SPECIAL KINDS OF PAPER
There are many varieties of paper products obtained by submitting
finished paper to a number of special processes. Of these only a few of
the more important will be described.
These products can be divided approximately into three classes:--
(1) Papers coated on one side or both sides with various substances,
such as "art," photographic papers, etc.
(2) Papers impregnated with chemicals, such as blue print, medicated,
and cheque papers.
(3) Paper pulp converted into modified products by chemical treatment,
such as vulcanised board, viscoid, etc.
Of the first class, the coated papers used for art and chromo
illustrations are the most important.
Of the second class, the blue prints and papers impregnated with
chemicals, chiefly employed for the production of engineers' drawings,
may be regarded as typical.
In the third class, vegetable parchment and vulcanised board are the
most familiar.
* * * * *
_Parchment Paper._--This is produced by the action of sulphuric acid
upon ordinary paper, the most suitable for this purpose being made
from unsized cotton rag, free from such additions as mechanical wood
pulp. The presence of the latter substance should be avoided, as it
is liable to char or burn, so that in the finished product it shows
itself in the form of small holes. The process depends upon the power
of sulphuric acid to change the surface of the paper into a gelatinous
mass, which has been shown to consist of a substance called amyloid.
The best parchment is made from pure cellulose such as rag or chemical
wood pulp. The quality of the parchment depends upon attention to the
strength of the acid, the temperature of the acid bath, the period of
immersion, the complete removal of the acid, and the careful drying of
the wet parchment.
[Illustration: FIG. 43.--Apparatus for making Parchment Paper.]
The acid is employed at a strength of 1·71 specific gravity, being
prepared by diluting the commercial concentrated acid in a leaden
vessel, with a sufficient quantity of water.
The parchment is generally prepared by passing a continuous sheet of
paper through a bath of acid of the proper strength at a speed which
ensures the correct period of immersion. As the treated paper leaves
the bath it passes through squeezing rolls which remove the excess of
acid, and the paper is then led through a series of tanks containing
fresh water, the last traces of acid being neutralised by small
additions of ammonia, or some alkali, to the last washing tank. The wet
parchment is then passed through suitable rollers and carefully dried
over cylinders heated internally by steam. The paper is kept perfectly
stretched as it dries, because it shrinks enormously, and would
otherwise become cockled and uneven.
Thick sheets of parchment paper are frequently made by passing three
sheets of paper through the acid bath and bringing them together
between the rollers before washing. The sheets unite when pressed
together; the remainder of the process being the same as that employed
for single sheets.
The parchment exhibits remarkable differences to the original paper,
the strength being increased three or four times, the density about 30
per cent., the latter being shown by the shrinkage, which amounts to at
least 30 per cent.
_Vulcanised Paper._--Zinc chloride has the property of parchmentising
paper in a manner similar to sulphuric acid. The product obtained when
this reagent is used is generally termed vulcanised fibre. The paper
is passed as a continuous sheet into a bath of strong zinc chloride,
having a density of 160-170 Twaddell, which causes the cellulose to
swell up and partly gelatinise. A very large excess of strong zinc
chloride is necessary, and the process is only rendered commercially
possible by careful recovery of the zinc from the washing waters, which
are submitted to chemical treatment.
The _vulcanised_ product is subsequently treated with nitric acid or
with a mixture of nitric and sulphuric acids to render them waterproof.
Dextrin is frequently employed to retard the chemical action to permit
of the necessary manipulation of the material before it is finally
washed. The complete removal of the excess of zinc and acid is a
necessary feature of the whole operation.
_Willesden Paper._--When paper is passed through an ammoniacal solution
of copper oxide, a superficial gelatinisation of the surface takes
place, so that the paper when washed and dried is impregnated with
copper oxide, which helps to preserve it, and it becomes waterproof.
Such material is well known as Willesden paper.
_Blue Print or Cyanotype Papers._--This name is usually given
to the process by means of which blue prints of engineers' and
architects' plans can be reproduced. It was discovered in 1842 by Sir
John Herschel. It is a useful method of reproducing drawings, and
incidentally is of great value to the amateur photographer because
of the facility with which it can be applied for getting proofs from
negatives quickly and easily without special baths and chemicals. The
process is based upon the reduction of a ferric salt to the ferrous
condition by light, and the formation of Prussian blue by the action of
potassium ferricyanide. The _negative cyanotype_ gives white lines on a
blue ground. Various formulæ are in common use.
----------------------------+---------+--------+--------+---------
-- |Herschel.| Clark. | Watt. |Rockwood.
----------------------------+---------+--------+--------+---------
| | | |
Solution 1. | | | |
Potassium ferricyanide | 16 | 27 | 48 | 10
Water | 100 | 100 | 100 | 100
Ammonia | -- | 2·3 | -- | --
Saturated solution of | | | |
oxalic acid | -- | 20 | -- | --
----------------------------+---------+--------+--------+---------
| | | |
Solution 2. | | | |
Ammonia-citrate of iron | 20 | 30 | 50 | 30
Water | 100 | 100 | 100 | 100
Boric acid | -- | -- | 0·5 | --
Dextrin | -- | -- | -- | 5
----------------------------+---------+--------+--------+---------
Equal parts of the two prepared solutions are mixed when required and
spread evenly over well-sized paper. The paper is hung up, dried, and
preserved in a dark dry place.
The _positive cyanotype_ gives blue lines on a white ground, being the
reverse of the ordinary blue print. That is, no image is formed where
the light acts, and the reaction is the formation of blue due to the
union of a ferrous salt with ferrocyanide of potassium.
Pizzighelli in 1881 gave the following formula:--
-----------------------+-----------+-----------+-----------+-----------
-- |Solution 1.|Solution 2.|Solution 3.|Solution 4.
-----------------------+-----------+-----------+-----------+-----------
Water | 100 | 100 | 100 | 100
Gum arabic | 20 | -- | -- | --
Ammonia-citrate of iron| -- | 50 | -- | --
Ferric chloride | -- | -- | 50 | --
Potassium ferrocyanide | -- | -- | -- | 20
-----------------------+-----------+-----------+-----------+-----------
Mix the first three solutions in the following order in the proportions
stated:--
Solution 1. 20 parts.
Solution 2. 8 "
Solution 3. 5 "
As soon as the solution, which at first gets thick and cloudy, is clear
and thin, it is spread over the surface of well-sized paper, which is
then dried in a warm room.
The print, which appears yellow on a dark yellow ground, is treated
with the developer (solution 4) by means of a brush dipped in the
solution. When the image is deep blue in colour, the print is washed in
water and then placed in dilute hydrochloric acid (1 part of acid to 10
parts of water) till the ground is quite white. A final washing with
water is then necessary.
Waterhouse gives the following formula:--
---------------------+-----------+-----------+-----------+-----------
-- |Solution 1.|Solution 2.|Solution 3.|Solution 4.
---------------------+-----------+-----------+-----------+-----------
Water | 650 | 150 | -- | 100
Gum arabic | 170 | -- | -- | --
Tartaric acid | -- | 40 | -- | --
Ferric chloride | | | |
solution 45° Baumé | -- | -- | 150 | --
Ferrocyanide of | -- | -- | -- | 20
potassium | | | |
---------------------+-----------+-----------+-----------+-----------
Solutions 1 and 2 are mixed and No. 3 added gradually with constant
stirring. The mixture is left twenty-four hours, and diluted with water
to a specific gravity of 1·100.
The paper is coated with the solution and used as already directed,
being developed in ferrocyanide of potassium solution and washed with
water, treated with weak hydrochloric acid, and then finally cleaned
from all traces of acid.
_Black Lines on a White Ground._--This modification of the ordinary
blue print is arrived at with the following formula:--
Water 96·0 parts.
Gelatine 1·5 "
Perchloride of iron (in syrupy condition) 6·0 "
Tartaric acid 6·0 "
Sulphate of iron 1·5 "
The paper is coated with the solution. After printing, the image is
developed with a solution containing
Gallic acid 1 part.
Alcohol 10 parts.
Water 50 "
A final washing of the print with water completes the operation.
COATED PAPERS.
This term should properly include all the varieties of special papers
which are coated with extraneous matter for particular purposes, such
as art, chromo, tinfoil, gilt, emery, carbon, photographic, marble, and
sand papers. In practice however, the term is almost entirely limited
to "art" papers used for illustration work and half-tone printing.
An "art" paper, using the definition given above, consists of an
ordinary sheet of paper, one or both sides of which have been coated
by the application of a mixture of a mineral matter, such as china clay
or satin white, and some adhesive, like casein or glue. The object
of the coating is to impart to the paper a perfectly smooth surface,
rendered necessary because of the conditions under which the printing
of the illustrations is carried out.
[Illustration: FIG. 44.--General arrangement of Plant for making "Art"
Paper.]
The machine used for coating the paper consists of a large hollow drum
about 40 inches diameter and 48 inches wide. The paper is brought over
upon the drum in a continuous sheet, and the coating mixture applied
to the surface by means of a revolving brush or an endless felt which
rotates in a copper trough containing a coating mixture which is
usually maintained at a temperature of 120° Fahr.
The amount of material put on to the surface of the paper is varied by
altering the proportion of water in the trough. As the wet coated paper
is drawn over the drum it comes into contact with a number of flat
brushes which move from side to side and brush the coating well into
the paper.
[Illustration: FIG. 45.--Sectional Elevation of "Coating" Plant.]
The last two or three brushes on the drum are made of very fine
bristles, so that when the coated paper leaves the machine the surface
is perfectly even and free from brush marks. The wet paper is then
drawn up an inclined ladder by an ingenious device, which causes the
paper to fall into festoons or loops, and these are carried bodily
forward by means of travelling chains. The process, somewhat difficult
to describe, is more easily understood by a study of the illustrations
given.
The paper is dried by a current of warm air which can be obtained by
means of steam pipes placed below the festoons or with a special air
blower. The dry paper is then led through guide rolls and wound up in
the form of a reel.
The paper at this stage has a dull coated surface, which is somewhat
rough and unfinished, and a high polish is imparted to it by a machine
known as a supercalender.
The supercalender consists of a number of alternate steel and cotton or
paper rolls placed vertically in a stack one above the other. When the
coated paper is led through this machine the friction of the alternate
steel and cotton rolls produces a high finish on its surface.
An art paper coated on both sides is manufactured by passing the paper
through the coating machine twice. Machines have been devised for
coating both sides of the paper at one operation, but these are not in
very general use.
Tinted art papers are prepared in the same manner, the desired colour
being obtained by the addition of pigments or aniline dyes to the
mixture in the trough containing the coating materials. When the two
sides of such tinted papers are coloured differently, they are often
described as duplex coated papers.
_Imitation Art Papers_ are prepared by quite a different process,
although they have the appearance, more or less, of the coated paper.
They are merely esparto papers very heavily loaded, containing
frequently as much as 25 to 30 per cent. of mineral matter prepared as
follows:--
Bleached esparto half-stuff is beaten together with any suitable
proportion of chemical wood pulp in an ordinary beating engine, and a
large quantity of china clay is added at the same time. The beating
is carried out under conditions which favour the retention of as much
china clay as the pulp will hold while being converted into paper on
the Fourdrinier machine.
After the paper passes over the drying cylinders of the machine it is
passed through the calenders in the usual way, but the surface of the
paper is damped by means of a fine water spray just before it enters
the calender rolls. The result is that a "water-finish," so called, is
imparted to the paper, and a close imitation of the genuine art paper
is obtained, the effect of this peculiar treatment being to compress
the fibres and bring the clay up, as it were, to the surface.
A paper containing such a large proportion of mineral matter intimately
mixed with the fibre is naturally very weak. It easily tears, and if
moistened with water goes all to pieces. At the same time it is a cheap
substitute for high-class art paper, being suitable for circulars,
temporary catalogues, and similar printed matter.
In an "art" paper the nature of the fibrous constituents is too often
regarded as a matter of secondary importance, because in the process
of printing the ink does not come into contact at all with the paper,
and an impression is produced merely on a layer of clay which is bound
together by the glue.
The illustrations are not absolutely permanent, and it is perfectly
easy to remove the whole of the impression and the coating itself by
immersing a sheet of the paper in warm water and rubbing the surface
gently with the fingers, or with a camel-hair brush.
In fact the amount of coating matter which has been brushed on to
a paper can be determined approximately by weighing a piece of the
coated paper, removing the mineral matter and glue from both sides as
indicated, allowing the paper to dry again, and then re-weighing, the
loss in weight representing the amount of coating.
It is not surprising to find that the true paper is merely regarded as
a convenient means of producing, so to speak, a smooth surface of clay,
and an examination of the material between the two clay surfaces often
reveals a paper of very low quality.
There are one or two empirical methods for testing the condition of
coating on an art paper. If the coating is firm and adherent, then on
pressing the moistened thumb on to the surface none of the coating
matter is removed, but in a badly-made art paper some of the coating
adheres to the thumb.
Another method is to crumple a sheet of paper between the fingers, and
if any of the coating comes away easily the paper is considered of poor
quality.
The complete examination of an art paper, apart from the practical test
of printing, involves the determination of the amount of coating matter
added to the paper, the proportion of glue in the coating, and the
usual analysis of the paper itself.
PACKING PAPERS.
This term may be applied to wrappings specially treated with substances
which render the paper air and water proof. They are principally used
for preserving food, or such articles as tobacco, which require to be
kept slightly moist.
_Waxed Paper._--The paper in the form of a continuous sheet is passed
through a bath of melted wax at a high temperature, any excess being
removed by squeezing rolls through which the hot waxed paper is passed.
The paper is led over skeleton drums and thoroughly cooled before being
cut into sheets.
_Butter Paper._--Ordinary parchment paper is generally used, but
for special purposes a solution containing albumen and saltpetre is
utilised for impregnating paper.
_Hardware Paper._--Needles and silver goods are frequently wrapped
in paper impregnated or mixed with substances which are supposed to
prevent deleterious fumes from coming into contact with them. The use
of black papers heavily loaded with pigment, sized with glue and an
excess of alum, is commonly resorted to. For silver ware, paper dipped
in a solution of caustic soda containing zinc oxide is used. A recent
patent suggests the impregnation of paper with heavy hydrocarbon oils,
which being slightly volatile cover the goods, such as needles, with a
thin film.
_Paraffin Paper._--Large quantities of this paper are consumed for
packing food and other articles which need protection from air and
moisture.
The paper is either passed through a bath of paraffin or passed over a
roller which rotates in a trough of paraffin.
If the paper is to be coated on both sides it is passed through the
bath containing the paraffin in a melted condition, the excess of which
is scraped from the paper as it leaves the bath. The paper is cooled by
exposure to air, and when the paraffin has solidified upon the sheet
the paper is wound up on a roller at the end of the machine.
If the paper is to be coated on one side only it is passed over a
heated roller which revolves in a bath of melted paraffin, the other
operations of drying and finishing being the same as in the case of a
paper coated on both sides.
_Tinfoil Papers_, required for packing tea, coffee, and similar
foodstuffs, are prepared by coating cheap paper with a solution of
gum and finely powdered tin. The manufacture of the fine powder
is accomplished by melting tin at a low temperature and shaking
it continually as it cools down, whereby a mixture of fine powder
and large particles is produced, the latter being separated out by
agitation of water.
Tin in a fine state of division can also be obtained by a chemical
process. Granulated tin is dissolved in strong hydrochloric acid, the
solution diluted with water, and a stick of zinc introduced into the
solution. The tin is gradually precipitated.
The dried powder is coated on to the paper with gum, and when the
paper is dry the necessary degree of brilliancy produced by suitable
calendering.
_Transfer Papers._--A number of important operations require the use
of what are known as _transfer_ papers, so that a design written or
printed upon a specially prepared surface can be _transferred_ to
another surface from which duplicate copies may be obtained. The
principle upon which all such operations are based is the coating of
suitable paper with starch, flour, and gum, singly or mixed, so as to
give a surface firm enough to take the design, but which readily breaks
up when the printed side is pressed against the wood, stone, or metal
object intended to receive the design.
Thus a paper may first be dusted over with dry starch, or coated with
starch paste and then dried. A layer of dextrine may then be put over
the starch coating, and the design printed upon the dextrine surface.
When the paper is turned face downward on a sticky metal plate the
design adheres to the metal, and the paper is easily pulled off, owing
to the dry starch layer between it and the dextrine being non-adhesive.
This principle is utilised in producing designs upon tins used for
packing, metal advertisement plates, domestic articles of every kind,
stoneware and earthenware goods.
It is further applied in the preparation of lithographic stones
required for printing.
Each class of work demands paper of a suitable character, but the
principle of an easily detached surface-coating is the same for all.
The main difficulty experienced is the liability of paper to stretch
when damped, and various methods are devised to obviate this, either by
employing paper which stretches very little when damp, or by making the
paper partially waterproof before use.
_Papier-mâché._--This name indicates a preparation of paper or paper
pulp mixed with various mineral substances firmly cemented together by
animal or vegetable adhesives.
The _paper pulp_ used for high-class goods consists of pure wood
cellulose, while for the commoner qualities mechanical wood pulp, waste
papers, and any similar fibrous material are employed.
The _mineral_ substances used are china clay, chalk, gypsum, barytes,
ochre, sienna, and other mineral pigments.
The _adhesive_ materials are glue, casein, gum, starch, paste,
dextrine, Iceland moss, or wax.
For experimental purposes, small quantities of papier-mâché may be
prepared in the following manner:--
When old newspapers or brown papers are used as the fibrous basis of
the papier-mâché, they are first torn up into small pieces, moistened
with hot water, tied up in a small cloth bag or sack, which must
only be half filled, and then immersed in a basin of warm water and
thoroughly kneaded by hand, so that the paper is gradually reduced
to the condition of pulp. If the kneading process is carried out
thoroughly the paper is entirely reduced to pulp. The excess of water
can be removed by pressure and the preparation of the final mixture
completed by the incorporation of clay, pigment, and adhesive.
In the preparation of papier-mâché for goods on a large scale a beating
engine is used in order to break up the old paper or wood pulp into a
fibrous condition.
The following formulæ can be used for making papier-mâché:--
-----------+-----------+---------------+-----------
(1) | (2) | (3) | (4)
-----------+-----------+---------------+-----------
Pulp 22 | Pulp 22 | Pulp 12 | Pulp 33
Clay 37 | Chalk 30 | Rosin size 22 | Starch 9
Casein 37 | Glue 4 | Flour 11 | Clay 9
Water 4 | Water 44 | China clay 11 | Water 49
| | Water 44 |
--- | --- | --- | ---
100 | 100 | 100 | 100
-----------+-----------+---------------+-----------
_Plaster Moulds._--Plaster of Paris or gypsum is the main article used
for moulds and pattern. The preparation of gypsum for casting is made
as follows:--The gypsum is gradually worked up into a creamy paste with
water, the mixing being done quickly yet thoroughly.
The pattern of which it is desired to form a mould must be coated with
oil. Around the pattern placed on a table a wall of wood or pasteboard
is fixed, so that a basin will be formed of suitable depth, preventing
the gypsum from flowing away. Patterns of figures or of curved articles
have to be made in two or more parts. For that purpose the pattern is
usually cut into two pieces. Two moulds are now readily obtainable by
first oiling the pattern and by pouring the gypsum in a thin state
gradually over the surface, to avoid the forming of air bubbles.
The rapid drying of the soaked gypsum is sometimes inconvenient, but
the addition of a saturated solution of borax in water to the gypsum
mixture can be resorted to as a check.
Various means are employed for hardening and strengthening the plaster
cast, such as the addition of coarse paper fibres, shreds of canvas,
iron filings, or wire.
_Colouring._--Usually a cheap water colour only is required; a light
coating of a cheap varnish may be sufficient. In other cases a water
colour serving as a filler for smoothing the surface may receive a
finish of one or more coats of resinous solutions in alcohol or of
copal varnish. Many goods are coated with asphaltum or Japan varnish
and dried in cold or hot air.
Some of the articles may be decorated with scrolls or arabesques in oil
colours or enamels, or the lines may be covered with bronze powder, or
with metal, gold, or aluminium leaf.
_Varnishing._--The following varnish recipes are suitable:--
--------------+---------------+--------------+-------------
(1) | (2) | (3) | (4)
--------------+---------------+--------------+-------------
Shellac 20 | Shellac 10 | Shellac 6 | Sandarac 15
Alcohol 70 | Rosin 10 | Sandarac 3 | Mastic 5
Lamp black 10 | Alcohol 60 | Mastic 18 | Turpentine 5
| Lamp black 20 | Alcohol 73 | Alcohol 75
--- | --- | --- | ---
100 | 100 | 100 | 100
--------------+---------------+--------------+-------------