SUMMARY OF THE MORE IMPORTANT PRINTING PROCESSES WITH CHROMATED
GELATINE.
These processes are all based upon the change in the hygroscopic
properties of chromated gelatine brought about by the action of light,
which renders the parts exposed insoluble.
=Carbon or Pigment Printing.=—In this interesting process finely ground
colouring matter is added to the solution of chromated gelatine, and
spread upon paper; or the paper may be coated with the gelatine and
pigment, and afterwards excited by immersion in a solution or bath
of one of the bichromates. If, after drying such paper or tissue, it
be exposed under a negative in an ordinary printing frame, and then
immersed in warm water (90° to 130° F.), the water will dissolve only
such portions as may not have been affected or hardened by the action
of light through the negative, while the parts that have been exposed
to its action will be insoluble, and remain attached to the paper
support, forming a positive picture. Images produced in such a manner,
however, are devoid of half-tone, and this procedure is only suitable
for line subjects, while even for such the paper must have been very
evenly coated, or portions of the picture may be washed away. For
the production of half-tone subjects it is necessary that the picture
be developed _from the back_. This may be effected by transferring
the printed tissue, after first soaking in cold water, to a surface,
usually either of paper or glass, and developing it in contact with,
and allowing it to remain upon, such surface. This procedure is
known in carbon printing as single transfer. Unless the negative be
a reversed one—or a film printed from the reverse side—the resulting
print will be subject to the disadvantage of being reversed as regards
left and right. In cases where ordinary negatives have to be used,
and this reversal cannot be tolerated, it is overcome by the “double
transfer” method of printing, the print in such cases being developed
upon a so-called “temporary support,” and again transferred to its
“final support,” when it appears as a non-reversed image in permanent
pigment.[D]
[D] For further details of this interesting process see the “Autotype
Manual,” by J. R. Sawyer; or Liesegang’s “Carbon Process.”
=The “Powder” Process.=—Take, instead of gelatine, a solution of
gum arabic and sugar, to which is added a solution of bichromate of
potassium or ammonium. With this a glass plate is coated, dried, and
exposed under a negative, if it be desired to produce a negative; or,
if a positive be desired, the exposure must be made under a positive.
After a short exposure the plate is dusted over with a powder in an
extremely fine state of division, such as graphite if for negatives; if
for burnt-in enamels, a specially-prepared enamel, ground extremely
fine, must be used. Such powders will only adhere to the plate in
exact proportion to the action of the light during the exposure in the
printing frame, and a reversed copy of the original will be produced.
=Collotype and Glasstype.=[E]—In these processes the printing depends
upon the property possessed by the bichromated gelatine film, which has
been properly exposed under a negative, of absorbing water in varying
proportions, in exact accord with, but in inverse ratio to, the action
of the light. The application of a roller charged with fatty ink of a
suitable consistency, will deposit upon the gelatine surface a layer
of ink in exact proportion to the action of the light and in inverse
proportion to the amount of water absorbed by the film. In this manner
a picture is produced in ink which by printing is transferred to paper,
and will exhibit all—even the finest—gradations of light and shade.
[E] Glasstype, more fully described later on, has never been practised
commercially in England.
=Woodburytype.=—If an especially thick film of gelatine, containing a
small proportion of pigment, be sensitised, exposed under a negative,
and subjected to prolonged soaking in warm water, a picture exhibiting
considerable relief will be obtained, which, after drying, may, by
pressing into lead or treated by the galvanoplastic method, yield
a matrix from which casts may be produced in a coloured gelatine
solution, or ink, which is poured into the mould; on this is placed
a waterproof paper; the press, which is of special construction, is
closed, and the ink allowed to set; the paper is then removed, bearing
upon its surface the printed Woodburytype, which is hardened by alum,
and then scarcely distinguishable from a silver print.
=Stannotype= is a modification of the last-described process by the
same inventor. A positive is first produced from the original negative,
and from the positive a print is made on a thick gelatine tissue. This
is developed by long washing in warm water, the resulting relief being
a negative; this is coated with tinfoil of extreme thinness, and used
as a mould for printing from with the gelatine ink, as previously
described. This obviates the use of hydraulic pressure and special and
expensive appliances.
=Heliogravure; Photogravure.=—If, instead of printing from the Woodbury
mould in the gelatinous solution, a fatty ink is used, the process is
known under these titles. The exact methods of roughing the depths of
the plates to facilitate their retention of the ink are regarded as
secrets by the most successful operators.
=Photo-Lithography and Photo-Zincography.=—In dealing with a subject
in lines, dots, or stipple, a paper may be coated with a solution of
chromated gelatine, dried, exposed under a suitable negative, coated
with a film of fatty ink, and allowed to soak in cold or tepid water;
this, penetrating the unhardened portions of the film, causes them
readily to part with the ink, which, however, adheres tenaciously
upon the hardened portions, representing the picture. This image, when
properly cleared, may be transferred to stone or zinc, and printed
from in the lithographic manner upon a suitable press, or if the image
upon the zinc be etched into relief by the process known as Gillotage,
and mounted on a wood block of suitable height, it presents a printing
surface which may be used in conjunction with type.
=Gelatinotype.=—It has long been felt as a desideratum to make use of
the photogenic properties of the gelatino-bichromate for letterpress
printing, _i.e._, to find out a simple and satisfactory method by which
collotype plates could be transformed in letterpress printing plates.
There have been made many attempts in this direction, but hitherto
without any practical results; the difficulties seemed to be too great.
Recently, however, Professor J. Husnik, of Prague, has overcome these
difficulties, his new patented process, called by him “Leimtypie,”
that is to say, Gelatinotype, being one of the greatest practical
value, quick, cheap, and giving most satisfactory results. The process
consists in the production of high relief blocks in gelatine. Husnik
exposes a gelatino-bichromate tissue under a negative, and then
develops the relief picture in a peculiar manner.
The method consists in the application of a saturated solution of
bichromate salts for developing, a new and previously unknown property
of the chrome salts thus coming into employment. These salts, in
comparison with the acids, have the advantage not only of dissolving
all unexposed gelatine during development, but also in hardening the
already exposed parts of the picture upon the copy to be developed, the
impression received from the light being increased by contact with the
chrome salt solutions. In this way the relief can be developed a longer
time, and becomes deeper.
Another new phase is the manner of the second development. The first
development is interrupted before the fine parts have been injured, the
relief allowed to dry, the white parts (the sub-ground) covered with
printing ink diluted with turpentine, using a fine brush quite near to
the lines, and then the whole relief exposed once more to light. The
relief, on account of its having taken up much chrome salt during the
first development, is very sensitive to light, and hardens not only on
the surface, but also on the sides of the lines. After removal of the
black ink it can be developed to any depth, particularly if the larger
white parts have before been cut out with a knife.
This process has lately been further improved by the inventor, by the
use of gutta-percha films, which are placed between the gelatine film
and the zinc plate on which the gelatine relief has to be fastened.
This is effected as follows:—The zinc plate, after having been well
cleaned and rubbed over with emery paper, is coated with a gutta-percha
solution and dried. It is then heated to about 212° F., and allowed
to cool. During this cooling process the bichromated gelatine film,
before being developed, is laid on at a temperature of between 111°
and 77° F., whereby the hardening gutta-percha film secures a perfect
combination with the zinc plate. When this has been done the gelatine
film is developed in the above described manner to a relief. By this
improvement the gelatine blocks become so solid that they will yield
more than 50,000 prints in the printing machine without being injured.