worked down the yard.
The butts pass first through the suspenders (20°-40° Bkr.) in about a
week, and are rocked in the first liquors. They next enter the handlers
(40°-55°) rounds of eight pits, six floaters and two dusters. Myrabs, or
a mixture with algarobilla is used as dusting material. The goods remain
in this set for two weeks, and should then be struck through. The
suspender handlers (55°-65°) are next entered, in which they remain up
to three weeks in suspension, being shifted forward on alternate days.
The goods now enter the layers, of which four are given: first 70° for
one week; second 75° for two weeks; third 80° for three weeks; and
fourth 90° for a month. The goods thus take sixteen weeks to tan, of
which ten weeks (62-1/2 per cent.) are in layers.
The system of working the liquors is expensive, and is only possible if
the butt liquors can be spent out by the offal. The best or fourth
layer, 90°, is made from the best leach liquor, 65°, and extract
(chestnut with some oakwood or mimosa bark). After use it becomes the
second layer, 75°. The third layer, 80°, is also made from fresh leach
liquor and extract (chestnut with some myrabs or mixed extract). After
being used thus it is used for the first layer, 70°. The used first and
second layers are mixed together and used partly to form the belly
layers, and partly to make a sharp liquor for the handlers (55°-40°) by
diluting with 40° leach liquor and adding quebracho extract. The old
handler liquor is run to the suspenders (40°-20°), and finally used for
colouring off the offal in drum or paddle 18°. The suspender handlers
(65°-55°) are made from fresh leach liquor and chestnut extract. They
are afterwards used to make shoulder layers. The course of the liquors
is shown in the diagram below.
It will be seen that fresh leach liquor and fresh material are used to
each set except the suspenders, which must have some mellowness to
ensure plumping and smooth grain. Layer liquors are used twice only, and
then (when only five weeks old) pass to the handlers. These are further
sharpened by fresh leach liquor and fresh extract and dry materials.
The forward handlers are fresh liquors with fresh extract. This tannage
is fairly typical of high-class sole leather, in which the liquors are
worked down the yard, but worked towards the offal, which thus receives
liquors with relatively greater proportions of mellow tans and soluble
non-tans.
----------Leach liquor-------------------
| / \ |
| V \ |
| 4th layer, 90° \ |
| | V |
| V 3rd layer, 80° |
| 2nd layer, 75° | |
| \ V |
| \ 1st layer, 70° |
\ \ / |
\ \ / |
\ V V |
\ (mixture)[-->bellies] |
\ / V
V V Suspender handlers-->(65°-55°)(shoulders)
Handlers, 55°-40°
|
V
Suspenders [-->offal]
=2.= The second type consists in a tannage of about four months, in
which the liquors are not worked down the butt yard. In this method also
there is an attempt to save much of the labour in handling, first by
shortening the time in the handlers by one week (as compared with the
above), and second by fusing the two progressive handler sets into two
sets of equal strength, through which the goods pass more slowly and
with less disturbance.
The goods go through the suspenders (10°-25°) in about a week, rocking
in the early liquors, and then into large rounds of handlers (30°-45°)
for one month. The handlers consist of floaters and several dusters, in
which the butts are laid away with 1-3 cwt. myrabs. The goods next enter
the layers, of the same strength as in Type 1, and in which they remain
the same time. The total tannage is thus 15 weeks, of which 10 weeks
(nearly 67 per cent.) are in layers.
The best or fourth layer is made up from leach liquor and extract, and
is then used successively as a third, second and first layer, and then
passes to the offal layers. The handler liquor is made entirely from
fresh leach liquor and quebracho extract, and is a sharp liquor of
greater strength than its Bkr. strength would indicate. The old handler
liquor is run to the butt suspenders. The course is represented thus:--
Leach liquor
/ |
/ 4th layer, 90°
/ |
/ 3rd layer, 80°
/ |
/ 2nd layer, 75°
/ |
/ 1st layer, 70°-->[offal layers]
V
Handlers (45°-30°)
|
V
Suspenders (25°-10°)
=3.= The third type consists of a short three-month's tannage in which
the liquors are worked straight down the yard. To compensate for the
short time it is necessary to have stronger layer liquors in which the
goods spend a still greater proportion of their total time. The stronger
liquors involve a greater proportion of extract, particularly of
quebracho, which fact causes the whole of the liquors to be sharper than
their Bkr. strength indicates, and justifies them being worked straight
down the yard.
The goods go through suspenders (20°-40°) as usual one week, and then
pass into suspender-handlers (40°-60°) for two weeks, and thence to the
layers. In the first two of these (65° and 70°) they are actually in
suspension, a week in each liquor. They are then dusted down for ten to
eleven days, first in 85° and then in a 95° liquor, and finally for a
month in a liquor of 110°. The total tannage is thus twelve weeks, of
which nine weeks (75 per cent.) are in layers. There is considerably
less handling than in Type 2, and it is more convenient, the goods being
in suspension.
=4.= The fourth type is also a three-month's tannage. In this it is
attempted to obtain even greater weight with still less labour. The
layer liquors are kept much stronger by the more extensive use of
extract, and this makes it impracticable as well as too costly to run
these liquors down the yard. They are therefore repeatedly strengthened
with extract and used again.
The goods go through suspenders (20°-40°) as usual one week, and then
through a round of suspender-handlers (40°-55°) consisting of fresh
sharp liquor from the leaches together with quebracho extract. They are
in this set two weeks, and then are laid away. They receive three
layers: first, 105° for 2 weeks; second, 110° for three weeks; and
finally, 120° for a month. Of the twelve weeks, therefore, nine weeks
(75 per cent.) are spent in layers. In this method the goods are
immersed in 3 per cent. sulphuric acid after passing through the
suspenders.
There is possible, of course, a tremendous number of variants of the
above types. The number of handler rounds is determined by the number of
butts being dealt with. With a large number it is more easily possible
to arrange for them to be in progressive strength as in Type 1. There
are also many systems of working the layers, of which the most notable
is to make the second or third layer from fresh leach liquor and
extract, and strengthen it with extract for the succeeding layers. It is
then used as a first layer and worked down the yard.
The bellies and shoulders often go through separate sets of liquors, but
it is common to put them through suspenders, and even handlers together.
They receive, of course, a distinctly shorter tannage, and are often
drummed with extract before laying away or after the first layer. By way
of illustration, the course of the offal and their liquors may be given
in the case of Type 1. The shoulders and bellies are coloured off in a
paddle or drum with old butt suspender liquor, which is then quite
exhausted. They then pass through suspenders (18°-40°) together in 4-5
days, and go through a handler round (40°-55°) for 3 weeks, including
one duster. The bellies are removed after 2 weeks, and given three
layers (60°, 70°, 80°) of a week each. They receive, therefore, nearly 6
weeks in all. The shoulders also have three layers (60°, 65° and 80°)
of 2, 3 and 4 weeks respectively.
The course of the liquors is shown thus:--
Butt layers Butt suspender Butt suspenders
(and + extract) Handlers (+ extract) |
| | |
V V |
Belly layers Shoulder layers |
(80°-60°) (80°-60°) |
\ / |
V V |
Offal handlers (55°-40°) |
| V
V Offal drums
Offal suspenders (40°-18°) (18°)
\ /
\ /
\ /
\ /
\ /
\ /
V V
Drain
The tanned butts are piled for 2-3 days, sometimes rinsed to remove
dusting material, and then scoured either by machine or by drumming with
sumac and extract. This removes bloom, but causes some loss of weight.
"Vatting" or "bleaching" now follows, in which it is attempted not only
to bleach the colour of the leather, but also to impart as much weight
as possible. The vat liquor is made several degrees stronger than the
last layer by means of quebracho bleaching extract and good coloured
chestnut or myrabs extract. The liquor is kept warm by a steam coil, at
about 100° F., but not much more without risk. The goods remain in the
bleach liquor 2-3 days and are then horsed or suspended to drain. Sumach
is sometimes used in the vats. A new vat liquor must be made up after
some weeks' use. The goods are sometimes rinsed in weak sumac liquor
before vatting to get good penetration, and sometimes after to ensure
good colour.
The butts are next oiled and hung up in a dark shed and allowed to dry
slowly and evenly to an "india-rubbery" consistency and rather slimy
feel. They are then "struck out" by machine, wiped, re-oiled and again
hung up to dry, preferably with sulphonated oil. After a short drying
to a suitable and even condition they are "rolled on," and, possibly
after further drying, "rolled off" with greater pressure, and then dried
for a day or two with the help of a little steam. Finally they are
machine-brushed and sent to the warehouse, where they are weighed and
classified.
The offal is often drum oiled. It needs more striking and is more
difficult to obtain in suitable condition for striking, rolling. It is
treated similarly to butts, but often also goes for dressing leather,
and may be split. It is of some interest to compare the above processes
with that once very popular manufacture of "bloomed butts" in the West
of England from South American salted hides. These receive a liming from
12-14 days, using 12-16 lbs. of lime per hide. They receive then a
tannage of about 9 months, comprising 3 weeks in suspenders
(20°-40°)--very sour and mellow liquors--4 weeks in handlers (40°-55°),
4 weeks in dusters (60°), 4 weeks in round made from hemlock extract
(60°), and 20 weeks in six layers (60°-90°) in which they were dusted
heavily with valonia. Oakwood extract was used for the layers, which
took 57 per cent. of the total time. The butts were scoured in a
much-dried condition, so that only the loose and surface bloom was
removed. No bleaching was given in the modern sense.
In the old oak-bark tannage of sole leather up to 12 months were taken
for tanning, two-thirds to four-fifths of which time the goods were in
layers. The strongest liquor rarely exceeded 50° even where valonia and
gambier were also used, and rather more than 30° if not.
It will be understood from the above that the tendency for many years
has been to shorten the time and the labour required for tanning. Drum
tanning is obviously the next stage in shortening the time. In one such
process the butts are put through suspenders (25°-40°) for 2 weeks,
drummed for 12 hours in an 80° extract liquor, and finally in a neat
extract 200° for 36 hours. Drum tanned sole leather, however, is not as
yet of good quality; the grain is not smooth, and the heavy weight
finish (striking and rolling) needed to counteract this tendency is
liable to cause poor "substance." The leather, too, readily wets and
goes out of shape. Possibly some drumming may be adopted to save time in
the early layers, but the most serious rival to the 3 months' tannage is
the waterproof chrome sole leather (Part III., Section V.).
REFERENCES.
Parker, _J.S.C.I._, 1902, 839.
Procter, "Principles of Leather Manufacture," p. 220.
Bennett, "Manufacture of Leather," pp. 179, 259.
Bennett, _J.S.C.I._, 1909, 1193.
SECTION VI.--BELTING LEATHER
The manufacture of belting leather is well illustrated by the tanning
and finishing of "strap butts." In general, the tannage presents many
points of great similarity with the tannage of sole leather; indeed, the
resemblance is so close that in some factories there is little
difference observed, and the currying and finishing operations are
relied on to produce the desired difference in final results.
Nevertheless, there is considerable difference in the type and ideal of
the two leathers, which may be expressed in trade parlance as a greater
"mellowness" for the belting leather, and in the best methods of
manufacture this fact is in evidence throughout the whole process of
manufacture.
In liming, there need be little difference between sole and belting
hides, and a sharp treatment of 9-10 days, by the three-pit system, with
a day or two extra in the coldest weather, would meet ordinary needs.
For the conservation of hide substance and for the saving of time a
shorter liming is sometimes given, in which more sulphide is employed
than is usual for sole leather. Even the very short processes of liming,
1 to 3 days, which involve the use of strong solutions of sodium
sulphide, have been successfully employed for belting leather. The
tendency to harsh grain with such processes is not so serious a defect
with belting as with sole leather, and can be minimized by careful
deliming. American and Continental factories tend to favour the use of
those quick processes which employ warm water in addition to sulphide.
The hides after a short liming in sulphide limes are immersed in warm
water, which greatly accelerates both the chemical and bacterial
actions. For example, after about 3 days' liming, in which both old and
new limes are used as usual, the hides may be thrown into water from
100°-105° F., and will be ready for depilation in 7 or 8 hours.
Even a stronger liming may be given, especially if the soaking is
unusually prolonged. Such processes undoubtedly save hide substance, and
the pelt is obtained more free from lime, but they have the disadvantage
that the natural grease of the hide is only imperfectly "killed" (_i.e._
saponified or emulsified), and may interfere with the normal course of
the tannage. The plumping is also apt to be insufficient. On the other
hand, liming processes are also used in which a mellower liming or a
longer liming is preferred in order to produce the desired degree of
softness and pliability in the finished leather. Belting must not be too
soft, of course, and it will be clear that the required difference from
sole leather can be produced either in liming or tanning or partly in
both. These considerations also decide whether bating is to be omitted
or not. A hard astringent tannage in sour liquors after a sharp liming
might make bating essential, but in these days it is usual to avoid it
and produce the effect in other ways. A light bating of a few hours is
sometimes given, but it is more unusual to delime the grain thoroughly
with boric acid, using up to 20 lbs. per 100 butts. Crackiness is a
fatal defect in strap butts, so that a sound grain must always be
obtained. Generally speaking, therefore, strap butts receive more
washing in water, and rather more deliming than sole leather, even when
they are not bated. It is also usual to scud much more thoroughly, and
to round a larger proportion of butt, especially in length.
The tannage is usually carried out with a blend which includes a much
greater proportion of the fruit tans, and correspondingly less of
extract.
Distinctly more myrabs are used than in sole leather tannages, in the
dry material, and amongst the extracts chestnut is preferred to
quebracho, and myrabs to mimosa bark, though all these may be used in
some degree. In the past the most favoured extract has been undoubtedly
gambier, which gives a tannage which is easily curried and imparts the
required mellowness to the uncurried leather. The great expense of this
material, however, together with the advent of drum stuffing and shorter
tannages in stronger liquors, have tended to cause a considerable
reduction in the proportion used for strap butts, and to limit its
employment to the earlier stages of tanning.
The same tendencies for reducing the time taken to tan, employing
stronger liquors, and securing economy of labour in handling, have been
evidenced in the tannage of strap butts as in sole butts. It is
nevertheless true that, broadly speaking, strap butts receive rather
more handling and rather weaker liquors than sole butts. A greater
amount of mechanical assistance is also employed with early stages
(paddling, drumming, rocking). This is less objectionable for curried
leather than for sole butts. The handling is more usually in suspension.
The liquors are usually worked straight down the yard as a greater
mellowness is needed in the early liquors than for sole butts. The offal
is given a separate tannage and often used for different purposes,
_e.g._ the shoulders for welting and the bellies for fancy goods.
Plumping with sulphuric acid is generally considered inadmissible for
strap butts. It has been shown that leather containing sulphuric acid
tends to perish after the lapse of a number of years. Sole leather will
be worn up before this effect is observed, but belting is an article
which is intended to last much longer, and the use of sulphuric acid is
consequently inadvisable. Plumping must be obtained, to a considerable
extent, but must be achieved by the organic acids (lactic, acetic,
formic and butyric acids). A few gallons of such acids are consequently
added to the handlers, especially in the winter and spring. Less may be
used in the autumn, when the layer liquors which fermented in the summer
months have worked down to the suspenders. A mixture of these acids is
usually better than any one alone, for they not only differ very
considerably in price, but also have different powers of neutralizing
lime and plumping the goods. Lactic acid (M.W. 90), Acetic acid (M.W.
60), and formic acid (M.W. 46) are each monobasic acids; consequently 3
lbs. formic will neutralize as much lime as 4 lbs. acetic or 6 lbs.
lactic. Their plumping powers are somewhat influenced by the anion. In
determining what quantities to take, the commercial strength of the
acids must also be considered. Formic is often 80-90 per cent. pure,
acetic 60-80 per cent., and lactic 40-60, but may be as low as 25 per
cent. The blend must be adjusted accordingly. As strap butts do not need
the firmness of sole leather, less of these acids may be used than for
sole butts.
The exact nature of the tannage and the strength of the liquors is
largely influenced by commercial considerations. If the manufacturer is
both tanner and currier, he need not go to such great expense in strong
liquors and in time in layers, for he can obtain some of this weight in
currying. If, however, the tanner sells the butts rough dried, he must
naturally aim at obtaining greater weight in tanning.
The actual details of the tanning processes are as usual very varied,
but may be classified according to type, just as in the case of sole
butts.
Illustrations will now be given.
=Type 1=, which may be compared with Type 1 for sole butts, is a tannage
of about 5 months. The goods pass through suspenders (8°-30°) in 2-1/2
weeks, and then pass to the handlers (30°-50°), in which they remain a
month; they are then put into suspension again and pass through the
suspender handlers (40°-55°), which takes 2-1/2 weeks. In this round
much gambier is added, and the goods are frequently handled. Four layers
are usually given, viz. first layer 55°, one week; second layer 60°, two
weeks; third layer 65°, four weeks; and fourth layer 75°, four weeks.
The tannage is thus 20 weeks, of which 11 weeks (55 per cent.) are in
layers. Extra layers may be given to heavier goods, using stronger
liquors made up with extract. All liquors work straight down the yard.
The tannage consists of 35 per cent. myrabs, 35 per cent. valonia, 10
per cent. Natal bark, and 20 per cent. extract, chiefly gambier, though
some chestnut and quebracho are used.
=Type 2= represents the modern tendency to use stronger liquors and a
shorter time. The strap butts pass through the suspenders (22°-50°) in
1-1/2 weeks, during about a third of which time they are rocked. They
next pass through two sets of suspender-handlers (50°-67° and 67°-80°),
which takes a month, and thence to the layers. Three layers are given
(85°, 90° and 100°), in which the goods remain one, three and four weeks
respectively. The tannage is thus 13-1/2 weeks, of which 8 weeks (nearly
60 per cent.) are in layers. The liquors work down the yard. Longer time
may be given to heavier goods. The tannage consists of 40 per cent.
myrabs, 35 per cent. valonia or Natal bark, and 25 per cent. extract,
chiefly chestnut, though some gambier may be added to the suspenders.
However tanned, strap butts are first dried out rough over poles. This
assists in making the tannage permanent, on account of secondary changes
discussed in Section III. They are next wet back for currying by
soaking in water or sumach liquor for a few hours and piling to become
soft and even. The first operation is "skiving," which is a light
shaving on the flesh side, carried out by a sharp slicker with a turned
edge. The butts are next scoured thoroughly by machine on both flesh and
grain, and sumached in a vat for several hours at 100° F., after which
they are slicked out and hung up in a cool shed to samm for stuffing.
Hand stuffing is often still preferred, with tallow and cod oil. The
butts are next set out, and it is important that this should be
thoroughly done. Machines are now generally used, and the goods are
often reset after further drying. After drying out completely they are
given a light coating of tallow and laid away till wanted for cutting up
into straps, which is now done by machinery.
A Continental method for making belting leather is to give 6 weeks in a
suspender set (70°-24°) of twelve pits arranged on the press system,
running two fresh liquors a week, and to give them two layers (24° and
28°) of 6 and 8 weeks. The material is chiefly pine bark, but some oak
bark, valonia, myrabs and quebracho are also used. The goods are stuffed
by "burning in," molten fat being poured on the flesh side.
REFERENCE.
Bennett, "Manufacture of Leather," pp 194, 295.
SECTION VII.--HARNESS LEATHER
When discussing the question of oak bark (Section III.), reasons were
advanced for its decreased use and popularity. These were quickly
appreciated in the sole leather trade, but the obsolescence of oak bark
in the dressing-leather section was much more prolonged, partly because
there was less pressing need to obtain good weight in the actual
tanning, and partly because in some branches of dressing leather, such
as belting and harness, a leather was required of great durability and
toughness, for which qualities oak bark tannage had a deservedly high
reputation. Hence harness leather manufacture affords a good
illustration of the transition between the methods of the late
nineteenth and those of the twentieth century. With the use of oak bark
lingered the old methods of liming, bating and tanning in weak liquors
for a long time with plenty of gambier. Hence in this section it will be
necessary to observe a gradual transition of method, both in wet work
and tanning. It should be pointed out that this transition has not been
and is not going on in all factories at the same rate. Many factories
remain in which the old methods are still preferred at some stages of
the manufacture, and some remain in which many of the changes indicated
below have not taken place at all. The leather trade has always been
considered conservative in its methods, but it should be realized that
much of the prejudice in favour of old methods is due to the public, and
that after all tanners and curriers, like other business men, have to
suit their customers. The march of industry is not like a regiment in
line; it is rather more like nature, a survival of the most adaptable.
Hides for harness leather are limed in various ways, of which the
following are types.
1. A rather mellow liming of 10-15 days (longer than for sole leather),
in which nothing but lime is used, and a certain amount of old liquor
used in making up the new limes. The liming was carried out by the
one-pit system, but the goods and liquors were kept clean by a good
soaking process. Hence the loss of hide substances was not very great;
goods so treated were bated before tanning.
2. A shorter liming than the above by the three-pit system. This saved
time (taking 9-10 days), saved hide substance, and ensured greater
regularity of treatment. The limes were about as mellow, but a little
sulphide (2-4 ozs. per hide) was used to assist the depilation,
especially during the short-hair season. These goods were also bated.
3. A distinctly longer liming, 15-16 days, in mellower limes. This
differed from Type 1 also in the respect that greater regularity was
ensured by the three-pit system; a foot or two of old liquor was used in
making up the new lime. More hide substance was lost than in either of
the above processes, but this was deliberate, the object being to
dispense with bating, which is always light for harness hides. Thus a
longer and mellower but systematic liming was used as a substitute for
shorter liming and bating. No sulphide was used in this process.
4. A short liming of 6-7 days, using up to 12 ozs. of sulphide per hide.
The object here is to save time and hide substance. The three-pit system
is preferred. Bating again becomes necessary, but the pigeon-dung bate
is replaced by artificial bates, less objectionable, quicker, and more
scientific in management.
5. A still shorter process of about five days, using still more sulphide
(about 16-20 ozs. per hide), together with some calcium chloride to
reduce harshness. In such a method there is a tendency to revert to the
one-pit system, which involves rather less labour. The three-pit system
shows to a great advantage in the longer processes of liming when the
process is reduced to five days; there is little difference between the
two, for a one-pit system is a two-liquor method. Hence again an
artificial bate is used.
The various methods of liming, together with analogous variations in
tannage, have resulted in great variety in bating. Sometimes up to three
days' bating has been given at 70° F., but more often the goods are
merely immersed overnight, and then delimed with boric acid, but with
sulphide processes it is an advantage to use some of the commercial
bates of the ammonium chloride type, and finish off with boric acid.
Scudding is always more thorough than for sole or belting, the hides are
rounded into long butts which include most of the shoulder "harness
backs." The goods are sometimes bate shaved.
A few tannages will now be outlined, in order of historic type.
=Type 1= may be taken to represent the so-called "high-class" process in
which oak bark myrabs and valonia are the staple materials. A good deal
of gambier is also used, and a little myrabs and chestnut extract are
helpful in attaining the desired strength of liquor. The "backs" go
first through suspenders (8°-30°), which takes up to three weeks, and
then in to handlers (30°-40°) for four weeks, consisting of rounds of
clear liquor. They next go through a duster round, in which they are put
for a week with oak bark and myrabs into a liquor of 45°. Four layers
are given (50°, 55°, 60° and 65°), in which the goods remain for two,
three, four and five weeks respectively, oak bark being the chief
dusting material. The tannage is thus for twenty weeks. Light backs
receive less time in the layers (only 11 weeks). If the tanner is also
the currier, the fourth layers are omitted. He then saves five weeks and
gets the weight in the stuffing.
=Type 2= is a tannage in which oak bark and valonia are replaced by
myrabs, mimosa bark and chestnut extract. It is therefore considerably
cheaper and probably no less durable. Expense is also curtailed in
handling. The harness backs go through suspenders (16°-30°) in two
weeks, handlers (30°-45°) in four weeks, and then receive four layers of
the same strength as in Type 1, but only one, two, three and four weeks
respectively. The last layer is omitted for light harness, and an extra
layer of 75° is given if the tanner is not the currier also. Thus the
usual tannage is 16-20 weeks, of which 10-14 weeks (63-73 per cent.) are
in layers.
=Type 3= is a tannage which may consist of myrabs (55 per cent.),
valonia or mimosa bark 25 per cent., and extract (26 per cent.). The
extract is chiefly quebracho, though some chestnut may be used. More
valonia and less myrabs may be used if desired (and when possible), and
myrabs extract will then replace quebracho and chestnut. The goods are
coloured off in drums or paddles, and then pass through two sets of
suspenders handlers (20°-55° and 55°-75°). They are handled up and down
very frequently in the first set and rapidly pass into stronger liquors.
The backs then receive three floaters at 80°, in each of which they
remain one week. The tannage is completed by three layers: first, 85°
for one week; second, 90° for one week; third, 95° for two weeks. The
tannage is thus 11 weeks, of which 7 weeks involve little labour. If the
tanner is not the currier, still stronger liquors may be used.
In all these tannages little or no acid is used for plumping, as the
natural acids of the liquors are sufficient to ensure what is necessary
in this direction for this class of leather. A little organic acid or
even boric acid may be used in the earliest liquors for deliming
purposes, when necessary. After tanning the goods are dried out and
sorted in the rough state. Harness is a somewhat broad term, and there
is scope for considerable variety in classification. The hides are
sometimes not rounded until after tanning. The finished article may be
any grade between heavy harness for artillery and leather for ordinary
bridles.
In currying heavy black harness, the backs are soaked and sammed for
shaving. Lighter goods may be machine-shaved, but the heaviest are
shaved lightly by hand over the beam or merely "skived" with the shaving
slickers. The neck needs most attention, and it is often advisable to
stone by machine and split. The scouring should be thorough, on flesh
and grain. This is done by machine, and not only cleans the goods from
bloom, dirt and superfluous tan, but also assists in setting out.
Sumaching may be for several days, merely overnight or even only for a
few hours, being stoned after wetting back to temper. Hand-stuffed goods
get a coat of cod oil first, and during the drying are often well set
out. Drum-stuffed goods are well set out by machine, and after some
drying, stoned and reset by hand. It is now usual to buff the grain,
_i.e._ remove the coarser parts by light shaving. This prevents cracking
in the finished article. The goods are blacked with logwood, iron and
ammonia, thinly dubbined again, again well set out and tallowed. Setting
out, indeed, may be done at any convenient opportunity. The superfluous
grease is removed by slicking, scraping, brushing with a stiff brush,
and finally with a soft brush.
For brown harness the goods are more carefully selected, more thoroughly
scoured and sumached, and bleached frequently with oxalic acid. They are
hand stuffed, stained twice, and after the usual setting out, glassing
and brushing, are finally rubbed with flannel.
For bridle leather the goods are carefully shaved but are not stuffed,
being merely oiled with cod oil on flesh and grain. They are dried out
before scouring, and then sized, set out, stained and resized. The goods
are heavily glassed during the finishing.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 195, 297.
Section VIII.--UPPER LEATHERS
The manufacture of leather for the uppers of boots and shoes embraces a
bewildering variety of goods, suitable for anything between a baby's
shoe and a man's shooting boot. Almost all degrees of lightness,
softness, and waterproofness are in demand. A great variety of finish is
also involved, determined by the ingenuity of the currier and the
ever-changing fancy of the public. Even greater is the variety of
methods by which all these results are obtained by methods which
superficially seem quite different; the desired qualities being imparted
in one case largely by the tannage and in another case almost entirely
by the currying. Under such circumstances the selection of types becomes
a problem.
The variety, moreover, commences from the earliest stages, the selection
of the raw material. Upper leather may be made from light calfskins,
heavy calfskins, kips (home and foreign), light dressing hides and heavy
dressing hides, which last may replace any of the former after splitting
to the required substance. In this section it will be necessary to take
kips as typical of the rest, and to use it in a rather broad sense,
including heavy calf and light dressing hides.
Speaking quite generally, kips for upper leather receive usually a long
and mellow liming, a thorough bating and a sweet and very mellow tannage
in weak liquors. In currying they are well scoured and set out, heavily
stuffed and stained black, being sometimes finished on the grain and
sometimes on the flesh. These outstanding features of upper-leather
methods will be further illustrated by a brief account of the tanning
of kips (light hide and heavy calf), and outlining the best known types
of finish for butt, shoulder and belly.
The goods receive usually a long and mellow liming of 14-16 days, using
only lime as a rule. In some factories lime liquors are used repeatedly
for successive packs to an almost indefinite extent. Dissolved hide
substance, ammonia, mud and dust, and bacteria accumulate for months and
sometimes for years. It is obvious that in such liquors "putrefaction"
is a more correct term than "liming" for the depilation. Such methods
have been used even in recent years, but there has now been a tendency
for some time to make the liming more methodical. Such old limes make a
leather which is empty, loose, and dull grained, but the defects are
minimized by the system of stuffing heavily and finishing the flesh, and
hence the ancient lime remained with surprising tenacity. Even so late
as 1903 we find that Procter with characteristic caution could write,
"Probably no lime ought to be allowed to go for more than three months
at the outside limit without at least a partial change of liquor." It is
within the writer's experience to find an upper leather factory with
limes which had never been emptied for over three years. In other
factories, however, there has been a revulsion of feeling with regard to
such processes, and it has been found advantageous to adopt a more
scientific routine, in which the lime pits are cleaned out at regular
intervals. There is little doubt that a mellow liming is desirable, but
this can be secured by blending some old lime liquors with fresh lime
liquor in a systematic manner. Similar considerations apply to the
question of working the various packs through the limes. It is clear
that with a mellow liming a one-pit system is quite possibly
satisfactory, but the revulsion of feeling against a lack of method
produced a method of liming more elaborate than usual, and it is now not
uncommon to find kips limed in a "round" of 6-8 pits, the goods passing
through each pit. They remain in one pit about two days, and are shifted
forward. In the green or old limes the goods are handled up and down.
The old limes are, of course, mellower than the new and exert the
desired softening effect. The working is quite analogous to that of a
round of handlers. Unhairing is sometimes assisted by the use of arsenic
sulphide. E.I. kips need a thorough soaking before any liming; several
days are usually needed. The old methods involving putrid soaks and
stocks may be considered out of date, and it is usual to soften back in
caustic soda or sulphide soaks with some assistance by drumming. A
little sulphide is sometimes added to the older limes to continue the
treatment.
The goods are next thoroughly bated and delimed. The hen or pigeon dung
bate is still usual, and probably gives the best results, though closer
approximations have been made of recent years on artificial lines. Some
bating with solution of hide substance seems necessary for these goods.
The lighter goods are often drenched also to complete the deliming,
using 6 per cent. bran on the weight of pelt. The heavier goods are more
often treated with boric acid after bating, which not only delimes
completely and gives a soft relaxed felt, but also acts as antiseptic
and stops the action of the bate, a matter of some importance (see
Section II.). Lactic acid may substitute boric, in which case about 2
per cent. on the pelt weight of 50 per cent. acid may be required. It is
important to avoid a strong solution and local excess, hence lactic acid
must be added gradually so that the liquor is never stronger than 0.2
per cent. Drumming and paddling is an advantage in deliming.
The tannage is light in most cases, partly because some of the finished
goods are sold by area, but partly also because even if sold by weight,
the weight is obtained quicker and more easily by stuffing, which course
is also often preferable to obtain the desired mellow feel,
waterproofness and durability. Hence it is seldom that strong liquors
are employed. The tannage is also mellow, on account of the softness and
pliability required; no acids are consequently employed, and no material
which is liable to yield sour liquors. Gambier is easily the first
favourite amongst the tanning materials, whilst oak bark comes second.
It should be observed, however, that a hypothetical tannage of equal
weights of cube gambier and oak bark is in reality a tannage by
four-fifths gambier and one-fifth oak bark, on account of the relatively
greater strength of the former. This observation is so apposite with
respect to some tannages that it is nearly correct to say that the
tannage is gambier and the oak bark an excuse for having leaches through
which the gambier liquors may be run occasionally to clear and to
sharpen slightly. No serious theoretical objection to such a method is
possible if the liquors are weak and the system of working the liquors
is scientific and the process carefully regulated. Upper-leather
tannages, however, have scarcely merited scientific praise. It is often
a case, not of poor methods, but of no method at all. The same lack of
system, principle, and regularity observed with regard to the limeyard
has been equally obvious in the tanyard, when perhaps the need was even
greater. Even a mellow tannage has varying degrees of mellowness
possible to it; there still remains the question of the soluble
non-tans. However, method in the upper-leather tanyard has often been
conspicuously absent. There has been many a factory where any one tan
liquor was as good as any other in the yard. In the writer's experience
are two such cases: in one the liquors were all 25° Bkr., in the other
they were all 0° Bkr. In such cases, handling the goods from pit to pit
is somewhat futile, and handling forward from set to set still more so.
Hence it is possible to find dressing leather tanned by putting it
slowly through one round of handlers, adding a few buckets of gambier
where it apparently is necessary. It is, from one point of view,
surprising to see what serviceable and excellent-looking upper leather
can be manufactured by such happy-go-lucky processes. It is, however,
also possible to see how this may occur. Gambier is a stable tan, and no
souring and little decomposition take place in gambier liquors. It is
also extremely mild and non-astringent, and is always used in weak
liquors. The hides, moreover, are completely delimed, and there is
little danger of bad or uneven colour. Tanning under these conditions is
at its easiest; it is almost more difficult to spoil the goods than make
them right. Under such conditions tanning deteriorated rather than
improved in method. When neglecting it made little difference to the
finished leather, it was neglected.
This state of affairs, however, was embarrassing whenever a tanner
wished to try any other tanning material. The expense of gambier and oak
bark made valonia and mimosa bark into obviously desirable alternatives
and substitutes. Methods which would tan with gambier, however, would
not work with Natal bark or valonia, and many a tanner has had to revise
his method of tanning from end to end. The use of myrabs also raised the
problem of souring, and it has become evident that "working the liquors
down the yard" is as desirable a method for dressing leather as after
all other tannages. It will be clear from the above that types of
upper-leather tannages are less typical than for other leathers, but
nevertheless the more progressive manufacturers have for some years now
been working on sounder lines, economically and scientifically. In such
cases it is now usual to pass the goods through at least two sets of
handlers, and through liquors of gradually increasing strength.
Occasionally dusters or layers are given, especially for the heavier
goods. The tannage is nearly always commenced now by paddling the goods
in the oldest liquor. This paddling may be anything from half an hour up
to twenty-four hours. It is sometimes desired to work up a "grain," and
the old liquor is then often sharpened by the addition of fresh gambier
or leach liquor.
The same tendency to save labour in handling is to be observed in upper
leather tannages as in sole and other dressing leather factories. There
is also a tendency to obtain rather more weight in tanning by using
stronger liquors, and in the heavier goods to shorten somewhat the time
taken. The following methods may be taken to illustrate modern processes,
in order of evolution. They all last about seven weeks.
_Type 1_.--In this process the kips are first paddled in an old liquor
(3°), and passed to the first handlers (3°-30°) for three weeks. After
working through this set they pass through the second handlers
(20°-30°), in which they are not handled quite so frequently. They are
in this set also three weeks. Heavy goods may then receive a floater
(30°) for another week.
_Type 2_.--In this process the goods are paddled, and then enter a large
handler round (8°-30°), through which they pass in five weeks. The goods
are handled frequently in the early stages. The tannage is completed by
one layer of two weeks (30°). The layer is made by the ancient method of
putting the goods and dust alternately into an empty pit, and then
filling up with liquor from the best leach. Oak bark, valonia and myrabs
are used as dust, though sumach and gambler have been used.
_Type 3_.--In this process an attempt is made to save handling and
obtain more complete tannage. The goods are paddled for three to five
hours in a rather sharp liquor of 10°, and are then handled well for a
week in the first handlers (5°-20°). The goods then go through the
second handlers (20°-45°) in six weeks, and heavy goods may then receive
an extra floater (45°) for one week.
In type 1 the leaching material is two-thirds oak bark and one-third
valonia; in type 2 it is half oak bark and half mimosa bark; in type 3
it is one-third oak bark, one-third valonia or Natal bark, and one-third
myrabolans. In all cases the strongest handler is obtained from the
leaches, and made up to the required with strong infusion of gambier.
When the liquor has passed through the forward handlers, it is returned
to the leaches to clear and sharpen, and then run to the green handlers.
After passing through this round it is returned to the paddle, from
which it passes to the drain. The rest of the paddle liquor may be from
the forward handlers. It is often customary to obtain the best liquor
from the second leach, and allow the best leach to stand for a few days.
This allows the bloom to deposit in the leaches. The system secures the
result desired, but the deposition of bloom involves a loss of tannin,
which waste makes the system expensive.
Heavier dressing hides are tanned by methods similar to the above, but
with floaters, dusters and occasionally layers added after they have
passed through two sets of handlers. Thus they may have first handlers
(8°-18°) two weeks; second handlers (40°-45°) for six weeks, making
twelve weeks in all. Lighter goods may receive two rounds, being two
weeks in each.
After tanning, the kips are rounded usually into butts, shoulders and
bellies, to which different finishes are given. The currying may be
illustrated by selecting types, but it must be borne in mind that there
is much elasticity in this matter. Thus kips may be made into waxed
butts, satin shoulders and lining bellies, but also may be cut down the
back in "sides," both of which are finished limings.
Waxed kip butts are a type of many similar upper leathers (waxed shoe
butts, waxed calf, waxed splits, etc.). The finish is on the flesh side.
The kip butts are soaked carefully, and shaved by machine. They are then
drummed in sumach for an hour or two, slicked out and sammed for
stuffing. The sumaching is also the scouring unless the goods be too
heavily bloomed. The samming is often done by machine. Drum stuffing
follows, wool fat and stearin being staple greases, with varying amounts
of degras and cod oil, and of tallow and cod oil. A little paraffin wax
and resin are also used sometimes. The goods are well slicked out and
dried. They may be now dubbined and laid away to mellow for whitening,
which consists of a careful shaving of the flesh by a turned-edge
slicker or by machine. The grain is stoned, set out and "starched," and
the butts grained by boarding the flesh. In the waxing, one of two
courses may be adopted. The butts may be blacked with lampblack and oil,
"bottom sized" with glue, soap and logwood, and then "top sized" with
glue, dubbin, beeswax and turpentine; or they may be given a
"soap-blacking" of soap and logwood and lampblack, applied by machine,
and sized once only.
Dressing hide butts may also be given a grain finish, such as the "memel
butts" for heavy uppers. The butts are soaked, shaved or split, sumached
in drum, and preferably thoroughly scoured on flesh and grain. They are
then sammed and heavily stuffed in the drum. The grain is buffed and
stained black with logwood, ammonia and iron solution (curriers' ink).
The butts are then dried, set out, thinly sized and slowly dried.
When dry on the face they are printed or embossed by machine to give the
characteristic memel pattern and dried out completely. They are then
grained four ways. The grain is finished by a coating of linseed oil
containing resin, and the flesh is whitened, French chalked and glassed.
Shoulders for "satin" receive a currying which strongly resembles the
"waxed" finishes, but the smooth finish is on the grain side. The grain
is buffed, and blacked, dubbined, set and reset, with intermediate
drying, and is sized and finished by compositions similar to those used
for waxed leathers. The flesh is whitened. Satin hide and satin calf are
dressed similarly.
Shoulders may also be finished for "levant." After soaking, splitting,
and shaving to substance, they are drum-sumached, machine-sammed, and
oiled up to dry. They are stained with logwood on the grain, and at once
printed with the typical "levant grain," blacked and dried out. They are
then softened by machine, seasoned with logwood and albumen, glazed,
grained and oiled lightly with mineral oil. It will be observed that
stuffing is omitted.
Bellies may be dressed for linings. After soaking and splitting to the
required substance, they are bleached in a weak and warm solution of
oxalic acid, and drum-sumached at 110° F. After slicking well out they
are hand-stuffed on the grain with dubbin and water, or merely oiled,
and hung up to samm. They are then set-out flesh and grain. If the grain
be coarse, it is buffed and reset. After drying out the flesh is fluffed
and the grain dusted with French chalk.
In this section may be conveniently discussed the manufacture of legging
leather. Whilst in many respects a typical dressing leather there are
some rather important differences from the average upper leather.
Broadly speaking, the differences are that legging leather needs a
smooth grain, greater firmness and more thorough tannage on account of
the absence of stuffing.
The liming and bating are somewhat similar to dressing leather, though a
shorter liming with sulphides and a milder bating would be in order. The
tannage is mellow, but not so much as is usual for upper leather. Thus
gambier is used, but more valonia and myrabs are employed, and the
liquors may be strengthened with chestnut and quebracho extracts. The
hides are rounded before tanning into long butts or backs, and the
tannage is commenced in suspenders (18°-40°), which are kept acid by the
addition of lactic or acetic acid, in order to obtain the required
firmness; the goods are three weeks in these liquors. The backs next go
through rounds of dusters (40°-50°), in which they are put down with oak
bark and Natal bark. They are six weeks in this section, and then pass
to the layers. Three layers are given, first 50° for one week; second
55° for two weeks; and third 60° for two weeks. The tannage thus takes
fourteen weeks.
In finishing, the goods are soaked and split, and then scoured flesh and
grain. They are heavily sumached, slicked out thoroughly, oiled up with
linseed oil and dried out. They are then next damped back, stoned and
flatted. After further wetting and tempering they are dressed with Irish
moss and tallow on the flesh, and with gum tragacanth on the grain.
They are glassed whilst drying out, and then stained twice and glassed
again. They are again brushed, seasoned and glassed by machine.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 197-201 and 301-308.
SECTION IX.--BAG LEATHER
Hides to be tanned for bag leather receive a treatment which is little
different in fundamental principle from that of dressing hides for upper
leather, except that the tannage is usually shorter. Hides for bags and
portmanteaux represent a type of dressing leather in which the
outstanding features are that the goods are split but not rounded. The
splitting is done at all stages, according to the requirements of the
tanner. Some tanners split "green," _i.e._ split the pelt itself. The
advantage of this is that the fleshes may then be treated in quite a
different way, _e.g._ pickled or given a much cheaper tannage. Other
manufacturers split after tanning, the advantage being that there is
much less material to handle. The general opinion, however, favours a
middle course in which the hides are split after being in the tan
liquors for a short time. The advantage of this course is that the hides
are easiest to split under these conditions--a great
consideration--being coloured through with tan, just a little plumped,
but not hard. A smoother flesh is obtained together with more even
substance. Here again, however, are differences; some tanners prefer to
split after two days, others after two weeks in tan. Much depends upon
the nature of the tan and the strength of the liquors.
For this class of work, flat, spready and evenly grown cowhides are
obviously the most suitable material, and are invariably used. It is
important, however, that the grain be good, and free from scratches and
similar defects. The tannage must be sweet and mellow, _i.e._ contain no
acid and little astringent tan. Hence myrabolans and gambier have always
been the favourite tanning materials. A soft and mellow tannage is the
more important, inasmuch as the leather is not heavily stuffed with
grease in finishing. These types of method for tanning split hides will
now be outlined, and the nature of the currying then indicated.
_Type 1_.--In this a long mellow liming of 15-16 days is given, much
like that described for harness leather in Section III., Type 3.
Only lime is used, but the liquors are not allowed to get dirty. The
three-pit system is much the best. The hides are trimmed at the rounding
tables, and then bated in hen or pigeon dung for three days at 75°-85°
F. The deliming is commenced by washing in tepid water before bating,
and is completed by a bath of boric acid, using up to 30 lbs. acid per
100 hides as necessary. In this and other processes for split hides it
is essential to obtain all the lime out, but to do no plumping with
acid. Lactic acid may also be used, but it is not so convenient to hit
the neutral point with it.
The tannage consists of oak bark and myrabs together with gambier. These
may be partly replaced by Natal bark, valonia, and quebracho
respectively. It is sometimes desired to have a smooth finish, but
sometimes to work up a "grain." In the latter case the hides are first
put through colouring pits containing fresh leach liquor. In these they
are constantly handled for a few hours. A little experience indicates
which leach liquor will serve the purpose. The hides then go through the
"green handlers" (8°-20°) in two weeks. The liquor is the old forward
handler liquor made up with gambier. The hides may be sammed and split
up at this stage, but the heavier goods may be tanned further. These
heavies and the grains of the split hides now go through the "forward
handlers" (20°-40°) for four weeks, and the heaviest goods given two
layers (40°) of two weeks each, and making ten in all.
_Type 2_.--In this a shorter liming of 8-9 days is given with the help
of sulphide. No dung bate is used, but the goods are washed with water
and bated with ammonium chloride and boric acid. The tannage is chiefly
of myrabs, but some valonia or Natal bark may be used together with
chestnut extract and some quebracho. Gambier is used in the early
liquors. The goods are coloured off in drum or paddle and tanned in
several sets of handlers, viz. green handlers (15°-35°) three or four
days; second handlers (35°-60°) two weeks; forward handlers (60°-80°)
1-1/2 weeks; and floaters (80°-90°) for three weeks. The tannage is thus
6-1/2 weeks in all. The arrangement of pits is a matter of local
convenience, and the number of sets of equal strength is determined by
the number of hides being tanned. The hides are split green or after
passing through the green set. After tanning they are oiled with cod oil
and dried out.
_Type 3_ is illustrated by American methods. The goods are tacked on
laths or racks with copper nails in order to ensure smooth grain. They
are then suspended in tan liquors. The tannage is largely with gambier
and in weak liquors, which also help to give smooth grain. The tendency
is to employ handler rounds involving a rather large number of pits, and
to work these on the press system. Handling is also saved by plumping
the liquors instead of shifting the goods forward, and by rocking the
suspenders instead of handling up and down. The hides are split after
about a month, and the heavier grains laid away in hemlock liquors.
_Type 4_.--This is a rapid process throughout. The hides are limed in
6-7 days with the help of sulphide, and "bated" by washing in warm water
and then in cold to which hydrochloric acid is gradually added,
finishing off again in tepid water. The hides are now coloured off in
paddles, put through a small handler round (11°-20°) for half a week,
and then split. The grains are drum tanned in a mixture of chestnut and
quebracho extract, over a period of about three days in which the liquor
is strengthened gradually from 30° to 50°. The fleshes are drum tanned
with the old grain liquors after strengthening with quebracho.
The split hide grains for bag work, after tanning, are drummed in
sumach, rinsed, drained, and oiled up to dry out, with some setting out.
After wetting back they are shaved if necessary, hand scoured, and
heavily sumached again to get a light even colour. The goods are slicked
out, oiled up to samm, reset and dried out. They are next stained,
sammed, printed by machine, dubbined or tallowed, "grained" (see Part
II., Section I.), brushed and rubbed with flannel.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 202, 308.
SECTION X.--PICKING BAND BUTTS
It is the paradox of vegetable tannage that the less the pelt is tanned
the stronger is the leather produced. The manufacture of butts for
picking bands affords a good illustration. What is required is a leather
of maximum toughness, pliability and durability. Any factor reducing the
tensile strength of the leather is fatal. Hence, compared with most
other tannages, picking band butts are under-tanned. To ensure the
desired softness and pliability, moreover, it is necessary to have a
mellow liming, rather heavy bating, and a soft mellow tannage in sweet
and weak liquors. The required durability and the necessity for weak
liquors both point to oak bark as the most suitable tanning material,
assisted by some gambier in the early stages.
A good quality hide is chosen, and given a long and mellow liming of
about 15-16 days. The one-pit system may be used, and the hides are put
into an old lime for about five days with frequent handling and then
placed in a new lime which is made up in a pit containing about a foot
depth of the old liquor. After about twelve days another 1/3 cwt. of
lime may be added.
After unhairing and fleshing the goods are bated in pigeon dung for four
days at a temperature of about 78° F., handling twice on the first and
last days. The bating is stopped and the deliming completed by paddling
with boric acid (15 lbs. per 100 butts).
The tannage is commenced by paddling in a spent handler liquor (4°) to
which a little gambier has been added. The butts then go through the
first handlers (5°-15°), which are rounds of ten pits in which the goods
are handled every day in the first week, and alternate days in the
second week, and are shifted forward twice a week in the next pit. The
goods are therefore in this set for five weeks. Gambier is added to
these liquors as needed. The butts next pass to duster rounds of four
pits, in which they are dusted down in a liquor of 20° for four weeks
with 1-2 cwt. of oak bark. The liquor is obtained from the leaches, and
afterwards run alternately to the leaches and to the first handlers. As
many as six layers are now given of 20°-25° strength, in which the butts
are dusted down with 2-3 cwt. oak bark for three weeks. The layer
liquors are received from and returned to the leaches, which are made
from the "fishings" from the layers. The tannage lasts, therefore, 27
weeks, of which 18 weeks (two-thirds) are in layers.
Shorter tannages are now often given, using stronger liquors, much as in
ordinary dressing leather.
The tanned butts are rough dried, and then wet in for shaving. They are
thoroughly scoured, flesh and grain. They are next drummed for
three-quarters of an hour in sumach, struck out and hung up to samm.
Hand stuffing is best, to avoid any tendering owing to high temperature,
but drum stuffing is also used. After setting out and stoning on the
grain they are stuffed with warm cod oil and laid away in grease for
several weeks, re-oiling occasionally. They may be stained before
stuffing.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 203, 310.
PART II.--SKINS FOR LIGHT LEATHERS
SECTION I.--PRINCIPLES AND GENERAL METHODS OF LIGHT LEATHER MANUFACTURE
The term "skin," like the term "hide," in its widest sense applies to
the natural covering for the body of any animal, but is generally used
with a narrower meaning in which it applies only to the covering of the
smaller animals. Thus we speak of sheep skins, goat skins, seal skins,
pig skins, deer skins, and porpoise skins. It is in this sense that it
will be used in this volume. The treatment of such skins to fit them for
useful purposes comprises the light leather trade. Whilst this branch of
the leather industry is certainly utilitarian, the artistic element is a
great deal more prominent in it than in the heavy leather branch. Thus
the light leathers are often dyed and artistically finished, and their
final purposes (such as fancy goods, upholstery, bookbinding, slippers,
etc.) have rather more of the element of luxury than of essential
utility. The total weight and value of the skins prepared, and of the
materials used in their preparation, are naturally considerably smaller
than those of the heavy leather trade. In the latter, moreover, one has
to consider the purpose in view from the very commencement of
manufacture and vary the process accordingly, but in light leather
manufacture one aims rather, in the factory, at a type of leather such
as morocco leather, and only after manufacture is it fitted to such
purposes as may be particularly suited to the actual result. These
results depend very largely upon the "grain pattern" which is natural to
the skin of any one species of animals. Hence in Part II. of this volume
it has been found most convenient to deal with the different classes of
skins in different sections. Just as the hides of ox and heifer were
much the most numerous and important of hides, so also naturally are
sheepskins the most prominent section of the raw material of the light
leather trade. This is the more true because the skin is valued for its
wool as well as for its pelt; indeed, the wool is often considered of
primary importance, and receives first consideration in fellmongering.
Unfortunately for the light leather trade, sheepskins, though most
numerous, do not give the best class of light leather, the quality being
easily surpassed in strength, beauty and durability by the leather from
goat or seal skins.
In the wet work for the preparation of skins for tannage much the same
general principles and methods are embodied as in the case of hides, but
with appropriate modifications. As soft leathers are chiefly wanted, a
mellow liming is quite the usual requirement for all skins. It is also
usual to have a long liming, for some skins (like those of sheep and
seal) have much natural fat which needs the saponifying influence of
lime and lipolytic action of the enzymes of the lime liquors; whilst
other skins (like those of goat and calf) are very close textured and
need the plumping action of the lime and a certain solution of
interfibrillar substance. In consequence of the long mellow liming,
sulphides are not usually necessary, and indeed sodium sulphide is not
usually desirable, on account of its tendency to make the grain harsh.
It is used, however, for unwoolling sheepskins, in such a manner that
the grain is not touched. Similarly caustic soda is seldom required, and
the yield of pelt by weight is usually a small consideration. Systems of
liming show some variety. The one-pit system is very common, and is less
objectionable for a long mellow liming, but rounds of several pits are
also used, and in some cases even more than one round. This is obviously
conducive to regularity of treatment, and as the work involved in
shifting the goods is much less laborious than in the case of heavy ox
hides, it would seem a preferable alternative. The depilation of
sheepskins involves very special methods of treatment (sweating and
painting) on account of the importance and value of the wool, the
quality and value of which would be impaired by putting the skins
through ordinary lime liquors. The pelts, however, are limed after
unwoolling.
In deliming light leathers the process of puering is widely used. This
consists in immersing the skins after depilation in a warm fermenting
infusion of dog-dung. In principle this disgusting process presents a
close analogy with bating, and indeed the two terms are both used
somewhat loosely, but there are nevertheless several points in which the
two processes are radically different. The dog-dung puer is a process
carried out at a higher temperature than the fowl-dung bate; it is also
a much quicker process, and the infusion employed is generally more
concentrated. Whilst the fowl-dung bate is always slightly alkaline to
phenolphthalein the dog-dung puer is always acid to this indicator, and
the course of the puering may be conveniently followed by testing the
pelts with it. The mechanism of the two processes is also probably
somewhat different. The mechanism of the dog-dung puer has been largely
made clear by the researches of Wood and others, and been found due
partly to a deliming action by the amine salts of weak organic acids and
partly to the action of enzymes from a bacillus of the coli class, which
received the name of _B. erodiens_, and which effects a solvent action
on the interfibrillar substance. As we have noted (Part I., Section
II.), the fowl-dung bate involves two fermentations, in each of which
(ærobic and anærobic) several species of bacteria are probably active.
Wood found the bacteria of the bate to be chiefly cocci, and ascribed
part of the difference in mechanism by the nature of the media, which in
the bate includes also the urinary products. In the dog-dung puer, also,
a lipolytic action is probably an essential part of the total effect.
The puer gives a much more complete deliming and a much softer and more
relaxed pelt than the bate, it is therefore particularly suited to the
needs of light leather manufacture. The puering action has been imitated
fairly successfully by artificial methods. "Erodin" (Wood, Popp and
Becker) involves the use of _B. erodiens_ and a suitable culture medium
including organic deliming salts: "Oropon," "Pancreol" and others
involve the use of ammonium chloride and trypsin, together with some
inert matter.
Light-leather goods are usually drenched after puering. They are also
often split green after the wet work. Sheepskins thus yield "skivers"
(the grain split), whilst the flesh split is often given an oil tannage
(see Part IV., Section III.). The greasy nature of sheep and seal skins
necessitates the processes of "degreasing." In the case of sealskins
this is done largely before liming, but with sheepskins either after
being struck through with tan, or after tannage is complete. Sheepskins
are often preserved in the pelt by pickling with sulphuric acid and
salt, which process forms a temporary leather. The fibres of the pelt
are dried in a separate condition, but the adsorption is easily
reversible and the pelts may be "depickled" by weak alkalies and
afterwards given an ordinary vegetable tannage.
In the vegetable tannage of skins for light leathers, the same
theoretical considerations have force as in the heavy-leather section,
but the former has its own rather special requirements and aims.
Generally speaking, a softer and more flexible leather is required, but
these qualities must not be imparted by stuffing with grease as in the
currying of dressing leather, because a bright and grease-free result is
usually required. Hence it is important that a sweet mellow tannage be
given. The durability of the leather is also a primary consideration for
goods intended for bookbinding, upholstery, etc., and the tannage must
be arranged to impart this quality and avoid anything tending to cause
the perishing of the fibre. Thus oak bark is a popular tanning material,
and sulphuric acid very definitely avoided. The tannage must be fast,
and take the dyestuffs well, and for the production of light shades of
colour in dyeing must be a light-coloured tannage. All these qualities
are imparted by sumach, which also fits in excellently with the other
general requirements, such as softness, brightness and durability.
Hence sumach is the principal light-leather tanning material, but the
tendency is to employ other materials--oak bark, myrabs, and chestnut
extract--to do much of the intermediate tanning, so that the expensive
and useful sumach may be used for setting the colour and grain at the
commencement, and for brightening, bleaching and mordanting the leather
at the end of the tanning process. Weight is generally no consideration,
but area is often a definite aim, partly because some goods are sold by
area and partly because the striking out, setting out and similar
operations improve the quality of the leather by giving evenness of
finish. Leather well struck out, moreover, is less liable to go out of
shape. As the grain pattern is so important in the finished leather,
appropriate care must be taken during tannage. If a smooth or a fine
grain finish is wanted, for example, the goods must not be allowed to
get wrinkled, creased, doubled or unduly bent to and fro during the
tanning. For such goods, suspension, careful handling and even the "bag
tannage" may be desirable, whilst for coarser and larger grains paddles
or drums may be more extensively used.
Amongst the finishing processes dyeing holds an important position. The
nature of the process has many points of similarity with that of
tanning. The great specific surface of pelt is probably more enhanced
than otherwise during tannage, at any rate with light leathers, owing to
the isolation of fibres, and consequently leather is as liable as pelt
to exhibit adsorption. The dyestuffs, on the other hand, are substances
very easily adsorbed. Some (like eosin and methylene blue) are
crystalloids, some (like fuchsin and methyl violet) are semi-colloids,
whilst others (like Congo red and night blue) are undoubted colloids
forming sols (usually emulsoid) with water as dispersion medium. The
crystalloids and semi-colloids may also be obtained in colloidal
solution, sometimes being so changed on the mere addition of salts to
the solution. In addition, the pelt has been mordanted with tannin. If,
however, leather has been kept long in the rough-tanned or "crust"
state, this may not be so effective, owing probably to the secondary
changes in tanning (Part I., Section III.), but such leathers are
usually "retanned" or prepared for dyeing by sumaching (which process
also incidentally bleaches). The tannin mordant assists materially in
the fixation of the dyes. In the case of basic dyestuffs, lakes also are
formed, _i.e._ there is a mutual precipitation of oppositely charged
colloids (+dye, -tannin). The dyeing of leather is thus a case of
colloid reactions even more complicated than that of tanning.
Another finishing operation typical of the light leathers is "graining"
or "boarding." In this the skins after dyeing and drying are worked by a
board which is covered by cork, rubber, perforated tin or other
material, and so grips or "bites" the leather. The object of "graining"
is to work up the grain pattern by pushing or pulling a fold on the skin
with the board. The nature of the grain varies with the thickness and
the hardness of the skin, with the amount of pressure applied, with the
nature of the board, with the direction of the boarding and with the
total number of directions boarded. There is thus infinite scope for
variety of finish, and hence arise bold grain, fine grain, hard grain,
straight grain, cross grain, long grain, etc. The operation requires
considerable skill and experience. In the case of skins with little
natural grain (such as sheepskin) embossing and printing machines
impress the desired pattern.
In seasoning, a dressing is applied containing essentially albumins and
emulsified fats, _e.g._ egg albumin and milk. Colouring matters are also
often added to intensify or modify the shade. After seasoning the goods
are usually "glazed" by a machine which rubs the seasoned grain with
considerable pressure, by a glass or hardwood tool, and so produces a
high gloss, for which the seasoning is very largely a preparation. Light
leathers are very lightly oiled with linseed or mineral oil.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 220, 394.
Bennett, "Manufacture of Leather," pp. 36-41, 55, 85-90, 92-112,
312, 332.
Wood, "Puering, Bating and Drenching of Skins."
Lamb, "Leather Dyeing and Finishing."
SECTION II.--GOATSKINS
Goatskins are amongst the most valued ra
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