GELATINE RELIEFS Probably more ingenuity has been displayed in devising processes for making gelatine reliefs to serve as printing surfaces than in any other form of photo-mechanical work, and yet there is not one of these processes that is in regular commercial use at the present day. These pro cesses date from the experiments of Fox Talbot and Poitevin. In general the basis of the pro cess is the preparation of a thick film of bichrom ated gelatine on plate glass, exposing it under a negative or positive when dry, and developing with warm water so that the unexposed parts wash away, leaving those portions standing that have been acted upon by light. The relief is hardened with alum or other agents, and dried. (This is the " Wash - out Gelatine Process," which see.) In some processes, however, the unaltered gelatine is not washed away, but is allowed to swell up, and thus form a matrix for casting in plaster. (See " Swelled Gelatine Process.")
The foregoing were chiefly used for line repro duction processes, but others are intended for making half-tone reliefs. Dallastype and Dallas tint, and Pretsch's photo-galvanography are processes of the kind that are described under separate headings; and Woodbury also devised a process of this nature for typographic printing. The half-tone image is either formed by printing through a screen, or by reticulating the surface of the gelatine. In Woodburytype (which see) the gelatine relief is utilised, but in an essentially different manner from the foregoing ; pigment is introduced into the film, and development is similar to that of carbon tissue. Stannotype is a variation of Woodburytype. Photo-ffiigrane (see "Flligrane") is another process depending on a gelatine relief. (See also " Acrograph," " Leimtype," " Mosstype," " Stannotype," etc.)