PHOTO-LITHOGRAPHY Under this term is included a large number of processes which may be classified into two leading divisions—namely, transfer processes and direct processes, both of which may be sub divided into line and half-tone.
The transfer processes are all based on the principle that light acting on a bichromatised colloid, such as gelatine, will render the lines or other elements of the image capable of attract ing greasy ink, whilst the other parts will repel it. The simplest method is to coat a good tough paper, such as bankpost, with gelatine, and after drying immerse it in a bath of potassium bichrom ate (usually about 5 per cent. strength). When dry, this sensitised paper is exposed under a line negative for a sufficient time. The paper is then thinly coated with transfer ink applied with a roller. On immersion in water, and by gentle rubbing with a tuft of cotton-wool, the surplus ink comes off, leaving the image in lines of ink. The print is dried, and is then ready for trans ferring to stone or metal. For half-tone work the paper may be exposed under a negative made through a ruled screen, or the half-tone may be formed by means of a reticulated grain, as in the papyrotint process.
In the direct process the stone or metal (usually zinc) is prepared with a sensitised coat ing of bichromatised glue, gelatine, or albumen, or with bitumen, exposed under either a line or half-tone negative, inked over if a bichromatised film, and developed with water ; if a bitumen film it need not be inked, and it is developed with turpentine. The bitumen attracts the greasy ink so that the image can be rolled up for printing. To obtain grained half-tone images, various ingredients are added to the colloid coating, as in the papyrotint or Pretsch processes ; or, in the case of bitumen, the image may be made to reticulate by dissolving the bitumen in a mixture of ether and alcohol.
In several processes of photo-lithography the image is prepared on a glass or metal plate similar to collotype, and transfers are pulled therefrom for re-transfer to stone or metal.