COLLOTYPE. A photo-mechanical printing process chiefly employed for the reproduction of originals containing fine detail such as paintings of old masters, tapestry, glassware, jewellery and ancient documents, with their stains and creases.
The process was first used at Metz, about 1865, and provides the means of producing a print in "full tones" like an ordinary photograph. It is unlike other printing processes, where reticula tion has to be provided by means of grain, stipple or screen.
The method adopted in preparing the printing surface is that a glass plate is ground and prepared with a substratum of potassium silicate, which, when dry, is coated with a gelatine film sensitized with either potassium bichromate or ammonium bichromate. The plate is then dried in a stove and is in a condition to have a photo graph of the subject to be reproduced printed upon it. A reversed continuous tone negative, which has been retouched with great care, is placed in contact with the sensitized film on the gelatine plate. Both are clamped in to a frame and exposed to light. The length of the exposure differs with the nature of the subject and character of negative. The plate is washed until all trace of the "free" bichromate has been removed. The "solids," which are transparent in the negative, become hard on the film and are not affected by water, whilst the other tones are washed away in varying degrees according to the intensity of the light which has passed through the negative on to the film, and are more or less water absorbent and ink resistant.
The plate is then ready for the machine, which is similar to a lithographic printing machine, but without damping apparatus. Not infrequently, when printing monochrome subjects, two sets of inking rollers are used, one set with a body ink and the other with a tint.
Before commencing printing, the plate is treated with glycerine and water and, to keep the plate in working order, this may have to be repeated several times during the printing of an edition.
There are many variations of this process, the most common being that of printing the key of a colour subject in collotype, and adding the colours by lithography; or a number of colours may be printed by collotype and the remainder added by other processes. Another variation, encouraged by the demand for speed, is to prepare the sensitized gelatine film so that it provides an "open grain." Impressions are taken in a special ink, and trans ferred to a lithographic stone or plate, and lithographically print ed. No variation of the process, however, gives results equal to that secured by pure collotype.
Experimenters have recently gone back to the method used by the original inventors, viz., floating the gelatine film on to a metal plate, which can be printed on a modern cylinder machine, running at a speed which quadruples the output of a collotype machine. (See COLOUR PRINTING.) (J. R. R.)