photographs of the same scene through coloured glass. Thus a screen of
yellow glass will intercept the blue and the red rays, and the sensitive
film will be impressed with images of objects containing yellow rays
only, and that in proportion to the quantity of these rays that enter
into any given tint. Similarly with images taken through red and blue
glasses. The positives from these partial images being projected by
three optical lanterns on the same space on a screen, and each being
coloured by passing through tinted glasses like the original, the
superposed images thus combined give a very lively impression of the
natural colours in all their gradations.
Among the many processes for reproducing photographs by non-photographic
processes, some have been more or less successfully combined with colour
printing. Some of these productions are very effective, and are more
attractive to many persons than the monochromatic tints of ordinary
photographs.
[Illustration:
FIG. 312.—_Portrait of Aloysius Senefelder._
]
PRINTING PROCESSES.
As it is beyond contradiction that printing is one of those inventions
which have most influenced the progress of mankind, so it will be
admitted that certain modern processes, by greatly facilitating the
operations, and vastly extending the resources, of the art, possess an
interest and importance surpassed by few of the subjects we have
discussed. In a former article the reader has been made acquainted with
the steam printing-press and other applications of machinery by which
the impressions of a form of type, or of a pattern, can be rapidly
multiplied. Here we have to describe some ingenious methods of preparing
the forms or originals for letterpress and other printing, and certain
beautiful processes for multiplying drawings, engravings, and pictures.
_STEREOTYPING._
This term is applied to the process of obtaining the impression of a
form of movable types, or of a woodcut, on a plate of metal which can be
printed from. These plates, after the required number of copies have
been printed, can be stored away; and they are ready for use whenever
another issue of the work is required. When the pages that are to be
stereotyped have been set up in ordinary type, there are several methods
by which the stereotype plates may be obtained from them; or rather,
there are several materials used to form the matrix or mould in which
the metal is cast. When plaster of Paris is used, the form is first
slightly oiled, to prevent adhesion of the plaster; a thin mixture of
plaster and water is then poured upon the form, which is surrounded by a
raised rim, to retain the plaster. The thin plaster is carefully led
into all the recesses of the type, and then some thicker material is
poured on. The plaster soon sets, and is lifted off the type, and, after
drying, is ready to receive the molten metal of which the stereotype
plate is formed. This metal is an easily fusible alloy of lead,
antimony, and other metals, which takes the form of the mould with great
accuracy, and is, when solid, sufficiently hard to print from.
[Illustration:
FIG. 313.—_Press for Stereotyping by Clay Process._
]
Another plan is to make use of prepared clay, spread upon an iron plate,
for the formation of the mould. The face of the type is brushed with
benzine, the plate with the clay is laid upon it, and pressure is
applied. The whole is then dried in a slow oven, and the clay, when
detached from the type, is ready to form the mould. The advantages of
the clay process are that the type does not require to be afterwards
cleaned from oil, and that the material does not fill up the deeper
spaces of the form, so that a thinner stratum of metal suffices to form
the stereotype plate.
A third mode of obtaining the mould has been already mentioned in
connection with the Walter Printing Press (page 313), in the working of
which the _papier maché_ process is ingeniously made to supply the
curved stereotype plates for the cylinders. This process is also largely
used for other newspaper presses, and sometimes for bookwork, as it
forms an invaluable means of expeditiously obtaining a number of
stereotype plates from the movable types. This production of a number of
similar forms makes it possible to strike off a very large number of
copies in a short time, for many presses can be employed simultaneously.
For the paper process a number of sheets of tissue-paper are pasted
together, and the moist paper is laid upon the form; then the operator,
by light strokes of a brush, beats down the paper into the hollows of
the type, beginning at the centre of the page, and going towards the
margins. A sheet of stout unsized paper, called “plate paper,”
constitutes the upper layer; and when the whole has been well beaten
down upon the type, pressure is applied by means of a screw acting upon
a plate of iron covering the whole. In this condition a gentle heat,
produced by steam, is made to completely dry and harden the paper
matrix, which is very soon fit to be used for casting the metal. The
apparatus for this purpose consists of a hollow iron table, within which
steam is made to circulate. On this the form is placed, and the platen
is pressed down upon it by means of a screw. In many cases the platen
also is heated by steam, to accelerate still further the drying of the
matrix, which is effected in about four or five minutes. One paper
matrix, by careful use, will serve for the production of a series of
casts without receiving any damage from the molten metal, as this is
fusible at a low temperature.
The mould for casting flat stereotype plates from the paper matrix is
made of iron, and has parallel surfaces, which admit of being so
adjusted that the thickness required in the plates may be obtained very
nearly. The paper matrix is laid on the horizontal iron bed of the
mould; gauge-bars are adjusted, which retain it in its position; and
then the second plate is folded down—the distance between that and the
paper being determined by the gauge-bars. The cover is secured by
clamping-screws, and then the mould is turned upright to receive the
metal, which is removed, when solid, after the mould has been turned
back into its horizontal position.
However the stereotype plates have been produced, it is necessary
accurately to adjust their thickness by planing off some of the material
from the back. The edges have also to be cut and trimmed to the exact
dimensions required by the press. Various machines have been devised for
effecting all these operations with accuracy and dispatch. The plates
are afterwards mounted on wooden or metal blocks to bring them to the
height of ordinary type.
A fourth method of producing plates for the same purpose as the
stereotype plates already described is by _electrotyping_. This method
appears to have been introduced as early as 1840, but the first results
were not without imperfections. Now, however, this plan is almost
universally applied to bookwork and woodcut illustrations. Many of our
popular illustrated periodicals have so large a circulation that the
wooden blocks would necessarily be spoiled by being used in steam
presses long before they had yielded the required number of impressions;
and the method has also the great advantage of securing the original
engraving from the chance of accidental damage, by which a block is
sometimes irretrievably injured. Hence woodcut illustrations are now
always printed from electrotype copies of the engraved blocks, whether
the work itself be printed from movable type or not. But the electrotype
or stereotype process is always resorted to in the case of a work,
whether illustrated or not, when it is foreseen that a re-issue will be
demanded. These processes are also of great advantage to the practical
printer, because when the pages set up in type have received their final
corrections, he can take the casts, and then the type may be
distributed—that is, returned to the cases ready for the compositors to
use for other work.
The electrotype process is almost as simple as those for producing
stereotype plates by casting, and its productions excel these by their
great durability and extreme exactness of reproduction. We may take it
for granted that the reader is familiar with the fact that ordinary
letterpress characters and woodcuts are printed from forms, in which the
black portions are in _relief_. For woodcuts the artist makes the
drawing, in reversed position, on a block of finely-grained boxwood, in
which the fibres of the wood are perpendicular to the surface. The
engraver hollows out all the parts which in the impression remain white,
while all the parts which are to receive the ink and produce the black
parts of the impression must be left at the original level. The wooden
blocks thus engraved would serve to produce a certain number of
impressions, which could be taken off by careful hand-printing without
perceptible damage to the block. But the pressure necessary for printing
inevitably crushes the projecting parts of the block; and the
impressions, after a certain number, lose their sharpness. This is
especially the case in machine printing; but not only does the
electrotype cast present a surface capable of bearing hard usage much
better than those of the hardest wood, but even if the number of
impressions required should wear out the metal plate, it can easily be
replaced by another cast from the original block.
The mould which serves to give the electrotype cast may be made either
of gutta-percha softened by a gentle heat and applied to the wood, or of
wax. In either case a powerful pressure is applied, in order to force
the yielding substance to take the forms of the engraved block or of the
metal type. Wax is now generally preferred; the yellow wax used for this
purpose is melted, and poured into a shallow pan; when it has become
solid, it is sprinkled over with finely-powdered pure blacklead, which
is brushed over the surface, and then the excess is removed by blowing
with bellows made for the purpose. Thus prepared, the wax is placed over
the type-form or wooden block in a powerful press, sometimes worked by
hydraulic power; but more frequently a _toggle_ press is employed, in
which the pressure is given by a screw and crank-wheel acting on two
_elbow joints_, or _toggles_. For the information of non-mechanical
readers it may be stated that a “toggle” consists of two bars jointed
together, and placed _nearly_ in a straight line: when a pressure is
applied to the joint, tending to bring the rods still more nearly into a
straight line, their extremities are thrust apart with a great force,
which increases indefinitely as the rectilinear position is approached.
In the electrotyper’s press there are two toggles constructed of very
broad bars, or rather thick plates, for they have nearly the width of
the bed of the press. With this machine a very powerful and regular
pressure is applied; and the wax in a few minutes takes a sharp
impression, embracing all the most delicate details of the work, and
becomes at the same time very hard. The impression, of course, has
hollows corresponding to the projections of the wooden block or
type-form, and _vice versâ_. The face of the wax mould is now very
carefully and completely blackleaded, a soft brush being used in the
process. It is then placed in the solution of sulphate of copper, and
the blacklead receives a deposit of copper, in the manner explained in a
former page (498). In about forty or fifty hours a firm, compact
deposit, about as thick as the finger-nail, covers the blackleaded
surface, forming a perfect reproduction of even the most minute details
of the engraved block or letterpress form.
The next operation has for its object the removal of the thin shell of
copper from the wax. This is effected by exposing the mould to a gentle
heat by immersing it in hot water, or by placing it on a hollow iron
table which is heated by steam. The wax is run off into a proper
receptacle for future use, and any portion adhering to the copper is
removed by the action of naphtha or of a solution of potash. The thin
copper shell is then tinned on the back, and an alloy of lead with some
tin and antimony, forming the _backing metal_, is poured on it, to the
depth of about one-eighth of an inch. When this has become solid the
backing is planed, so that the compound plate may have a certain regular
thickness, and that the back surface may be parallel to the face. The
edges are cut by a circular saw and trimmed by machine-tools, and the
plate is rendered perfectly even, and adjusted with the greatest
possible exactness to the required thickness. It is prepared for the
press by being screwed down upon a block of wood of a certain thickness,
so that the face of the plate may have the same height as common type,
the screws passing through the margin or other hollow parts of the face
of the cast. No more enduring surface than the copper of these
electrotype casts, backed up by the hard alloy, has yet been discovered.
_LITHOGRAPHY._
To Aloysius Senefelder, a musician attached to one of the theatres in
Munich, whose portrait appears at the head of this article, is due the
invention of the art of lithography. It is said he used to arrange his
musical compositions on a kind of slates, formed of flakes of the
limestone which is found in the neighbourhood of Munich. One day a
memorandum which he had made in this manner happened to fall into a
slop-bucket full of greasy water; on withdrawing the piece of stone, he
noticed with surprise that the grease had attached itself to the
characters, while the rest of the stone remained quite clean. Such an
incident might have happened to each one of a thousand men, and its
significance might not be perceived; but it suggested great
possibilities to Senefelder, who, applying himself for some years with
ingenuity and perseverance to experiments with the Munich limestone,
became, in the year 1800, the inventor of a new art. Though he was no
chemist, and was unskilled in mechanics and in drawing, yet within four
years from his first observation he had succeeded in finding the proper
materials for his crayons and the appropriate acids for acting on the
stone, in contriving a suitable press for taking the impressions, and in
producing samples of lithographic work in various styles of art. He
endeavoured to keep his processes secret, and having obtained the
exclusive right of exercising his invention in his own country, he
attempted to carry on all the operations himself. Little by little,
however, the general nature of the process became known, and although
the details were jealously concealed, ingenious persons in France and
elsewhere, by force of experiment, succeeded in re-inventing the art for
themselves, and Senefelder never profited by his invention as he should
have done.
The first lithographic press in London was established by Mr. Hullmandel
in 1810. The value of lithography as a means of multiplying works of art
was soon afterwards proved by the publication of a magnificent series of
picturesque delineations of the quaint architecture of the old towns of
Flanders and Germany, drawn on the stone by Samuel Prout. The late Mr.
J. D. Harding largely contributed to the popularity of lithography by
the landscapes which he drew on the stone, and thus placed in the hands
of every one, prints in which all the freedom and force of the artist’s
work were secured. The French designers excel in fine-art lithography,
and many beautiful productions of their crayons have been published in
every department of pictorial illustration.
The best lithographic stones come from Germany; but for some kinds of
work stones from other localities are used, on account of their less
cost. Thus, in England, a stone yielded by the white lias formation near
Bath has been found to possess the requisite qualities. The stones for
lithography are prepared in much the same way as slabs of marble are
polished; that is to say, by rubbing one slab against another with sand
and water. When the stones have thus been brought to a plane surface,
they are finished according to the purpose for which they are intended.
If they are intended to receive written characters, they are polished to
a very smooth surface by means of pumice-stone. But if they are to take
drawings, then a certain uniform grain is given by means of
finely-sifted sand, the operation being performed in a similar manner to
that in which the stones are dressed, only pressure is not applied to
the upper stone. The stones, after being washed and dried, are carefully
covered on their prepared surfaces with thin paper, and are sent out for
use.
When the stone is employed to reproduce written characters, or drawings
imitating those done with a pen, _lithographic ink_ is made use of with
an ordinary pen, a ruling-pen, a fine brush, or a pen which the
lithographer makes for the occasion out of thin metallic plates. The
composition of the ink varies much: the usual ingredients are wax,
gum-mastic, gum-lac, soap, and lampblack. This composition forms a
solid, which is rubbed down with water to a thick liquid when required
for use. The characters have, of course, to be written on the stone in a
reversed position, and the lithographer acquires the habit of doing this
with neatness and dexterity. He is provided with a looking-glass for
viewing his work, in order to see the effect which will be given by the
impression, for the looking-glass shows the characters in their usual
position, just as the image of ordinary writing seen in it is reversed,
showing, in fact, the very appearance the characters present on the
stone. For a drawing, a _lithographic crayon_ is used, made of wax,
soap, grease, lampblack, and other ingredients. With this the drawing is
made on the stone exactly as on paper, save the necessary reversals.
When the design has been placed on the stone, a liquid containing nitric
acid and gum is poured over it. This liquid acts on all the parts of the
stone not protected by the ink or crayon: they are thus rendered
incapable of receiving printing-ink, while the protected parts have the
impression more strongly fixed; for when the stone has been well washed
with water, and turpentine has afterwards been applied, so that all the
matter used in marking the design is dissolved away, the seemingly
obliterated characters reappear when—after the stone has been lightly
wiped with a damp sponge—the roller charged with printer’s ink is
applied. The ink is taken up by the stone only at those places which
have not been acted on by the acid. The impression is obtained by laying
a sheet of damp paper on the inked stone and applying pressure by means
of a roller, under which the stone passes. The stone is moistened with
water after each impression before the inking-roller is again applied.
The lithographic stone, like other originals used in printing, is liable
to deteriorate when large numbers of impressions are taken from it. This
would be a serious drawback in lithography, but for a method of renewing
the impression, which renders it unnecessary for the artist to retouch
his work. This is the process of _transferring_, which is practised by
the aid of a certain kind of paper specially prepared by a coating of
paste. On this a proof is taken from the original drawing on the stone,
and the still moist sheet is then applied to another stone, with the
face downwards, and passed under the press. The effect of the pressure
is to cause the adherence of the layer of paste to the stone; and when
the paper has been thoroughly wetted at the back, it may be removed,
leaving the paste still adhering to the stone, with the impression
beneath it. When water is applied, the paste is washed off, while the
ink of the impression remains attached to the stone, there reproducing
the design drawn on the first stone. The transferred design is treated
in exactly the same manner as the original drawing, acid being poured
over the stone, &c., and the impressions obtained by the same method of
successively sponging, inking, and pressing. The transferred drawing may
be made to yield another transfer, and so on indefinitely; but when a
large number of impressions from one design are required, it is usual to
make at once from the original as many transfers to separate stones as
will yield the required number of impressions without deterioration. In
this way as many as 70,000 copies have been taken from a single drawing
without their showing any marked difference in the character of the
impressions.
The transfer process is also applied to place on the stone characters
which have been written with a pen in the ordinary manner on prepared
paper. In this way a person’s handwriting is so accurately reproduced in
the impressions that it is often very difficult to detect the
interposition of the lithographic stone, and the impression often passes
as the immediate production of the writer’s pen. It is obvious that
drawings etched with the pen on transfer-paper can be printed from in
the same manner. And line engravings, which have been originally
produced by cutting hollow lines on polished plates of copper, can be
printed lithographically by transferring an impression to the stone. By
transfer also the impressions of raised types or of woodcuts can be
printed from the stone when desirable.
A beautiful and important application of lithography to the reproduction
of pictures in colours has been so successfully carried out that a new
branch of the art, termed _chromo-lithography_, now gives facsimiles of
water-colour drawings and of paintings in oil. The copies of
water-colour drawings especially are remarkable for their artistic
qualities, and it is undeniable that these cheap reproductions of good
paintings have done much to extend the knowledge of art. It is not
contended that a chromo-lithograph, for example, after one of old
William Hunt’s rustic figures, or birds’ nests with banks of primroses,
can possess the wonderful refinement of the original; but it will
nevertheless convey much of the artist’s sentiment. Such transcripts of
the works of our best artists adorn the homes of thousands who have
never perhaps had the opportunity of even seeing the painter’s original
handiwork. In many a remote settlement in distant colonies, as in many
an English home, the chromo-lithograph is the brightest of the household
art treasures.
The principle of chromo-lithography consists in printing on the same
paper with inks of various colours from different stones successively,
so as to produce, by the juxtaposition and superposition of the various
tints, the effect of a coloured drawing or painting. The artistic
effects of the best chromo-lithographs require a great number of
printings for their production, in some cases as many as twenty
different stones being employed. The stones and colours for such
productions require true artists to prepare them, persons who can
thoroughly understand and enter into the spirit of the original work.
The first operation consists in the preparation of a faithful but
spirited outline of the original, etched on transfer-paper, from which
the outline is placed on a lithographic stone. This sketch we have
called an outline, but it is in reality something more; for it should
suggest all the markings and limits of tints which belong to the
original. This first sketch has some points marked on the margin by dots
or crosses, which serve to secure true register in the subsequent
processes; that is, the impressions of the successive tints are so
placed on the press that these points coincide in each impression.