COLLOTYPE IN NATURAL COLOURS.
A short chapter may be devoted to the various suggested processes for
producing Collotype in natural colours, with which are associated the
names of Vidal, Albert, and Obernetter. To give a complete _resume_
is altogether impossible, as up to the present time the inventors
have only published their method of procedure with very considerable
reservations, and notwithstanding the fact that a great deal has been
written and published in current literature on the subject, little is
actually known. The methods of Vidal and Albert are essentially alike,
both being founded upon an opti-chemical basis. By careful registration
and printing from three Collotype plates, representing fragments of
the same subject upon one sheet of paper, and making use of inks
corresponding with the three primary colours, a total effect should be
produced which ought, theoretically, to resemble the coloured original.
In preparing the three negatives for reproducing the three colour
plates a special mode of procedure must be adopted. Each of the three
negatives must give a resulting Collotype plate capable of retaining
varying amounts of ink when rolled up, according to the predomination
in the original of one or the other of the primary colours—red, blue,
or yellow.[M] This is effected in the production of the negative for
the red plate, by making the exposure through a green glass screen,
while the negative intended to produce the plate from which the yellow
is to be printed is exposed through a screen of violet.
[M] Pictures produced in the three primary colours are extremely
crude. Dr. Vogel has proposed a large number of negatives,
using for their production sensitisers corresponding with
every region of the spectrum. For example—Napthol blue for
red, cyanin for orange, rozin for yellow, and fluorescein for
bluish green, the ordinary sensitiveness of the plate being
sufficient for blue and violet, the latter, however, being
exposed through a yellow screen. The images thus obtained are
printed from lithographic stones or Collotype plates, each
of which is printed in a colour complimentary to that part
of the spectrum to which the particular plate was sensitive.
The greater the number of separate images produced in this
way the more complete will be the reproduction of the various
shades in the original, and the more pleasing the resulting
print.
The green screen allowing to pass so few chemically active rays the
exposure has to be prolonged to such an extent that gelatine bromide
dry plates stained with eosine must of necessity be used.
Obernetter’s method is entirely different, depending more upon
mechanical assistance of a non-photographic character in the production
of the unlimited number of negatives he uses for producing his
printing surfaces, and the results more closely resemble those of
chromo-lithography. What particulars Obernetter himself has from
time to time communicated are here given. He first produces, by the
dusting-on process, as many copies of the original negatives as
there are colours to be reproduced. The negative intended to produce
the plate from which the blue will be printed is obtained by a long
exposure and only slight dusting of the reproduced negative. That
intended to reproduce the plate for the non-actinic colours—yellow or
brown—should be correctly exposed and dusted in; while that intended
for the red should be subjected to a shorter exposure and more vigorous
dusting. By retouching, the densities of the various negatives may
be modified by strengthening them in parts, other portions are
stopped out, and the negatives so manipulated that they will produce
plates capable of giving impressions—when printed in their proper
colours—giving a facsimile reproduction of the original work. Much will
depend upon the artistic qualifications of the operator, who may, by
extending the number of plates, finally produce copies of considerable
artistic value.
Hösch, of Munich, has patented a process in which the same result is
attained in a somewhat different manner. A negative is produced, and
from that a Collotype plate from which a number of impressions are
taken equal to the number of printing desired or necessary in the
finished chromo-collotype picture. Each print has certain portions
painted over in a neutral tint, such portions depending upon the colour
of the ink in which it is intended to print the corresponding plate.
Other portions are stopped out white in the print, or black in the
negative, and from the prints so treated a set of negatives are taken;
these are utilised for the production of a set of Collotype plates,
which, printed from in suitable colours, yield impressions of great
excellence.