PHOTO “GLASS” PRINTING.[N]
[N] This process has never been practised in this country so far
as I am aware.—TRANS.
There remains to be mentioned another variation of the Collotype
process known under the above vague title, the distinctive
characteristic of which method of preparing the plate is that the
negative is taken upon a sheet of glass sufficiently thick to stand the
necessary pressure in printing when converted direct into a Collotype
plate. After the completion and drying of the collodion negative it is
coated upon the film side with the chromated gelatine solution, dried
in the drying-box, laid face downwards upon a black velvet surface, and
exposed through the back of the glass. The first publication of this
method of preparing plates emanated from the State Printing Works at
Berlin, and the following is an outline of the method there practised;
it is scarcely necessary to state that it is only available for line
subjects. The collodion used in the preparation of the negative should
be one capable of producing clear, hard negatives entirely free from
fog; probably some modification of the dark slide may be necessary to
enable it to receive a plate of sufficient strength to stand printing
from. The collodion negative being completed it is flooded with the
chromated gelatine as applied to Collotype plates, dried, and exposed
through the negative until the print layer has assumed a dark brown
colour; it is well washed in cold water and damped before printing by
an application of gum water containing a little ox-gall; this is done
to facilitate the production of perfectly clear impressions. The same
end may be attained by the application of dilute ammonia and glycerine.
The author has experimented for several years in the same direction
with the object of producing heliographic printing plates in copper,
for which purpose this method of producing reliefs in gelatine is well
suited. From the negative printing plate described, which is treated
in all respects as a Collotype plate, impressions may be taken upon
transfer paper, and transferred to stone or zinc. Before coating the
negative with the chromated gelatine the plate should be allowed to
dry. In consequence of printing from the back the lines will always be
slightly wider than the original, to meet which difficulty Brandt has
proposed putting the negative, when fixed and washed, but still wet,
into a weak solution of chromated gelatine, which will, if a suitable
collodion has been employed, entirely penetrate the latter. This
variation appears worthy of commendation; it is, therefore, here given
in the abstract:—“Having produced a negative in the usual manner, on
a plate which has been lightly but carefully albumenised, and having
fixed and washed the same, it is placed for five minutes in a solution
prepared as follows: Allow 45 parts of gelatine to swell in water,
and pour off the surplus, add 50 parts of distilled water, and apply
sufficient heat to completely liquify the gelatine, adding a solution
of 5·5 parts of bichromate of ammonium in 100 parts of water, and heat
again to 60° Réamur. With plates of large size the solution may be
poured over the plate, and it may be necessary to repeat this operation
several times to ensure an even and sufficient coating. Immediately
previous to the application of the chromated layer the negative should
be rapidly washed over with warm water. A sufficient quantity of the
chromated solution must be allowed to remain upon the plate, and the
latter warmed in a horizontal position over a spirit lamp. When steam
arises the plate has to be held in a perpendicular position for a
minute, restored to its horizontal state, and again warmed until the
coating of the plate is quite dry, care being taken that at no time
the plate is so heated that it becomes unpleasant when placed upon the
hand. Place the plate face downwards upon a black support, print until
the fine lines are plainly visible, when the plate may be washed and
further treated as in Collotype.”