[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.
CHAPTER XIV.
Magic Prints.
TO Professor Husnik (to whom collotypists are greatly indebted for so many unselfish publications), we owe the origination of this interesting method of producing “magic” prints. It is founded on the reaction well known to chemists that two substances, each colourless in itself, when mixed, enter into fresh chemical combinations and produce other substances of intense colour. Herschel and Hunt made use of many of these reactions, both to develop the invisible image, and to give to others, already visible a more agreeable colour or tone. Impressions other than those in fatty ink may be obtained from the Collotype plate—as in the process patented by Edwards—by means of colour, soluble in water, to which the addition of a small amount of gum has been made. In such cases it will not be those portions of the layer which have been rendered insoluble by the action of light which will give the impression (as it would if ink were used) but on the contrary, the unexposed parts, or those which remain soluble. On this account the resulting print will be a negative, and to obtain a positive, the printing must take place under a positive. If a Collotype plate so produced be damped with a linen rag which has been moistened with an aqueous solution of a salt readily absorbed by gelatine, and which will produce in combination with a second solution an intensely coloured compound (the use of the ink roller being entirely omitted), the impression on the printing paper will be almost or even entirely invisible until brought in contact with the second solution, when the picture will be immediately developed and rendered visible. For instance, dampen the plate with an aqueous solution of red prussiate of potash, surface dry it, print upon paper, allow to dry thoroughly, and place upon a solution of protosulphate of iron, when the picture will immediately appear of an intense blue colour. Solutions of bichromate of potassium or nitrate of silver will produce brown images; the former developed with extract of logwood, alazarine or sugar of lead, all produce different colours. Not only is this process interesting, applied in the manner already described, but it is of use in calico printing. Care must however be exercised in the selection of a suitable solution for damping the plate, as many salts would tan the gelatine layer, and such are of course utterly unsuited for the purpose. Among the mordants in common use, salts of tin, alum, iron, &c., may be mentioned as exercising a hardening influence upon gelatine, and therefore useless for moistening the layer. There remain, however, many others which produce no damaging effect upon the plate, and at the same time produce useful colours.