CHAPTER XII_The Manufacture of Paper With Illustrations, and a Bibliography of Works Relating to Cellulose and Paper-Making

THE DETERIORATION OF PAPER

Recent complaints about the quality of paper and the rapid decay of

manuscripts and papers have resulted in arousing some interest in the

subject of the durability of paper used for books and legal documents,

and in the equally important question of the ink employed. The Society

of Arts and the Library Association in England and the Imperial Paper

Testing Institute in Germany have already appointed special committees

of inquiry, and from this it is evident that the subject is one of

urgent importance.

It is sometimes argued that the lack of durability is due to the want

of care on the part of manufacturers in preserving the knowledge of

paper-making as handed down by the early pioneers, but such an argument

is superficial and utterly erroneous. The quality of paper, in common

with the quality of many other articles of commerce, has suffered

because the demand for a really good high-class material is so small.

The general public has become accustomed to ask for something cheap,

and since the reduction in price is only rendered possible by the use

of cheap raw material and less expensive methods of manufacture, the

paper of the present day, with certain exceptions, is inferior to that

of fifty years ago.

The causes which favour the deterioration of paper are best understood

by an inquiry into the nature of the fibres and other materials used

and the methods of manufacture employed.

_The Fibres Used._--Cotton and linen rags stand preeminent amongst

vegetable fibres as being the most suitable for the production of

high-class paper capable of withstanding the ravages of time. This

arises from the fact that cotton and linen require the least amount

of chemical treatment to convert them into paper pulp, since they are

almost pure cellulose, cotton containing 98·7 per cent. of air-dry

cellulose, and flax 90·6 per cent. The processes through which the raw

cotton and flax are passed for the manufacture of textile goods are

of the simplest character, and the rags themselves can be converted

into paper without chemical treatment if necessary. As a matter of

fact certain papers, such as the O. W. S. and other drawing papers,

are manufactured from rags without the aid of caustic soda, bleach, or

chemicals. The rags are carefully selected, boiled for a long time in

plain water, broken up and beaten into pulp, and made up into sheets by

purely mechanical methods.

The liability of papers to decay, in respect of the fibrous

composition, is almost in direct proportion to the severity of

the chemical treatment necessary to convert the raw material into

cellulose, and the extent of the deviation of the fibre from pure

cellulose is a measure of the degradation which is to be expected. The

behaviour of the fibres towards caustic soda or any similar hydrolytic

agent serves to distinguish the fibres of maximum durability from

those of lesser resistance. It may be noted that in the former the

raw materials, viz., cotton, linen, hemp, ramie, etc., contain a high

percentage of pure cellulose, while in the latter the percentage of

cellulose is very much lower, such fibres as esparto, straw, wood,

bamboo, etc., giving only 40-50 per cent. of cellulose. The two

extremes are represented by pure cotton rag and mechanical wood pulp.

Other things being equal, the decay which may take place in papers

containing the fibre only, without the admixture of size or chemicals,

may be considered as one of oxidation, which takes place slowly in

cotton, and much more rapidly with mechanical wood pulp. Experimental

evidence of this oxidation is afforded when thin sheets of paper made

from these materials are exposed to a temperature of 100° to 110° C.

in an air oven. The cotton paper is but little affected, while the

mechanical wood pulp paper soon falls to pieces.

The order of durability of various papers in relation to the fibrous

constituents may be expressed thus: (1) rag cellulose; (2) chemical

wood cellulose; (3) esparto, straw, and bamboo celluloses; (4)

mechanical wood pulp. The rate and extent of oxidation is approximately

shown by the effect of heat as described. The differences between

the celluloses are also shown by heating strips of various papers in

a weak solution of aniline sulphate, which has no effect on wood or

rag cellulose, dyes esparto and straw a pinkish colour, and imparts a

strong yellow colour to mechanical wood pulp and jute.

_Physical Qualities._--The permanence of a paper depends not only upon

the purity of the fibrous constituents and the freedom from chemicals

likely to bring about deterioration, but also upon the general physical

properties of the paper itself. Other things being equal, the more

resistant a paper is to rough usage the longer will it last. The reason

why rag papers are so permanent is that not only is the chemical

condition of the cellulose of the highest order, but the physical

structure of the fibre is such that the strength of the finished paper

is also a maximum.

The methods of manufacture may be modified to almost any extent, giving

on the one hand a paper of extraordinary toughness, or on the other

hand a paper which falls to pieces after a very short time. Thus a

strong bank-note paper may be crumpled up between the fingers three

or four hundred times without tearing, while an imitation art paper is

broken up when crumpled three or four times.

A thorough study of the physical qualities of a paper is therefore

necessary to an appreciation of the conditions for durability. The

physical structure of the fibre, the modifications produced in it by

beating, the effect of drying, sizing, and glazing upon the strength

and elasticity of the finished paper, are some of the factors which

need to be considered.

_Strength._--The strength of a paper as measured by the tensile strain

required to fracture a strip of given width, and the percentage of

elongation which the paper undergoes when submitted to tension, are

properties of the utmost importance. The elasticity, that is, the

amount of stretch under tension, has not received the attention from

paper-makers that it deserves. If two papers of equal tensile strength

differ in elasticity, it may be taken for granted that the paper

showing a greater percentage of elongation under tension is the better

of the two.

The strength of a paper, as already indicated, is greatly influenced by

the conditions of manufacture. This has been explained in the chapter

devoted to the subject of beating, and other examples are briefly given

in the following paragraphs.

_Bulk._--The manufacture during recent years of light bulky papers for

book production has accentuated the problem in a marked degree, and the

factor of _bulk_ as one of the causes of deterioration is therefore

a comparatively new one. It is interesting to notice that the rapid

destruction of such books by frequent use is in no way related to the

chemical purity of the cellulose of which it is composed, or to the

influence of any chemical substance associated with the fibre. It is

purely a mechanical question, to be explained by reference to the

process of manufacture.

This paper is made from esparto entirely, or from a mixture of esparto

and wood pulp. The pulp is beaten quickly, and for as short a time as

possible, little or no china clay being added, and only a very small

percentage of rosin size. The wet sheet of paper is submitted to very

light pressure at the press rolls, and the bulky nature is preserved by

omitting the ordinary methods of calendering.

The paper thus produced consists of fibres which are but little felted

together. The physical condition and structure of the paper are readily

noticeable to the eye, and when these peculiarities are reduced

to numerical terms the effect of the conditions of manufacture is

strikingly displayed.

The effect of this special treatment is best seen by contrasting the

bulky esparto featherweight paper with the normal magazine paper made

from esparto. In the latter case a smoother, heavier, stronger sheet

of paper is made from identically the same raw material. But the pulp

is beaten for a longer period, while mineral matter and size are added

in suitable proportions. The press rolls and calenders are used to the

fullest extent.

The difference between these two papers, both consisting, as they do,

of pure esparto with a small proportion of ash may be emphasised by

comparing the analysis by _weight_ with analysis by _volume_. The two

papers in question when analysed by weight proved to have the following

composition:--

--------------------------------------------

| Parts by Weight.

+----------------+------------

-- | Featherweight. | Ordinary.

--------------+----------------+------------

Esparto fibre | 96·0 | 95·4

Ash, etc | 4·0 | 4·6

| ----- | -----

| 100·0 | 100·0

--------------+----------------+------------

But if the papers are compared in terms of the _composition by volume_,

it will be found that the featherweight contains a large amount of air

space.

--------------+-----------------------------

| Composition by Volume.

+----------------+------------

-- | Featherweight. | Ordinary.

--------------+----------------+------------

Esparto fibre | 28·0 | 65·5

Ash, etc | 0·7 | 1·8

Air space | 71·3 | 32·7

| ----- | -----

| 100·0 | 100·0

--------------+----------------+------------

In other words, the conditions of manufacture for the bulky paper are

such that the fibres are as far apart from one another as possible, and

the cohesion of fibre to fibre is reduced to a minimum.

While paper of this description is agreeable to the printer, and

probably to the general reading public, yet its strength and physical

qualities, from the point of view of resistance to wear and tear, are

of the lowest order. It is very difficult to rebind books made from it,

which is not altogether to be wondered at, seeing that the bookbinder's

stitches can hardly be expected to hold together sheets containing 60

to 70 per cent. of air space.

This concrete case emphasises the necessity for including in a schedule

of standards of quality a classification of papers according to

strength and bulk.

_Surface._--The introduction of new methods of printing has brought

about some changes in the process of glazing and finishing paper which

are not altogether favourable to the manufacture of a sheet having

maximum qualities of strength and elasticity, two conditions which are

essential to permanence. In other words, the very high finish and

surface imparted to paper by plate-glazing, supercalendering, water

finish, and other devices of a similar character is carried to excess.

All papers are improved in strength by glazing up to a certain point,

but over-glazing crushes the paper, renders it brittle and liable to

crack. Unfortunately, the maximum strength of a paper is generally

reached before the maximum of finish, with the result that the former

is frequently sacrificed to the latter. The usual result of glazing

is found in an increase of 8 to 10 per cent. in the tensile strength,

but a diminution of elasticity to the extent of 8 to 10 per cent. With

supercalendered magazine papers, the high surface is imparted for the

sake of the illustrations which are produced by methods requiring it.

The addition of considerable quantities of clay or mineral substances

improves the finish, so that the question of the relation of glazing

to strength, surface, and loading is one which affects the subject of

deterioration of paper very materially. With writing paper the false

standard of an "attractive" appearance is almost universally accepted

by the public as the basis of purchase without any reference to actual

quality.

_Mineral Substances._--China clay, sulphate of lime, agalite and other

inert mineral substances are important factors in lowering the quality

of paper, not so much in promoting the actual deterioration of paper

by any chemical reaction with the fibres, as in making the paper less

capable of resistance to the influence of atmospheric conditions and

ordinary usage. Clay in small, well-defined quantities serves a useful

purpose, if added to some papers, because it favours the production

of a smooth surface, but when the combination of mineral substances

is carried to an extreme, then the result from the point of view of

permanence is disastrous. This is well recognised by all paper-makers,

and in Germany the limits of the amount of clay or loading in

high-grade paper have been rigidly fixed. In the case of _imitation

art_ paper, which contains 25 to 30 per cent. of its weight of clay,

the strength and resistance of the sheet is reduced to a minimum. The

paper falls to pieces if slightly damped, the felting power of the

fibres being rendered of no effect owing to the weakening influence of

excessive mineral matter. This paper is used chiefly for catalogues,

programmes, circulars, and printed matter of a temporary and evanescent

character, and so long as it is confined to such objects it serves

a useful purpose, being cheap, and suitable for the production of

illustrations by means of the half-tone process; but its lasting

qualities are of the lowest order. The addition of 10 per cent. of any

mineral substance must be regarded as the maximum allowance for papers

intended for permanent and frequent use.

_Coating Material._--The ingenious method for producing an absolutely

even surface on paper by the use of a mixture of clay or other mineral

substance and an adhesive like glue or casein brushed on to the surface

of the paper, is responsible for many of the complaints about the

papers of the present day.

The sole merit of this substance is the facility with which half-tone

process blocks can be utilised for the purpose of picture production.

Beyond this, nothing can be said. The paper is brittle, susceptible to

the least suspicion of dampness, with a high polish which in artificial

light produces fatigue of the reader's eye very quickly, heavy to

handle, and liable to fall to pieces when bound up in book form.

As the fibrous material is completely covered by mineral substances, it

is frequently considered of secondary importance, with the result that

the "value" of the paper is judged entirely by the surface coating,

with little regard to the nature of the body paper. In such cases, with

an inferior body paper, the pages of a book very quickly discolour, and

the letterpress becomes blurred.

ANALYSIS OF A TYPICAL ART PAPER.

---------+-----------+-----------+------------

| Per Cent. | | Volume

-- | by | -- | Composition

| Weight. | | per Cent.

---------+-----------+-----------+------------

Fibre | 77·5 | Fibre | 68·3

Ash, etc.| 22·5 | Ash | 12·0

| | Air space | 19·7

| ----- | | -----

| 100·0 | | 100·0

---------+-----------+-----------+------------

_Rosin._--The presence of an excess of rosin is a well-known factor in

the disintegration of the paper, even when the fibrous composition is

of the highest order. The decomposition is largely due to the action

of light, many experiments having been made by Herzberg and others to

determine the nature of the reactions taking place. One of the chief

alterations is the change brought about in the ink-resisting qualities

of the paper.

The actual character of the chemical reactions as far as the effect

on the fibre is concerned is not accurately known. The degradation

of a hard-sized rosin paper by exposure to strong sunlight, for

example, is probably due to the alteration in the rosin size, and not

to any material change in the cellulose. It is hardly conceivable

that in a pure rag paper sized with rosin and yielding readily to ink

penetration, after about one year's exposure to light, the cellulose

itself had undergone any chemical changes capable of detection.

_Gelatine._--Papers properly sized with gelatine are preferable

to those sized with rosin for the majority of books and documents

preserved under normal circumstances. But the nature of a tub-sized

paper may be, and often is, greatly altered by unusual climatic

conditions. In hot, damp countries papers are quickly ruined, and

high-class drawing papers sized with gelatine often rendered useless.

The change is scarcely visible on the clean paper, and is only observed

when the paper is used for water-colour work, the colour appearing

blotchy in various parts of the sheet where the gelatine has been

decomposed by the united action of heat and damp.

The artist is frequently compelled in such cases to put a layer of

heavy white colour on the sheet of paper before proceeding to paint the

picture.

The storage of books under favourable conditions has a great deal to

do with the permanence of the paper, and the degradation of a paper in

relation to the tub-sizing qualities is much hastened by the presence

of moisture in the air.

_Starch._--The same is true of starch, which is largely employed as

a binding or sizing material in paper. The degradation of gelatine,

starch, and similar nitrogenous substances is due to the action of

organisms, and the following experiments, suggested by Cross, are

interesting in this connection.

If strips of paper are put into stoppered bottles with a small quantity

of warm water and kept at a temperature of about 80° F., fungus

growths will be noticed on some of them after the lapse of fourteen

days. Rag papers sized with gelatine will show micro-organisms of all

kinds. A pure cellulose paper, like filter paper, will not produce any

such effects. The result in the first case is due to the nitrogenous

substance, viz., the gelatine used in sizing, since the two papers

are identical as far as the cellulose fibres are concerned. High-class

wood pulp papers, unless sized with gelatine, would not show similar

results. The action of the organisms upon the nitrogenous material by a

process of hydrolysis is in the direction of the production of soluble

compounds allied to the starch sugars capable of being assimilated by

organisms.

The cellulose of esparto and straw are readily attacked, and it is

on this account that the tissues of the various straws are digested

more or less when eaten by animals. It is for this reason that the

celluloses from straw and esparto are inferior to the cotton cellulose

in producing a paper likely to be permanent.

_Chemical Residues._--The necessity for manufacturing a pure cellulose

half-stuff is fully recognised by paper-makers. This was not the case

in the early days of the manufacture of wood pulp, for it is a matter

of common experience that many of the books printed on wood pulp paper

between 1870 and 1880 are in a hopeless condition, and it is quite easy

to find books and periodicals of that date the pages of which crumble

to dust when handled. This serious defect has been proved to be due to

the presence of traces of chemicals used in manufacture which have not

been thoroughly removed from the pulp.

The precautions necessary in bleaching pulp by means of chloride of

lime, in order to prevent (1) any action between the fibre and the

calcium hypochlorite, (2) the presence of residual chlorine or soluble

compounds derived from it, and (3) the presence of by-products arising

from the use of an antichlor, are also well known to paper-makers. The

subject has been closely studied by chemists, who have shown that the

deterioration of many modern papers may be ascribed to carelessness in

bleaching.

The questions relating to the chemical residues of paper can only be

adequately dealt with by a discussion of actual cases which arise from

time to time. There are certain conditions in manufacture, common to

all papers, which may give rise to the presence of chemical residues,

of which two have already been mentioned.

The acidity of papers is frequently quoted as an instance. It is true

that the presence of free acid in a paper is most undesirable, as it

seriously attacks the cellulose, converting it into an oxidised form.

This in course of time renders the paper so brittle as to destroy its

fibrous character.

The change is brought about by the acid, which itself suffers no

material alteration, so that the process of deterioration is continued

almost indefinitely until the cellulose is completely oxidised. Most

papers, however, show an acid reaction when tested with litmus, the

usual reagent employed by those not familiar with the proper methods

of testing paper. All papers which have been treated with an excess

of alum for sizing purposes would show an acid reaction with litmus

without necessarily containing any free acid.

The presence of iron is undesirable, particularly in photographic

papers, and since cellulose has a remarkable affinity for iron, the

conditions of manufacture which tend to leave iron in the pulp have to

be taken into consideration. The presence of minute quantities of iron

in the form of impurities must not be confused with the presence of

iron in large quantities derived from the toning and colouring of paper

by means of iron salts.

The fading of colour is frequently observed when coloured papers are

tested on boxboards, particularly those made of straw. This fading may

often be traced to the presence of alkali in the straw board which has

not been completely removed in the process of manufacture.

The blurring of letterpress is a defect which often occurs with

printing papers made of chemical wood pulp. The oil in the ink

seems to separate out on either side of the letter, producing a

discoloration. In such cases the paper itself frequently exhibits an

unpleasant smell.

These defects are usually determined by the presence of traces of

sulphur compounds in the paper resulting from incomplete washing of

the pulp in manufacture. The presence of sulphur compounds sometimes

associates itself with papers which have been coloured by means of

ultramarine, which in presence of alum is slightly decomposed by the

heat of the drying cylinders.

Some knowledge of the effect of chemical residues in paper is

important, not only in regard to the deterioration which takes place

in the fibre itself, but also in relation to the fading of the ink

which is used. The subject of the ink has received much attention

from chemists on account of the serious difficulties which have been

experienced by State departments in various countries.

The United States Department of Agriculture have devised certain

methods for ascertaining the suitability of stamping ink used by the

Government and suggest the qualities desirable in such an ink. The ink,

first of all, must produce an indelible cancellation; that is, it must

be relatively indelible as compared with the ink used for printing

the postage stamps. The post-mark made with the ink must dry quickly

in order that the mail matter may be handled immediately without any

blurring or smearing of the post-mark.

Both this property and the property of the indelibility involve the

question of the rate at which the ink penetrates or is absorbed by the

fibre of the paper. A satisfactory ink does not harden or form a crust

on the ink-pad on exposure to air. There must be no deposition of solid

matter on the bottom of the vessel in which the ink is stored, and the

pigments on which the indelibility of the ink depends, if insoluble,

must not settle out in such a way as to make it possible to pour off

from the top of the container a portion of the ink which contains

little or none of the insoluble pigment or pigments.

_Colour._--If the subject of deterioration of paper is to be considered

in its broadest sense as including changes of any kind, the fading of

colour must be taken into account. The use of aniline dyes which are

not fast to light results in a loss of colour in paper just as with

textiles, and the fading may be regarded as a function of the dye and

not as arising from its combination with the paper.

The gradual fading of some dyes, however, and of many water-colour

pigments may be traced to the presence of residual chemicals in the

paper and to the presence of moisture in an atmosphere impregnated

with gaseous or suspended impurities. In fact the latter is a greater

enemy to permanence of colour than light, since it has been proved by

experiment that most colours do not fade when exposed to light in a

vacuum. The oxygen of the air in combination with the moisture present

is the principal agent in bringing about such changes. The dulling

of bronze, or imitation gold leaf, on cover papers is a practical

illustration of this, though this can hardly be quoted as an instance

of actual deterioration of the paper.

The maintenance of the original colour can only be assured by the

careful selection of pure fibrous material, the use of fast dyes,

and the preservation of the book or painting from the conditions

which favour the fading as described above. For common papers such

precautions become impossible, but for water-colour drawings and

valuable papers they are essential.

The demand for an abnormally white paper is indirectly the cause of

deterioration in colour, but in this case the ultimate effect is not a

fading but a discoloration of white to a more or less distinct yellow

or brown colour, due to changes in the fibre which may often be traced

to excessive bleaching. In this case the fading of colour is directly

due to deterioration of the paper itself, and may occur in celluloses

of the best type. With lower-grade papers containing mechanical wood

pulp the degradation of colour and fibre is inevitable.

_Air and Moisture._--The exact effects produced on paper freely

exposed, or in books as ordinarily stored, depend upon the condition of

the atmosphere. Pure air has little or no action upon paper, cellulose

being a remarkably inert substance, and even in impure mechanical wood

pulp, if merely exposed to pure dry air, the signs of decay would be

delayed considerably. The combined action of air and moisture is of a

more vigorous character in promoting oxidation changes in the fibres,

or a dissociation of the sizing and other chemical ingredients of the

paper. The presence of moisture is, indeed, absolutely essential for

the reaction of some substances upon one another, and it is easy to

show that certain chemical compounds can be left in ultimate contact,

if absolutely dry, for a lengthened period without reacting, but the

addition of a little moisture at once produces chemical union. This may

be shown by a simple experiment.

Thus a piece of coloured paper which may be bleached immediately

if suspended in an atmosphere of ordinary chlorine gas will remain

unbleached for several hours if first thoroughly dried in an oven and

exposed to dry gas.

In the case of books and papers, these conditions which promote slow

disintegration are aggravated by the presence of impurities in the

air, such as the vapours of burning gas, the traces of acidity in

the atmosphere of large manufacturing towns, the excessive dampness

and perhaps heat of a climate favouring the growth of organisms. All

these factors are of varying degrees in different places, so that the

deterioration of papers does not proceed in the same measure and at the

same rate everywhere.

_Moisture._--It may not be out of place to discuss some important

relations between moisture and the physical qualities of a sheet of

paper. A paper in its normal condition always contains a certain

proportion of water as one of its ingredients, and the presence of this

moisture has much to do with the strength, elasticity, and use of the

paper, the absence of moisture giving rise to defects and troubles in

the use of the paper which to a certain extent lower its commercial

value and deteriorate it, though not perhaps in the sense of permanent

degradation of quality.

One trouble frequently experienced by stationers and others is that

known as wavy edges. The edges of a stack containing sheets of paper

piled upon one another frequently twist and curl, producing what are

known as wavy edges. This arises from the fact that the paper when

manufactured was deficient in natural moisture, and that when stacked

it has gradually absorbed moisture, which is taken up first by the

edges exposed to the air. This causes unequal expansion of the fibres

with the production of the so-called wavy edges. The only remedy in

such cases is the free exposure of the sheets before printing, so that

the moisture is absorbed equally all over the sheet. The cracked edges

of envelopes may be explained by reference to the same conditions. The

paper is worked up into envelopes in an over-dry condition, and the

fibres, being somewhat brittle, readily break apart from one another.

If the paper is kept in stock for some time before use this defect can

be very largely remedied.

With supercalendered papers it is only possible to obtain the best

results by allowing the paper to stand for several days after making

before it is glazed.

It is evident from these few examples that many of the troubles

experienced by printers are due to the fact that orders for paper

are frequently accompanied by an instruction for immediate delivery,

under which circumstances it is impossible to obtain the best results.

The expansion of papers used for lithography, and the bad register

frequently seen in colour work, may be explained by reference to the

behaviour of the individual fibres towards moisture. The expansion is

usually greater in one direction of the paper than in the direction

at right angles to it, and this is due to the fact that fibres have a

greater ratio of expansion in the diameter than in the length.

The behaviour of papers when damped is a peculiarity well known to

paper-makers and printers. For certain purposes it is desirable that

paper should not show any material alteration when damped, since

any expansion of the sheet is liable to throw the printing out of

"register." The liability of papers to such stretch or expansion is

largely minimised by careful manipulation of the pulp during the

process of beating, and also by a proper regulation of the web of paper

as it passes from the wet end of the paper machine over the drying

cylinders to the calenders. The paper which fulfils the necessary

qualifications as to a minimum stretch is prepared from pulp which has

not been beaten for too long a period, so that the pulp obtained is

fairly light and bulky. By this means the expansion of the fibres takes

place in the sheet itself without making any material alteration in its

size. That is to say, as the sheet of paper is fairly _open_, there is

sufficient room for expansion, which thus takes place with the least

alteration of the total area of the sheet. The paper which is allowed

to shrink on the machine during the process of drying, without undue

tension, usually exhibits a minimum amount of expansion subsequently in

printing.

It is important to notice that the expansion of paper is different for

the two directions, that is for the machine and cross directions.

This arises from the fact that in the machine-made paper the greater

proportion of the fibres point in the direction of the machine while

the paper is being made. In consequence of this the expansion of the

paper is greatest in what is known as the cross direction of the paper,

that is, in the direction at right angles to the flow of the pulp along

the machine wire.

This is to be explained by reference to the behaviour of fibres when

damped or brought into contact with an excess of water. The question

of the exact changes in the dimensions of a fibre due to absorption of

water has been dealt with in an interesting manner by Hohnel. He points

out that the well-known peculiarity of the shrinkage of ropes which

have been lying in the water can be explained by an examination of the

behaviour of the single fibres. He relates in detail the experiment

which can be carried out for the exact observation of the fibres when

in contact with water. A dry fibre when soaked in water appears to

become 20 to 30 per cent. greater in diameter, whereas in length it is

usually only increased by one-tenth per cent.

The method adopted by Hohnel was to place a fibre of convenient length

on a glass slip down the centre of which was a fine narrow groove

capable of holding water, so that the fibre could be wetted. Over the

fibre was a cover glass with a small scale marked on it. The loose

end of the fibres passed over a small roller and was stretched by a

light weight. The movements of the fibre were measured by means of an

eye-piece micrometer.

In this way it is possible to determine alterations in length to within

0·005 per cent., and this variation can be directly seen under the

microscope.

Hohnel observes in his account of the experiments that all fibres

become thicker when wetted, that vegetable fibres are more susceptible

than animal fibres.

Animal fibres expand about 10 to 14 per cent. in diameter, but

vegetable fibres as much as 20 per cent., as shown in the following

table:--

-------------+----------+------------------+----------

Animal Fibre.| Per Cent.| Vegetable Fibre. | Per Cent.

-------------+----------+------------------+----------

Human hair | 10·67 | New Zealand flax | 20·0

Angora wool | 10·2 | Aloe hemp | 25·8

Alpaca wool | 13·7 | Hemp | 22·7

Tussah silk | 11·0 | Cotton | 27·5

-------------+----------+------------------+----------

The reverse is the case when the expansion of the fibres in regard to

length is considered, since animal fibres expand 0·50 to 1·00 per cent.

of their length, and vegetable fibres only 0·05 to 0·10 per cent.

The maximum amount of expansion in the case of the vegetable fibres is

obtained by gently breathing upon them rather than by the use of an

excess of water.

These figures are important as explaining many of the peculiar

characteristics of vegetable and animal fibres. Advantage is taken of

the greater expansion of the latter in the manufacture of instruments

for the measurement of moisture, such as the hair hygrometer, in which

the elongation of a stretched hair registers the variation in the

moisture of the atmosphere.

_Quality of Book Papers._--The Committee of the Society of Arts in

dealing with the evidence as to the permanence of finished papers

suggest the following classification as indicating the desired

standards of quality:--

(A) CLASSIFICATION AS TO FIBRES.

A. Cotton, flax, and hemp.

B. Wood celluloses, (_a_) sulphite process, and (_b_) soda and

sulphate process.

C. Esparto and straw celluloses.

D. Mechanical wood pulp.

The Committee find little fault with the Principles which govern the

trade in the manufacture of high-class papers, and limit the result of

their investigation to the suggestion of a normal standard of quality

for book papers required in documents of importance according to the

following schedule:--

_Fibres._--Not less than 70 per cent. of fibres of Class A.

_Sizing._--Not more than 2 per cent. rosin, and finished with the

normal acidity of pure alum.

_Loading._--Not more than 10 per cent. total mineral matter (ash).

With regard to written documents, it must be evident that the proper

materials are those of Class A, and that the paper should be pure,

sized with gelatine and not with rosin. All imitations of high-class

writing papers, which are in fact merely disguised printing papers,

should be carefully avoided.

These recommendations are good as far as they go, but in order to

establish the proper standards of quality some specifications must be

laid down with regard to the strength of the paper and its physical

properties, together with a reference to the use for which the paper

is intended. The physical condition of the paper itself apart from the

nature of the fibre has much to do with its resistance to wear and

tear, and this is easily proved by comparing modern book papers made

from esparto with book papers of an earlier date made from the same

material.

The only official schedule of requirements in relation to public

documents is that issued by the Stationery Office.

The details set out relate chiefly to questions of weight and strength,

the limits being expressed in definite form and not allowing much

margin for variation in respect of strength or fibrous constituents.

Mechanical wood pulp is excluded in all papers except common material

as stated in the schedule. The papers required for stock are divided

into twelve classes. In each class the trade names of various sized

papers are given, the size of the sheet and the weight of the ream,

and, where required, any special characteristics are set out. The

schedule is as follows:--

_Class 1. Hand-made or Mould-made._

_General Specification._--Hand-made or mould-made. Animal tub-sized.

("Hand-made" or "Mould-made" to be marked on the wrapper.)

Where special water-marking is required mould will be supplied by the

Stationery Office for those papers made by hand.

_Class 2. Writings, Air-dried._

_General Specification._--Plate rolled. Machine made. Animal tub-sized.

Air-dried. (Must bear ink after erasure.)

_Note._--The mean breaking strain and mean stretch required are given

for each paper. The figures represent the mean of the results obtained

for both directions of the sheet, and are calculated on a strip of

paper five-eighths of an inch wide and having a free length of seven

inches between the clips.

_Class 3. Writings, Ordinary._

_General Specification._--Rolled. Machine-made. Animal tub-sized.

_Class 4. Writings, Coloured._

_Specification._--Highly rolled. Machine-made. Animal tub-sized.

_Class 5. Blotting Papers._

_Specification._--All rag. Machine-made. Free from loading.

_Class 6. Printing and Lithographic Papers._

_General Specification._--Rolled. Machine-made. Engine-sized. Loading

not to exceed 15 per cent.

_Class 7. Coloured Printings._

_General Specification._--Rolled. Machine-made. Engine-sized.

_Class 8. Copying and Tissue Papers._

_Specification._--Machine-made. Free from loading. (Copying papers are

required to give three good copies.)

_Class 9. Brown Papers, Air-dried._

_Specification._--Air-dried. Machine-made.

_Note._--The mean breaking strain and mean stretch required are given

for each paper. The figures represent the mean of the results obtained

for both directions of the sheet, and are calculated on a strip of

paper two inches wide and having a free length of seven inches between

the clips.

In the case of papers indicating a larger breaking strain than the

minimum required, a proportional increase in the stretch must also be

shown.

_Class 10. Brown Paper, Cylinder-dried._

_General Specification._--Machine-made.

_Note._--The mean breaking strain required is given for each paper. The

figures represent the mean of the results obtained for both directions

of the sheet, and are calculated on a strip of paper two inches wide

and having a free length of seven inches between the clips.

_Class 11. Smallhands._

_General Specification._--Machine-made. Engine-sized.

_Class 12. Buff Papers._

_Specification._--Highly finished both sides. Machine-made. Hard

engine-sized.

Mechanical wood pulp must not be used in the manufacture of any papers,

with the exception of engine-sized coloured printings, and buff papers,

where an addition up to 25 per cent. will be allowed.

All animal tub-sized papers are required to be as far as possible free

from earthy matter; and, except where specially stated, the amount of

_loading_ added to other papers must not exceed 6 per cent.

When sulphite or soda pulps are used, either separately or conjointly,

in the manufacture of printing papers, the quantity of neither material

shall separately exceed 50 per cent.

The most complete specification as to the requirements for standard

papers is that published by the Paper Testing Institute in Germany, and

used as the basis of most contracts, at least for public and official

documents.

_Standards of Quality in Germany._--The classification of papers

according to the raw materials used and the nature of the finished

paper is very complete. The classification is made under three

headings: (_A_) Raw Material; (_B_) Strength; (_C_) Uses.

_(A) Classification according to Material._

(1) Paper made from rags only (linen, hemp, and cotton).

(2) Paper made from rags with a maximum of 25 per cent. of cellulose

from wood, straw, esparto, manila, etc., but free from mechanical wood

pulp.

(3) Paper made from any fibrous material, but free from mechanical wood

pulp.

(4) Paper of any fibrous material.

_(B) Classification according to Strength._

----------------------+-------+-------+-------+-------+-------+------

Class | 1. | 2. | 3. | 4. | 5. | 6.

----------------------+-------+-------+-------+-------+-------+------

in metres | 6,000 | 5,000 | 4,000 | 3,000 | 2,000 | 1,000

| | | | | |

Elasticity per cent. | 4 | 3·5 | 3 | 2·5 | 2 | 1·5

| | | | | |

number of foldings) | 190 | 190 | 80 | 40 | 20 | 3

----------------------+-------+-------+-------+-------+-------+------

The tests for tearing length, resistance to folding, elasticity,

etc., are made in air showing relative humidity of 65 per cent. The

calculations for tearing length are made on strips of paper dried at

100° C.

_(C) Classification according to Use._

------+-------------------+------+----------+-----------+---------------

| |Fibre.| Strength.| Size of +-------+-------

| | | | | Kg. | Grms.

--+---+-------------------+------+----------+-----------+-------+-------

1 | Writing papers for | | | | |

| important documents | 1 | 1 | 33 × 42 | 15 | --

| | | | | |

| documents | 1 | 1 | 26·5 × 42 | 12 | --

| | | | | |

2 | Paper for registers, | | | | |

| | | | | |

| (_a_) First quality | 1 | 2 | 33 × 42 | 14 | --

| | | | | |

| (_b_) Second quality | 1 | 3 | 33 × 42 | 13 | --

| | | | | |

3 | Documents intended to | | | | |

| | | | | |

| (_a_) Foolscap paper | 2 | 3 | 33 × 42 | 13 | --

| | | | | |

| (quarto size) | 2 | 3 | 26·5 × 42 | 10·4 | --

| | | | | |

| (octavo size) | 2 | 3 | 26·5 × 21 | 5·2 | --

| | | | | |

| paper | 2 | 3 | 33 × 42 | 7 | --

| | | | | |

| writing paper | 2 | 4 | 33 × 42 | 13 | --

| | | | | |

4 | Paper for documents of| | | | |

| | | | | |

| (_a_) Foolscap paper | 3 | -- | 33 × 42 | 12 | --

| | | | | |

| (quarto size) | 3 | -- | 26·5 × 42 | 9·6 | --

| | | | | |

| (octavo size) | 3 | -- | 26·5 × 21 | 4·8 | --

| | | | | |

| writing paper | 3 | 4 | 33 × 42 | 12 | --

| | | | | |

5 | Envelopes and | | | | |

| | | | | |

| (_a_) First quality | -- | 3 | -- | -- | --

| | | | | |

| (_b_) Second quality | -- | 5 | -- | -- | --

| | | | | |

6 | Writing paper of | | | | |

| medium quality | -- | 5-6 | -- | -- | --

| | | | | |

7 | Covers for documents--| | | | |

| | | | | |

| for frequent use| 1 | Tearing | 36 × 47 | 81·2 | 480

| | | 2,500 | | |

| | | 3·5% | | |

| | | | | |

| purposes | 3 | Tearing | 36 × 47 | 42·3 | 250

| | | 2,500 | | |

| | | 2·5% | | |

| | | | | |

8 | Printing paper-- | | | | |

| | | | | |

| printed matter | 1 | 4 | -- | -- | --

| | | | | |

| printed matter | 3 | 4 | -- | -- | --

| | | | | |

| (_c_) For common use | -- | 5-6 | -- | -- | --

--+-----------------------+------+----------+-----------+-------+-------

The Manufacture of Paper With Illustrations, and a Bibliography of Works Relating to Cellulose and Paper-Making · 第 12 / 13 章