_LOADING.--SIZING.--COLOURING._
Loading.--Sizing.--French Method of Preparing Engine
Size.--Zinc Soaps in Sizing.--Colouring.--Animal or
Tub-Sizing.--Preparation of Animal Size.--American Method of
Sizing.--Machine-Sizing.--Double-sized Paper.--Mr. Wyatt's
Remarks on Sizing.
=Loading.=--The very finest qualities of paper are usually made
without the addition of any _loading_, as it is called, but for
most other papers more or less loading material is added, according
to the quality of paper to be produced. The loading material used
for ordinary qualities is kaolin, or china clay, and for the better
qualities sulphate of lime or _pearl hardening_, as it is termed in
the trade. China clay, as it occurs in commerce, is in the form of
soft lumps and powder, is nearly white, and when rubbed between the
finger and thumb should present no hard particles of gritty matter.
To prepare it for mixing with the pulp it is first worked up into a
thin cream with water, which is usually done in a vessel furnished
with an agitating arrangement by which the clay becomes intimately
mixed with the water. The cream is then strained through a fine
sieve to separate any impurities present, and is then allowed to
flow into the beating-engine containing the stuff while in motion,
by which it soon becomes mingled with the pulp. The proportion of
china clay or other loading material which is to be introduced into
the pulp depends upon the quality of the fibre and the requirements
of the manufacturer, some makers using less of the material than
others. From 3 per cent. to 10 or 15 per cent. appears to be about
the extreme range for employing the material as a necessary
ingredient, in the production of various classes of paper, above
which figures the addition of loading material may be considered as
an adulteration. Sometimes nearly twice the largest amount named is
employed, no doubt to meet the exigences of keen competition--from
foreign sources especially.
One effect of the loading, whether it be china clay or sulphate
of lime, is to close the pores of the paper, whereby a smoother
surface is obtained, while at the same time, if the material has
been used in proportions suited to the quality of the fibre, and
not in immoderate excess, a stronger paper is produced. A species
of asbestos termed _agalite_ has been introduced as a loading
material, and since it has a fibrous texture, it blends with the
fibres of the pulp, forming, as it were, a vegeto-mineral paper. It
is stated that as much as 90 per cent. of the agalite used in the
beating-engine enters into the manufactured paper, while not much
more than half the china clay used is held by the pulp.
=Sizing.=--"Engine sizing," as it is termed, consists in adding
certain ingredients to the pulp while in the beating-engine. The
materials generally used are alum and resin soap, in proportions
suitable to the paper to be produced. Resin soap is formed by
boiling ordinary resin in a jacketed pan such as is used by
soapmakers for preparing small quantities of fancy or other soaps,
with a solution of soda crystals in the following proportions:
Resin, 16 lbs.; soda crystals dissolved in water, 8 lbs.; and the
boiling is kept up for about two hours, or until a soap is produced
which is perfectly soluble in water. The method of preparing this
soap as conducted at the soapworks has been described in the
author's work on soap-making,[22] p. 64, from which the following
abstract is taken: "Put into a pan capable of holding about 12
gallons, 2¼ gallons of fresh caustic soda ley at 30° B. Apply
gentle heat, and when the ley begins to boil throw in, every few
minutes, in small quantities at a time, finely powdered and sifted
resin until 37 lbs. have been introduced. The mixture must be well
stirred the whole time to prevent the resin from 'clogging' and
adhering to the pan. It is important to moderate the heat, as the
resin soap has a great tendency to expand and an excess of heat
would cause it to boil over. The heat, however, must be kept to
near the boiling point, otherwise the mass will become thick and
of a very dark colour. When kept at near the boiling point it is
always clear and its colour of a reddish yellow. If, during the
boiling, the resin soap rises and threatens to overflow, the heat
must be checked by throwing in a little cold water, only using
sufficient to effect this object. It is absolutely necessary to
stir the mass continually, otherwise the resin will agglomerate in
masses and thus prevent the alkali from acting freely upon it. The
boiling takes about two hours, when the soap is run into an iron
frame and allowed to cool. It is very important that the resin used
is freed from particles of wood, straw, etc., for which purpose it
should be passed through a tolerably fine sieve."
Respecting the preparation of resin soap, Davis says:--"The
proportion of resin used to each pound of soda ash varies in
different mills, 3, 4, or even 5 lbs. of resin being used to each
pound of soda ash. The proportion of resin, soda ash, and water,
can be best determined by practical experience, as no prescription
could be devised which would be suitable to every case." M.
d'Arcet, who modified the proportions recommended by M. Bracconot,
recommends for the preparation of resin soap--
Powdered resin 4·80 parts.
Soda crystals at 80° (French, alkalimeter) 2·22 "
Water 100 "
Theoretically speaking, only 2·45 parts of alum would be required
to precipitate the resin; but the waters, which are almost always
calcareous, neutralise part of the alum. Crystals of soda are
much more expensive than soda ash, but on account of their greater
purity they are sometimes preferred to the latter. At the present
day the resin soap is preferably made by dissolving ordinary
resin with a solution of carbonate of soda under boiling heat in
a steam-jacketed boiler, the class of paper to be made governing
the quantity of resin to be employed. The boiling usually requires
from two to eight hours, according to the relative proportions of
soda ash and resin used--the greater the proportion of soda used
the less time is required for boiling--the process being completed
when a sample of the soap formed is completely soluble in water....
About 3 lbs. of resin to 1 lb. of soda is the usual proportion. The
resin soap is cooled after boiling by running it into iron tanks,
where it is allowed to settle, the soap forming a dense syrup-like
mass, and the colouring matters and other admixtures of the resin
rising to the top are easily removed. It is important to run off
the mother liquor (ley) containing the excess of alkali, for when
the soap is used it consumes the alum to neutralise it."
When the impurities and ley have been removed the soap is dissolved
in water, and if, from imperfect boiling, a portion of the resin
is found not to have been saponified, a small quantity of a strong
solution of soda crystals is added to the water used for dissolving
the soap.
Where starch is used for stiffening purposes, the soap is mixed
with a quantity of starch paste in the proportion of 1½ part of
starch to 1 part of resin soap. Some manufacturers, Mr. Davis
states, mix the starch paste with the kaolin in lieu of mixing
it with the resin soap. In either case the materials should be
thoroughly strained before being added to the pulp. From 3 to 4
lbs. of the mixture of resin soap and starch paste to each 100
lbs. of dry pulp are about the proportions in which the size is
generally used, but the quantity added to the pulp in the beater
depends upon whether the paper is to be soft-sized or hard-sized.
Sizing is chiefly applied to papers which are to be written upon
with ordinary inks, and also, with a few exceptions, to printing
papers, the object being to close the pores of the paper and render
it non-absorbent, by which the spreading or running of the ink is
effectually prevented. While the finest lines may be written upon a
well-sized paper (as ordinary writing paper, for example) without
spreading in the least degree, a similar stroke of the pen upon
blotting paper, tissue, or unsized printing paper would spread in
all directions, owing to the highly absorptive property of the
cellulose.
The sizing of the pulp is conducted as follows:--After the
loading material has been introduced and well mixed, the resin
soap, previously dissolved in water, a little carbonate of soda
being sometimes added, is mixed with a paste of starch prepared
by dissolving starch in boiling water, and the mixture of soap
and starch is then passed through a fine sieve to keep back
any particles or lumps that may be present. The proportion of
the materials used in sizing vary at the different mills, each
manufacturer having formulæ of his own; about 1 part of resin size
to 3 of starch paste, and, say, from 9 to 12 lbs. of the mixture,
may be used for 300 lbs. of pulp; and, if preferred, the respective
ingredients may be put into the engine separately, a method adopted
at some mills. Some manufacturers of the finest papers, instead of
dissolving the starch in hot water, make it into a thin paste with
cold water, in which condition it is introduced into the pulp, the
object being to impart to the paper a particular feeling to the
touch which is not obtainable by other means.
The mixture of resin size and starch paste, with or without the
addition of water, is added to the pulp in the beater, in which
the pulp is circulating, and the engine allowed to run until the
materials are well incorporated in the pulp. At this stage a
solution of alum (about 28 to 30 lbs. for 300 lbs. of pulp), or
of sulphate of alumina,[23] is introduced, which causes the resin
soap to become "separated," the sulphuric acid of the alum uniting
with the alkali of the soap and setting the resin and alumina
free in the form of minute particles; the resin in the subsequent
drying on the calenders becomes fused, as it were, and thus cements
the fibres and alumina together, at the same time rendering them
non-absorbent and improved in whiteness by the precipitated
alumina. Sometimes ordinary soap is added to the resin soap, which
is said to impart a higher finish to the paper in the operation of
calendering.
The so-called "concentrated alum," which contains a higher
percentage of sulphate of alumina than the crystallised alum,
is considered the most economical in use, being proportionately
cheaper, and the variety known as "pearl alum" is specially
recommended. "Aluminous cake" is another preparation which has
found favour in many mills, but since it sometimes contains a
large excess of free sulphuric acid it requires to be used with
caution, since this acid, although it will brighten the colour of
some aniline dyes, will discharge the colour from others, while
at the same time it may injuriously affect the brass-wire cloths
of the paper machine. The alum solution should be prepared in a
lead-lined tank, fitted with a steam pipe for heating the contents
when required.
The proportions of the materials used in sizing differ considerably
in different mills, but the following may be taken as an average
for common writing and printing papers:--
Per 100 parts of dried pulp 10 to 12 parts of resin.
" " " 20 " 30 " starch.
" " " 10 " 12 " alum.
To the sizing solution is generally added from 30 to 50 parts
of kaolin. When a colour is present on which alum would have a
prejudicial effect this is usually replaced by about one-third of
its weight of sulphate of zinc. Many mineral substances have from
time to time been added to paper stock, principally to increase its
weight, and in 1858 Sholl took out a patent for adding carbonate
of lime, a substance which, however, had long been fraudulently
used in order to increase the weight, but he found it to have
the property of fixing the ink in the pores of the paper, thus
rendering it immovable. The only useful addition is kaolin, or some
similar aluminous compound, as it attaches itself to the fibre,
and, while giving the required opacity and a good surface, takes
both printing and writing ink well, and has the advantage, from a
manufacturer's point of view, of increasing the weight. It has been
proposed that small quantities of glycerine be added to the pulp,
in order to give the paper greater flexibility, and especially to
give copying-paper the quality of taking up colour readily.[24]
=French Method of Preparing Engine Size.=--Thirteen pails of water
are boiled in a copper-jacketed pan capable of holding about 150
gallons; 90 lbs. of soda crystals are then introduced and allowed
to dissolve, when 200 lbs. of finely-powdered resin are gradually
introduced, with constant stirring, and the boiling is sustained
for about two hours after the last portion of resin has been added.
A further addition of water is now made by putting in five pails
of cold water, and the water is then boiled for an hour and a half
longer. The resin soap is then transferred to stock-chests, in
which it is allowed to remain for ten days or longer, fresh batches
being prepared in rotation, to meet the requirements of the mill.
To determine whether an excess of resin soap or of alum has been
added to the pulp, red and blue litmus papers should be employed,
the former turning blue if an excess of resin soap be present, and
the latter red when alum or sulphate of alumina is in excess. For
uncoloured papers the aluminous material should be added until the
pulp becomes faintly acid, which will be indicated by the blue
litmus paper turning slightly red when immersed in the pulp.
Besides resin soap, various substances have been proposed as sizing
materials, including wax dissolved in a strong solution of caustic
soda and precipitated with alum, but the cost would be an objection
to the use of this material except for the highest classes of
paper. It is stated that 12 lbs. of gum tragacanth to each 500 lbs.
of resin has been used in preparing some kinds of engine-sized
papers, and is said to impart to them an appearance equal to that
of tub-sized papers.
=Zinc Soaps in Sizing.=--According to a paragraph in the
_Papermakers' Monthly Journal_, a somewhat novel method of sizing
is employed in Germany, which consists in the precipitation in the
stock of zinc soaps. Cottonseed oil soap or Castille soap is worked
up in the engine with the stuff, and after it has become well
mixed with the pulp a solution of sulphate of zinc is added, which
results in the formation of a white and heavy zinc soap, which is
insoluble, and adheres well to the fibres. The weight and whiteness
of the zinc soap are the main points in favour of this method,
which is said to yield good results.
=Colouring.=--The pulp, after passing through the various processes
described, although apparently white, invariably presents a yellow
tinge when converted into paper. To obviate this it is usual to
"kill" the yellow tint by adding to the pulp small quantities of
blue and pink colouring matters. The blue colours generally used
are ultramarine, smalts, and various aniline blues, and the pinks
are usually prepared from cochineal, either in a liquid form or as
"lakes" (compounds of cochineal and alumina) or aniline dyes, the
former being preferable, as it is not injuriously affected by the
alum used in sizing. The ultramarine should be of good quality,
otherwise it will become decomposed, and its colouring property
destroyed by the action of the alum, but more especially so if
the alum contains an excess of free acid. Smalts blue, which is a
kind of coloured glass, is not affected by acids. In preparing the
colouring matters for mixing with the pulp they must first be mixed
with water, and the liquid should then be strained, to keep back
any solid particles that may be present in the material. Aniline
blues should be dissolved in hot water, or alcohol, and then
diluted. Samples of the pulp are examined from time to time until
the desired effect is produced, which the practised eye of the
beater-man can readily determine.
=Animal or Tub-sizing.=--Another process of sizing, termed
"animal-sizing," "tub-sizing," or "surface-sizing," is also
adopted in the manufacture of certain classes of paper, and is
either accomplished by hand or on the machine. The former method
having been elsewhere described (p. 132) we will now describe the
operation of sizing on the machine, to which the term tub-sizing is
also applied. The size employed, which is prepared from what are
called "glue pieces," or clippings of "limed" and unhaired skins of
animals, requires to be as colourless as possible, in order that
the colour of the paper may not be injuriously affected by it.
=Preparation of Animal Size.=--This operation is generally
conducted at the mill, the materials from which the size is
produced being the cuttings or parings of animal skins and hides,
or _pelts_, which have undergone the processes of "liming" and
unhairing preparatory to being tanned. The cuttings, or _pates_,
commonly called "glue pieces," are first soaked in a mixture of
lime and water, placed in large tubs for several days, after which
they are put into a wooden cylinder, or drum, five or six feet
in diameter, and about ten feet in length, which revolves upon
a horizontal shaft, which, being hollow, admits the passage of
water to the interior of the drum. The drum is perforated, and
revolves in a large tank, while a continuous stream of water is
allowed to pass through it, and the dirty water escapes through
the perforations in the drum. When the cuttings are sufficiently
cleansed in this way, they are transferred to an iron copper,
furnished with a false bottom and steam-pipe, or a jacketed pan.
The cuttings are next covered with water; steam is then turned
on, and the liquid brought to a temperature below boiling point,
or say, about 180° to 190° F., it being very important that the
liquid should not actually boil. This operation is carefully kept
up for twelve to sixteen hours, according to the nature of the
cuttings, by which time all the material excepting any membranous
or fatty matters that may be present, will have become dissolved
and a solution of gelatine obtained. The liquor is then allowed
to settle for a short time to allow fatty matters to rise to the
surface and membranous substances to deposit, and the fatty matters
must afterwards be carefully removed by skimming. The liquor should
next be strained to separate any floating particles of a membranous
character. Sometimes the gelatine solution is clarified by adding
a small quantity of powdered lime, which is thoroughly mixed by
stirring, after which it is allowed to rest. When it is found that
the impurities and lime deposit too slowly, a little weak sulphuric
acid is added, which, forming an insoluble sulphate of lime, the
solid matters quickly subside, leaving the liquor quite clear.
The solution is next filtered through felt, and is afterwards
treated with a solution of alum, which at first causes the liquid
to thicken and become nearly solid, but it becomes fluid again,
however, on the addition of more alum solution. When this condition
is finally attained, the liquid is ready for use in the process of
sizing. The addition of the alum (which should not contain any free
acid) to the gelatine greatly improves its sizing property, besides
preserving it from decomposition. The treatment of the glue pieces
for the purpose of obtaining gelatine solutions is fully described
in the author's work on "Leather Manufacture," p. 401.[25]
=American Method of Sizing.=--Another method of preparing size,
and which is adopted in America, is the following:--In large paper
mills the size is generally prepared in a room devoted to the
purpose, and is commonly situated near the machine. The finest
grades of light hide and skin clippings are used for No. 1 letter
papers, but less costly stock is employed for the lower grades of
animal-sized papers. To preserve the glue pieces the tanners and
tawers macerate the clippings in milk of lime and afterwards dry
them. As the clippings require to be freed from the lime, the first
treatment they receive at the paper-mill is to put them in large
wooden tubs partly filled with water, in which they are allowed to
soak for several days. They are afterwards more perfectly cleansed
by means of a drum-washer, such as we have before described. Fresh
hide and skin clippings, that is, those which have not been limed
and dried at the tanneries, and which are occasionally purchased
by the paper manufacturers, require to be used as soon as possible
after they arrive at the mill as they readily decompose, and are
placed in tubs partly filled with water, in which 2 per cent. by
weight of caustic lime has been dissolved. The pieces, if from
calfskins, are allowed to remain in the lime bath for ten to
fifteen days, clippings of sheepskins fifteen to twenty days, and
trimmings from heavy hides, as ox, etc., twenty-five to thirty
days, the milk of lime being renewed once or twice a week, and
the material well stirred from time to time. The glue-stock, as
it is sometimes termed, is afterwards thoroughly washed in the
drum-washer, and when this operation is complete the material is
spread out in the yard to drain, and when sufficiently dried is
ready for boiling, or may be stored until required for use.
To prepare size from the material treated as described, it is
placed in a boiler of cast or wrought-iron or copper, furnished
with a perforated false bottom, and capable of holding from 100
to 400 lbs. of the raw material, according to the requirements of
the mill. Several such boilers may be placed close to each other.
At the bottom of the boiler is a stop-cock for drawing off the
gelatine solution when required. When the requisite charge of
glue-stock has been introduced into the boiler, water is poured
over it and steam turned on, which passes through a pipe fixed
beneath the false bottom, and care is taken that the temperature
of the contents of the boiler should not exceed 200° F., which
heat is kept up for ten to eighteen hours, according to the
nature of the materials treated. The gelatine solution is drawn
off from the boiler as it is formed, into wooden tubs, and at the
same time carefully strained to remove membranous matters and
suchlike impurities. Several boilings are made from the same batch
of glue-stock, and all the solutions are afterwards mixed together
in the receiving tubs, and a solution of alum is added in such
proportions as to be recognised by tasting the liquor. One object
in adding the alum being to prevent the gelatine from decomposing,
more of this substance should be added in warm than in cold weather.
When the solutions are cool they are ready for use, and the
gelatine is removed from the receiving tubs and dissolved in a
separate tub as required for use, the dissolving tub being provided
with a steam-pipe. The proportion of water--which should only be
lukewarm--used in dissolving the gelatine varies from a quarter to
half the bulk of the latter, the nature of the fibre and thickness
of the paper regulating the proportion of water to gelatine, the
strength of the size liquors being greater for thin papers and weak
fibres than for thick papers and strong fibres.
The operation of sizing is considered one of the most difficult
and uncertain with which the paper-maker has to deal, since the
material (gelatine) is greatly influenced by the conditions of the
atmosphere, both as regards its temperature and humidity, while
the temperature of the liquid size itself has also an important
influence on the success of the operation. The condition of
the paper, again, also affects the result, for if it be highly
porous it will probably be weak, and consequently there may be
considerable waste during the process of sizing from the necessary
handling it is subjected to; moreover, should the paper have
been blued with ultramarine, a strongly offensive odour is often
imparted to it; this, however, may be obviated by employing fresh
size and drying the paper as completely as possible. There are two
systems of animal-sizing employed at the mill, namely, hand-sizing
and machine-sizing, which is also called tub-sizing, the former
being applied to papers of the finest quality. Papers that have
been made by the machine, after being cut into sheets, are
hand-sized, as described in the next chapter.
=Machine-Sizing.=--The lower-priced papers, to be machine-sized,
are first partly dried over a few cylinders, after which the paper
passes through a tank containing liquid size, from whence it passes
between two rollers, which squeeze out the superfluous size; it is
then wound on to a reel on which it remains some time to enable the
size to thoroughly permeate the paper, after which it is wound on
to another reel, and from thence it passes over a series of wooden
drums or cylinders, each of which is furnished with a revolving
fan; by this means the paper becomes dried slowly, whereby a more
perfect sizing of the material is effected.
=Double-Sized Paper.=--This term is applied to paper which, after
being sized in the engine in the usual way, is afterwards "surface
sized," as it is called, with animal size in the manner described.
Respecting the drying of paper after it has been tub-sized there
seems to be some difference of opinion as to whether it is best
to hang it in a loft to dry or to dry it over the cylinders of a
drying machine. Upon this point the New York _Paper Trade Journal_
makes the following remarks:--"When the paper is passed through
the size-tub, it is again wet; the fibres expand, and their hold
on each other is relaxed. Now it must make a difference to the
subsequent strength and quality of this paper whether it be hung
up in a loft to dry or run over a drying machine. If it is hung in
the loft no strain is put upon it and the fibres are at liberty
to shrink, or slowly contract, in all directions; whereas if it
is run over a drying machine, consisting of from 50 to 100 reels,
the longitudinal strain prevents the fibres from shrinking and
reassuming their normal position in that direction. Attempts have
been made to obviate this defect by regulating the speed of each
section of the machine in such a manner as to allow for the
shrinking, but this only remedies the evil by preventing the paper
from breaking as it travels over the machine. Everything else being
equal, it would seem that loft-dried paper must be superior to that
dried over the drying machine. Our home manufacturers endorse this
view, inasmuch as they continue to prefer the system of loft-drying
to the less expensive machine methods."
=Mr. Wyatt's Remarks on Sizing.=--Mr. James W. Wyatt, in a
paper on the "Art of Paper-making,"[26] makes the following
observations on engine-sizing and animal-sizing which will be
read with interest:--"Engine-sizing renders the paper fully as
non-absorbent as animal size. The latter penetrates the sheets only
slightly and forms a coating or skin on each surface, whereas the
engine size surrounds each fibre and impregnates the whole mass.
Surface-sizing, however, produces a stronger, firmer sheet, and is
smoother for the pen to travel over; the manufacturer also gets
the benefit in the price of the paper of the additional weight of
the size, amounting to 7 per cent. on the average. On the other
hand, as the animal size is mostly a skin on the surface, if the
coating be broken anywhere by the use of a knife in scratching,
the paper will only imperfectly resist ink in that place, a great
disadvantage for account and office-books and ledgers. Engine-sized
paper is much cheaper to produce than animal sized, and is
therefore used principally for the lower qualities of writings and
for almost all kinds of printings where firmness and smoothness is
not so much a desideratum. Most tub-sized papers have a certain
portion of engine size mixed with the pulp. This not only ensures
the thorough sizing of the sheet, but also is a measure of economy
in reducing the absorbing power of the paper for the animal
size. Papers for ledgers and office-work are best given an extra
proportion of engine size to ensure their ink-resisting properties,
and they are also sized by hand in animal size and loft dried." The
following rough estimate of the comparative cost in materials and
wages of engine-sizing and animal-sizing paper may be of interest:
Engine-sizing, per 20,000 lbs.:--
£ s. d.
Materials 5 2 0
Wages 0 12 6
---------- d.
Total £5 14 6 Cost per lb. = 0·068
==========
Animal-sizing, per 20,000 lbs.:--
£ s. d.
Materials 36 0 0
Wages 4 10 0
-----------
Total £40 10 0
===========