in light ink to other stones as there are colours required in the
reproduction. To each colour a special stone is assigned, on which the
lithographer, guided by the slight impression of the sketch, draws with
the ordinary black crayon the form which that colour is to produce on
the paper. Much artistic skill and judgment are required to do this in
such a manner as to obtain a clear and harmonious final result. The
gradations of the colours, and their blendings by superposition, must be
carefully regarded. When the form and limits of each colour have been
skilfully laid down upon its own stone, the surface is acted on by the
acid, it is washed, the ink is dissolved off by turpentine, the stone is
sponged, and the roller charged with ink of the appropriate tint is
passed over it. The ink, as before, adheres only to the parts over which
the crayon has passed, and an impression may be drawn off. Each of the
other stones is similarly treated, and when the whole are ready, a proof
is taken by giving the same sheet of paper the whole series of
impressions in their proper order and colours, with the greatest
possible accuracy of register. If any alterations appear desirable, they
are made accordingly, by aid of certain devices which need not be here
described, and when a satisfactory result has been obtained, the
printing of the whole series of impressions is proceeded with. When the
number of these is very large, transfers of each stone are taken as in
ordinary lithography, only with certain extra precautions for obtaining
precision in the register.
The brilliant effects produced by using gold and silver in lithography
are obtained by using a kind of varnish, instead of coloured ink, for
printing those parts where the metal is to appear. When this varnish has
acquired a certain stickiness by partial drying, powdered gold or silver
is applied, and this attaches itself only to the varnish; when the sheet
is dry it is passed under a burnished steel roller, the pressure of
which imparts a brilliant lustre to the metal.
A method of colour-printing, in some respects resembling that of
chromo-lithography, is practised by printing in variously coloured inks
from a series of wooden blocks. This admits of far greater expedition in
working off the impressions than the process with stones. The gradations
of the coloured inks and powdered tints are produced in the same manner
as those of ordinary woodcuts in black and white; and when the colours
are well chosen, and care is taken to secure the accurate superposition
of the impressions, very pleasing effects can be produced by this means.
The coloured prints which are from time to time issued as supplements to
the “Illustrated London News” are produced by this process, and are no
doubt well known to the reader. Our plate of spectra, No. XVII., is an
example of another method of printing in colours.
_OTHER PROCESSES._
In recent times a great number of printing processes have been devised,
but only a few have found their way into practical use, and some of
these have scarcely been so extensively applied as their merits appear
to deserve: either because the public demand has been insufficient to
bring these inventions into common use, or the cost of working them has
been too great. There is no doubt of their scientific success, whatever
may be their commercial value as competing with cheaper and readier
methods. We shall first describe the plan which has been termed _Nature
Printing_.
This process is applicable only to certain objects which possess, or may
be made to assume, a flat form. It has been most successfully applied to
botanical specimens, the impressions of the leaves, flowers, and other
parts of plants being given with an accuracy and minuteness of detail
which the finest work of an engraver could never attain. In fact, the
prints may be examined with a microscope, and they then reveal the
minute structure of the object with wonderful clearness and delicacy.
The notion of nature printing originated with M. Auer, the
Superintendent of the Imperial Printing Office at Vienna; but the
process was introduced into England, with certain improvements, by Mr.
H. Bradbury. Supposing the object to be printed is a plant or the frond
of a fern, it is first thoroughly dried by being pressed between folds
of blotting-paper by means of a screw-press. The paper is changed
several times, and, when necessary, the drying is accelerated by a
gentle heat. When the specimen is perfectly dry, it requires very
careful handling, for it is then generally extremely brittle. It is laid
upon a sheet of pure soft lead, the face of which has been formed into a
perfectly even surface, smooth and bright as a mirror. Mr. Bradbury
encountered some difficulties in attempting to produce a surface of this
kind, for small irregularities of the lead surface showed themselves;
but Mr. James Wood succeeded in preparing for him a machine by which the
lead is planed and polished in one operation. The object having been
carefully laid upon the bright and smooth surface of the lead, a
powerful pressure is applied by passing the plate between a pair of
polished steel rollers. The effect of this is to embed the plant in the
soft metal, which thus receives even the most delicate markings of the
object. The next operation is the careful and patient removal of the
object from the plate; and as this is very brittle, it will be easily
understood that it does not in general come away entirely, but portions
will be left embedded in the metal. The skill of the operator is shown
by destroying these by means of a blowpipe-flame, without in the least
fusing the lead, which would of course ruin the impression.
When the whole has been removed, the leaden plate will have been
engraved, as it were, by the object itself; and in this state the plate
will yield impressions with ink in the same manner as an engraved copper
plate. But in the soft metal the image would soon be obliterated, and
therefore a facsimile of its impression is obtained in copper by the
electrotype process. For this end the lead is covered with a varnish,
except on the face, and thus the deposit of copper takes place only
where it is required, and the current of electricity is continued until
a proper thickness of deposit has been obtained. This electrotype has
all the hollow forms of the lead plate in relief, and it is used only
for the preparation of another electrotype. For this purpose its face is
brushed over with fine, pure blacklead, in order to prevent the deposit
from becoming incorporated with it, while the rest of the plate is
varnished. When it is placed in the electrotyping solution the copper is
deposited on the blackleaded face, and the action is continued until the
layer of metal has acquired the thickness of one-eighth of an inch. It
is then removed from the matrix, and is ready for the printer, who deals
with it in the ordinary manner of copper-plate printing, except that he
uses a softer paper, and this is forced by the pressure into the
depressions in the plate, so that the impression is really embossed on
the paper. Coloured inks are also used instead of black; for instance,
to the leaves green-coloured ink is applied, and to the stems, &c.,
brown ink.
Several works on certain branches of natural history have been very
appropriately illustrated in this way; among these, perhaps, no more
beautiful example is to be found than in “The Ferns of Great Britain and
Ireland,” with text by Lindley and Moore. The merits of the
nature-printing process appear to be the accuracy of outline in the flat
form, and the delicacy of detail in parts projecting from the surface.
The impressions cannot present artistic or natural shading in the
objects; for the depth of colour will be in proportion to the projection
of the part, whereas in nature the darkest shades are seen in the
deepest recesses.
A copper plate, cut in the ordinary manner—as a line engraving, for
example—soon deteriorates, as the pressure applied for each impression
taken from it tends to close up the lines. It has therefore been
necessary, where a plate has to yield a large number of impressions, to
make use of steel instead of copper. But the electrotype has given the
means of multiplying indefinitely facsimiles of engraved copper plates,
so that in many cases a number of these are prepared, and used so long
as they continue to yield clear impressions, the original plates
engraved by the artist only furnishing the matrix. The mode of
reproducing the plates by electrotyping from the original engraved
plates is identical with that just described for obtaining the plates
for nature printing from the leaden plates.
Another process of wider interest, and producing very beautiful results,
is known as the Woodbury printing process, from the name of its
inventor. It is a mode of photographically forming a picture in relief,
from which printing blocks are obtained in much the same manner as in
the nature-printing process. But the subject which is thus printed is a
photograph; and it is only because in the actual production of the
impression on paper the agency of light is not called into play that it
is not described under the head of photography, for it is an ingenious
mode of causing the photograph to engrave its own image on a metal
plate. It is founded on a fact which has already been noticed, namely,
the insolubility of gelatine which has been mixed with a bichromate and
exposed to the action of light. Mr. Woodbury has obtained the best
results with a solution of Nelson’s opaque gelatine, 1 oz. of which is
dissolved in 5 oz. of water, and to each ounce of the solution 15 grains
of ammonium bichromate are added. When a layer of this mixture, which is
of course prepared in the dark, is exposed to the action of light under
a negative photograph, the gelatine is rendered insoluble under those
parts of the negative through which the light passes, that is, in the
parts corresponding with the dark shades in the original object, and the
depth of the layer thus rendered insoluble in each part will depend on
the relative thickness of the silver deposit in the negative photograph.
Thus, in the half-tints the insoluble layer will not be so deep as under
the parts of the negative through which the light passes without
interruption. But the differences of depth will appear when the soluble
gelatine has been dissolved away on the side of the layer which is
farther from the negative. Hence, Mr. Woodbury spreads his layer of
bichromated gelatine on a sheet of plate-glass, previously coated with
collodion, and when the gelatine has become dry, the double film is
detached from the glass and exposed under a negative, the collodion side
being uppermost and in contact with the photograph. After exposure the
film is temporarily attached to another piece of glass, by means of a
solution of India-rubber, and is then immersed in warm water, which
quickly dissolves the soluble parts of the gelatine. Thus a counterpart
in relief of the photograph is obtained. This is allowed to dry, and the
next operation consists in obtaining an impression from it in metal:
this Mr. Woodbury at first obtained by electric deposition, but he has
discovered a much more expeditious process, which one would hardly have
supposed possible before actual trial. The dry hard gelatine is placed
upon a flat, truly-surfaced steel plate, with the collodion surface
downward, a plate of soft metal is placed upon the gelatine, and the
whole is subjected to a pressure of about four tons per square inch in a
hydraulic press. In one minute a perfect impression of the gelatine
relief, down to the smallest detail, is formed in the soft metal; and,
strangely enough, the delicate sculpture which the light has executed on
the gelatine is not in the least injured, but will stamp its image on an
indefinite number of metal plates in the same manner.
The reader will understand that the impressed plate of metal now bears a
hollow sculpture representing the image of the original object from
which the negative photograph was taken, the darkest shades of the
object being represented by the deepest depressions in the plate, while
the highest lights are represented by portions of the metal at the
level, or nearly so, of the surface of the plate. From this plate the
prints on paper are obtained as follows: The plate is placed
horizontally, with its impressed face upwards, and a quantity of a
certain kind of ink is placed upon it. The composition of this ink, if
ink it may be termed, is one of the ingenious parts of this elegant
process. It is made of gelatine, coloured with some suitable transparent
or semi-transparent pigments, and it is poured on the plate in a warm
and fluid state, and in quantity more than sufficient to fill all the
hollows. A sheet of paper is placed over the plate, and a moderate
pressure is applied, when the excess of ink is squeezed out and escapes.
That which remains in the hollows of the plate, becoming set by cooling,
adheres to and is removed with the paper, giving in each part a force of
tint proportional to its quantity, that is, according to the depth of
the hollow in the plate. The paper is laid aside to dry, and although
the picture has at first a certain relief, yet the gelatine ink dries
down, the picture becoming so flat that no difference of the surface is
perceptible. It will be observed that this mode of printing rests upon a
distinctly new principle—namely, the production of shades and gradations
of tints by the varying quantity of the ink laid upon the different
parts of the paper. The method is in this respect identical with that by
which the water-colour painter produces his gradations; for the colour
is applied in transparent layers, and the depth of the tint produced
depends upon the mass of the pigment laid on, and is greater or less
according as the white of the paper is more or less visible through the
film of colouring matter. The gradations of tint in wood and steel
engraving and in lithographs are dependent upon quite another
principle—namely, the varying distribution of spots, patches, or lines
in black ink of uniform intensity. The Woodbury print has all the detail
and clearness of the photograph, together with a certain softness,
produced by the transparency of the colouring matter, not found in the
ordinary photographic print. The method admits of any desired tint being
given to the prints, and these are perfectly unchangeable by light. Thus
the result is a print which secures every good quality of a photograph
without any of the unpleasant ones, such as hardness, harsh tints,
opacity, fugaciousness. The prints may be taken on plates of glass, and
they then form beautiful transparencies. Such prints constitute most
admirable slides for the magic lantern, since the semi-transparent
colouring matter, and the soft gradations, produce charming effects.
Another ingenious invention of Mr. Woodbury’s provides a means of making
the sunbeam engrave a mezzotint copper plate from a photograph. The
action of light on bichromated gelatine is here again taken advantage
of. A film is prepared similar to that used in the above-described
Woodbury process proper, but the gelatine is mixed with some powdered or
granular material, so that it may give rise to a granulated texture in
the resulting plate. This film is treated exactly in the same way as
before with regard to exposing, washing with warm water, drying, &c. The
product is a very thin sheet, having a mezzotint-like surface, with more
or less grain according to the action of the light. The white parts are
perfectly freed from the granular matter by the solution of the
gelatine, while in the darkest parts there is the greatest accumulation.
The dry film in this condition is pressed into soft metal, and by a
double process of electrotyping and subsequent facing with steel, a
plate is obtained fit for printing at the copper-plate press. The firm
of Messrs. Goupil and Co., of Paris, extensively employ this process for
the preparation of the illustrations in that elegant publication, “The
Portfolio.” Another method of photographic engraving lately projected by
Mr. Woodbury is the following: a plate of steel is covered with a layer
of gelatine, mixed with a certain proportion of gum and glucose, and
dried in a dark room. This is exposed to the action of light under a
transparent photograph on glass. When afterwards this gelatine layer is
breathed upon, the moisture attaches itself to the portions which have
not been acted on by the light, and these become more or less sticky.
Sand or emery sifted to three different degrees of fineness is then
sprinkled over the plate, beginning with the coarsest, which attaches
itself to the most sticky parts. The less sticky parts are incapable of
retaining these larger particles; while the finest sand, which is
sprinkled on last, is held by parts of the plate that are even very
slightly sticky; but the places where the light has been intense are
dry, and none of the sand adheres. The gelatine layer is then completely
dried, and the plate, being covered with another of soft metal, is
placed in a press, by which a granular impression is produced on the
soft metal, and this may then be copied in copper by the electrotype
process. The larger particles of sand produce deeper depressions in the
plate, and thus a gradation of tint is obtained.
Amongst other applications of the gelatine relief devised by Mr.
Woodbury is that of producing a watermark in paper. A very delicate
relief is firmly attached to a plate of steel or zinc, and when paper is
rolled in contact with these plates, it receives an impression of the
design, all the delicate half-tints being represented in the slight
opacity of the paper. Mr. Woodbury is at present engaged in perfecting a
method for wedding his own process to that of chromo-lithography, by
first printing the different tints on the paper, and then transferring
the Woodbury prints to the top of the colours. The transparency of the
gelatine and ink is such that the most brilliant effects are attainable
in this way.
Bichromated gelatine is also the agent employed in _photolithography_,
the image of a negative photograph being thus rendered insoluble in a
layer of gelatine spread on the stone, which is acted on by acids, &c.,
in the usual way, after the soluble portions have been removed by water.
As there are also methods of using the lithographic process with plates
of zinc instead of stones, so there are processes of impressing the
image photographically upon the zinc. Of the general nature of the
processes of _zincography_, _photolithography_, and _photozincography_
the reader will now probably be able to form some idea, but the details
need not here be described. The last two, and some other processes for
printing photographic effects mechanically, all labour under the defect
of imperfectly rendering the _half-tints_ of a picture. This remark does
not apply to the Woodbury process. The photo-lithographic process gives
marvellous results in cases where no gradations are required. Thus a
whole page of the _Times_ newspaper may be lithographed in a space not
exceeding half of this page, and although the characters may be
indistinguishable to the naked eye, a lens will show them perfectly.
Similarly, we may obtain within the compass of an octavo page a
photo-lithograph of one of Hogarth’s large engravings, which will show
every touch of the original artist’s _burin_.
There is reason to hope that the time is not far distant when all our
tedious mechanical methods of reproducing drawings by wood or steel
engravings will be superseded by processes which will give us absolute
facsimiles of every touch of the artist’s pencil; and when some process,
giving all the delicacy and truthfulness of Mr. Woodbury’s prints, will
supply us with faithful transcripts of nature for book illustration at a
cost not exceeding that of the ordinary methods. So far as relates to
one style of drawing, these requirements appear to be nearly realized in
the process termed the _graphotype_, which reproduces mechanically, in
the form of a metal plate with all the lines in relief, a design which
the artist has etched on a flat surface. This is effected in the
following manner: Chalk is powdered very finely, and sifted through wire
gauze having very narrow meshes. A quantity of this is spread upon a
smooth plate of metal, and subjected to an intense pressure by means of
an hydraulic press. The particles of the chalk cohere into a mass,
having sufficient firmness to admit of its surface being drawn upon in
the same manner as a block of boxwood. The drawing is effected with an
ink composed of lampblack and glue, a finely-pointed camel’s-hair brush
being employed; but the shades must be produced by lines and strokes as
in wood engraving. When the ink is quite dry, the surface is rubbed with
a fitch brush or with velvet; and by this brushing the particles of
chalk not protected by the inked strokes are loosened and carried off.
In a short time the chalk between the strokes becomes quite hollowed
out; and when a depth of about one-eighth of an inch has been attained,
every line remains standing in relief exactly as in an engraved wood
block. A strong solution of silicate of potash is then poured upon the
chalk, which its chemical action converts into a kind of stone without
in any way altering the forms. Although this artificial stone is quite
hard, so that impressions may at once be taken from it, yet it is
incapable of enduring the wear and tear of the printing-press.
Accordingly a mould is taken from it, and this is made, by some of the
processes of casting or electrotyping already described, to furnish a
metal stereotype plate.
_THE LINOTYPE MACHINE._
[Illustration:
FIG. 313_a_.—_The Linotype Machine._
]
Among recent inventions in connection with printing, the _linotype
machine_ calls for special mention. In this machine a great number of
actions are combined and co-ordinated with the utmost ingenuity, but
such mechanism does not lend itself to popular description, and we must
confine ourselves to a statement of what it effects, recommending the
reader to avail himself of some opportunity of seeing the apparatus at
work. It will not then be needful to give details of every one of the
very numerous parts, which present in the _ensemble_ a great appearance
of complication, the more so that much ingenuity has been exerted to
make the machine compact, which is a practical point of great
importance. The disposition of parts is not, therefore, that which is
calculated to show each movement clearly to the spectator, but that by
which the least space is occupied. The machine is driven by belting from
a main shaft, turned by a steam-engine, gas-engine, electro-motor, or
other regular source of power, and rotated at such a rate that the main
pulley of the machine itself (14½ inches in diameter) shall make about
60 revolutions per minute. Fig. 313_a_ shows the general aspect of the
machine and seat for the one operator required, but as we are not
undertaking a detailed and complete description of the whole mechanism,
no letters of reference are given; but the reader will be able, from the
following diagrams, to identify the more important parts, and form a
general idea of their action and purpose. In this machine great use is
made of the contrivances called _cams_, several of which may be observed
in the sketch towards the side of the machine on the left, being fixed
on and turning with its main shaft. They consist of plates, or open rims
of various forms, which move levers, etc., in any required way, and at
any required period of the revolution.
[Illustration:
FIG. 313_b_.—_A Matrix._
]
[Illustration:
FIG. 313_c_.—_Diagram of Movements._
]
[Illustration:
FIG. 313_d_.—_A Line of Matrices._
]