Photo-Lithography
Photo-lithography is now so largely employed for the reproduction of all classes of work that a few words in explanation of it may be very desirable. The process is of a somewhat technical nature, but simplicity itself when understood! It is founded upon the fact that gelatine, by the addition of a certain chemical, is rendered insoluble on exposure to light.
A negative photograph from the original drawing in black lines having been taken by the wet or collodion plate, it is intensified to the required degree, so that the lines of the design appear perfectly clear upon a dense, opaque, black film. It is then “exposed” in the printing frame, upon paper that has been coated (in the dark) with the prepared gelatine—now extremely sensitive to light. Taken out of the printing frame in the dark room, the exposed print (which barely shows any trace of the design at this stage) is then covered with a thin film or coating of printers’ transfer ink. The lines exposed to the light have been rendered insoluble, while the white ground of the design protected by the negative is still in a soluble condition. Floated in a bath of warm water, the soluble gelatine (not acted on by the light), with its coating of ink, is washed away—the insoluble lines of the design alone remaining, coated with printers’ transfer ink. This “photo-transfer” is then ready to be put down to the lithographic stone and printed from, or it may be transferred to a polished zinc plate, and etched to the requisite depth as a block for type printing.