PRACTICE OF PHOTO-LITHOGRAPHY.
In photo-lithographic operations, quite a different set of conditions
has to be considered as compared with Collotype. An explanation is, of
course, not far to seek, as both the chemical and physical properties
of the lithographic stone and the exposed chromated gelatine layer are
so extremely different. To the better understanding of the operations,
the composition of the lithographic stone and the principles underlying
the process may be first considered.
Senefelder is a name inseparably connected with lithography, as it
was he who, after a continuous and laborious research, discovered the
remarkable properties of the Solenhofen limestone, who thoroughly
worked out the conditions under which printing with fatty inks could
be carried out, and so thorough and conclusive were whose researches
that the process as now practised may be said to be actually identical
with those of the discoverer. He was by inclination a dramatic author,
but failing to find a publisher for the productions of his brain, he
conceived the idea of publishing his own works. At a printer’s he
acquired the necessary knowledge to perform the work, but lacked
the funds to purchase press or type. He then turned his attention
to copperplate, made many unsuccessful attempts with an old one he
possessed, and when that was unfit for further experiment, extended his
operations to the pewter plates of his mother’s dining service, but
still without success.
By accident, his next experiments were made upon a stone procured from
Cellheim (Solenhofen), and which up to the present had served him as
a slab for his ink. For want of paper and ink to write a domestic
account for his mother, he casually used the surface of this stone in
lieu of paper, using as an ink a mixture of soap, wax, and soot, with
the intention of copying upon paper later on. When about to clean the
slab, the thought flashed across his mind why not etch the stone with a
dilute solution of nitric acid? The experiment was tried, and succeeded
beyond expectation. Upon this simple accident has been built up the
whole process of lithography. It was only in 1799, after a prolonged
and severe struggle, that he was granted protection for his art in
Bavaria.
The acid etches away a very thin layer of the unprotected stone,
while the writing is not attacked, but left slightly in relief.
Senefelder—first with a dabber, and later with a piece of wood covered
with cloth—inked in the writing, and so produced impressions, the
earliest of which were sheets of music. The etching of a lithographic
stone bearing a drawing or transfer in lithographic ink has a twofold
action, each of which facilitates the printing. In addition to the
slight relief already mentioned, as resulting from the application
of the acid, the latter forms upon the surface of the stone, where
attacked, nitrate of calcium, which resists and refuses the fatty ink
upon the surface of the roller, and if the surface of the stone be kept
constantly damp it becomes possible to roll up the drawing, to which
alone the ink adheres.
Lithographic stones do not consist merely of carbonate of lime, but
contain also silica, alumina, and some iron; the colour is a useful
indication of quality, those presenting a bluish grey appearance
being most highly prized, while those of a yellow colour are somewhat
inferior. The stone may be split into various thicknesses, and prepared
for use by grinding either one or both surfaces. They are placed face
to face, and the first grinding is completed with coarse sand, for the
second grinding a finer sand is employed between the two surfaces, and
this is succeeded by a grinding with pumice stone and water, the stone
being finally polished with snake stone. The so-called lithographic
chalk, with which the drawing on stone is sometimes executed, is
composed of finest lamp black (merely added for colour), soap, wax,
tallow, and shellac; during the etching, these ingredients undergo a
change, and form with the stone a chemical compound of great stability.
Since the fatty acids of the soap and tallow-namely, oleic and
margaric acid—set free by the nitric acid, combine in the nascent state
with the lime to form oleo-margarate of lime. This adheres tenaciously
to the stone, and the whole picture may, by the application of
turpentine, to all appearances be completely removed from the stone. If
the ink roller be again applied, it will be found that even the finest
details remain intact, and will reappear of their original strength,
the ink only adhering to the oleo-margarate of lime, and not to the
surface of the stone, which owing to the etching is wet with nitrate of
calcium.
This is the theory of lithographic etching and printing, to which it
has only to be added that soap and tallow must always be present in the
inks or chalks used in drawing to produce the combinations referred
to. Wax and shellac are only present as binding material. Transfer
ink, as used in photo-lithography, must be slightly thinned, and this
is best done by means of turpentine, and, if necessary, the occasional
addition of a few drops of olive oil. Should the ink be too stiff, or
be allowed to dry too much upon the transfer paper, the transfer will
probably be faulty, as the ink will only partially leave the paper. If
the ink remains in a soft condition it will be absorbed very readily
by the clean dry litho stone (not etched). Owing to the avidity with
which all grease is absorbed by the stone, it is necessary to keep it
perfectly free from all such contaminations. To keep the stone in a
clean condition while printing it must be etched, kept damp during
working, and, on completion or suspension of work, the surface should
be coated with a thick solution of gum arabic. Before resuming printing
operations, it is of course necessary to remove the gum from the
surface of the stone, which is done with a damp sponge.
As an introduction to the practical work of lithographic transferring
and printing the following description of the autographic method
will be found of interest, especially as it bears some similarity
to the more difficult process of photo-lithography, and enables
everyone possessing a lithographic press and stones to print their own
circulars, price lists, &c.