The Manufacture of Paper
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Title: The Manufacture of Paper
Author: R. W. Sindall
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| are indicated by $bold$. Passages in small caps are shown in |
| title case without explicit indication. Subscripted characters |
| are indicated by _{subscript}. Superscripted characters are |
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THE MANUFACTURE
OF PAPER
BY
R. W. SINDALL, F.C.S.
CONSULTING CHEMIST TO THE WOOD PULP AND PAPER TRADES; LECTURER
ON PAPER-MAKING FOR THE HERTFORDSHIRE COUNTY COUNCIL, THE
BUCKS COUNTY COUNCIL, THE PRINTING AND STATIONERY
TRADES AT EXETER HALL, 1903-4, THE INSTITUTE
OF PRINTERS; TECHNICAL ADVISER TO THE
GOVERNMENT OF INDIA, 1905
AUTHOR OF "PAPER TECHNOLOGY," "THE SAMPLING OF WOOD PULP"
JOINT AUTHOR OF "THE C.B.S. UNITS, OR STANDARDS OF PAPER
TESTING," "THE APPLICATIONS OF WOOD PULP," ETC.
WITH ILLUSTRATIONS, AND A BIBLIOGRAPHY OF WORKS
RELATING TO CELLULOSE AND PAPER-MAKING
[Illustration]
NEW YORK
D. VAN NOSTRAND COMPANY
23 MURRAY AND 27 WARREN STREETS
1908
PREFACE
Paper-making, in common with many other industries, is one in which
both engineering and chemistry play important parts. Unfortunately
the functions of the engineer and chemist are generally regarded as
independent of one another, so that the chemist is only called in by
the engineer when efforts along the lines of mechanical improvement
have failed, and _vice versa_. It is impossible, however, to draw a
hard and fast line, and the best results in the art of paper-making are
only possible when the manufacturer appreciates the fact that the skill
of both is essential to progress and commercial success.
In the present elementary text-book it is only proposed to give an
outline of the various stages of manufacture and to indicate some of
the improvements made during recent years.
The author begs to acknowledge his indebtedness to manufacturers and
others who have given permission for the use of illustrations.
CONTENTS
PAGE
PREFACE v
LIST OF ILLUSTRATIONS ix
CHAPTER
I. HISTORICAL NOTICE 1
II. CELLULOSE AND PAPER-MAKING FIBRES 20
III. THE MANUFACTURE OF PAPER FROM RAGS 47
IV. ESPARTO AND STRAW 72
V. WOOD PULP, AND WOOD PULP PAPERS 95
VI. BROWN PAPERS AND BOARDS 126
VII. SPECIAL KINDS OF PAPER 137
VIII. CHEMICALS USED IN PAPER-MAKING 153
IX. THE PROCESS OF "BEATING" 175
X. THE DYEING AND COLOURING OF PAPER PULP 199
XI. PAPER MILL MACHINERY 214
XII. THE DETERIORATION OF PAPER 229
XIII. BIBLIOGRAPHY 253
INDEX 273
LIST OF ILLUSTRATIONS
FIG. PAGE
1. SHEET OF PAPYRUS, SHOWING THE LAYERS CROSSING ONE
ANOTHER 3
2. AN EARLY PAPER MILL (FROM "KULTURHISTORISCHEN BILDERBUCH,"
A.D. 1564) 10
3. THE PAPER MILL OF ULMAN STROMER, A.D. 1390 (SUPPOSED
TO BE THE OLDEST KNOWN DRAWING OF A PAPER MILL) 12
4. THE FIRST PAPER MACHINE, A.D. 1802. PLAN AND ELEVATION 17
5. THE IMPROVED PAPER MACHINE OF A.D. 1810 18
6. A RAG SORTING HOUSE 47
7. A RAG DUSTER 49
8. A RAG CUTTER 50
9. INTERIOR OF PAPER MILL FOR HAND-MADE PAPER (R. BATCHELOR
& SONS) 51
10. VIEW OF A RAG BOILER, SHOWING CONNECTIONS 52
11. A BREAKING AND WASHING ENGINE 54
12. OETTEL AND HAAS' APPARATUS FOR THE MANUFACTURE OF
ELECTROLYTIC BLEACH LIQUOR 58
13. THE "HOLLANDER" BEATING ENGINE 59
14. THE HAND MOULD, SHOWING FRAME AND DECKLE 61
15. APPARATUS FOR SIZING PAPER IN CONTINUOUS ROLLS 63
16. A SUPERCALENDER 65
17. THE FIRST WATERMARK IN PAPER 67
18. COTTON 69
19. LINEN 70
20. AN ESPARTO DUSTER 74
21. SINCLAIR'S "VOMITING" ESPARTO BOILER 75
22. A PORION EVAPORATOR 76
23. SCOTT'S MULTIPLE EFFECT EVAPORATOR 79
24. A PRESSE-PÂTE FOR ESPARTO PULP 85
25. ESPARTO PULP 88
26. A CYLINDRICAL DIGESTER FOR BOILING FIBRE 89
27. STRAW 93
28. A PAIR OF BARKERS FOR REMOVING BARK FROM LOGS OF WOOD 98
29. VIEW OF HORIZONTAL GRINDER (A), WITH SECTION (B) 99
30. A VERTICAL GRINDER FOR MAKING HOT GROUND MECHANICAL
WOOD PULP 101
31. CENTRIFUGAL SCREEN FOR WOOD PULP 102
32. SECTION OF CENTRIFUGAL SCREEN FOR WOOD PULP 103
33. WOOD PULP DIGESTER, PARTLY IN ELEVATION, PARTLY IN
SECTION 106
34. VIEW OF ORDINARY SULPHUR-BURNING OVENS 108
35. SPRUCE WOOD PULP 114
36. MECHANICAL WOOD PULP 115
37. THE SCREENS FOR REMOVING COARSE FIBRES FROM BEATEN
PULP 118
38. THE PAPER MACHINE (WET END SHOWING WIRE) 119
39. PAPER MACHINE SHOWING WIRE, PRESS ROLLS, AND DRYING
CYLINDERS 123
40. SINGLE CYLINDER OR YANKEE MACHINE 130
41. SECTION OF WET PRESS, OR BOARD MACHINE 131
42. DOUBLE CYLINDER BOARD MACHINE 133
43. APPARATUS FOR MAKING PARCHMENT PAPER 138
44. GENERAL ARRANGEMENT OF PLANT FOR MAKING "ART" PAPER 143
45. SECTIONAL ELEVATION OF "COATING" PLANT 144
46. COTTON PULP BEATEN 8 HOURS 179
47. COTTON PULP BEATEN 37 HOURS 180
48. PLAN AND SECTIONAL ELEVATION OF A "HOLLANDER" 185
49. BEATING ENGINE WITH FOUR BEATER ROLLS 186
50. UMPHERSTON BEATER 188
51. SECTION OF UMPHERSTON BEATING ENGINE 189
52. NUGENT'S BEATING ENGINE WITH PADDLES FOR CIRCULATING
THE PULP 190
53. A "TOWER" BEATING ENGINE WITH CENTRIFUGAL PUMP FOR
CIRCULATING PULP 191
54. WORKING PARTS OF A MODERN REFINING ENGINE 192
55. CONVENTIONAL DIAGRAM OF A WATER SOFTENING PLANT 216
56. AN "ENCLOSED" STEAM ENGINE 220
57. AN ELECTRICALLY DRIVEN PAPER MACHINE 222
58. DIAGRAM OF THE "EIBEL" PROCESS 223
THE MANUFACTURE OF PAPER
CHAPTER I
HISTORICAL NOTICE
_History._--The art of paper-making is undoubtedly one of the most
important industries of the present day. The study of its development
from the early bygone ages when men were compelled to find some means
for recording important events and transactions is both interesting and
instructive, so that a short summary of the known facts relating to the
history of paper may well serve as an introduction to an account of the
manufacture and use of this indispensable article.
_Tradition._--The early races of mankind contented themselves with
keeping alive the memory of great achievements by means of tradition.
Valiant deeds were further commemorated by the planting of trees, the
setting up of heaps of stones, and the erection of clumsy monuments.
_Stone Obelisks._--The possibility of obtaining greater accuracy by
carving the rude hieroglyphics of men and animals, birds and plants,
soon suggested itself as an obvious improvement; and as early as B.C.
4000 the first records which conveyed any meaning to later ages were
faithfully inscribed, and for the most part consigned to the care of
the priests.
_Clay Tablets._--The ordinary transactions of daily life, the writings
of literary and scientific men, and all that was worthy of note in
the history of such nations as Chaldea and Assyria have come down to
us also, inscribed on clay tablets, which were rendered durable by
careful baking. On a tablet of clay, one of the earliest specimens of
writing in existence, now preserved in the British Museum, is recorded
a proposal of marriage, written about B.C. 1530, from one of the
Pharaohs, asking for the hand of the daughter of a Babylonian king.
_Waxed Boards._--Bone, ivory, plates of metal, lead, gold, and brass,
were freely used, and at an early period wooden boards covered with
wax were devised by the Romans. In fact, any material having a soft
impressionable surface was speedily adopted as a medium for the
permanent expression of men's fancy, so that it is not strange to find
instances of documents written on such curious substances as animal
skins, hides, dried intestines, and leather. The works of Homer,
preserved in one of the Egyptian libraries in the days of Ptolemæus
Philadelphus, were said to have been written in letters of gold on the
skins of serpents.
_Leaves, Bark._--The first actual advance in the direction of paper,
as commonly understood, was made when the leaves and bark of trees
were utilised. The latter especially came speedily into favour, and
the extensive use of the inner bark (_liber_) made rapid headway.
Manuscripts and documents written on this liber are to be found in many
museums.
_Papyrus._--The discovery of the wonderful properties of the Egyptian
papyrus was a great step in developing the art of paper-making. The
date of this discovery is very uncertain, but one of the earliest
references is to be found in the works of Pliny, where mention is made
of the writings of Numa, who lived about B.C. 670. This celebrated
plant had long been noted for its value in the manufacture of mats,
cordage, and wearing apparel, but its fame rests upon its utility in
quite a different direction, namely, for conveying to posterity the
written records of those early days which have proved a source of
unending interest to antiquaries.
[Illustration: FIG. 1.--Sheet of Papyrus, showing the layers crossing
one another (Evans).]
The Egyptian papyrus was made from the fine layers of fibrous matter
surrounding the parent stem. These layers were removed by means of a
sharp tool, spread out on a board, moistened with some gummy water, and
then covered with similar layers placed over them crosswise. The sheets
so produced were pressed, dried, and polished with a piece of ivory or
a smooth stone. Long rolls of papyrus were formed by pasting several
sheets together to give what was termed a _volumen_.
_Roman Papyri._--The Romans improved the process of manufacture,
and were able to produce a variety of papers, to which they gave
different names, such as _Charta hieratica_ (holy paper, used by
priests), _Charta Fanniana_ (a superior paper made by Fannius), _Charta
emporetica_ (shop or wrapping paper), _Charta Saitica_ (after the city
of Sais), etc. The papyrus must have been used in great quantities
for this purpose, since recent explorations in Eastern countries have
brought to light enormous finds of papyri in a wonderful state of
preservation. In 1753, when the ruins of Herculaneum were unearthed, no
less than 1,800 rolls were discovered. During the last ten years huge
quantities have been brought to England.
_Parchment._--Parchment succeeded papyrus as an excellent writing
material, being devised as a substitute for the latter by the
inhabitants of Pergamus on account of the prohibited exportation of
Egyptian papyrus. For many centuries parchment held a foremost place
amongst the available materials serving the purpose of paper, and even
to-day it is used for important legal documents. This parchment was
made from the skins of sheep and goats, which were first steeped in
lime pits, and then scraped. By the plentiful use of chalk and pumice
stone the colour and surface of the parchment were greatly enhanced.
Vellum, prepared in a similar manner from the skins of calves, was
also extensively employed as a writing material, and was probably the
first material used for binding books. Until comparatively recent times
the term "parchment" comprehended vellum, but the latter substance is
much superior to that manufactured from sheep and goat skins.
_Paper._--The Chinese are now generally credited with the art of making
paper of the kind most familiar to us, that is from fibrous material
first reduced to the condition of pulp. Materials such as strips of
bark, leaves, and papyrus cannot of course be included in a definition
like this, which one writer has condensed into the phrase "Paper is an
aqueous deposit of vegetable fibre."
A.D. 105.--The earliest reference to the manufacture of paper is to be
found in the Chinese Encyclopædia, wherein it is stated that Ts'ai-Lun,
a native of Kuei-yang, entered the service of the Emperor Ho-Ti in A.D.
75, and devoting his leisure hours to study, suggested the use of silk
and ink as a substitute for the bamboo tablet and stylus. Subsequently
he succeeded in making paper from bark, tow, old linen, and fish nets
(A.D. 105). He was created marquis in A.D. 114 for his long years of
service and his ability.
A.D. 704.--It has been commonly asserted that raw cotton, or cotton
wool, was first used by the Arabs at this date for the manufacture
of paper, they having learnt the art from certain Chinese prisoners
captured at the occupation of Samarkand by the Arabs. The complete
conquest of Samarkand does not, however, seem to have taken place until
A.D. 751, and there is little doubt that this date should be accepted
for the introduction of the art of paper-making among the Arabs.
_Recent Researches._--Professors Wiesner and Karabacek have ascertained
one or two most important and interesting facts concerning the actual
manufacture of _pure_ rag paper. In 1877 a great quantity of ancient
manuscripts was found at El-Faijum, in Egypt, comprising about 100,000
documents in ten languages, extending from B.C. 1400 to A.D.
1300, many of which were written on paper. The documents were closely
examined in 1894 by these experts, at the request of the owner, the
Archduke Rainer of Austria.
Researches of a later date resulted in the discovery of some further
interesting documents which appear to establish with some degree of
certainty the approximate date at which _pure_ rag paper, that is,
paper made entirely from rag, was manufactured.
Chinese documents dated A.D. 768-786, which have been reported upon
by Dr. Hoernle, and others dated A.D. 781-782-787, reported upon by
Dr. Stein as recently as 1901, appear to show what materials were used
by the Chinese paper-makers in Western Turkestan. The manuscripts
mentioned were dug out from the sand-buried site of Dandan Uilig, in
Eastern Turkestan.
Professor Wiesner found that all the papers of the Rainer collection
were made of _linen_ rag, with an occasional trace of _cotton_,
probably added accidentally. The earliest dated paper was a letter A.D.
874, but two documents, which from other reasons could be identified as
belonging to A.D. 792, proved that at the end of the eighth century the
Arabs understood the art of making linen paper on network moulds, and
further that they added starch for the purpose of sizing and loading
the paper.
Professor Karabacek advances some ingenious explanations as to the
origin of the idea that raw cotton was first used for paper-making,
and he suggests that the legend owes its origin to a misunderstanding
of terms. In mediæval times paper was known as _Charta bombycina_, and
sometimes as _Charta Damascena_, the latter from its place of origin.
Paper was also made in Bambyce, and a natural confusion arose between
the terms, since the word _bombyx_ was used as a name for cotton, and
the paper commonly in use suggested that material to the mind of the
observer, and the name became corrupted to _bombycina_.
The suggestions of Professor Karabacek, together with the microscopical
investigations of Dr. Wiesner, appear to show that paper made entirely
from raw cotton fibre was not known.
_Invention of Rag Paper._--Dr. Hoernle, in discussing this question,
points out that, taking A.D. 751 as the date when the Arabs learnt
the art of paper-making, and A.D. 792 as the date when paper made
entirely of linen rag was produced, the date of the invention of rag
paper must lie between these two dates. The documents discovered in
Eastern Turkestan and bearing the dates mentioned, which papers fill
up the gap between the years A.D. 751 and A.D. 792, were found to
contain certain raw fibres, such as China grass, mulberry, laurel, as
the main constituents, and macerated flax and hemp rags as the minor
constituents.
The addition and substitution of rag evidently increased in course of
time, and since the improvement thus effected soon became an obvious
and established fact, the raw fibres were omitted. Hence the credit
of the manufacture of pure rag paper would be given to the people of
Samarkand, the date being between the years A.D. 760 and A.D. 792; and
further the constitution of such paper has been shown by Dr. Wiesner to
be linen, and not cotton, as commonly stated.
These researches are of such interest that we quote Professor Hoernle's
translation of the summary of the principal results of Dr. Wiesner's
examination of the Eastern Turkestani papers so recently discovered:--
"Taking into account the dates assigned to the papers on palæographic
grounds, the following conclusions may be drawn from the examination of
their material:--
"(1) The oldest of the Eastern Turkestani papers, dating from the
fourth and fifth centuries A.D., are made of a mixture of raw fibres
of the bast of various dicotyledonous plants. From these fibres the
half-stuff for the paper was made by means of a rude mechanical process.
"(2) Similar papers, made of a mixture of raw fibres, are also found
belonging to the fifth, sixth, and seventh centuries. But in this
period there also occur papers which are made of a mixture of rudely
pounded rags and of raw fibres extracted by maceration.
"(3) In the same period papers make their appearance in which special
methods are used to render them capable of being written on, viz.,
coating with gypsum and sizing with starch or with a gelatine extracted
from lichen.
"(4) In the seventh and eighth centuries both kinds of papers are of
equal frequency, those made of the raw fibre of various dicotyledonous
plants and those made of a mixture of rags and raw fibres. In this
period the method of extracting the raw fibre is found to improve from
a rude stamping to maceration; but that of preparing the rags remains a
rude stamping, and in the half-stuff thus produced from rags it is easy
to distinguish the raw fibre from the crushed and broken fibre of the
rags.
"(5) The old Eastern Turkestani (Chinese) paper can be distinguished
from the old Arab paper, not only by the raw fibres which accompany the
rag fibres, but also by the far-reaching destruction of the latter.
"(6) The previous researches of Professor Karabacek and the author had
shown that the invention of rag paper was not made in Europe by Germans
or Italians about the turn of the fourteenth century, but that the
Arabs knew its preparation as early as the end of the eighth century.
"The present researches now further show that the beginnings of the
preparation of rag paper can be traced to the Chinese in the fifth or
fourth centuries, or even earlier.
"The Chinese method of preparing rag paper never progressed beyond its
initial low stage. It was the Arabs who, having been initiated into the
art by the Chinese, improved the method of preparing it, and carried it
to that stage of perfection in which it was received from them by the
civilised peoples of Europe in the mediæval ages.
"(7) The author has shown that the process of sizing the paper with
starch in order to improve it was already known to the Arabs in the
eighth century. In the fourteenth century the knowledge of it was lost,
animal glue being substituted in the place of starch, till finally in
the nineteenth century, along with the introduction of paper machines,
the old process was resuscitated. But the invention of it was due to
the Chinese. The oldest Eastern Turkestani paper which is sized with
starch belongs to the eighth century.
"(8) The Chinese were not only the inventors of felted paper and the
imitators of rag paper--though in the preparation of the latter they
made use of rags only as a surrogate by the side of raw fibres--but
they must also be credited with being the forerunners of the modern
method of preparing 'cellulose paper.' For their very ancient practice
of extracting the fibre from the bark and other parts of plants by
means of maceration is in principle identical with the modern method of
extracting 'cellulose' by means of certain chemical processes."
[Illustration: FIG. 2.--An Early Paper Mill (from "Kulturhistorisches
Bilderbuch," A.D. 1564).]
_Paper-making in Europe._--The introduction of the art into Europe
seems to have taken place early in the eleventh century, when the Moors
manufactured paper at Toledo. The early authorities who have studied
this subject express the opinion that the paper produced in Europe at
this time was made from cotton rags and from raw cotton, but, in view
of the recent researches into the composition of paper, it is difficult
to say how this idea arose, unless we accept the explanation offered by
Professor Karabacek. In standard encyclopædias the following statements
are made as to existing early documents printed on paper made in
Europe:--
A.D. 1075. Syriac manuscripts of early date in the British
Museum.
A.D. 1102. A document printed on cotton, being a deed of King
Roger of Sicily, now at Vienna.
A.D. 1178. A treaty of peace between the Kings of Aragon and
Spain, said to be printed on linen paper, preserved at
Barcelona.
A.D. 1223. The "Liber Plegierum," printed on rough cotton
paper.
One of the most interesting books on this subject is the "Historical
Account of the Substances used to describe Events from the Earliest
Date," by Matthias Koops, published in 1800. This writer appears to
have obtained most of his information from German authorities.
The industry of paper-making passed through Spain into Italy, France,
and the Netherlands. In 1189 paper was being manufactured at Hainault,
in France, and the industry rapidly spread all over the Continent.
In 1390 Ulman Stromer established a mill at Nuremberg, in Germany,
employing a great number of men, who were obliged to take an oath that
they would not teach anyone the art of paper-making or make paper on
their own account. In the sixteenth century the Dutch endeavoured
to protect their industry by making the exportation of moulds for
paper-making an offence punishable by death.
The bulk of the paper used in England was imported from France and
Holland, and it was many years before the industry was established
in England. This is not surprising in view of the protective and
conservative policy of the Continental paper-makers.
[Illustration: FIG. 3.--The Paper Mill of Ulman Stromer, A.D. 1390
(supposed to be the oldest known drawing of a Paper Mill).]
_Paper-making in England._--The actual period at which the manufacture
of paper was first started in England is somewhat uncertain. The
first mention of any paper-maker is found in Wynkyn de Worde's "De
Proprietatibus Rerum," printed by Caxton in 1495, the reference being
as follows:--
And John Tate the younger, joye mote he brok,
Which late hathe in England, doo
Make thys paper thynne,
That now in our Englyssh
Thys booke is prynted inne.
John Tate was the owner of a mill at Stevenage, Hertfordshire. In the
household book of Henry VII. an entry for the year 1499 reads, "Geven
in rewarde to Tate of the mylne, 6_s._ 8_d._"
In 1588 a paper mill was erected by Sir John Spielman, a German, who
obtained a licence from Queen Elizabeth "for the sole gathering for ten
years of all rags, etc., necessary for the making of paper." This paper
mill was eulogised by Thomas Churchyard in a long poem of forty-four
stanzas, of which we quote two:--
I prayse the man that first did paper make,
The only thing that sets all virtues forth;
It shoes new bookes, and keeps old workes awake,
Much more of price than all the world is worth:
It witnesse bears of friendship, time, and troth,
And is the tromp of vice and virtue both;
Without whose help no hap nor wealth is won,
And by whose ayde great works and deedes are done.
Six hundred men are set to worke by him
That else might starve, or seeke abroad their bread,
Who now live well, and goe full brave and trim,
And who may boast they are with paper fed.
Strange is that foode, yet stranger made the same,
For greater help, I gesse, he cannot give
Than by his help to make poore folk to live.
The industry made but little progress for some time after Spielman's
death, and up till 1670 the supplies of paper were obtained almost
entirely from France. The first British patent for paper-making was
granted to Charles Hildeyard in 1665 for "the way and art of making
blue paper used by sugar bakers and others." The trade received a great
impetus on account of the presence of Huguenots who had fled to England
from France in consequence of the revocation of the edict of Nantes in
1685.
In 1695 a company was formed in Scotland for the "manufacture of white
and printing paper."
Improvements in the art were slow until 1760, when Whatman, whose name
has since become famous in connection with paper, commenced operations
at Maidstone. Meantime the methods by which the rags were converted
into paper were exceedingly slow and clumsy, so that the output of
finished paper was very small.
Some interesting details as to the early manufacture of paper in
England are given by Mr. Rhys Jenkins, and from his account of "Early
Attempts at Paper-making in England, 1495-1788," the following extracts
have been made:--
About
1496. First attempts at paper-making by John Tate at Hertford.
1496. Tate's paper used by Wynkyn de Worde in "De Proprietatibus
Rerum."
1557. A paper mill in existence at Fenditton, Cambridge.
1569. A mill at Bemmarton, Wilts.
1574. Mill erected at Osterley, Middlesex, by Sir Thomas Gresham.
1585. Richard Tottyl asked for sole right to make paper for
thirty-one years, which was not granted.
1588. John Spilman erected a mill at Dartford, Kent. Granted a
patent for sole manufacture of paper.
1588. Churchyard's poem on the "Paper Myll built near Darthford by
Master Spilman."
1612. Robert Heyricke's mill at Cannock Chase, Staffordshire.
1636. The three or four paper mills in the neighbourhood of Hounslow
and Colnbrook temporarily shut down on account of the plague,
the collection of rags having been forbidden.
1665. Patent granted to Charles Hildeyard for an invention, "the way
and art of making blew paper used by sugar bakers and others."
1675. Approximate date of erection of mills at Wolvercote, Oxford,
where the Oxford India paper is now made.
1678. Mill at Byfleet, Surrey, mentioned by Evelyn in his diary.
1682. Bladen--A patent for an engine and process whereby rags are
wrought into paper.
1684. Baysmaker--A patent for "the art and mistery of making paper
in whole sheets."
1684. Jackson--A patent for "an engine, either for wind or water,
which prepareth all materials whereof paper may be made."
Evidently Jackson was acquainted with the "Hollander" beating
engine.
1686. A charter granted to the "White Paper Makers' Company" for the
sole right of making paper exceeding 4_s._ a ream in value.
1674. Annual importation of paper, presumably from France, stated to
be 160,000 reams, of average value of 5_s._ (Somers).
1689. Trade with France prohibited by royal proclamation.
1696. Price of paper very high owing to scarcity, being 11_s._ per
ream.
1712. Duties levied on all kinds of paper, manufactured or imported.
1725. Monopoly of making paper for Bank of England notes granted to
De Portal, of the Laverstoke mills, Hampshire. This paper is
still made by the firm of Messrs. Portal.
1739. Galliott and Parry estimated that there were 600 paper mills
in England, making 6,000 reams a day. The Commissioner of
Excise reported only 278.
1739. James Whatman erected a mill at Boxley, Maidstone.
1758. Baskerville printed an edition of Virgil on so-called "woven"
paper.
_Early Methods._--The most rapid development of the industry appears
to have taken place in Holland. The rags used for paper-making were
moistened with water and stored up in heaps until they fermented
and became hot. By this means the dirt and non-fibrous matter was
rendered partially soluble, so that on washing a suitable paper pulp
was obtained. The washed rags were then placed in a stamping machine
resembling an ordinary pestle and mortar. The mortars were constructed
of stone and wood, and the stamps were kept in motion by levers which
were raised by projections fixed on the shaft of a waterwheel. The
operation of beating thus occupied a long period, but the paper
produced was of great strength.
The invention of the "Hollander," a simple yet ingenious engine which
is deservedly known by the name of the country in which it first
originated, gave a tremendous impetus to the art of paper-making,
as by its means the quantity of material which could be treated in
twenty-four hours was greatly increased. Unfortunately the date of the
invention of this important machine has not been definitely traced. The
earliest mention of it seems to occur in Sturm's "Vollständige Mühlen
Baukunst," published in 1718. It was in extensive use at Saardam in
1697, so that the invention is at least some years previous to 1690.
On this point Koops says: "In Gelderland are a great many mills, but
some so small that they are only able to make 400 reams of paper
annually, and there are also water mills with stampers, like those
in Germany. But in the province of Holland there are windmills,
with cutting and grinding engines, which do more in two hours than
the others do in twelve. In Saardam 1,000 persons are employed in
paper-making."
THE FIRST FOURDRINIER PAPER MACHINE.
Up till the year 1799 paper was made entirely in sheets on a hand
mould, but during the last few years of the eighteenth century a
Frenchman, Nicholas Louis Robert, manager for M. Didot, who owned a
paper mill at Essones, had been experimenting for the purpose of making
paper in the form of a continuous sheet, and eventually produced some
of considerable length.
The idea was taken to England by Didot's brother-in-law, Gamble, and
introduced to the notice of Messrs. Fourdrinier, wholesale stationers,
of London.
[Illustration: FIG. 4.--The First Paper Machine, A.D. 1802. Plan and
Elevation.]
The first machine was naturally a very crude affair. It consisted of an
endless wire cloth stretched in a horizontal position on two rollers,
one of which rotated freely in a bearing attached to the frame of the
machine, the other being fitted in an adjustable bearing so that the
wire could be tightened up when necessary.
The beaten pulp, contained in a vat placed below the wire, was thrown
up in a continual stream upon the surface of the wire, and carried
forward towards the squeezing rolls. A shaking motion was imparted to
the travelling wire so as to cause the fibres to felt properly. A great
deal of the water fell through the meshes of the gauze, and further
quantities were removed by means of the press rolls. The wet paper was
then wound up on to a wooden roller, which was taken out as soon as
sufficient paper had been made.
[Illustration: FIG. 5.--The Improved Paper Machine of A.D. 1810.]
The whole process was carried on under great difficulties, but
substantial improvements were soon made by the enterprising
Fourdriniers, who commenced operations in Bermondsey, employing Mr.
Bryan Donkin, then in the service of Messrs. Hall & Co., of Dartford,
who had shown himself keenly interested in the machine. In 1803 the
first "Fourdrinier," so called, was built at Bermondsey, and erected at
Two Waters Mill in Herefordshire.
In this machine the mixture of pulp and water was carried forward
between two wires, and, after passing through the couch rolls,
transferred to an endless felt. This arrangement proved to be faulty
because the water did not escape freely enough from the wire, and a
great deal of the paper was spoilt.
Donkin, however, hit upon a simple but effective device for curing this
fault by altering the relative position of the two couch rolls. Instead
of keeping the two rolls exactly in a vertical position one over the
other, he placed them at a slight angle so that the upper one should
bear gently on the web of paper carried by the wire before receiving
the full pressure of the rolls, and thus remove a greater proportion of
the water. In this way the paper was firmer and less liable to break
when pressed between the couch rolls, an additional advantage being
secured in the fact that the upper wire could be dispensed with.
The various improvements effected resulted in a machine the details of
which appear in the appended diagram, the device of the inclined couch
rolls being fitted about 1810.
The mixture of water and pulp flowed from a stuff chest into a small
regulating box and on to the wire over a sloping board. The pulp at
once formed into a wet sheet of paper, the water falling through the
meshes of the wire, being caught in a bucket-shaped appliance, and
conveyed back to the regulating box. The stream of pulp was confined
upon the wire by means of a deckle. Further quantities of water were
removed by the aid of a pair of squeezing rolls before the web passed
through the couch rolls after which the paper was reeled up on a wooden
spindle.
From this date the success of the machine was assured, though the
inventor and his colleagues were practically ruined, an experience only
too common with the early pioneers of many great and useful industrial
enterprises. In fact, the firm of Messrs. Donkin were the only people
to profit from the invention, for they manufactured a number of
machines, as stated in the report of the Jurors of the Exhibition of
1851, and from 1803 to 1851 no less than 190 Fourdriniers were set to
work.
CHAPTER II
CELLULOSE AND PAPER-MAKING FIBRES
When plants such as flax, cotton, straw, hemp, and other varieties of
the vegetable kingdom are digested with a solution of caustic soda,
washed, and then bleached by means of chloride of lime, a fibrous mass
is obtained more or less white in colour.
This is the substance known to paper-makers as paper pulp, and the
several modifications of it derived from different plants are generally
known to chemists as cellulose.
Although plants differ greatly in physical structure and general
appearance, yet they all contain tissue which under suitable treatment
yields a definite proportion of this fibrous substance. The preparation
of a small quantity of cellulose from materials like straw, rope,
hemp, the stringy bark of garden shrubs, wood, and bamboo can easily
be accomplished without special appliances. Soft materials, such as
straw and hemp, are cut up into short pieces, hard substances like
wood and bamboo are thoroughly hammered out, in order to secure a fine
subdivision of the mass. The fibre so prepared is then placed in a
small iron saucepan, and covered with a solution made up of ten parts
of caustic soda and 100 parts of water. The material is boiled gently
for eight or ten hours, the water which is lost through evaporation
of steam being replaced by fresh quantities of hot water at regular
intervals. When the fibrous mass breaks up readily between the fingers,
it is poured into a sieve, or on a piece of muslin stretched over
a basin, and washed completely with hot water until clean and free
from alkali. Hard pieces and portions which seem incompletely boiled
are removed, and the residual fibres separated out. These fibres are
placed in a weak, clear solution of ordinary bleaching powder, left for
several hours, and subsequently thoroughly washed. This simple process
will give a more or less white fibrous material.
The purest form of cellulose is cotton. A very slight alkaline
treatment, followed by bleaching, is sufficient to remove the
non-fibrous constituents of the plant, and a large yield of cellulose
is obtained. For this reason the cotton fibre ranks high as an almost
ideal material for paper-making, possessing the quality of durability.
Cellulose is an organic compound, containing carbon, hydrogen, and
oxygen in the following proportions:--
Carbon 44·2
Hydrogen 6·3
Oxygen 49·5
-----
100·0
Its composition is represented by the formula C_{6}H_{10}O_{5}.
The celluloses obtained from various plants are not identical either in
physical structure and chemical constitution, or as to their behaviour
when employed for paper-making. In fact, the well-known differences
between the raw materials used for paper-making, and also between the
numerous varieties of finished paper, are to be largely accounted for
and explained by a careful study of the cellulose group, particularly
with reference to the microscopic characteristics and the chemical
composition of the individual species.
The only vegetable substance which may be regarded as a simple
cellulose is cotton, all others being compound celluloses of varying
constitution, the nature of which cannot be appreciated without a
considerable knowledge of chemistry. The classification of such plants,
therefore, in a book of this description must be limited to certain
distinctions having some immediate practical bearing on the question of
paper manufacture.
_Cotton._--Regarded as the typical simple cellulose, containing 91 per
cent. of cellulose, and remarkable for its resistance to the action of
caustic soda.
_Linen._--The cellulose isolated from flax by treatment with alkali or
caustic soda cannot readily be distinguished from cotton cellulose by
chemical analysis or reactions. The difference is almost entirely a
physical one.
Flax is a typical compound cellulose, to which has been given the name
pecto-cellulose on account of certain properties. Other well-known
plants of this class are ramie, aloe, "sunn hemp," manila.
_Esparto._--The cellulose isolated from esparto differs in composition
from cotton cellulose:--
Carbon 41·0
Hydrogen 5·8
Oxygen 53·2
-----
100·0
-----
It is regarded as an oxycellulose, being readily oxidised by exposure
to air at 100° C. Other oxycelluloses familiar to the paper-maker are
straw, sugarcane, bamboo.
_Wood._--The difference between wood and the plants already mentioned
is expressed by the term lignified fibre or ligno-cellulose. This term
is used to indicate that the wood is a compound cellulose containing
non-fibrous constituents, to which has been given the name lignone.
Jute is another example of this class.
* * * * *
These distinctions may be exemplified by reference to a simple
experiment. If three papers, such as a pure rag tissue or a linen
writing, an ordinary esparto printing, and a cheap newspaper containing
about 80 per cent. of mechanical wood, are heated for twenty-four
hours in an oven at a temperature of 105° C., the first will undergo
little, if any, change in colour, while the others will be appreciably
discoloured, the mechanical wood pulp paper most of all.
This change is due to the gradual oxidation of the constituents of
the paper, the ligno-cellulose of the mechanical wood pulp being most
readily affected by the high temperature, and the pure cellulose of the
rag paper being least altered.
The process of oxidation, brought about rapidly under the conditions of
the experiment described, takes place in papers of low quality exposed
to air in the ordinary circumstances of daily use, but of course at an
extremely slow rate. The deterioration of such paper is not, however,
due to the simple oxidation of the cellulose compounds, because other
factors have to be taken into account. The presence of impurities in
the paper on the one hand, and of chemical vapours in the air on the
other, hastens the decay of papers very considerably.
_Percentage of Cellulose in Fibrous Plants._--The value of a vegetable
plant for paper-making is first determined by a close examination
of the physical structure of the cellulose isolated by the ordinary
methods of treatment. If the fibres are weak and short, the raw
material is of little value, and it is at once condemned without
further investigation, but should the fibre prove suitable, then the
question of the percentage of cellulose becomes important.
There are several methods employed for estimating the amount of
cellulose in plants. The process giving a maximum yield is known as the
chlorination method, the details of which are as follows:--About ten
grammes of the air-dried fibre is dried at 100° C. in a water oven for
the determination of moisture. A second ten grammes of the air-dried
fibre is boiled for thirty minutes with a weak solution of pure caustic
soda (ten grammes of caustic soda in 1,000 cubic centimetres of water),
small quantities of distilled water being added at frequent intervals
to replace water lost by evaporation. The residue is then poured on to
a piece of small wire gauze, washed thoroughly, and squeezed out. The
moist mass of fibre is loosened and teased out, placed in a beaker,
and submitted to the action of chlorine gas for an hour. The bright
yellow mass is then washed with water and immersed in a solution of
sodium sulphite (twenty grammes of sodium sulphite in 1,000 cc. of
water). The mixture is slowly heated, and finally boiled for eight to
ten minutes, with the addition of 10 cc. of caustic soda solution. The
residue is washed, immersed in dilute sodium hypochlorite solution
for ten minutes, again washed, first with water containing a little
sulphurous acid and then with pure distilled water. It is finally dried
and weighed.
The second process for estimating cellulose is based upon the use
of bromine and ammonia. About ten grammes of the air-dried fibre is
placed in a well-stoppered wide-mouthed bottle with sufficient bromine
water to cover it. As the reaction proceeds the red solution gradually
decolourises, and further small additions of bromine are necessary. The
mass is then washed, and boiled in a flask connected to a condenser
with a strong solution of ammonia for about three to four hours. The
fibrous residue is washed, again treated with bromine water in the
cold, and subsequently boiled with ammonia. The alternative treatment
with bromine and ammonia is repeated until a white fibrous mass is
obtained.
In practice the paper-maker is confined to two or three methods for the
isolation of the fibres, viz., alkaline processes, which require the
digestion of the material with caustic soda, lime, lime and carbonate
of soda, chiefly applied to the boiling of rags, esparto, and similar
pecto-celluloses; acid processes, in which the material is digested
with sulphurous acid and sulphites. The latter methods are at present
almost exclusively used for the preparation of chemical wood pulp.
_Yields of Cellulose in the Paper Mill._--The object of the paper-maker
is to obtain a maximum yield of cellulose residue at a minimum of cost.
Usually the amount of actual bleached paper pulp obtained in the mill
is less than the percentage obtained by careful quantitative analysis,
for reasons easily understood.
In the first place, the raw material is digested for a stated period
with a carefully measured quantity of caustic soda, for example, at
a certain temperature. Now the conditions of boiling may be varied
by altering one or more of these factors, the period of boiling, the
strength of solution, or the steam pressure, and the paper-maker must
exercise his judgment in fixing the exact relation between the varying
factors so as to produce the best results.
In the second place, the mechanical devices for washing the boiled
pulp and for bleaching cause slight losses of fibre, which cannot be
altogether avoided when operations are conducted on a large scale.
Frequently, also, a greater yield of boiled material may involve
a larger quantity of bleaching powder, so that it is evident the
adjustment of practical conditions requires considerable technical
skill and experience.
The percentage of cellulose in the vegetable plants employed more or
less in the manufacture of paper is given in the following table:--
TABLE SHOWING PERCENTAGE OF CELLULOSE IN FIBROUS PLANTS.
------------------+-------------------------
Fibre. | Cellulose, per cent.
------------------+-------------------------
Cotton | 91·0
Flax | 82·0
Hemp | 77·0
Ramie | 76·0
Manila | 64·0
Jute | 64·0
Wood (pine) | 57·0
Bagasse | 50·0
Bamboo | 48·0
Esparto | 48 to 42
Straw | 48 to 40
------------------+-------------------------
_The Properties of Cellulose._--Cellulose is remarkably inert towards
all ordinary solvents such as water, alcohol, turpentine, benzene,
and similar reagents, a property which renders it extremely useful in
many industries, with the result that the industrial applications of
cellulose are numerous and exceedingly varied.
_Solubility._--Cellulose is dissolved when brought into contact with
certain metallic salts, but it behaves quite differently to ordinary
organic compounds. Sugar, for example, is a crystalline body soluble
in water, and can be recovered in a crystalline state by gradual
evaporation of the water. Cellulose under suitable conditions can be
dissolved, but it cannot be reproduced in structural form identical
with the original substance.
If cellulose is gently heated in a strong aqueous solution of zinc
chloride, it gradually dissolves, a thick syrupy mass being obtained,
which consists of a gelatinous solution of cellulose. If the mixture
is diluted with cold water, a precipitate is produced consisting of
cellulose hydrate intimately associated with oxide of zinc, which
latter can be dissolved out by means of hydrochloric acid. The
resulting product is not, however, the original substance, but a
hydrated cellulose, devoid of any crystalline structure.
Cellulose is also soluble in ammoniacal solutions of cupric oxide, from
which it can be precipitated by acids or by substances which act as
dehydrating agents, _e.g._, alcohol.
_Hydrolysis._--An explanation of the behaviour of cellulose towards the
solvents already mentioned, and towards acid and alkali, requires a
reference to its chemical composition.
The substance is a compound of carbon, hydrogen, and oxygen represented
by the formula
C_{6}H_{10}O_{5}
being one of a class of organic compounds known as carbohydrates,
so designated because the hydrogen and oxygen are present in the
proportions which exist in water.
Water = Hydrogen + Oxygen
H_{2} + O.
The H_{10}O_{5} in the cellulose formula corresponds to 5 (H_{2}O).
When cellulose is acted upon by acid, alkali, and certain metallic
salts, it enters into combination with one or more proportions of
water, forming cellulose hydrates of varying complexity. This change is
usually termed hydrolysis.
With mineral acids like sulphuric and hydrochloric acids, cellulose,
if boiled in weak solutions, is converted into a non-fibrous brittle
substance having the composition
C_{12}H_{20}O_{10} 2 H_{2}O
to which the name _hydra-cellulose_ has been given. Similar changes
occur, but at a much slower rate, when cellulose is in contact with
free acids at ordinary temperatures. For this reason it is important
that paper, when finished, should not be contaminated with free acid.
The nature and extent of the chemical change can be varied by altering
the strength of the acid and the conditions of treatment. The
manufacture of _parchment_ paper is an example of the practical utility
of the chemical reaction between cellulose and acid. A sheet of paper
is dipped into a mixture of three parts of strong sulphuric acid and
one part of water, when it becomes transparent. Left in the solution it
dissolves, but if taken out and dipped into water in order to wash off
the acid the reaction is stopped, and a tough semi-transparent piece of
_parchment_ is obtained. The cellulose is more or less hydrated, having
the composition
C_{12}H_{20}O_{10} H_{2}O,
a substance having the name _amyloid_.
_Oxidation._--Cellulose is only oxidised to any appreciable extent by
acid and alkali if treated under severe conditions. It is remarkable
that the processes necessary for isolating paper pulp from plants when
digested with these chemical reagents do not act upon or destroy the
fibre, and this capacity for resisting oxidation has rendered cellulose
extremely valuable to many of the most important industries.
The resistant power of the cellulose is, however, broken down by the
use of acid and alkali in concentrated form.
Oxalic and acetic acids are obtained when cellulose is heated strongly
at 250° C. with solid caustic soda.
Oxy-cellulose, a white friable powder, is produced by means of strong
mineral acids. Nitric acid at 100° C. attacks the fibre very readily
and produces about 30-40 per cent. of the oxidised cellulose.
CELLULOSE DERIVATIVES.
The great number of compounds and derivatives, _i.e._, substances
obtained by chemical treatment, may be judged from the following list.
The substances of commercial importance are suitably distinguished from
those of merely scientific interest by the printing of the names in
small capitals.
ACETIC ACID.--An important commercial product obtained by the
destructive distillation of wood. The crude pyroligneous acid is
first neutralised with chalk or lime, and the calcium acetate
formed then distilled with sulphuric acid. Wood yields 5 to 10
per cent. of its weight of acetic acid according to the nature of
the wood.
ACETONE.--A solvent for resins, gums, camphor, gun cotton, and other
cellulose products. Prepared by distilling barium or calcium
acetate in iron stills, the acetate being obtained from the crude
acetic acid produced by the dry distillation of wood.
_Acid Cellulose._--(See Hydral-Cellulose.)
_Adipo-Cellulose._--A distinct compound cellulose present in the
complex cuticular tissue of plants, and separated easily by
suitable solvents from the wax and oily constituents also present.
ALKALI CELLULOSE.--When cotton pulp is intimately mixed with strong
caustic soda solution, this compound is formed. It is utilised in
the manufacture of _Viscose_.
_Amyloid._--Strong sulphuric acid acts upon cellulose and converts it
into a gelatinous semi-transparent substance to which the name
amyloid has been given. (See Parchment Paper.)
BALLISTITE.--A smokeless powder composed of nearly equal parts of
nitro-glycerine and nitrated cellulose, with a small quantity of
diphenylamine.
_Carbohydrate._--A large number of important commercial products,
such as cellulose, sugars, starches, and gums, consist of the
elements carbon, hydrogen, and oxygen, associated in varying
proportions. The ratio of hydrogen to oxygen in these compounds
is always 2:1 (H_{2} and O).
Cellulose C_{6}H_{10}O_{5}.
Sugar C_{6}H_{12}O_{6}.
Dextrin _n_ (C_{6}H_{10}O_{5}).
To all these substances the term carbohydrate is applied.
_Celloxin_ (Tollens).--A substance having the stated composition
C_{8}H_{6}O_{6} considered to be present in oxidised derivatives
of cellulose.
CELLULOID.--This well-known material is made by incorporating camphor
with nitro-cellulose, a plastic ivory-like substance being
produced. In practice the process is as follows:--Wood pulp or
wood pulp paper is saturated with a mixture of sulphuric acid
(five parts) and nitric acid (two parts), which produces nitrated
cellulose. The product is washed, ground, and mixed with camphor,
the mastication being effected by heavy iron rollers. The mass
thickens and can be removed in the form of thick sheets. These
sheets are submitted to great pressure between steam-heated
plates. The cake obtained is cut into sheets of any desired
thickness, seasoned by prolonged storage, and afterwards worked
up into boxes, combs, brush-backs, and many other domestic
articles of a useful and ornamental character.
CELLULOSE ACETATE (Cross).--If cellulose is heated with acetic
anhydride at 180° C., viscous solutions of the acetates are
obtained. The process yielding a definite acetate of commercial
value is based upon the following reaction:--100 parts of
cellulose prepared from the sulpho-carbonate are mixed with
120 parts of zinc acetate, heated and dried at 105° C. Acetic
anhydride is added in small quantity, and 100 parts of acetyl
chloride. At a temperature of 50° C. the mixture becomes liquid,
and cellulose acetate is subsequently obtained as a white powder.
The compound can be used in the place of cellulose nitrate, and,
being non-explosive, may gradually replace the latter in many
industrial applications.
_Cellulose-Benzoate._--When alkali cellulose is heated with benzoyl
chloride and excess of caustic soda, this substance is obtained.
_Cellulose Hydrate._--The substances produced by the action of acid
and alkali on cellulose under certain strictly defined conditions
are bodies containing cellulose united with water to form
hydrates. The industrial applications of cellulose based upon
this reaction are described under the special headings.
CELLULOSE NITRATE.--A considerable number of derivatives are obtained
by bringing cellulose into contact with nitric acid. Variations
in the strength of the acid, the temperature of reaction, and the
time of contact determine the nature of the product. The best
known nitrates are:--
Cellulose di-nitrate.
Cellulose tri-nitrate and tetra-nitrate, present chiefly in
pyroxyline.
Cellulose penta-nitrate.
Cellulose hexa-nitrate, the chief constituent of gun-cotton.
CHARCOAL.--Not a cellulose derivative in the strict sense of the
term, charcoal being a residue obtained in the dry distillation
of wood.
COLLODION.--A soluble nitrate of cellulose used in photography. (See
Pyroxyline.)
CORDITE.--A smokeless powder consisting mainly of nitro-glycerine
and gun-cotton mixed with acetone. The materials are thoroughly
incorporated and the resultant paste formed into threads which
are dyed and then cut up into suitable lengths for cartridges.
_Cuto-Cellulose._--Synonymous with adipo-cellulose.
_Dextron._--A compound prepared from the waste liquors of the
bisulphite process used for the manufacture of wood pulp.
Resembles dextrin in its physical properties.
DEXTROSE.--A carbohydrate which can be obtained by the action of
mineral acids on cellulose. Commercial dextrose, or glucose, is
prepared by the conversion of starch with sulphuric acid. The
starch is mixed with dilute acid at a fixed temperature, and the
starch milk obtained poured gradually into a vessel containing
dilute acid, which is maintained at boiling point. The conversion
is complete and rapid.
EXPLOSIVES.--The production of the several cellulose nitrates has
given rise to a great number of highly explosive substances.
_Blasting Gelatine._--A mixture of nitro-glycerine with
cellulose nitrates.
_Amberite_, _Ballistite_, _Cordite_, and other smokeless
powders, consisting of nitro-glycerine and cellulose nitrates in
about equal proportions.
_Sporting powders_ made by mixing nitro-cellulose with
barium nitrate, camphor nitro-benzene, such as _indurite_,
_plastomenite_, etc.
GLUCOSE.--(See Dextrose.)
GUN-COTTON.--An explosive prepared by the action of nitric acid on
cotton. Selected cotton waste suitably opened up is immersed
in a mixture of three parts of nitric acid by weight (1·50 sp.
gr.) and one part of sulphuric acid by weight (1·85 sp. gr.) and
submitted to a number of processes by which the nitration is
properly effected so as to produce a nitro-cellulose of uniform
composition. The material is washed, reduced to pulp, and moulded
into various forms.
_Hemi-Cellulose._--The constituents of plant tissues are extremely
varied in character. Many plants contain substances which
resemble true cellulose, but differing from it in being easily
converted by hydrolysis, and by the action of dilute acids,
into carbohydrates. Plants which contain a large proportion of
such constituents are termed hemi-celluloses. In some cases
certain crystallisable sugars can be obtained by hydrolysis under
suitable conditions.
_Hydral-Cellulose_ (Bumcke).--A compound of merely scientific
interest, resulting from the treatment of cellulose with hydrogen
peroxide. When acted upon by alkali it is decomposed into
cellulose and acid cellulose, the latter a derivative of unstable
composition.
_Hydro-Cellulose._--This product, a white, non-structureless, friable
powder, is obtained by treating cellulose with hydrochloric or
sulphuric acid of moderate strength. The substance itself has
no commercial value, but the reaction is useful in separating
cotton from animal fabrics. If a woollen cloth containing cotton
is soaked in dilute sulphuric acid, washed, and dried at a gentle
heat, the cotton is acted upon, and can be beaten out of the
fabric, the wool resisting the acid treatment.
_Lignin._--The complex mixture of substances which is associated
with cellulose in wood, jute, and other _ligno-celluloses_. The
conversion of wood into chemical pulp effects the removal of this
material more or less completely. The well-known "phloroglucine"
test for mechanical wood in papers is based upon the presence of
_lignin_ in the wood.
_Ligno-Cellulose._--Wood and jute are typical bodies consisting of
cellulose and complex non-cellulose, generally described as
lignin, associated together in the plant tissue. The chemistry
of the non-cellulose portion of wood is a matter still under
investigation, its importance from a commercial point of view
being obvious from the fact that the removal of the _lignin_
during the conversion of the wood into wood-cellulose results in
a loss of 50 per cent. of the weight of wood.
_Lustra-Cellulose._--Synonymous with and suggested as a more
appropriate name for the material usually described as
_artificial silk_.
MERCERISED COTTON.--When cotton is immersed in strong solutions of
caustic soda a remarkable change sets in. The physical structure
of the fibre is entirely altered from the long flattened tube
having a large central canal to a shorter cylindrical tube in
which the canal almost disappears. Hydration of the cellulose
takes place, and these changes are taken advantage of in the
production of mercerised cloth (so named from the discoverer of
the reaction, Mercer). Cotton goods, particularly those made of
long stapled cotton, when mercerised, exhibit a beautiful lustre,
and some magnificent crêpon effects are obtained by the process.
_Methoxyl._--A constituent of the complex compound known as
ligno-cellulose, which is present in wood and similar fibres.
The amount of methoxyl in lignified tissue can be accurately
determined, and it has been suggested that the proportion of
methoxyl found in a cheap printing paper could be used as a
measure of mechanical wood pulp present.
_Muco-Cellulose._--This term is applied to certain compound
celluloses present chiefly in mucilages, gums, and in
seaweeds (Algæ). The natural substances are all of commercial
importance--Iceland moss, Carragheen, Algin, etc.
NAPHTHA.--One of the products of the dry distillation of wood,
usually described as wood-naphtha, or wood spirit.
NITRO-CELLULOSE.--The treatment of cellulose with nitric acid gives
a number of nitro-celluloses according to the conditions of the
process. (See Cellulose Nitrates.)
OXALIC ACID.--A substance of great commercial importance prepared
by heating the sawdust of soft wood, such as pine, fir, and
poplar, with strong solutions of mixed caustic soda and potash
to dryness. The wood yields after six hours a greyish mass
containing about 20 per cent. of the acid, which is separated out
by water and then crystallised.
It is used for bleaching, and as a _discharge_ in calico printing
and dyeing.
_Oxy-Cellulose._--A white friable powder produced by treating
cellulose with nitric acid at 100° C. The oxidation of cellulose
is brought about by several reagents such as chromic acid,
hypochlorites of lime and soda, chlorine, and permanganates. The
extent to which cloth has been damaged by overbleaching may be
determined by a simple test with methylene blue solution, which
is readily absorbed by oxy-cellulose present in such fabrics.
PARCHMENT.--A tough paper prepared by the action of sulphuric acid on
unsized paper. (See page 137.)
_Pectins._--(See Pecto-Cellulose.)
_Pecto-Cellulose._--A generic term applied to many important fibrous
materials, such as flax, straw, esparto, bamboo, phormium,
ramie, &c., which on alkaline treatment yield cellulose for
paper-making, and a non-fibrous soluble residue of complex
composition. These soluble derivatives are known as pectin
(C_{32}H_{48}O_{32}), pectic acid (C_{32}H_{44}O_{30}), and
metapectic acid (C_{32}H_{28}O_{36}). Although the soluble
constituents of the pecto-celluloses amount to 50 per cent. by
weight in most cases, no process for the recovery of the product
in a commercial form has yet been devised. (See description of
Soda recovery, page 78.)
PYROXYLINE.--A substance prepared by nitrating cotton. The cotton is
immersed in a mixture of nitric and sulphuric acids of carefully
regulated strength, and subsequently washed free of the acid.
Three volumes of nitric acid (sp. gr. 1·429) are diluted with
two volumes of water and nine volumes of strong sulphuric acid
(sp. gr. 1·839) added. To the solution when cool the cotton is
added in small quantities at a time. The resultant pyroxyline
is soluble in a mixture of equal quantities of alcohol and
ether, and in the soluble form is utilised as _collodion_ for
photography.
SILK, ARTIFICIAL.--A remarkable substance made from wood or cotton
cellulose, closely resembling silk in appearance and physical
properties.
Nitrated cellulose is dissolved in a mixture of equal parts of
alcohol and ether.
The solution is forced through five capillary tubes under
high pressure, and the filament so obtained solidifying at once
is wound together with other similar filaments upon suitable
bobbins. Various modifications of this general process are in
use, such as the solidification of the solution into threads
by passing it into water; the application of solvents less
inflammable than ether and alcohol; the use of other forms of
dissolved cellulose such as those prepared by means of zinc
chloride, ammoniacal copper oxide, or acetic anhydride. In
all cases the yarn or thread is submitted to further chemical
treatment for the removal of nitric acid and to render the
material non-explosive and less inflammable. The finished product
is soft and supple, can be easily bleached and dyed, and is
capable of acquiring a high lustre.
SMOKELESS POWDERS.--(See Explosives.)
_Sulpho-Carbonate._--(See Viscose.)
SULPHATE CELLULOSE.--Chemical wood pulp prepared by the sulphate
process. (See page 107.)
SULPHITE CELLULOSE.--Chemical wood pulp prepared by the sulphite
process. (See page 107.)
VISCOSE.--A soluble sulpho-carbonate of cellulose, prepared by
treating cellulose with a 15 per cent. solution of caustic soda,
and shaking the product with carbon bisulphide in a closed
vessel. The mixture forms a yellowish mass soluble in water,
giving a viscous solution which has some remarkable and valuable
properties.
This _viscose_, on standing, coagulates to a hard mass which
can be turned and polished.
If spread on glass and coagulated by heat, films are obtained
from which the alkaline by-products can be washed out. These
films are transparent, colourless, very tough and hard.
VULCANISED FIBRE.--Fibre or pulp treated with zinc chloride in acid
solution, or otherwise, for the manufacture of hard boards. (See
page 139.)
WILLESDEN GOODS.--Paper, fibre, and textiles when treated with
cuprammonium oxide are partially gelatinised on the surface and
rendered waterproof. (See page 139.)
_Wood Spirit._--(See Naphtha.)
_Xylonite._--(See Celluloid.)
FIBRES FOR PAPER-MAKING.
Although the vegetable world has been explored from time to time for
new supplies of cellulose, and some plants have been found serviceable
in certain directions, yet the number of fibres in actual use is very
limited.
The following table indicates the principal sources of the material
required for paper-making:--
--------+----------------------------+--------------------------------
Fibre. | Source of the Fibre. | Application of the Fibre.
--------+----------------------------+--------------------------------
Linen | Rags, textile waste. | High class writings and
| | printings.
Cotton | Rags, textile waste. | High class writings and
| | printings.
Esparto | Natural grass. | Writings and printings.
Straw | Straw from various | Printings, box and card boards.
| cereals--wheat, barley, |
| oats, etc. |
Wood | Mechanically ground wood. | Cheap papers, boxboards,
| | middles, tickets and cards,
| | writings and printings.
" | Chemically prepared wood. | Writings and printings.
Flax | Threads, waste from | Wrappings, boards, cable
| spinning mills. | papers.
Hemp | Spinning refuse, old rope, | Wrappings, boards, cable
| sailcloth, etc. | papers, strong writings.
Jute | Waste, old gunny bags. | Wrappings, boxboard, cards.
Bamboo | Natural stems. | Writings and printings (not in
| | Europe, and only limited
| | quantities elsewhere).
Ramie | Bast fibres of the plant; | Rarely used, except in special
| textile refuse. | cases.
Bagasse | Sugar-cane refuse. | Common papers (chiefly
| | experimental results).
Manila | Textile and rope refuse. | Wrappings, cable papers.
Hemp | |
--------+----------------------------+--------------------------------
_Exploiting New Fibres._--The exploitation of any new paper-making
fibre requires attention to certain important details, which may be
fairly considered in the following order:--
(1) _Supply._--The supply of material must be plentiful and obtainable
in large quantities. Too often this question is entirely neglected by
those who bring new fibres to the notice of paper-makers, probably
because they do not realise that enormous quantities of material are
necessary to supply even a very small section of the paper trade,
the fact being that few plants yield more than half their weight of
paper-making fibre.
(2) _Suitability._--The fibre should be properly examined as to
its chemical and physical properties in a laboratory equipped with
appliances for its conversion into bleached paper pulp on a small
scale. The examination of the fibre would include tests as to the
amount of pulp which can be obtained from one ton of raw material, the
approximate cost of treatment, and details as to the value of the fibre
for paper-making.
(3) _Cost of Raw Material._--If the supply of material seems to be
sufficient, and the paper pulp obtained possesses suitable qualities,
then it is necessary to get accurate information as to the cost of the
fibre delivered to some given spot at or near the place of collection.
The exploitation of any new fibre for paper-making purposes will
involve a recognition of the fact that the raw material must be
converted into pulp at or near the place where the material is most
abundant.
The only interesting exception to this is the case of esparto
fibre, which is imported into England in large amount, but this is
only possible because esparto possesses most valuable paper-making
qualities, and is obtained in countries close to England, where large
quantities are consumed. It is doubtful whether other fibres could be
utilised in the same way.
(4) _The Cost of Manufacture_ at or near the place of collection
requires to be carefully worked out, due consideration being given
to the actual cost of chemicals on the spot, cost of labour, and the
conditions under which the maintenance of machinery can be efficiently
looked after.
(5) _Carriage and Freight Charges_ are the last, but by no means the
least, items of importance. It is not too much to say that the whole
success of the exploitation of new paper-making fibre hangs entirely
upon this item, the majority of many fibres which have been brought to
the notice of the trade being suitable, but impracticable, solely on
account of these and similar commercial considerations.
In the pages of the trade press for the last few years the following
fibres have been noticed:--
(1) _Flax Pulp._--This material was to be obtained from flax straw.
Attempts were made on a commercial scale to produce quantities of flax
fibre, but so far the efforts made have not been very successful.
(2) _Ramie Fibre._--This material has been exploited over and over
again, chiefly for textile trades, its application as a paper-making
material being limited to small quantities used for special purposes
such as bank notes. The fibre is too valuable, except for textile
industries, and can only come into the paper trade as a waste material
from such sources.
(3) _Tobacco Fibre_ has been before the trade for some years, the
idea being to utilise tobacco stems and other tobacco waste for
the manufacture of paper suitable for use as wrappers for cigars,
cigarettes, and similar purposes.
(4) _Agave Fibre._--This name is given to a large and important genus
of fibre-yielding plants found chiefly in Central America. It is also
found in India, and in 1878 an experiment was made for the manufacture
of paper at a mill near Bombay, but this did not give any satisfactory
results, probably on account of the primitive methods used in treatment.
(5) _Bagasse._--The waste material from sugar-cane has been looked upon
for many years as a desirable fibre, much time and labour having been
given to the utilisation of this material. In spite of these efforts
bagasse still remains an almost useless and unworkable material. This
is partly due to inferiority of the pulp and partly due to difficulties
connected with its treatment. Probably cultivation of the plant for the
sake of its fibre instead of the sugar might give better results.
(6) _Peat._--The attempts made to utilise peat for paper-making are
probably fresh in the minds of those paper-makers interested in the
production of wrappers and boxboards. The nature of peat, however, is
such as to exclude the hope of making any useful article. The material
has been exploited by companies in Austria, Ireland, and Canada on a
fairly large scale, with but a limited amount of success.
(7) _Cotton-seed Hulls._--Many patents have been taken out for the
chemical treatment of cotton-seed waste and having for their object the
removal of the particles of seed hulls, so as to obtain a pure cotton
pulp. The scheme sounds attractive, but there are so many conditions
which have to be taken account of that the commercial success of any
undertaking based on the use of cotton-seed hulls is very questionable.
The fact is that the hulls have a market value quite apart from the
possibility of their application to paper-making, and this initial cost
would prevent paper-makers from buying the material owing to the large
quantity necessary for the manufacture of one ton of pure pulp.
(8) _Apocynum._--This plant is said to be utilised to some extent by
the Russian Government in the manufacture of bank notes, the plant
being cultivated at Poltava. This is an instance of the particular
application of a fibrous material in limited quantities, a proposition
which is always feasible in the case of special requirements.
(9) _Cornstalk._--This fibre has been chiefly exploited in America,
experts having been attracted by the enormous quantities of cornstalk
available in the several wheat-producing States. The manufacture
of paper pulp from this material on a large scale has yet to be
established.
(10) _Japanese Paper Fibres._--In Eastern countries a great number of
fibrous plants are utilised in small quantities for the manufacture
of special papers. It is obvious that in these Eastern countries the
employment of fibres which are not cultivated in large bulk is readily
possible when the question of price obtained for the paper and the
cost of production are considered. Of such fibres may be mentioned the
_Mitsumata_ and _Kodzu_, easy of cultivation and giving a good yield
of material per acre of ground. The waxed papers used for stencils in
duplicating work on the typewriter are made from these fibres. The
_paper Mulberry_ is also a well-known fibre; while a third species
particularly valuable for thin papers is the _Gampi_.
(11) _Antaimoto Fibre._--The bark of this shrub is utilised in
Madagascar in very small quantities for local purposes and possesses
little interest for paper-makers.
(12) _Refuse Hempstalk._--The suggestion of the use of this material
comes from Italy, the hempstalk having been experimented with at San
Cesario Mill. This also is a fibre of a local interest only. The
percentage of cellulose is very high, being over 50 per cent.
(13) _Papyrus._--The revival of this celebrated material is of
comparatively recent date. It should be noted that the manufacture
of papyrus as carried out by the Egyptians, by smoothing out layers
of bark in order to utilise them as sheets of paper, and the present
day proposals which involve the production of paper pulp from papyrus,
are two entirely different propositions, and the success of the old
Egyptian method cannot be referred to as any assurance of success
for the production of paper from papyrus along modern lines. The
exploitation of this fibre must follow the lines of modern research
and commercial investigation, and its value, if any, could then be
established.
(14) _Pousolsia._--This is a fibre of the same family as hemp and
ramie. The value of this material is at present unknown, but the
ultimate fibre appears to possess a most extraordinary length. Very
little information is available at present as to its value for
paper-making.
(15) _Bamboo._--This material has been before the paper trade for many
years, having first been exploited seriously by Mr. Thomas Routledge in
1875. Since that date a good deal of work has been done in connection
with the fibre, but not until recently has the investigation been made
of a sufficiently extensive character to enable paper-makers to form
some conclusions as to the best methods of obtaining a reliable paper
pulp. The researches of the writer in India go to prove that with any
fibre it is necessary to take into account all the factors likely to
affect the final cost of the paper pulp delivered to any given paper
mill.
The figures given in a report recently published, "The Manufacture
of Paper and Paper Pulp in Burma," show the necessity of thorough
investigation into all the points likely to affect the final results,
viz., the price at which the paper pulp can be sold in England,
assuming that the fibre in question is suitable for the manufacture of
paper.
* * * * *
_Examination of Fibres._--The exact chemical analysis of a new fibre
is necessary in order to establish completely its value for textile and
paper-making purposes, but the investigation of the suitability of the
fibre for paper-making may be simplified by simple reduction of the raw
material with caustic soda. The following process is sufficient for all
practical purposes:--
_Condition of Sample._--A record should be made of the general
appearance of the sample, its condition and the amount available for
the investigation. Any information available as to the source of supply
and the growth of the plant should also be noted.
_Preparation of Sample._--The material is cut up into small pieces. The
most convenient appliance for this purpose is a mitre cutter as used by
picture-frame makers. If the sample is a piece of wood, sections one
inch thick cut across the grain of the wood are most suitable, as they
can be readily cut up into thin flakes by this machine.
_Moisture in Sample._--A small average sample should be dried at 100°
C. for the determination of moisture.
_Treatment with Caustic Soda._--About two hundred grams of the raw
material is closely packed into a small digester or autoclave and
covered with a solution of caustic soda having a specific gravity of
1·050. A perforated lead disc should be placed above the sample in the
digester to prevent any of it from floating above the level of the
solution. The material should be digested for five or six hours at a
pressure of 50 lbs. The conditions of treatment here given will need to
be varied according to the nature of the fibre. Some materials can be
readily converted into pulp with weaker liquor and at a lower pressure,
while others will require prolonged treatment. These conditions must be
varied according to judgment or according to the effects produced by
the conditions already set out.
_Unbleached Pulp._--The contents of the digester are emptied out into
an ordinary circular sieve provided with a fine copper wire bottom,
having a mesh of about sixteen to the inch. The sieve is immersed in
water and the contents partially washed with hot water. The partially
washed material is squeezed out by hand and tied up in a strong cloth
and then kneaded thoroughly by hand in a basin of water which is
repeatedly renewed until the fibre is thoroughly washed. The process of
kneading at the same time reduces the fibre to the condition of pulp.
The water is carefully squeezed out of the pulp by hand, and the moist
pulp is then divided into two equal parts, the first of which is made
up into sheets of any convenient size, care being taken that none of
the fibre is lost. These sheets are then dried in the air and preserved
as samples of unbleached pulp, a record being made of the weight
produced.
_Bleached Pulp._--The second portion of the moist pulp is mixed with
a solution of bleach, the strength of which has been accurately
determined by the usual methods. The amount of bleach added should
be about 20 per cent. of the weight of air-dry fibre present in the
moist sample of pulp. The pulp should be bleached at a temperature
not exceeding 38° C., and when the colour has reached a maximum the
amount of bleach remaining in solution is ascertained by titration with
standard arsenic solution. In this way the amount of bleaching powder
required to bleach the pulp is determined. The product is then made up
into sheets of pulp which are dried by exposure to air and subsequently
weighed.
_Yield of Pulp._--The percentage yield of finished pulp obtained from
the raw material is determined from the figures arrived at in the
experiment described, and the weight of raw material necessary to
produce one ton of bleached pulp is readily calculated.
_Examination of Bleached Fibre._--The fibre should be carefully
examined under the microscope and a record made of general microscopic
features, especially with reference to the length and diameter of the
fibres, and the proportion of cellular matter present, if any.
_Sample of Paper._--It is only in the case of short-fibred material
similar to esparto and straw that sheets of paper capable of giving
comparative results as to strength can be made. The figures obtained
with fibrous materials of this kind are only comparative, because it is
possible in practice to make a much stronger sheet of paper when the
material is beaten properly under normal conditions.
A similar investigation should be made by submitting the fibre to
treatment with bisulphite of lime, that is to say, if the fibre lends
itself to such a process. A lead-lined digester is necessary, and the
solution employed is bisulphite of lime prepared according to the
directions given on page 160.
The preparation of sulphite pulp requires more attention than the
manufacture of soda pulp. It is most important that the digester
should be absolutely tight in order to prevent the escape of any free
sulphurous acid gas, and the contents of the digester must be heated
slowly until the maximum pressure has been reached.
CHAPTER III
THE MANUFACTURE OF PAPER FROM RAGS
[Illustration: FIG. 6.--A Rag Sorting House.]
The word rag is used to designate a very wide range of raw material
suitable for conversion into paper. In the case of high-class hand-made
writing papers only the best qualities are employed, such as new linen
and cotton cuttings from factories, or well-sorted rags of domestic
origin. The usual classification adopted by merchants who supply the
paper mills is somewhat as follows:--
New white linen cuttings (from textile factories).
New white cotton cuttings (from textile factories).
Fine whites (domestic rags).
Outshots (a quality between fines and seconds).
Seconds (a grade inferior to fines).
Thirds (inferior and dirty well-worn rags).
Coloured prints (of all grades and colours).
Fustians and canvas.
Manila and hemp rope.
Baggy, gunny, and jute.
The total amount of rag used in England for paper-making is not known.
The only figures available refer to rags _imported_; and these cannot
be regarded as a measure of consumption, which could only be arrived at
by first ascertaining the quantity of _home rags_ used. The imports of
rag at stated periods are given in the appended table:--
RAGS IMPORTED INTO ENGLAND.
--------------+----------+----------+----------+----------+----------
-- | 1872. | 1882. | 1892. | 1902. | 1905.
--------------+----------+----------+----------+----------+----------
Weight (tons) | 22,254 | 21,200 | 23,032 | 18,692 | 23,681
| | | | |
Value | £373,035 | £303,349 | £214,065 | £173,732 | £224,232
--------------+----------+----------+----------+----------+----------
_Sorting and Cutting._--All rags on arrival at the mill are carefully
sorted. This process is conducted entirely by women, who sort and cut
up the rags at special tables provided with cutting knives curved in
shape similar to a scythe. These are fixed at an angle in the centre
of the table, with the back towards and in front of each work-woman.
The top of the table is made of thick coarse wire so that some of the
dirt and foreign impurities may fall through. All buttons, hooks and
eyes, pins, leather, pieces of rubber, and other articles are carefully
removed, while seams and hems are also opened out. The rags are cut
into slips 3-5 inches long and then recut crosswise, and thrown into
suitable baskets or receptacles standing round the table, by which
means the _sorting_ operation is effectually carried out. The care and
attention given to the sorting is an important item in the manufacture
of papers of uniform quality, and in the best mills the sorting is
carried out to such an extent that twenty or twenty-five grades are
obtained.
[Illustration: FIG. 7.--A Rag Duster.]
_Dusting._--The rags are next passed through a machine which removes
dirt. This is a hollow cylindrical or conical drum having an external
covering of coarse wire cloth, which rotates inside a wooden box.
The shaft is provided with projecting spikes, so that the rags are
violently agitated in their passage through the machine. The dirt and
other impurities fall through the wire on to the floor of the room,
while the clean rags are discharged from the lower end of the drum. The
loss in weight varies according to the condition of the rags. With good
materials the loss may only be 1-2 per cent., while with dirty common
rags the loss during cleaning and dusting may amount to 10 per cent.
[Illustration: FIG. 8.--A Rag Cutter.]
_Boiling._--The further purification of the rags is effected by a
chemical treatment, viz., boiling at a high temperature with alkaline
substances, which process removes fatty, glutinous, and starchy matter
from the material.
[Illustration: FIG. 9.--Interior of Paper Mill for Hand-made Paper (R.
Batchelor & Sons).]
For this purpose a spherical digester is used, generally 7-9 feet
diameter, and capable of holding 2-2½ tons of rag. The boiler or
digester is filled with dusted rags, and the requisite amount of
alkaline solution added. The manhole is then closed, and steam admitted
through the hollow trunnions until the pressure reaches 20 or 30 lbs.,
at which pressure the boiling is continued for three to six hours
according to requirements, the digester rotating slowly the whole time
in order that the rags may be evenly and thoroughly boiled.
[Illustration: FIG. 10.--View of a Rag Boiler, showing connections.]
The liquor employed for boiling is a solution of caustic soda,
carbonate of soda, or milk of lime. In the case of caustic soda the
amount required varies from 5 to 10 per cent. of the weight of rag.
Caustic soda is preferable to lime, because it acts upon the grease
and other fatty matters, forming a soluble compound which is freely
removed in the subsequent process of washing. Many paper-makers,
however, use milk of lime, carefully strained through fine cloth,
almost exclusively. Considerable experience and skill are necessary
in this operation in order to avoid injury to the fibre not only as
regards its strength, but also its colour.
_Washing._--When the rags have been sufficiently boiled, the steam
is turned off and the pressure allowed to fall. This can be effected
quickly by blowing off from a valve fixed at the bottom of the boiler
opposite to the manhole. The cover is removed from the boiler and the
boiler slowly rotated in order that the contents may be discharged
into a tank placed below. The "black liquor," as it is called, is
then drained away from the rags, which are immediately subjected to a
preliminary washing. The process of washing must be carried out in a
thorough manner in order to remove all soluble compounds, which if left
would cause an unnecessary waste of bleach in the subsequent stages of
purification. There are many schemes employed for washing, most of them
being devised with the idea of using a minimum quantity of water.
The most general practice, in the absence of special machinery, is the
preliminary treatment in the tank below the digester, followed by a
more complete washing process in a machine known as a breaking engine.
This apparatus is a shallow oval-shaped vessel with circular ends,
divided lengthwise by a partition called a mid-feather, which, however,
does not extend the full length of the apparatus. In one of the two
channels into which the vessel is thus divided a heavy roll is fitted,
which is provided with a number of steel knives. On the floor of this
channel there is fixed a "bed-plate," also provided with projecting
knives which are parallel with the knives in the roll. The distance
between the knives in the roll and those in the "bed-plate" may be
altered as required by means of an adjusting screw. In the other
channel of the breaking engine there is fitted a "drum-washer," which
serves for the removal of the dirty water from the machine. This drum
is divided into sections by means of partitions which reach from the
centre to the circumference. The surface of the "drum-washer" consists
of a fine brass wire cloth supported by a coarser material placed
underneath.
[Illustration: FIG. 11.--A Breaking and Washing Engine.]
The breaking engine is half filled with clean water, and the rags are
thrown into the engine until it is suitably filled. The rotation of the
heavy roll causes the mixture of rags and water to circulate round the
vessel, the floor of which is so constructed that the pulp is drawn
between the roll and "bed-plate" and discharged over the "backfall,"
which is that portion of the sloping floor behind the "bed-plate."
The "drum-washer" rotates with its surface in contact with the mixture
in the engine, so that the dirty water passes through the wire cloth
and is caught in the curved sections or buckets inside the drum and
discharged into a trough adjacent to the centre, and thereby conveyed
away from the engine. Clean water is allowed to run into the vessel
at one end while the dirty water is discharged by means of the
"drum-washer." At the same time the rags are broken up by means of the
knives on the roll, so that when the rags are sufficiently washed, a
process which usually occupies four hours, they are also partially
disintegrated.
_Bleaching._--The clean disintegrated rag is next bleached by means of
ordinary bleaching powder solution. Bleaching powder is a substance
prepared by the action of chlorine gas on dry slaked lime, resulting
in the formation of a compound which has the property of bleaching or
"whitening" vegetable matters. The clear solution obtained by treating
the powder with water is utilised by the paper-maker for bleaching the
rag pulp.
Various methods are used for this purpose. Sometimes the requisite
volume of clear bleach liquor is added to the pulp in the breaker,
and the material kept in constant circulation until the operation
has been completed. In other cases the broken pulp is transferred to
a "potcher," which is a vessel similar in shape to the breaker, but
merely provided with paddles for keeping the pulp in circulation, and
bleached by the addition of chloride of lime solution.
Another method frequently adopted is to discharge the pulp from the
breaker, immediately after the addition of the bleach, into brick or
cement tanks, allowing the bleaching action to proceed spontaneously
without prolonged agitation.
In some instances the process is hastened by adding dilute sulphuric
acid to the pulp after the bleach liquor has been run in, or by
heating the mixture with steam. For high-class papers such devices as
this are seldom resorted to, as experience shows that the colour of
pulp bleached by drastic methods does not maintain a high standard.
The pulp is then thoroughly washed in order to remove every trace
of residual bleach, and also the soluble compounds which have been
formed during the operation. Very large quantities of water, clear
and free from suspended dirt, are necessary. In some mills any excess
of bleach is neutralised by the use of an "antichlor" such as sodium
hyposulphite, or sodium sulphite, but the best results are undoubtedly
obtained when the quantity of chemicals used is kept at a minimum.
If the pulp is bleached in a breaker or potcher, the washing is
effected by the aid of the drum-washer. With pulp treated in steeping
tanks, fresh water is allowed to percolate or drain slowly through the
mass.
ELECTROLYTIC BLEACHING.
The substitution of a sodium hypochlorite solution for the ordinary
calcium hypochlorite solution obtained from common bleaching powder has
been the aim of specialists for many years. As early as 1851 a patent
was taken out by Charles Watt for decomposing chlorides of the alkali
metals and the formation of hypochlorites. It was not until 1886 that a
practical method was devised for producing an electrolysed solution of
salt, but in that year Hermite introduced a continuous process in which
an electrolysed solution having a strength of three grammes chlorine
per litre was passed continuously into the potcher.
Many patents for the electrolysis of salt have been taken out during
the last twenty years, of which the Bird-Hargreave process is in
operation in England, the Rhodin process in America, the Siemens and
Halske in Norway, and the Oettel and Haas apparatus in Germany. The
figures relating to the latter apparatus may be mentioned as typical of
the present condition of electrolytic bleaching. The apparatus consists
of a narrow rectangular trough divided into a number of chambers
through which a solution of brine flows at a constant and steady
rate. The electric current is passed through the solution by suitable
electrodes, the temperature being kept down by means of a cooling coil.
The cost of producing the bleach liquor as given by the inventors of
the apparatus from the results of actual working are shown in the
following table:--
TABLE GIVING ANALYSIS OF COST FOR PRODUCING BLEACH LIQUOR.
Capacity of tank 750 litres = 166 gallons.
Strength or density of brine 1·5 Baumé, or 23 Twaddell.
286 lbs. of common salt required for 166 gallons.
--------------------+-------+-------+------+-------+--------+--------
Hours worked | 2 | 4 | 6 | 8 | 10 | 12
Grammes of chlorine | | | | | |
per litre produced| 4·35 | 7·38 | 9·9 | 12·42 | 14·31 | 16·20
Temperature C. of | | | | | |
brine during | | | | | |
operation | 20 | 21 | 20 | 21 | 20 | 20
Ampères of 110 volts| 55 | 50 | 46 | 52 | 47 | 43
Power in h.p. hours | 16 | 31 | 45 | 61 | 75 | 89
Cost of the h.p. at | | | | | |
·22_d._ per h.p. | | | |1_s._ |1_s._ |1_s._
hour | 3½_d._| 6¾_d._|10_d._| 1½_d._| 4½_d._| 7½_d._
Cost of salt |1_s._ |1_s._ |1_s._ |1_s._ |1_s._ |1_s._
| 6_d._| 6_d._| 6_d._| 6_d._ | 6_d._ | 6_d._
Total cost |1_s._ |2_s._ |2_s._ |2_s._ |2_s._ |3_s._
| 9½_d._| 0¾_d._| 4_d._| 7½_d._| 10½_d._| 1½_d._
Total chlorine | | | | | |
obtained in kilos.| 3·262 | 5·535 | 7·425| 9·315 | 10·732 | 12·150
Cost of chlorine per| | | | | |
kilo. |6·6_d._| 4½_d._|3¾_d._|3·4_d._| 3·2_d._| 3_d._
Salt used per kilo. | | | | | |
chlorine | 35 | 20 | 15 | 12 | 10 | 9
--------------------+-------+-------+------+-------+--------+--------
The above costs have been estimated on prices as follows:--
Coal 10_s._ per ton.
Salt 12_s._ per ton.
After 12 hours the 166 gallons (750 litres) are converted into
electrolytic bleach liquor containing 26¾ lbs. of active chlorine
(12·15 kilos.).
_Beating._--Although the rags are reduced by the breaking engine to a
condition of fibrous lint, called "half-stuff," they are not fit for
conversion into paper. They have to be _beaten_ in special machinery
until a complete separation of the single fibres has been effected,
and this process is rightly regarded by many paper-makers as the most
important stage of manufacture.
[Illustration: FIG. 12.--Oettel and Haas' Apparatus for the manufacture
of Electrolytic Bleach Liquor.]
The beating engine is similar in construction to the breaking engine,
but there are certain essential differences in arrangement and
manipulation. There is usually no drum-washer; the roll contains a
large number of knives which are fixed in clumps or sets of three
round the circumference; the lowering of the roll upon the bed-plate
is carefully watched and controlled, and the desired effects are only
obtained by strict attention to the condition of the pulp during the
whole process.
[Illustration: FIG. 13.--The "Hollander" Beating Engine.]
The beater is first partially filled with water, and the drained
half-stuff added gradually until the "furnish," a convenient term
applied to the contents of the engine, has the proper consistency,
which varies according to the nature and quality of paper required.
The mass is circulated steadily round the engine by the action of the
beater roll, which is lowered from time to time until the distance
between the knives on the roll and those on the bed-plate has been set
to the desired adjustment. This lowering of the roll and its proper
adjustment call for the greatest care.
_Influence of the Beating._--The importance of this operation can
easily be judged from one or two specific examples. In the case of rag
papers the two extremes of variation are represented by the ordinary
blotting paper on the one hand and a hard strong writing paper known
as a _loan_ on the other. Now the great difference in these papers may
be traced to the careful selection of the rag and the treatment in the
beater as the two primary causes of the final results.
For blotting papers it is essential that the rags should be old and
tender. In the beating operation subsequent to the usual boiling and
bleaching processes the half-stuff is beaten quickly with sharp knives,
the roll being lowered soon after the engine is filled, so that the
beating is finished in about one to one and a half hours.
For the strong writing paper new strong rags are selected. In the
beating process the knives used are dull, the roll is lowered slowly
and cautiously, and the beating goes on for eight to ten hours.
The effect of such difference in treatment is easily seen by
examination of the fibres of the papers under the microscope. In
the first case the fibres appear short with clean cut ends, the
shape little distorted, the structure well defined, bearing a strong
resemblance to the unbeaten material. In the case of the well-beaten
paper the ends of the individual fibres appear to be drawn or frayed
out, the fibres do not possess the sharp well-defined outline
characteristic of blotting paper; they are partly split up into
fibrillæ which lie together in a confused mass.
In the _blotting_ paper these effects are produced because the knives
being sharp cut up the material quickly, and in the _writing_ paper
because the dull "tackle" tends to draw out the fibres and tear them up
lengthwise.
The practical result is a spongy, soft, and bulky blotting and a hard,
strong, heavy writing paper. Of course the great difference between a
blotting and a writing paper is not all due to this one operation,
but is obtained by a series of operations, of which one of the most
important is, however, the beating.
_Colouring the Paper._--The pulp is brought to any desired tint by
the addition of mineral pigments or aniline dyes to the contents of
the engine. The latter soluble dyes, however, are seldom used for
high-class rag papers. Prussian blue, ultramarine, and smalts are
chiefly used for this purpose, giving toned blue, azure, and blue laid
papers.
[Illustration: FIG. 14.--The Hand Mould showing Frame and Deckle.]
_Making the Paper._--The beaten pulp, when duly prepared, is run from
the engine into store tanks known as stuff chests, ready for the
actual manufacture. The pulp properly diluted with water is strained
through special screens to remove any insufficiently beaten material
and any impurities present, after which it is run off into the vat, a
square-shaped vessel built of wood or stone.
The apparatus used in forming the sheets is called a _hand mould_.
The mould is a rectangular frame of mahogany upon which is stretched
tightly a fine wire cloth, the surface of the latter being kept flat
by a coarser wire cloth fixed underneath, supplemented by wedge-shaped
pieces of wood. A second frame called the _deckle_ fits on to the mould
in such a manner as to form a shallow tray, the bottom of which is the
fine wire cloth.
The vatman takes up the mould with both hands and dips it into the
vat full of pulp in a slanting position, drawing it through the stuff
towards him in a peculiar manner and lifting it out from the vat with
a definite quantity of the mixture in the frame. As the water drains
away from the pulp, through the wire cloth, he imparts a shaking motion
to the mould in order to cause the fibres to "felt" properly, this
felting or interlacing of the fibres being an essential feature in the
manufacture of a good sheet of paper. When the water has drained away
sufficiently from the pulp, the vatman removes the deckle from the
mould and passes the latter over to the coucher, who takes the mould,
reverses it, and presses the contents, which may now be described as a
wet sheet of paper, down on to a damp piece of felt, by which means the
paper is transferred to the felt. He returns the mould to the vatman,
who meanwhile has made another sheet with a duplicate mould, and then,
having laid a second felt upon the wet sheet of paper, he proceeds to
transfer the next sheet of paper to the second felt. This process is
continued until a pile is formed consisting of wet sheets of paper
alternated with pieces of felt.
The pile is at once submitted to great pressure in the hydraulic press,
and the excess water slowly forced out, while at the same time the
sheets are compressed and thus "closed up," as it is termed. When all
the excess water has been removed as far as possible, the pile is taken
away and the sheets of damp paper taken out, the felts being placed in
one pile ready for further use, and the sheets of paper in a second
ready for the next process.
The papers are put back into the press without felts between the sheets
and left for some time. In most cases the sheets are turned round or
mixed in with the sheets of another pile, before pressing. In this way
any unevenness or irregularity in the sheets is counteracted and a more
uniform result obtained.
When these changes are repeated several times the paper acquires an
even texture and becomes firm and hard.
_Drying the Paper._--The sheets are hung up in the _loft_, as the
drying room is called, upon poles or ropes. The moisture gradually
evaporates, and the paper is thus dried by exposure to air. In winter
it is necessary to warm the air in the loft, as the air is then
saturated with moisture. In lofts of limited capacity the air is heated
in order to hasten the process, but the best paper is allowed to dry
naturally, as by this means the shrinkage is gradual and a maximum
strength is attained.
[Illustration: FIG. 15.--Apparatus for Sizing Paper in continuous
Rolls.]
_Sizing the Paper._--The dried paper as it leaves the loft is termed
_Waterleaf_ because, being unsized, it readily absorbs water, and
therefore before it can be used it must be sized. For this purpose
it is dipped into a solution of gelatine, an operation described as
_tub-sizing_ or _animal-sizing_, the former term being used on account
of the tub in which the size is kept, and the latter on account of
the fact that the gelatine is made from animal matter such as hides,
cartilage, hoofs, and other refuse.
_Animal Size._--This is prepared from hide pieces, skins, and the
like by a simple process, which, however, requires a good deal of care
in order to obtain the best results. The material is first thoroughly
washed in plenty of clean water, and then heated with a definite
quantity of water in a steam jacketed copper pan. The pieces slowly
dissolve until a solution of gelatine is produced, and after the dirt
and impurities have settled to the bottom of the pan the clear liquid
is drawn off into store vessels. There are many details of a technical
character to be attended to in the manufacture of good gelatine, and as
the process is expensive, considerable attention is demanded at this
stage in the completion of a sheet of paper.
The dry sheets of paper are sized by the simple expedient of dipping,
or by the passage of the paper through a long trough. In the first
case the workman takes up a number of sheets and dips the bunch into a
vat of size at the proper temperature, about 100° Fahrenheit. He then
allows the surplus size to drain off, and the sheets are submitted to a
slight pressure in order to remove the excess of gelatine that will not
drain off.
In the second case a different method is adopted in that the sheets
of paper are carried by travelling felts through a bath of heated
size, the excess gelatine being removed by the action of rubber or
wooden rollers through which the papers are passed before leaving the
apparatus. The papers are quickly and evenly sized by this method,
which is now most generally used.
_Glazing._--When the sheets of paper are quite dry they are ready for
glazing, a process which turns the dull rough surface of the sized
sheet into a highly polished smooth surface fit for use. The sheets are
placed singly between copper or zinc plates, and a pile of these passed
several times through heavy iron rollers, great pressure being applied
to the latter during the operation.
[Illustration: FIG. 16.--A Supercalender.]
The amount of polish imparted by this plate-glazing process, as it
is termed, can be varied considerably. With a light pressure and
few rollings, the sheet of paper can be turned out having a fairly
smooth surface, and without a conspicuously shiny appearance. By
employing a great pressure and repeated rolling a much higher surface
is attainable. If the plates are hot a still higher finish is
possible. Machine-made rag papers are glazed usually by means of the
supercalender, which is a stack of alternate steel and paper rolls
placed one above the other in a vertical position. The reel of paper
passes between these rolls and becomes highly surfaced.
This operation effects many changes in the paper, besides imparting
a good finish. The thickness of the sheet is reduced by about 40 per
cent., the fibres being compressed much closer together. The tensile
strength of the paper is also materially increased, and in every
way the paper is improved. Moderation is essential in this as in
everything, because excess of glazing weakens a paper, rendering it
brittle and liable to crack when folded.
_Laid and Wove Papers._--When certain papers are held up to the light
and carefully examined it will be noticed that they appear to contain
delicate transparent lines running parallel with one another at equal
distances of about an inch, and that these are intersected by similar
transparent lines running at right angles, which are much closer
together. Such papers are known as _Laid_ Papers, and the peculiar
formation of the transparent lines is due to the construction of the
mould used in the making. The wire surface of this mould consists of a
number of somewhat stout wires placed about one inch apart, interwoven
with finer wires running across and at right angles, which are threaded
much closer together. When the mould is dipped into the vat and
withdrawn, the water drains away from the under surface of the wire,
and the moist pulp settles down on the upper surface; but since the
coarser wires project a little from the finer threads, the paper is
slightly thinner along those wires, though to an almost infinitesimal
extent, with the result that on drying the sheet appears to contain
transparent lines.
_Wove_ papers are so called from the nature of the mould used. The
surface of the mould in this case consists of fine wires equally
distributed, being woven in such a manner that the wires are
equidistant from one another, as in ordinary wire gauze. A wove
paper, on being examined in the light, simply shows a number of small
diamond-shaped spaces, which in the majority of instances are difficult
to detect.
_The Watermark._--The transparent device observed in many papers when
held up to the light is known as the watermark, a term probably derived
from the conditions existing at the time the sheet of paper is made on
the mould. The effect is produced by means of a raised design sewn or
soldered to the surface of the mould, the design being fashioned out of
fine wire.
[Illustration: FIG. 17.--The First Watermark in Paper.]
When a mould thus fitted with the design is dipped into a vat of pulp
and lifted out, the water falls through the wire, and the pulp sinks
down on to the surface of the mould, forming a replica, so to speak,
of the design, which is easily seen when the dry paper is held up to
the light, because the paper is thinner just at those points where the
wires forming the design come into contact with the wet pulp.
Some of the watermarks are very elaborate and interesting. A familiar
illustration of a beautiful design of this description is to be found
in the Bank of England notes. As a general rule the ordinary watermark
consists of a mere trade term such as "Vellum," "Zenobia," or of the
name of the manufacturer, such as "J. Whatman," "R. Batchelor," and
so on. In the earlier days of paper-making many highly interesting
designs were used, and some of these are still extant. In fact many of
the names by which certain standard sizes of paper are known owe their
origin to the watermarks employed.
The earliest known watermark bears the date A.D. 1301, being in the
form of a globe and cross, as shown. Of equal interest are those
designs from which certain papers are called foolscap, crown, pott,
post, royal, columbier, and so on. The watermarks are now little used,
but the terms are still retained, as indicating the size of the sheet.
MICROSCOPIC FEATURES OF COTTON AND LINEN FIBRES.
The _cotton_ fibre is about 30 mm. long, with an average diameter of
·025 mm. of tube-like shape, and having a prominent central canal.
There are no cross markings on the cell walls, and the ends of the
fibre are rounded off into a somewhat blunt point. It exhibits a marked
tendency to twist itself, especially if dry, and this peculiarity is
readily observed with the raw material.
The process of paper-making alters the characteristic structure of the
fibre very greatly. The ends of the fibre are seldom to be seen; the
curious twist is less prominent, and the fibres are torn and destroyed.
The effect of the beating process, for example, on cotton is easily
to be noticed by comparing the fibres of a blotting paper under the
microscope with the fibres of a _bank_ or _loan_ paper.
The distortions produced by prolonged beating renders the
determination of the exact percentage of cotton in a rag paper rather
difficult, but the features to be looked for are the absence of pores,
cross markings, the existence of a central canal, striations produced
in many cases on the cell walls parallel to the length of the fibre.
The structural features are more readily observed when the fibres are
stained with a suitable reagent. (See page 71.)
[Illustration: FIG. 18.--Cotton.]
The _linen_ fibre has an average length of 27 mm. with a diameter of
·02 mm. The raw flax is very different from raw cotton and is easily
distinguished. The fibre is slender in shape, having thickened knots
at regular intervals throughout its length, the general appearance of
which may be compared to a stick of bamboo. The central canal of the
fibre is extremely narrow, running like a small thread through the
length of the fibre. The cell walls are further marked by numerous
pores, which appear as small dark lines running from side to side, but
not meeting in the centre.
[Illustration: FIG. 19.--Linen.]
In the treatment necessary for making paper these characteristics are
largely destroyed, and while it is quite easy to ascertain that a paper
is of linen, or of cotton, or that a paper is mainly cotton with a
small percentage of linen, yet there are conditions under which it is
difficult to determine the exact percentage of cotton or linen in a
rag paper. If, for example, a paper contains nearly equal quantities
of cotton and linen, the exact proportions cannot be determined closer
than 10 per cent., especially in well-beaten papers.
REAGENT FOR STAINING FIBRES.
_Preparation._--Dissolve 2·1 grams potassium iodide and 0·1 grams
iodine in 5 c.c. of water. Mix this solution with a solution containing
20 grams of dry zinc chloride in 10 c.c. of water. Allow the mixture to
stand; pour off the clear liquid into suitable bottles.
COLORATION PRODUCED.
Cotton, linen, hemp.--Wine red.
Esparto, straw and wood cellulose.--Bluish violet.
Mechanical wood, unbleached jute.--Yellow.
Manila hemp.--Blue, bluish grey to yellow.
CHAPTER IV
ESPARTO AND STRAW
ESPARTO PAPERS.
The value of Esparto for the manufacture of high-class printing and
medium quality writing paper is well known. This material has qualities
which cannot readily be obtained from other fibres, such as rag and
wood pulp. It is chiefly used in papers required for lithographic
printing, books, and art illustration, since it gives a sheet having a
good surface and one which is soft and flexible.
The grass is obtained from Spain, Morocco, Algeria, Tunis, and Tripoli,
in which countries it grows wild, requiring very little cultivation.
The condition of the crop is improved by proper treatment, and in
districts where the grass is cut for export as a paper-making material
attention is given to cultivation.
The plant grows to a height of three or four feet, and when mature the
long blades of grass curl up into the form of a cylinder resembling a
piece of wire. The leaf consists of two parts, the stalk and a sheath,
which are easily separated when harvested. The grass is pulled up by
hand and stacked into heaps in order that it may be dried by the heat
of the sun, after which process it is carefully picked over for the
removal of all extraneous matter and impurities. It is then graded,
the best sorts being kept for weaving, and the remainder being sold
for paper-making. It is packed up into large bales of about 4 cwt.
capacity, compressed into small bulk by powerful presses, and shipped
to England.
_Esparto Pulp._--The first process in the manufacture of the paper
is cleaning. The bundles of grass are opened up, shaken out, and put
through a willowing machine. This consists of a hollow conical drum,
the outer surface of which is a coarse wire cloth. Inside the drum is
fitted a shaft provided with wooden teeth, and as the grass passes
through it is tossed about and the dust removed. The clean grass is
conveyed by travelling belts to the digester house. For the production
of a high-class paper the grass is often examined by girls, who stand
on either side of the travelling conveyer and take out any coarse root
ends and foreign material not removed by the willowing machine.
_Boiling._--The object of submitting esparto to chemical treatment is
to obtain a pure paper-making fibre known as cellulose. The composition
of this raw material is shown by the following analysis:--
_Spanish Esparto._
Cellulose 48·25
Water 9·38
Aqueous extract 10·19
Pectous matter 26·39
Fatty matter 2·07
Ash 3·72
------
100·0
------
Yield of dry cellulose obtained in actual practice
from good raw material 45 to 48%
By boiling the esparto with caustic soda under pressure for a stated
time, the non-fibrous constituents are removed, leaving the cellulose
in a more or less pure form according to the severity of the chemical
treatment.
[Illustration: FIG. 20.--An Esparto Duster.]
In practice the grass is packed tightly into upright stationary
digesters and a definite quantity of caustic soda solution added,
the amount of chemical used being equal to 15-18 per cent. of the
weight of grass packed into the digester. The form of digester almost
universally employed is that known as the Sinclair's "vomiting" boiler,
which is constructed so that a continuous circulation of the liquid
is maintained by means of what are called "vomit" pipes. These are
fitted to the sides of the digester in such a manner that the caustic
soda solution circulates from the bottom of the digester, up through
the "vomit" pipes, and is discharged downwards upon the contents of
the boiler through a perforated plate fixed in the upper part of the
digester. The requisite quantity of caustic soda solution is placed in
the digester, and steam admitted into the bottom of the vessel while
the grass is being thrown in. In this way a much larger weight of grass
can be boiled at one operation, since the bulk is greatly reduced when
the grass has become thoroughly soft and wet.
[Illustration: FIG. 21.--Sinclair's "Vomiting" Esparto Boiler.]
When the boiler is loaded the inlet is closed up and steam turned on
to the full pressure of about 40 or 50 lbs., this being maintained
for a period of about four hours. The non-fibrous constituents of the
esparto are gradually dissolved out by the caustic soda, and when the
operation is completed the black liquor is run off from the digester
into large store tanks, and the esparto grass which remains in the
digester is then completely washed until the soda is almost entirely
washed out.
[Illustration: FIG. 22.--A Porion Evaporator.]
The conditions for boiling and bleaching esparto are varied by the
paper-maker as circumstances require. A maximum yield of fibre is
obtained when the least possible quantity of caustic soda is used, but
a larger percentage of bleaching powder may be necessary to ensure a
well bleached pulp. The use of an excess of caustic soda is probably
the general practice for several reasons, amongst which may be noted
the advisability of guarding against irregularities in the quality
of the esparto, and consequent insufficient boiling, as well as the
advantage of having some free caustic in the spent liquors to prevent
the furring up of the tubes of the evaporating apparatus in the soda
recovery department.
The following experiments, given by a contributor to the _Paper Trade
Review_ some years ago, are interesting as showing the effect of
varying proportions of caustic soda used per unit of grass:--
EXPERIMENTS _RE_ YIELD OF AIR-DRY BLEACHED PULP FROM ORAN ESPARTO.
Air-dry Pulp containing 10 per cent. water.
---+--------+----------------+------------------+-------+--------+-------
|Esparto.| Soda Liquor. | Conditions of | | Dry |
| | | Boiling. |Weight | Pulp |Bleach-
---+--------+-------+--------+------+-----+-----+ of |on Dry | ing
No.| Wt. |Volume,| Per |Time. |Temp.|Pres-|Air-dry|Esparto.|Powder.
| taken. | C.C. | cent. |Hours.|° C. |sure.|Pulp. | Per | Per
| Grams. | |Na_{2}O.| | | Lbs.|Grams. | cent. | cent.
---+--------+-------+--------+------+-----+-----+-------+--------+-------
1 | 200 | 800 | 1·58 | 3 | 142 | 55 | 87·30 | 43·65 | 29·5
2 | 200 | 800 | 2·13 | 3 | 142 | 55 | 80·67 | 40·33 | 18·5
3 | 200 | 800 | 2·69 | 3 | 142 | 55 | 72·00 | 36·00 | 10·5
---+--------+-------+--------+------+-----+-----+-------+--------+-------
No. = No. of Experiment.
PRACTICAL DATA CALCULATED FROM EXPERIMENTS.
---+------------+----------+------------+-------------+----------------
| Boiling. |Weight of |60 per cent.| Bleaching |For One Ton of
No.+------+-----+Esparto to|Caustic Soda| Powder |Esparto used.
|Time. |Pres-|give 1 ton|required to |required to +----------------
|Hours.|sure.| Pulp. | Digest |Bleach 1 ton | 60 per |Bleach-
| |Lbs. | Cwts. | Esparto. |Air-dry Pulp.| cent. |ing
| | | | Cwts. | Cwts. |Caustic.|Powder.
| | | | | | Lbs. | Lbs.
---+------+-----+----------+------------+-------------+--------+-------
1 | 3 | 55 | 45·8 | 4·30 | 5·26 | 210 | 260
2 | 3 | 55 | 49·5 | 6·27 | 3·39 | 282 | 156
3 | 3 | 55 | 55·5 | 8·90 | 1·96 | 358 | 79
---+------+-----+----------+------------+-------------+--------+-------
No. = No. of Experiment.
_Recovery of Spent Liquor._--As it is possible to recover 75 to 80 per
cent. of the soda originally used in digesting the esparto, the washing
of the boiled grass is conducted on scientific principles in order to
ensure a maximum recovery of soda at a minimum cost.
The recovery is effected by evaporating down the black liquor, together
with the washing waters, to a thick syrupy mass, which can be burnt.
The organic and resinous constituents of the esparto which have been
dissolved out by the caustic soda, forming the soluble soda compounds,
ignite readily, and during combustion the organic soda compounds are
converted more or less completely into crude carbonate of soda.
It is obvious, then, that the cost of recovery depends mainly on the
quantity of weak washing water which has to be evaporated. Consequently
methods are devised by means of which the grass is thoroughly washed
with as little water as possible, and some of the methods are very
ingenious.
The spent liquors and washing waters are evaporated to a small bulk in
a vacuum multiple effect apparatus, and the thick liquid mass obtained
by evaporation is burnt either in a rotary furnace or on an ordinary
hearth. Every precaution is taken to effect this operation with a
minimum quantity of coal. The burning off of this mass results in the
formation of a black substance which is taken away from the furnace
and allowed to char or slowly burn until the impure white soda ash, or
carbonate of soda, is obtained.
Two systems of recovery are in general use, which deserve a brief
notice:--
_Direct Evaporation._--The liquors may be evaporated to a small bulk
ready for incineration by treatment in long shallow pans or furnaces,
the heat necessary for the process being obtained mainly from the
combustion of the thick concentrated liquor. The most familiar type of
this form of apparatus is the Porion evaporator.
[Illustration: FIG. 23.--Scott's Multiple Effect Evaporator.]
The combustion of the concentrated liquor is started by a coal furnace
at one end of the apparatus. The thick viscous mass catches fire and
burns with a fierce flame, and the heat is utilised in evaporating the
weaker liquors which flow continuously through shallow brick troughs,
the surface of which is freely exposed to the heat and flames from the
hearth where the organic soda compounds produced in the boiling of
esparto are being incinerated and converted into soda ash.
Under suitable conditions this evaporator is most economical in its
results. It can be erected cheaply, and when all the heat is fully used
in every possible direction it can be worked at a low cost compared
with the more modern multiple effect evaporators.
_Vacuum Multiple Effect Evaporation._--Advantage is taken of the
fact that water boils at a lower temperature in a vacuum than at the
ordinary pressure of the atmosphere. There are many forms of apparatus
based on this principle, amongst which the most recent is Scott's
evaporator. The black liquor from the boilers is pumped through tubes
heated externally by high-pressure steam. The liquor is passed into a
chamber in which a slight vacuum is maintained, so that immediately on
entering, the liquor parts with a good deal of water in the shape of
steam. The steam liberated is utilised in producing further evaporation
of the partially concentrated liquor, and this operation is repeated
several times until the concentration is effected to the desired point.
In most cases the actual incineration of the thick liquor is carried
out in a rotary furnace when such an apparatus as this is used.
EVAPORATION TABLE.
Showing the volume of liquor obtained by evaporating 1,000 gallons of
weak black lye of density _d_ to a higher density D.
-------------+-----------------------------------------------------
Lower | Higher Density D (Twaddell) at 100° F.
Density _d_ +-----------------------------------------------------
(at 100° F.).| 20. | 25. | 30. | 35. | 40. | 45. | 50. | 55. | 60.
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----
2 |100 | 80 | 66·6| 57·1| 50 | 44·4| 40 | 36·3| 33·3
3 |150 |120 |100 |85·7 | 75 | 66·6| 60 | 54·5| 50
4 |200 |160 |133·3|114·3|100 | 88·8| 80 | 72·7| 66·6
5 |250 |200 |166·6|143 |125 |111·0|100 | 90·9| 83·3
6 |300 |240 |200 |171·4|150 |133·3|120 |109 |100
7 |350 |280 |233·3|200 |175 |155·5|140 |127 |116·6
8 |400 |320 |266·6|228·6|200 |177·6|160 |145·5|133·3
9 |450 |360 |300 |257 |225 |200 |180 |163·5|150
10 |500 |400 |333·3|286 |250 |222 |200 |181·8|166·6
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----
EXAMPLE:--1,000 gallons of weak liquor at a density of 7° Twaddell
are reduced to a volume of 200 gallons having a density of 35°
Twaddell, or to a volume of 140 gallons with a density of 50°
Twaddell, by evaporation.
_Preparation of Caustic Soda._--The crude soda ash recovered from
previous boiling operations is dissolved in large lixiviating tanks
and extracted with hot water. The clear solution obtained after
all impurities have been allowed to settle is pumped up into the
causticising tanks, where it is converted into caustic soda, the loss
due to the amount of soda not recovered being made up by the addition
of ordinary soda ash. The causticising pans are large circular iron
vessels usually 9 feet diameter and 8 or 9 feet deep, into which a
known volume of the recovered carbonate of soda solution is placed.
A weighed quantity of ordinary quicklime is then put into a perforated
iron cage which is fixed inside the causticising pan at such a level
that the whole of the lime is immersed in the solution. The liquor is
kept in constant circulation by means of an agitator and heated to
boiling point, with the result that the chemical reaction sets in,
the carbonate of soda being converted into caustic soda and the lime
being thrown out as chalk. When the operation is completed, the steam
is turned off and the chalk allowed to settle. The clear liquor is
carefully strained off and pumped up into store tanks from which the
required quantities are drawn off into the digesters as circumstances
demand.
_Washing._--The grass which has been partially washed in the digester
is dug out by the workmen and discharged through a manhole fitted on
one side of the digester near the bottom. It is then conveyed in any
convenient manner to the breaking engine, in which the grass is more
completely washed. This important machine has already been described
on page 53. The floor of the vessel slopes slightly upward towards
the front of the roll and falls suddenly behind the roll, in order to
promote a circulation of the contents of the engine round and round the
vessel.
A definite weight of boiled grass is thrown into the engine together
with a large quantity of fresh water. The circulation of the roll draws
the mixture of pulp and water between the knives, breaking it up and
at the same time discharging it behind the beater roll, and producing
a continuous circulation of the mixture in the two sections of the
vessels.
The dirty water is continuously removed from the vessel by means of a
"drum-washer." This is a large hollow drum, the outer surface of which
consists of a fine wire cloth, the interior of the washer being fitted
with specially curved scoops. The drum-washer is lowered until it is
half immersed in the mixture of pulp and water, and as it rotates the
dirty water finds its way through the wire cloth, being caught up by
the internal scoops and discharged through a pipe to a drain outside
the breaking engine. At the same time fresh water is run into the
vessel at one end, and the continuous washing of the pulp thus effected.
_Bleaching._--The clean boiled grass is bleached by means of a solution
of chloride of lime.
There are several methods used for this purpose, each of which has
special advantages of its own, though this is largely a question of
local conditions:--
(A) The pulp can be bleached in the washing engine directly the grass
has been sufficiently cleaned. In this case the flow of fresh water
is stopped and as much water as possible removed by means of the
drum-washer. The drum-washer is then raised out of the pulp and a
known volume of bleaching powder solution corresponding to a definite
weight of dry powder is added to the contents of the breaking engine.
The amount used depends on the quantity of dry grass in the breaking
engine, the usual proportion being 8 to 10 per cent. on the calculated
air-dry weight of raw grass. As the stuff circulates round the engine
the colour gradually changes from dark yellow to white.
The process is sometimes hastened by blowing a small quantity of steam
into the mixture and thereby raising its temperature. Considerable care
must be exercised in using heat, because pulp bleached quickly by this
means is liable to lose colour at the later stages of manufacture.
When the pulp has been bleached to the required extent, the drum-washer
is again lowered into contact with the bleached pulp, and the latter
is thoroughly washed so as to be quite free from traces of bleach and
other soluble impurities.
(B) Esparto is often bleached in a "Tower" bleaching engine which
consists of a tall cylindrical vessel of 9 feet diameter, and 15 or 16
feet deep, at the bottom of which is fixed a small centrifugal pump.
The boiled grass together with sufficient water and clear bleaching
powder solution is placed in the engine; the centrifugal pump draws
the mixture from the bottom of the vessel and discharges it, by means
of a large external pipe, direct into the top of the vessel, where, as
it falls, it comes into contact with a circular baffle-plate, which
distributes the pulp evenly over the surface of the mixture in the
vessel. A continuous and rapid circulation is thus maintained, and the
process is said to be very effective. The bleached pulp is subsequently
washed free from any traces of bleach.
(C) Esparto is frequently bleached by the "steeping" process. In this
case the pulp is washed in the breaking engine, mixed with the required
quantity of bleach, and at once discharged through the outlet pipes
of the engine into large brick tanks, where the bleach is allowed to
act quietly upon the boiled grass. This method produces a pulp of good
colour and is economical.
Whichever process of bleaching is adopted, it is necessary to remove
all the by-products formed during the process, as these soluble
by-products if left in the mixture produce a lowering of colour.
The presence of small traces of bleaching powder solution can be
detected by the use of starch and potassium iodide test papers. If a
handful of the pulp after bleaching, when squeezed out, does not turn
the test paper violet or blue, then the absence of any free bleach is
taken for granted. The slightest trace of bleach will turn such test
papers blue or violet according to the amount present. This is the test
usually applied by the men in charge of the bleaching operations.
_Making Sheets of Esparto Pulp._--For convenience in handling, it is
usual to work up the washed and bleached pulp into the form of moist
sheets. This is effected on a machine known as a "presse-pâte," an
apparatus which closely resembles the wet end of a paper machine. It
consists of a set of flat strainers or screens, a horizontal wire
similar to the paper machine wire, provided with deckles, the usual
couch rolls, and press rolls.
[Illustration: FIG. 24.--A Presse-pâte for Esparto Pulp.]
The pulp diluted with water is passed through the screens and on to
the horizontal wire, where it is formed into a moist sheet, the water
draining away from the wire, and also being removed by vacuum pumps.
The thick sheet of pulp is carried through the couch rolls and press
rolls, being finally wound up on a wooden roller at the end of the
machine. In this moist condition it is ready for use in the mill.
_Dry Esparto Pulp._--When the bleached pulp is intended for export
a more elaborate machine is used--to all intents and purposes a
paper-making machine--by means of which the continuous sheet of moist
pulp is dried and cut up into smaller sheets of suitable size. These
dried sheets are packed up in bales containing 2 cwt. or 4 cwt. of
dried pulp, then wrapped in hessian and bound with iron wires.
_Other Methods._--Since the yield of esparto pulp from the raw material
is less than 50 per cent. and it requires 45 cwt. of grass to make
one ton of finished pulp, methods have been devised for treating the
grass in the green state in the districts where it is grown, but so far
nothing has been done on a large scale.
_The isolation of the cellulose by alkaline treatment in the cold_ has
been suggested, but the method never passed beyond the experimental
stage. This process was indeed first mentioned by Trabut, who many
years ago considered that the removal of non-fibrous constituents from
fresh grass could be readily accomplished by the less drastic treatment
of the esparto with alkaline carbonates of soda and potash at ordinary
temperatures.
_The production of esparto pulp by bacteriological fermentation_ is an
idea of later date. According to the inventor, the grass is crushed
mechanically by means of rollers and then immersed in sea water
inoculated with special bacillus obtained from esparto, and gradually
resolved into cellulose and soluble by-products by fermentation which
is complete in about eleven days. The commercial value of this idea has
not yet been demonstrated.
ESPARTO PULP: MICROSCOPICAL FEATURES.
The pulp of esparto when examined under the microscope is easily
recognised, first by the characteristic appearance of the long slender
cylindrical-shaped fibres, and secondly by the numerous cells always
present. These cells consist of cuticular vessels with serrated edges,
and also of small pear-shaped seed hairs, the shape of which is a ready
means of identifying esparto. An examination of the transverse section
of the raw material indicates the source of these pear-shaped vessels.
_Test for Esparto in Papers._--Paper containing esparto fibre may be
tested by means of a weak solution of aniline sulphate. The suspected
paper is gently heated in the test reagent, and if esparto is present
the paper turns a rose-red or pink colour, the depth of colour being
a measure of the amount of esparto. Most of the modern book papers
are prepared from chemical wood pulp and esparto mixed in varying
proportions, and while this test can be used as a means of detecting a
small or a large proportion of esparto, a microscopical examination is
required for a more accurate estimation.
The proportions used by the paper-maker depend upon the weighing
out of the wood pulp and esparto more or less accurately, while
the microscopical test is based upon the relative proportions as
represented by the volume of fibres of each class on the glass slip
placed under the microscope. Since the wood pulp consists of a number
of broad flat ribbon-like fibres, and the esparto of small cylindrical
fibres, considerable practice is necessary in making a proper analysis
of the two constituents in paper.
[Illustration: FIG. 25.--Esparto Pulp.]
STRAW.
The use of straw for the manufacture of paper was first brought
prominently into notice about the year 1800 by Matthias Koops, who
published a book printed on paper made from straw, but it was not until
1860 that this material was used in any large quantity.
[Illustration: FIG. 26.--A Cylindrical Digester for Boiling Fibre.]
Straw is now converted into a bleached paper pulp for news and
printings, and is also utilised for the manufacture of straw boards.
The production of a white paper pulp from straw is carried out in a
manner similar to that used in the case of esparto fibre, viz., by
digestion with caustic soda under pressure and subsequent bleaching.
As the straw contains considerable quantities of siliceous matter, the
chemical treatment necessary to reduce the material to paper pulp is
more severe, a stronger solution of caustic soda being used, and the
process of digestion being carried out at a higher temperature.
For the best quality of straw cellulose, the material is cut up into
small pieces by machines which resemble an ordinary chaff-cutter, and
the knots taken out by a separating machine. In most cases, however,
the whole straw is simply cut up into small lengths of about one to
two inches long, and placed at once in the digester. When the straw is
contaminated with foreign weeds, sand, husks, and similar substances,
as is usually the case, it is carefully hand-picked by girls, who
remove these impurities, which tend to produce particles of unbleached
matter in the finished pulp. The expense of this preliminary cleaning
process is more than compensated for by the enhanced value of the
bleached straw pulp.
_Digesting._--The cut straw is boiled in rotary cylindrical or
spherical vessels, stationary upright boilers of the vomiting type
being seldom employed because the circulation of the caustic soda
liquor does not take place freely with straw packed in the latter.
As the material is very bulky, some of the liquor is first put into the
boiler and the steam admitted while the straw is being thrown in. By
this means the straw is softened and reduced in bulk, so that a larger
quantity can be added before the digester is quite full. The full
amount of caustic soda is then made up by further additions of liquor,
and the contents of the digester heated by high-pressure steam for four
to six hours.
The conditions of treatment are shown by the following trial:--
Amount of straw 5,600 lbs.
Caustic soda, 20 per cent. 1,120 lbs.
The caustic soda was added in the form of a liquor, having a volume of
2,012 gallons and a specific gravity of 1·055.
Time of boiling 5 hours.
Pressure 60 lbs.
_Washing._--The boiled straw is discharged into large tanks placed
below the digester and washed with hot water, the smallest possible
quantity being used consistent with complete washing in order to
prevent the accumulation of large volumes of weak lye. The spent liquor
and washing waters are drained off into store tanks and evaporated in a
multiple effect apparatus by the same process as that used for esparto
pulp. The last washings are usually run away because the percentage of
soda in them is too small to pay for the cost of recovery.
The final washing of the straw pulp is completed by the use of a
breaking engine or potcher. As straw pulp contains a large proportion
of cellular matter which cannot be regarded as true fibres, there
is always a danger of considerable loss in yield if the use of the
breaking engine is extensively adopted, because the short cells escape
through the meshes of the drum-washer. The washing is most economically
effected in the tanks if a good yield of pulp is required.
_Separating out Knots._--The broken pulp from the breaking engines is
diluted with large quantities of water and pumped over sand traps in
order to remove knots and weeds which have resisted the action of the
caustic soda. These traps consist of long shallow trays, perhaps sixty
to eighty yards long, one yard wide, and nine inches deep, containing
boards which stretch from side to side, sloping at an angle, and nailed
to the bottom of the trays. The dilute pulp flows through the trays,
leaving the heavy particles, knots, and foreign matter behind the
sloping boards, and finally passes over the strainers, which retain
any large coarse pieces still remaining.
_Making Sheets of Pulp._--The mixture from the strainers contains a
large excess of water which has to be removed before the pulp can be
bleached. For this purpose a wet press machine (see page 103) or a
presse-pâte (see page 85) is employed, and the wet sheets of pulp are
then ready for bleaching.
_Bleaching._--The process by which the pulp is bleached is exactly
similar to that used for treating esparto.
From 1870 to 1890 large quantities of straw were used for the
manufacture of newspaper in conjunction with esparto and wood pulp, but
the price of the material was gradually advanced so that it could not
be used with advantage, especially as the production of wood pulp gave
a material which was much cheaper, and which could be utilised at once
without chemical treatment.
In the manufacture of newspaper the tendency during recent years has
been to make the paper mill operations as mechanical as possible and to
dispense with the preliminary operations which are essential for the
manufacture of half-stuff, the chemical processes being left in the
hands of the pulp manufacturers.
The manufacture of straw cellulose is now practically confined to
Germany, but small quantities of the bleached straw cellulose are
imported because the pulp imparts certain qualities to paper which
improve it, notably in making cheap printing papers harder and more
opaque.
MICROSCOPICAL FEATURES OF STRAW.
The paper pulp obtained from straw consists of a mixture of short
fibres together with a large proportion of oval-shaped cells. The
fibres are short and somewhat resemble esparto, but the presence of
the smaller cells is a sure indication of the straw pulp. The fibres
themselves closely resemble the fibres of esparto, but as a rule the
latter are long slender fibres, while the straw fibre is very often
bent and twisted or slightly kinked.
[Illustration: FIG. 27.--Straw.]
The only method of distinguishing between straw and esparto is by
examination with the microscope. There is no chemical reagent known
which will produce a colour reaction on a paper containing straw that
will serve to distinguish it from a paper containing esparto. If such
papers are gently heated in a weak solution of aniline sulphate a pink
colour is slowly developed, the intensity of which is to some extent a
measure of the amount of straw or esparto present.
Straw and esparto are usually described in text-books under one
heading, partly because the fibres possess strong resemblances in
physical and chemical constitution, and partly because the methods of
manufacture are identical. At the same time the qualities of the two
pulps are so different that they cannot be used indiscriminately, the
one for the other. Straw cellulose cannot be utilised in the place
of esparto, particularly for light bulky papers. Hence in magazine
and book papers containing a fibre which gives a pink coloration with
aniline sulphate it is fairly safe to assume that esparto pulp is
present.
CHAPTER V
WOOD PULP AND WOOD PULP PAPERS
THE MANUFACTURE OF MECHANICAL WOOD PULP.
Wood is converted into pulp suitable for the manufacture of paper by
methods which produce two distinct varieties. The first is _mechanical
wood pulp_, so called because it is made by a purely mechanical
process. The second is termed _chemical wood pulp_ from the fact that
the material is submitted to chemical treatment.
_Ground Wood and Cellulose._--The two varieties of pulp are sometimes
distinguished by the use of the terms ground wood and cellulose. In
the former case the description implies a product consisting of pulp
obtained by grinding wood into a fibrous condition, while in the second
the word suggests a purified chemical product freed from the resinous
and non-fibrous constituents found in wood. This is, in fact, the
essential difference, for mechanical wood pulp consists of fibres which
have been torn away from wood by means of a grindstone; it differs
but slightly in chemical composition from the original raw material
and contains most of the complex substances natural to wood. Chemical
wood pulp, on the other hand, consists of fibre isolated from wood in
such a manner that the complex non-fibrous substances are more or less
entirely removed. The difference between these two pulps is shown in
the following approximate analysis of spruce wood, and of the pulp
derived from it. The composition of the mechanical pulp is practically
identical with that of the wood itself.
COMPOSITION OF SPRUCE WOOD, AND OF CHEMICAL WOOD PULP (SPRUCE).
----------------+---------+----------
-- | Wood | Chemical
|(Spruce).|Wood Pulp.
----------------+---------+----------
Cellulose | 53·0 | 88·0
Resin | 1·5 | 0·5
Aqueous Extract | 2·5 | 0·5
Water | 12·0 | 8·0
Lignin | 30·5 | 2·5
Ash | 0·5 | 0·5
+---------+----------
| 100·0 | 100·0
----------------+---------+----------
The use of mechanical wood pulp is generally confined to the
manufacture of news, common printings and packing papers, cardboards,
and boxboards. It possesses very little strength, quickly discolours
when exposed to light and air, and gradually loses its fibrous
character. The chemical wood pulp is a strong fibre, from which
high-class papers can be manufactured, the colour and strength of which
leave little to be desired.
_Species of Wood._--The woods most commonly used for the manufacture
of wood pulp belong to the order Coniferæ, or cone-bearing trees.
In Europe the spruce and silver fir are the chief species, while in
America spruce, balsam, pine, and fir are employed. The harder woods,
such as hemlock, beech, larch and others, are not converted into pulp
by the mechanical process.
_Timber Operations._--The trees are cut down in the early part of
winter by gangs of men specially trained to the work. The organisation
of a lumber camp when the operations are of an extensive character is
very complete and carefully arranged, every detail being attended to
in order to get out the wood as cheaply and expeditiously as possible.
The branches and small tops are removed from the trees when they are
fallen, and the trunks cut into logs of 12, 14, or 16 feet in length,
and afterwards piled up on the banks of the nearest river, or on the
ice, ready for the breaking up of the winter.
As soon as the ice breaks up and the rivers become navigable the logs
are floated down to their destination, in some cases hundreds of miles
from the scene of operations. Where rivers are not available the timber
is brought out by horses or bullocks, or by means of a light railway.
_Log Cutting._--As the timber arrives at the mill it is carefully
measured, both as to its diameter and length, in order that a record
may be kept of the quantity used. Some of the logs are piled up in
the storeyard for use in the winter, and the remainder converted into
pulp day by day. The logs are first cut into short pieces about 2 feet
long by means of a powerful circular saw, the arrangements for this
work being devised so as to keep down the cost of labour as much as
possible. All waste pieces are thrown aside to be utilised as fuel.
_Barking._--The bark on the logs is removed in one or two ways. Much
of it is knocked off during the transfer from the forest to the mill,
but even then the wood requires to be cleaned. In Norway and Sweden
the wood is treated in a _tumbler_ or a _barker_, while in America and
Canada the use of the tumbler is practically unknown.
The barker consists of a heavy iron disc fitted with knives, usually
three in number, which project from the surface of the disc about
half or three-quarters of an inch. The barker rotates in a vertical
position, and the short pieces of wood are brought one by one into
contact with the disc in such a manner that the bark is shaved off by
the knives. The machine is provided with conveniences for pressing the
wood against the disc and for turning the logs as they are barked.
[Illustration: FIG. 28.--A Pair of Barkers for removing Bark from Logs
of Wood.]
The machine is encased in a strong cast-iron cover, and all the bark
shaved off is carried away by the strong current of air set up by the
rapid motion of the disc, and subsequently burnt.
The tumbler system is quite different. In this case the short pieces
are thrown into a large circular drum with hot water, and the bark
taken off by the friction of the pieces as the drum rotates. The loss
of material is of course less in this process, but the wood is not
cleaned quite so effectively.
[Illustration: FIG. 29.--View of Horizontal Grinder (A), with Section
(B).]
The wood at this stage can be used either for the manufacture of
mechanical or chemical pulp. As a general rule the pieces are taken
indiscriminately for either process, but sometimes the wood is sorted
out, the clean stuff free from knots and blemishes being reserved for
high quality chemical pulp.
_Grinding._--The main feature of the grinding process is the attrition
of the wood when held against the surface of a rapidly revolving
grindstone, the fibres as they are rubbed off being instantly carried
away from the stone by a current of water. A complete description of
the machines used and the modifications of the process practised by
manufacturers is impossible in this book, but the following points will
be sufficient.
The machine consists of a large grindstone about 54 inches in diameter,
and 27 inches thick. It rotates in a vertical or in a horizontal
position at a high speed. The stone revolves inside a casing which is
provided with a number of _pockets_, so called, into which the pieces
of wood are thrown at regular intervals, as fast as the wood is ground
by the friction of the stone.
A continual stream of water playing upon the surface of the stone
washes away the pulp into a tank or pit below the machine.
The quality of the pulp may be varied by the conditions under which it
is made. By limiting the proportion of water so that the wood remains
in contact with the stone for a longer time the temperature of the mass
in the pockets rises. Such _hot ground pulp_, as it is termed, is tough
and strong.
When the fibres are washed away from the stone as fast as they are
produced the temperature does not rise, and _cold ground_ pulp is made,
which is not characterised by the somewhat leathery feel of the pulp
made at the higher temperature.
The surface of the stone plays an important part also. If the stone
is smooth the wood is rubbed away slowly, but if the surface has been
roughened and grooved by means of a special tool the fibres are torn
away quickly. In the first case the pulp comes from the stone in a
finely-ground state and in a uniform condition, while in the second
the pulp is coarse and chippy.
The output of the machine is, however, much increased by the use of
sharp stones and by the application of considerable pressure to the
blocks of wood.
[Illustration: FIG. 30.--A Vertical Grinder for making Hot Ground
Mechanical Wood Pulp.]
_Screening._--The mixture of water and pulp leaving the grinder falls
into a tank below the stone, all large chips being retained by means
of a perforated plate. The finer pulp, still too coarse for use, is
then pumped to the screens, which serve to remove all chippy and coarse
fibres and produce a uniform material. The _shaking sieve_ consists
of a shallow tray, the bottom of which is a brass plate or series of
plates perforated with small holes or slits. The pulp flows on to the
tray, which is kept in a state of violent agitation, the fine pulp
passing through the holes and the coarser pieces working down to the
lower edge of the tray into a trough which carries them away. The _flat
screen_ is somewhat different in construction, but the principle of
separation is the same. It consists of brass perforated plates forming
the bottom of a shallow cast-iron tray, continually agitated by means
of cams fixed to the under surface of the trays.
[Illustration: FIG. 31.--Centrifugal Screen for Wood Pulp.]
The _centrifugal screen_ is a cage made of finely perforated brass
sheeting which revolves at a very high rate of speed inside a circular
cast-iron vessel. The pulp flows into the interior of the cage, the
fine fibres being forced through the screen by the centrifugal action
of the machine, and the coarse material is retained.
[Illustration: FIG. 32.--Section of Centrifugal Screen for Wood Pulp.]
_Wet Pressing._--The pulp leaving the screens is mixed with such a
large quantity of water that it is necessary to concentrate it. This
is effected by means of the wet press machine (Fig. 41). The pulp and
water are pumped into a wooden box in which revolves a large hollow
drum, the surface of this drum consisting of a fine wire cloth of about
60 or 70 mesh. The drum is not entirely immersed in the mixture, so
that as it rotates the pulp forms a skin or thin sheet on the surface,
and the water passes away through the wire into the interior of the
hollow drum. The drum carries the thin sheet out of the box and above
the level of the mixture until it comes into contact with an endless
blanket or felt, which is pressed against that part of the drum not
immersed in the liquid.
By this means the thin sheet is transferred to the felt and carried
between squeezing rolls to the finishing rolls. The felt, carrying on
its upper surface the thin sheet of pulp, passes between two rolls,
usually 16 to 20 inches in diameter, the upper being made of wood and
the lower one of cast iron. The pulp adheres to the upper drum and the
felt passes round the lower drum back to the box containing the mixture
of pulp and water; the thin sheet is continuously wound on the upper
roll until a certain thickness is reached.
When this occurs the attendant removes the thick sheet by a dexterous
movement of a sharp stick across the face of the roll. The wet pulp at
this stage consists of 30 per cent. air-dry pulp and 70 per cent. of
water.
_Hydraulic Pressing._--The sheets taken from the wet press machine are
folded into a convenient shape and piled up, coarse pieces of sacking
being placed between the sheets. At stated intervals the piles are
submitted to pressure in hydraulic presses in order to remove further
quantities of water, which slowly drains away through the sacking. In
this way a mass of pulp in the form of thick folded sheets containing
50 per cent. of dry wood pulp is produced.
The pieces of sacking are taken out and the sheets put up in bales of
any required weight, usually 2 cwt. or 4 cwt.
THE MANUFACTURE OF CHEMICAL WOOD PULP.
Most vegetable fibres are converted into pulp by alkaline processes,
that is by digesting the raw material with caustic soda and similar
alkaline substances. Wood may be treated in two ways, one of which is
the ordinary soda process, and the other an acid treatment requiring
the use of sulphurous acid.
_Preparation of the Wood._--The logs of wood are cut up and barked
exactly as in the case of mechanical pulp. The short two-foot pieces
are then cut up into small flakes about one inch square and half an
inch thick by means of a machine known as a _chipper_. This is similar
in construction to a barker, consisting of a heavy iron disc rotating
at a high speed inside a stout cover. The disc revolves in a vertical
position, and three projecting knives slice up the logs into flakes.
For this purpose the disc is provided with three slots which radiate
from the centre towards the circumference for about 12 inches. The
knives can be adjusted so that they stand up through the slots and
above the surface of the disc to any required distance.
In order to ensure uniformity in the size of the chips, the practice
is frequently adopted of sifting the wood leaving the chipper. The
sieve is a large skeleton drum, the outer surface of which is made of a
coarse wire cloth capable of passing all pieces of the size mentioned.
Larger chips and pieces are retained in the drum as it revolves in a
horizontal position and only fall out on reaching the extreme end of
the machine.
_The Digesters._--The object of boiling the wood under pressure with
chemicals is to dissociate the valuable fibrous portion of the plant
from the resinous and non-fibrous portion. In this process the wood
loses half its weight, the yield of pulp being about 50 per cent., and
the remainder is dissolved out by the chemical solution. The conditions
of treatment are extremely varied in character, the quality of the pulp
produced varying in proportion.
The digesters are either spherical, cylindrical, or egg-shaped, being
constructed to revolve at a slow rate of speed, or fixed permanently
in an upright position. Spherical boilers are usually 9 or 10 feet in
diameter, the cylindrical digesters being 40 or 50 feet high and 12 or
15 feet diameter, the larger ones being capable of taking 20 tons of
wood for each operation.
[Illustration: FIG. 33.--Wood Pulp Digester, partly in elevation,
partly in section.]
For the alkaline process the interior of the digester does not require
any special treatment, but with the acid process the internal portion
of the boiler is carefully lined with a thick layer of acid-resisting
brick and cement.
The contents of the digester are heated by means of high-pressure
steam, which is blown direct into the mass or passed through a coil
lying at the bottom of the vessel. In the former case the steam is
condensed by the liquor, the volume of which is consequently increased,
while in the latter case the condensed steam is drawn off continuously
from the pipes. Each system has its own particular advantages.
_Different Kinds of Chemical Wood Pulp._--According to the method of
treatment so the quality of the pulp varies. The chemicals used, the
system of boiling, the temperature of digestion, the strength of the
solutions, the duration of the cooking period, and, last but not least,
the species of wood, are all determining factors in the value of the
ultimate product.
_Soda Pulp._--This is prepared by digesting wood with caustic soda in
revolving boilers for eight or ten hours at a pressure of 60 to 80 lbs.
_Sulphate Pulp._--Prepared by digesting the wood with a mixture of
caustic soda, sulphide of soda, and sulphate of soda.
_Sulphite Pulp._--The process most generally adopted for the
manufacture of wood pulp is the treatment of the material in
brick-lined digesters with bisulphite of lime for eight to nine hours
at a pressure of 80 lbs.
_Mitscherlich Pulp._--This is sulphite pulp prepared by digesting the
wood at a much lower temperature and for a longer period than the
ordinary sulphite. The steam is not blown direct into the mass of wood,
and the pressure seldom exceeds 45 or 50 lbs., the time of boiling
occupying 45 to 50 hours. So called from the name of the inventor.
_Sulphite Wood Pulp._--This name is given to pulp prepared by digesting
wood with solutions containing sulphurous acid, or salts of sulphurous
acid. The acid is produced by burning sulphur or certain ores
containing sulphur, such as copper or iron pyrites, in special ovens.
The most modern form of oven consists of a cylindrical cast-iron drum
revolving slowly in a horizontal position on suitable bearings. The
sulphur is thrown at intervals, or fed automatically, into the oven,
the amount of air being carefully regulated to avoid the formation of
sulphuric acid in the later stages of preparation. The sulphur is also
burnt in stationary ovens which consist of flat shallow closed trays.
[Illustration: FIG. 34.--View of ordinary Sulphur-burning Ovens.]
The hot sulphurous acid gas passes through pipes and is cooled, after
which it is brought into contact with water and lime for the production
of the bisulphite of lime. This is accomplished by one of two methods
as follows.
_Tower System._--The cool gas is drawn into high towers usually
built of wood, 7 or 8 feet diameter, which are filled with masses of
limestone. From tanks at the top of each tower a carefully regulated
quantity of water flows down upon the limestone and absorbs the
ascending column of gas, this being drawn into the tower from the
bottom. The limestone is simultaneously dissolved, and the liquid
which flows out from the pipes at the bottom of the tower consists of
lime dissolved in sulphurous acid, together with a certain proportion
of free sulphurous acid. This is generally known as a solution of
bisulphite of lime.
_Tank System._--The somewhat costly tower system has in many cases been
superseded by the use of a number of huge wooden vats, 10 to 12 feet
diameter and 8 to 10 feet high. These tanks are filled with water and
a known quantity of slaked lime. The gas is forced into the tanks by
pressure or drawn through by suction, and the conversion of the milk of
lime into bisulphite of lime proceeds automatically. In order to ensure
complete absorption the gas passes through the tanks in series, so that
the spent gases leaving the vats do not contain any appreciable amount
of sulphurous acid.
In order to obtain pulp of uniform quality it is necessary that the
liquor should be of constant composition. The formula differs in the
various mills according to the conditions which are found most suitable.
_Sulphite Digesters._--The almost universal form of boiler employed in
cooking wood by the sulphite process is a tall cylindrical vessel of
about 50 feet in height, and 14 to 15 feet internal diameter, lined
with acid-resisting brick.
This form of digester is capable of holding 20 tons of wood at one
charge, yielding 10 tons of finished pulp.
The chipped wood is discharged into the digesters from huge bins
erected just above the openings to the digesters, so that the latter
can be filled without any delay and the requisite quantity of sulphite
liquor added.
The manhole or cover is at once put on, securely fastened, and steam
turned on gradually until the pressure reaches 70 or 80 lbs., at which
pressure the cooking is steadily maintained. The progress of the
operation is watched and samples of the liquor drawn from the boiler
at intervals to be tested, so that the boiling may be stopped when the
results of the testing show the wood is sufficiently cooked.
There is no special difficulty in this operation, provided the
necessary conditions are observed. It is important that the wood should
be dry, and that the proportion of sulphite liquor per ton of dry wood
should be constant. If the wood happens to be wet, due allowance must
be made for the excess water and a somewhat stronger liquor used in
order to compensate for this. Other precautions of a similar character
are observed in order to minimise the danger of an insufficiently
cooked pulp.
_Washing._--When the pulp has been boiled, a process which generally
occupies seven or eight hours, the steam is shut off and the contents
of the boiler blown out into large vats known as blow-out tanks, the
pressure of steam remaining in the digester being sufficient to empty
the softened pulp in a few minutes. Much of the spent sulphite liquor,
now containing the dissolved resinous and non-fibrous portions of the
original wood, drains away from the mass in the tank, and then copious
supplies of clean water are added in order to wash out the residual
liquors which it is essential to remove.
Numerous other devices are employed to ensure the complete washing of
the boiled pulp.
_Screening._--The production of a high-class pulp necessitates proper
screening to eliminate coarse pieces of unboiled wood and the knots,
the latter not being softened completely. The methods adopted vary
according to requirements.
For uniform clean pulp that can be bleached easily the material from
the blow-out tanks is, after washing, mixed with large quantities of
water and run through sand traps, which consist of long shallow wide
boxes provided with slanting baffle-boards to retain knots and large
pieces of unsoftened wood, the pulp thus partially screened being
subsequently treated in the proper screening apparatus.
Sometimes the washed pulp is sent direct to the screens and the
well-boiled fibres sorted out by a system of graded screens, which
separate the completely isolated fibres from the bulk and retain the
larger pieces, these being broken down in a suitable engine and put
back on the screens.
The machinery employed for screening chemical pulp is identical with
that used for the treatment of mechanical wood pulp.
_Finishing._--The ordinary sulphite pulp is worked up into the form
of dry sheets for the market and not sent out in a wet state as the
mechanical wood. There are several practical disadvantages in preparing
the latter in a dry condition which do not, however, occur with
chemical pulp.
Hence the pulp after being screened is not pressed but submitted to a
different process. From the screens the mixture of pulp and water, the
latter being present in large quantity, is pumped into a concentrator,
or slusher, as it is termed, by means of which some of the water is
taken out.
The slusher consists of a wooden box divided into two compartments by
a vertical partition. In the larger compartment a hollow drum covered
with a fine wire cloth revolves, the construction and purpose of which
are precisely the same as that of the wet press machine used for
mechanical pulp.
As the drum revolves the pulp adheres to the outer surface, while
the water passes through the wire cloth. The drum is not completely
immersed in the mixture, so that the skin of pulp is brought out of the
water by the rotation of the drum. When this takes place the contact of
a wooden or felt covered roll which revolves on the top of the drum
causes the pulp to be transferred from the drum to the roll. The wet
pulp is continuously scraped off by an iron bar or _doctor_, as it is
called, resting on the surface of the roll, and it finally drops into
the second compartment of the slusher in a more concentrated form ready
for the drying machine.
_Drying._--The mass of wet pulp from the slusher is conveyed into a
circular reservoir or _stuff chest_, which serves to supply the machine
used for converting the pulp into dry sheets.
The machine is to all intents and purposes a Fourdrinier paper machine,
and the process is similar to that used for the manufacture of paper.
The pulp flows in a continuous stream on to a horizontal endless wire,
which carries it forward as a thin layer; the water drains through
the meshes of the wire, further quantities being removed by _suction
boxes_, which draw away the water by virtue of the vacuum produced by
special pumps. The wet sheet then passes between the _couch rolls_
which compress the pulp, squeezing out more water, and then through
_press rolls_, which finally give a firm adherent sheet of pulp
containing 70 per cent. of water. The sheet is dried by passing over
a number of steam heated cylinders, which cause all the moisture to
evaporate from the pulp. At the end of the machine the dry pulp is cut
up into sheets of any convenient size, and packed up in bales of two or
four cwts.
_Mitscherlich Sulphite Pulp._--This term is applied to sulphite wood
prepared by submitting the chipped wood to a comparatively low pressure
for a long period. The wood is placed in the stationary upright form of
digester with the requisite amount of liquor, and the heating produced
by the passage of steam through a leaden coil lying at the bottom of
the digester, so that the steam does not condense in the liquor but
in the coil, from which it is drawn off. The pressure seldom exceeds
45 lbs. but the duration of the cooking is thirty-six to forty-eight
hours. The boiler is not emptied under pressure, but the pulp is
discharged from the digester after the pressure has been lowered, and
the manhole taken off. The contents are usually shovelled out by the
workmen.
The pulp is carefully washed, screened and made up into wet sheets
on the ordinary wet press machine. This pulp is never dried on the
Fourdrinier like the common sulphite, as its special qualities can only
be preserved by the treatment described. This pulp is particularly
suitable for parchment papers, grease proofs and transparent papers.
_Soda Wood Pulp._--The chipped wood is boiled in stationary or
revolving digesters for eight or nine hours at a pressure of 70 or 80
lbs. A solution of caustic soda is employed, about 16 to 20 per cent.
of the weight of the wood being added to the contents of the digester.
Live steam is blown direct into the mass, and after the operation the
spent liquor is carefully kept for subsequent treatment. The pulp is
washed in such a manner that the amount of water actually used is kept
down to the smallest possible volume consistent with a complete removal
of soluble matters. This is done in order that the spent liquors may be
treated for the recovery of the soda.
_Recovery of Spent Liquors._--When wood is cooked by the soda and
sulphate processes the solutions containing the dissolved organic
matter from the wood can be evaporated, and the original chemical
recovered. In the case of soda pulp the method of treatment is as
follows: the spent liquors and the washings are evaporated by means of
a multiple effect vacuum apparatus to a thick syrup. The concentrated
liquor produced is then burnt in special furnaces, all the organic
matter being consumed, leaving a black mass which consists mainly
of carbonate of soda. The mass is washed with water to remove the
carbonate which is afterwards converted into caustic soda by being
boiled with lime.
[Illustration: FIG. 35.--Spruce Wood Pulp.]
The spent liquors from the sulphite process have no value, for they
cannot be recovered by this method. At present the whole of the sulphur
used and the organic matter dissolved from the wood is lost. This means
the loss of about 250 to 350 lbs. of sulphur and nearly 50 per cent. of
the weight of wood for every ton of pulp produced.
WOOD PULP; MICROSCOPIC FEATURES.
[Illustration: FIG. 36.--Mechanical Wood Pulp.]
Mechanical and chemical pulps are readily distinguished under the
microscope. The former consists of fibres of irregular shape and size,
mixed with a large proportion of structureless particles, all bearing
evidence of having been torn apart and separated by mechanical methods.
The chemical pulp, on the other hand, consists of fibres isolated by a
process which preserves them in perfect condition and form. The pulp
from the various woods can be differentiated by minute details in fibre
structure, some of the woods being determined from the presence of
characteristic cells.
The use of aniline sulphate can also be resorted to, and for
microscopic work the most useful reagent is a mixture of zinc chloride
and iodine. This produces an intense yellow colour with mechanical pulp
and a bluish colour with sulphite and other chemical wood pulps.
THE DAILY NEWSPAPER.
The newspapers of the present day are made almost exclusively of wood
pulp. The use of the latter material for paper-making has steadily
increased from the date of its introduction about A.D. 1870, when wood
pulp was imported into England in considerable quantities.
News and cheap printings consist of mechanical and chemical wood pulps
mixed in varying proportions determined chiefly by the price paid
for the finished paper. In some cases the proportion of mechanical
wood pulp is as much as 85 per cent., though the average composition
of a cheap wood paper is represented by the following proportions:
Mechanical pulp, 70 per cent.; sulphite pulp, 20 per cent.; loading, 10
per cent.
Some idea of the enormous quantity of material used for the daily
press may be judged from one or two examples. A certain popular weekly
newspaper having a circulation of one and a quarter million copies per
week requires every week 137 tons of paper produced from 170 tons of
wood. A popular halfpenny newspaper boasting a circulation of about
one-half million copies per day consumes 185 tons of paper manufactured
from 230 tons of wood, every week.
It is easy also from these facts to estimate the amount of timber
which must be cut down to supply the demand for newspapers and cheap
printings.
The manufacture of news calls for considerable skill and able
management, owing to the keen competition amongst the paper mills
devoted to this class of paper. The process as carried on in England is
as follows:--
The mechanical pulp, reaching the mill in the form of thick sheets
suitably packed up into bales, is first broken up again into moist
pulp. Various machines are used for this, such as Wurster's kneading
engine, Cornett's breaker, or some similar contrivance. An old potcher,
such as is used for the breaking and washing of rags, makes a good pulp
disintegrator. The broken pulp is discharged into beating engines in
any suitable or convenient manner and the right proportion of chemical
wood pulp added in the form of dry sheets. The beating process only
occupies thirty to forty minutes in the case of the common news, a
marked contrast to the eight or nine hours required by rags. China clay
is added to the contents of the beater, ten to twelve per cent. being
the general practice. This is followed by a measured quantity of rosin
size, and after thorough incorporation the size is precipitated upon
the fibres by means of alum.
In the commoner qualities of these papers the materials are added in
the dry state, but for finer grades of newspaper the china clay is
mixed with water, and carefully drained through a fine sieve before
use. The alum cake is also dissolved and treated in a similar manner in
order to keep out dirt and coarse particles likely to produce holes in
the paper.
The paper machine used for the manufacture of cheap printings is
constructed to produce as much as 100 to 180 tons of finished paper per
week, every detail being arranged for a large output at a very high
speed. In the modern machine it is possible to produce paper at the
rate of 450 to 550 feet per minute, the width of the sheet being from
120 to 160 inches.
Careful attention is paid to economy of every kind with regard to the
power required for driving the machine, the amount of steam consumed
in drying the paper, recovery of excess of fibre and china clay which
escapes from the machine wire, and similar details of a mechanical
order.
[Illustration: FIG. 37.--The Screens for removing Coarse Fibres from
Beaten Pulp.]
The beaten pulp, after being sized and coloured, is discharged into
huge circular brick tanks, or stuff chests, two of which are found
with each paper machine. The supply of pulp and water for the machine
is taken from one stuff chest while the second is being filled up
from the beating engines, in order to secure a mixture of constant
composition.
[Illustration: FIG. 38.--The Paper Machine (wet end showing wire).]
The pulp is pumped from the stuff chest into a small regulating box
placed above the machine wire, and this box is kept full of beaten
pulp so that the supply of pulp and water to the machine is perfectly
constant. The pulp, diluted with the proper quantity of _back-water_,
is carefully strained through rotary screens and allowed to flow
through a distributing box on to the machine wire, where it rapidly
forms a sheet of paper.
The excess of water, together with a certain proportion of fine
fibre and china clay, falls through the wire, and is caught below
in a shallow box, called the save-all. This _back-water_, as it is
called, is used over again for diluting the beaten pulp to the right
consistency, as already described.
The whole of the water obtained in this way is not all utilised in the
regulating box, and any surplus is pumped up continually into large
store tanks and used in the beating engines for breaking down the dry
pulp.
In many cases, where a large quantity of water is used on the machine,
special methods have to be adopted for the recovery of all the fibre
and clay, which would otherwise be lost, and there are many ingenious
systems in use whereby this saving is effected.
The most usual practice is to allow the excess of water, which contains
from 8 to 15 lbs. of suspended matter per thousand gallons, to flow
through a series of brick tanks at a slow rate of speed. The clay and
fibre settle to the bottom of the tanks, and the water passes away from
the last tank almost clear and free from fibre and loading.
The drying of the moist paper leaving the press rolls of the machine is
effected in the usual manner by means of drying cylinders. On account
of the great increase of speed at which the paper is produced, the
number of drying cylinders has also been increased, and at the present
time a machine of this description is provided with 28 or 32 cylinders,
the object being to dry the paper economically.
MECHANICAL WOOD PULP IN PAPER.
The presence of mechanical wood pulp in paper is detected by means of
several reagents, which produce a definite colour when applied to a
sheet of paper containing mechanical wood. The depth of colour obtained
indicates approximately the percentage present, but considerable
practice and experience is necessary to interpret the colour exactly. A
more reliable method of estimating the percentage of mechanical wood in
a paper is by microscopic examination.
The reagents which can be used are--
(1) _Nitric Acid._--This produces a brown stain on the paper, but it is
not a desirable reagent for ordinary office purposes.
(2) _Aniline Sulphate._--A solution of this is prepared by dissolving 5
parts of aniline sulphate in 100 parts of distilled water. When applied
to the surface of news a yellow coloration is produced, more or less
intense according to the amount of mechanical wood present. It can only
be used with white papers, or papers very slightly toned.
(3) _Phloroglucine._--This sensitive reagent, which gives a rose-pink
colour when brushed on to the surface of the paper, is prepared by
dissolving 4 grammes of phloroglucine in 100 c.c. of rectified spirits,
and adding to the mixture 50 c.c. of pure concentrated hydrochloric
acid.
There are several other aniline compounds which give colour reactions
of a similar character, but they are not often used. The phloroglucine
reagent fails as a test for mechanical wood in papers which have
been dyed with certain aniline colours, for example, metanil yellow.
Paper which has been coloured with this dye will, when moistened with
the phloroglucine reagent, give an intense pink colour, even if no
mechanical wood is present. This is due to the fact that the dye itself
is acted upon by the hydrochloric acid in the test reagent. The same
colour is produced on the paper with hydrochloric acid _per se_.
There is little difficulty in distinguishing between the colour arising
from the presence of such a dye, because the effect is instantaneous,
whereas the coloration due to mechanical wood develops gradually.
Moreover, the reaction due to the presence of metanil yellow gives a
perfectly even coloured surface, whereas with mechanical wood pulp the
fibres appear to be more deeply stained than the body of the paper.
_Output of a Paper Machine._--The quantity of paper which can be
produced on the paper machine is readily calculated from the following
data:--
Speed of machine in feet per minute _F_
Nett deckle width in inches _D_
Width of sheet of paper in inches _W_
Length of sheet of paper in inches _L_
Number of sheets in ream _S_
Weight of paper per ream _R_
The general formula for the output of paper per hour is
720 × _F_ × _D_ × _R_
Output in lbs. per hour = -----------------------.
_S_ × _L_ × _W_
When the number of sheets in the ream is 480, this formula simplifies to
1½ × _R_ × _F_ × _D_
Output in lbs. per hour = --------------------.
_L_ × _W_
The term "nett deckle width" applies to the width of the trimmed
finished paper at the end of the machine. The formula takes no account
of the allowance required for trimming edges. In most cases the deckle
width of the machine is arranged so that the paper is cut into strips
of equal width when leaving the calenders, _e.g._, a deckle of 80
inches will give 4 sheets, each 20 inches wide.
[Illustration: FIG. 39.--Paper Machine showing Wire, Press Rolls, and
Drying Cylinders.]
The method by which the general formula is obtained may be explained by
an example.
What is the output of a machine having a speed of 100 feet per minute,
with an 80-inch deckle, producing a sheet of paper 20 inches by 30
inches, weighing 30 lbs. per ream of 480 sheets?
The machine produces every minute a sheet of paper 100 feet long and 80
inches wide.
Hence output per minute in square inches
= 12 × 100 × 80.
Output per hour in square inches
= 60 × 12 × 100 × 80.
Now each (20 × 30 × 480) square inches is area of one ream.
Output of paper per hour in reams
60 × 12 × 100 × 80
= ------------------.
480 × 30 × 20
Output of paper per hour in lbs.
720 × 100 × 80 × 30
= -------------------
480 × 30 × 20
= 600 lbs.
The general formula may be applied for the purpose of calculating the
speed at which the machine must be driven.
_Example._--A machine with 75-inch deckle is required to produce
6 cwts. per hour of a paper 25 inches by 18 inches (500 sheets),
weighing 19 lbs. to the ream. At what speed is the machine to be driven?
Output in lbs. per hour
720 × _F_ × _D_ × _R_
= ---------------------
_S_ × _L_ × _W_
720 × F × 75 × 19
672 = -----------------
500 × 18 × 25
_F_ = 148 feet per minute.
CHAPTER VI
BROWN PAPERS AND BOARDS
_Common Browns._--The raw material used in the manufacture of common
brown papers is chiefly jute and waste fibres of every description,
such as waste cuttings from boxboard factories, old papers, wood pulp
refuse, and other substances of a like nature. The jute, in the form
of sacking or old gunny bags, and the hemp refuse, in the shape of old
rope and string, are subjected to a slight chemical treatment just
sufficient to isolate the fibres to a condition in which it is possible
to work them up into paper. The bagging and string are cut up in a rag
chopper and boiled in revolving boilers with lime or caustic soda for
several hours at a pressure of 20-30 lbs., the lime being used when it
is desired to manufacture a harsh paper, and the caustic soda being
employed for the production of paper having a softer feel. The pulp is
not always washed very completely after the process of digestion, as is
the case with white papers, and it is often possible to extract from
brown papers of this class a considerable proportion of the alkaline
matter which has not been thoroughly removed from the boiled pulp.
The presence of this alkaline residue does not affect the quality of
ordinary brown paper, but is frequently a serious defect in the case
of middles or straw boards, which are afterwards utilised for boxes
and covered with coloured papers. The colour of the paper pasted on to
such incompletely washed boards is frequently spoilt by the action of
the alkali when moistened with the paste used, many aniline dyes being
susceptible to the small proportion of alkali present.
The stronger materials, such as jute or old rope and string, are either
used by themselves or blended with inferior raw material according
to the quality of the paper being made. The jute and hemp fibres are
generally beaten by themselves in the engine before the other materials
are added. The pulp is mixed with the required amount of loading, while
the sizing and colouring operations are carried out in the usual way.
The common brown papers are known by a variety of trade names which
at one time indicated the nature of the fibrous constituent, but at
the present day the name is no guide or indication of the material
used for the manufacture of the paper. The common heavy brown used for
wrapping sugar and sundry groceries made in heavy grey and blue shades
is a coarse paper made from cheap materials and containing a large
proportion of mineral matter. It is usually supplied under the trade
name of _royal_.
A somewhat lighter and stronger wrapping paper of a white or buff
colour, used for wrapping groceries, tea, and cotton goods, is that
known as _casings_, a name probably derived from the application of
this paper originally to the lining of cases.
_Manila papers_ so called were originally made from rope, but the term
is now applied to papers which may be made entirely of wood pulp.
_Rope browns_ are common papers made of fairly strong material of a
miscellaneous character, this name having been derived from the fact
that rope and similar fibre were at one time used exclusively.
_Wood Pulp Wrappers._--Most of the papers of the present day are made
from wood pulp, this material giving a thin, light, tough paper, which
is pleasant to handle and forms a great contrast to the dense, opaque,
heavily loaded, and inartistic specimens produced some years ago. Paper
of this kind, though apparently more expensive than common browns, is
really more economical in use. The paper is not only stronger, but it
is possible to obtain a larger number of sheets for a given weight.
The great advantage in the improvement of brown papers dates from
the introduction of the now well-known kraft papers, which are of
comparatively recent origin.
_Kraft Paper._--The term Kraft, meaning "strength," is applied to
a remarkably strong cellulose paper prepared from spruce and other
coniferous woods by the soda treatment, the special feature of the
process being an incomplete digestion of the wood.
The wood previously chipped into pieces 1 inch to 1½ inches in length,
is boiled with caustic soda, the digestion being stopped before the
wood pulp has been quite softened, and while the pulp is still too
hard to be broken up into isolated fibres by simple agitation in
water. The pulp after thorough washing is disintegrated by means of
an edge-runner, or some form of breaking engine, the first mentioned
probably giving the most satisfactory results, and converted into paper
by the usual methods.
The wood can also be reduced by the sulphate process, in which case the
chipped wood is boiled in a liquor to which about 25 per cent. of spent
lye from a previous cooking is added.
The best results are obtained by attention to the cooking process to
ensure an under-cooked pulp, by careful isolation of the fibres in a
kollergang, or edge-runner, which machine is capable of separating the
fibres without shortening them, and by proper manipulation on the paper
machine.
The paper produced under favourable conditions in this direction is
wonderfully tough and strong and may be quoted as the most recent
example of the fact that the latent possibilities of wood pulp have by
no means been exhausted or even thoroughly investigated.
_Imitation Kraft Paper._--If wood is boiled in water at high
temperatures the fibre is softened and much of the resinous matter is
removed. Such wood, if ground in the same way and by the same methods
as ordinary mechanical wood pulp, is readily disintegrated, and a
long-fibred pulp may be obtained. The process of boiling short 2 feet
logs of wood in a digester under a pressure of 20-50 lbs. has long
been known. The wood after boiling is partly washed and then worked
up into pulp by the usual mechanical process. The wood is easily
ground and yields pulp containing long fibres which in their physical
properties closely resemble those of pure wood cellulose, but the
original constituents of the wood are present almost unchanged, just
as in mechanical pulp. The product obtained by grinding is a very
tough flexible material of a brownish yellow colour, and the paper is
known as _Nature brown_. It is chiefly used for the preparation of
tough packing papers, for the covers of cheap pocket-books, and other
miscellaneous purposes. When this brown mechanical wood pulp paper is
glazed on both sides it is then known as _ochre glazed_, the word ochre
referring to the colour. When made up into light weight papers it is
sold as _imitation kraft paper_.
A great variety of wrapping papers are now made from wood pulp, such
as _sealings_, _sulphite browns_, _manilas_, _sulphite caps_, but the
distinctions between these papers relate chiefly to the amount of
finish, the colour and size of the sheet. The methods of manufacture
only differ in small details as indicated by these distinctions.
_Fine Wrappings._--The papers used for packing small goods such as
silver ware and other delicate articles are generally tissues, the
better qualities of which are made from rag, and the cheaper qualities
from wood pulp. These papers are known as tissue, crêpe, crinkled
tissue, manila tissue, and by a variety of trade terms.
[Illustration: FIG. 40.--Single Cylinder or Yankee Machine.]
Many of the fine wrappings of the tissue class and the somewhat heavier
papers known as M. G. Caps are manufactured on the single cylinder
machine, which produces a paper having a highly polished surface on one
side and a rough unglazed surface on the other side.
In the single cylinder machine the beaten pulp passes from the
stuff-chest on to the wire of the ordinary Fourdrinier machine and
through the press rolls, but instead of being dried over a number of
cylinders the paper is led over one single cylinder of very large
diameter which is heated internally with steam. The paper is usually
pressed against the surface of the cylinder by means of a heavy felt,
which is, however, sometimes omitted. The side of the paper coming
into contact with the cylinder becomes highly polished, the surface in
contact with the felt remaining in an unfinished rough condition. This
paper is said to be machine glazed and is known as an M. G. paper.
[Illustration: FIG. 41.--Section of Wet Press, or Board Machine.]
_Boards._--Cards, millboards, middles, boxboards, carriage panels,
and similar paper products are manufactured either on a _single board
machine_, by means of which single sheets of any required thickness can
be obtained, or on a _continuous board machine_, which is capable of
producing cards and plain or duplex boards of moderate thickness.
The raw material used consists, as in the case of browns and wrappers,
of every conceivable fibrous substance mixed with mineral matter and
then suitably coloured. The preliminary processes for the treatment of
the pulp are exactly the same as those employed in the case of brown
papers up to the point at which the beating has been effected.
SINGLE BOARD MACHINE.
The beaten pulp, diluted with large quantities of water, is pumped
continuously into a large wooden vat of rectangular shape. Inside
this vat revolves slowly a hollow cylindrical drum, the circumference
of which is covered with wire gauze of fine mesh. The drum is not
completely immersed in the mixture of pulp and water, so that as it
revolves the water passes through the wire, while the pulp adheres to
the surface. The water flows regularly into the interior of the drum
and runs away through pipes fitted at each side of the vat near the
axis of the drum, and the pulp is brought up out of the water until it
comes into contact with a travelling felt. The thin moist sheet of pulp
adheres to this felt, passes through squeezing rolls which remove part
of the water, and is finally carried between two wooden or iron rollers
of large diameter. The pulp adheres to, and is wound up on the upper
roller, the felt being carried back by the lower roller to the vat.
When the sheet on the upper roller has attained the desired thickness,
it is immediately cut off and transferred to a pile of similar sheets,
a piece of coarse sacking or canvas being interposed between every wet
board. The dimensions of the full-sized board are determined by the
diameter of the upper roller and its length. A roll 74 inches wide and
14 inches diameter will give a board 74 inches by 44 inches.
As soon as a sufficient number of wet boards has been obtained they are
submitted to pressure in order to remove the excess of water and at the
same time compress the material into dense heavy boards. The pieces of
sacking are then taken out and the boards dried by exposure to air at
the ordinary temperature or in a heated chamber.
[Illustration: FIG. 42.--Double Cylinder Board Machine.]
The dried boards are finished off by glazing rolls. These rolls
compress the boards still further and impart a polished surface. The
amount of "finish" may be varied by the pressure, number of rollings,
temperature of the rolls, and by damping the surface of the dry boards
just before they are glazed. The boards are cut to standard sizes
before or after glazing.
_Duplex Boards._--If the single board machine is fitted with two vats
instead of one, it is possible to manufacture a board with different
coloured surfaces. A board coloured red on one side and white on the
other is manufactured by having one vat full of pulp coloured red and
the second vat full of white pulp. The thin moist sheets from the two
vats are brought together and passed through the glazing rolls, which
cause the moist sheets to adhere closely to one another, the double
sheet of pulp so formed being wound up on the rollers at the end of the
machine. The board is then dried, glazed, and finished in the usual way.
The same principle is occasionally adopted on the Fourdrinier
machine for duplex wrappers. Thus a common brown pulp is worked up
in conjunction with a dyed pulp to produc
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