COLLOTYPE (Fr., Phototypie ; Ger., Licht druch) A process known also as " phototype," and, in slight variations, as " Albertype," " Arto type," etc. It is based on the principle that if a film of bichromated gelatine is exposed to light under a negative, and the unaltered bichromate is washed out, the film will have a similar property to that possessed by a litho graphic stone of attracting ink in some parts and absorbing water in others, the water repelling the ink. It differs essentially, however, from lithographic work in the fact that the attraction for ink and water in the different parts is pro portionate to the action of the light, so that the strength of the ink image varies in proportion to the light and shade of the picture. The dis covery of this property was made by Pox Talbot in 1853, and his researches were followed up by A. Poitevin, of Paris, from 1856 onward. The first practical collotype process was intro duced by Tessie du Motay and Ch. R. Marechal, of Metz, in 1865 ; and the perfection of the present-day process of collotype is due to the labours of Josef Albert, Husnik, and Obernetter. Although the process is still largely worked, its commercial success has been much retarded of late years by the progress of half tone, photogravure, and other etching methods, and it has to a considerable extent fallen into disfavour, especially in England and America.
The general method of working the process is as follows :—A thick glass plate is ground on one side with fine emery powder, and is then placed on a levelling stand or levelling screws, and having first been coated with a suitable substratum and dried, is flowed over with a measured quantity of bichromated gelatine. When the film is set the plate is placed in a dry ing oven, which is brought up to a temperature not higher than i 3o° F. (54° C.), at which the dry
ing takes about two hours. When cool, the plate is placed with the negative in a special printing frame, pressure being applied by wedges. The plate is next washed to remove the unaltered bichromate, and allowed to dry. To prepare the image for printing, the surface is flooded with a mixture of glycerine, water, and sometimes other ingredients, allowing it to stand for thirty minutes. Then the excess is removed and the plate is rolled up with a lithographic roller charged with a special collotype ink, which is similar to lithographic ink, but stiffer. When com pletely inked, paper is laid on the plate and pressure applied in a press. An ink image re producing the tones of the original is thus obtained. Success depends on the proper form ation of a grain caused by reticulation of the gelatine during the drying, and the grain is modified by exposure according to the action of the light passing through different parts of the negative.
Many modifications of detail have been made by different workers, but the above general out line applies to all the methods, except that in some cases attempts have been made to form the image on aluminium, copper, lead, and other plates instead of glass.
Collotype in colours has been worked with success for some years in Germany, and to some extent in England. The number of negatives made varies with the number of different colours required. A collotype plate is made from each negative, and all its parts are blocked out except those required for that particular colour. The printing is then done as in chromo-lithography, the impressions of each colour being super imposed in exact register.