GB316364A - Process for the production of screen negatives for producing photo-mechanical printing surfaces - Google Patents

Abstract

316,364. Gill, H. A., (Amira Trust). May 14,1928. Photo-mechanical printing-surfaces; developing. -A screen negative on a transparent base from which a printing-surface is to be made, is produced by exposure through a screen and in the form of hardened silver dots with no soluble gelatin adjacent the base, and the dots of the image are then modified by intensification or reduction by ordinary photographic methods. By one method a dry plate is exposed through the base using a half-tone screen, and developed with a hardening developer, or developed and treated with a reagent to harden the gelatin in the presence of the exposed silver image, prior to a wash-out treatment to remove the unhardened gelatin. The image is not fixed with hypo, and to induce solubility of the unexposed gelatin the negative is immersed for a short time in potassium meta-bisulphite. In order to prevent the production of very high dots, the emulsion may have colouring-matter in it : alternatively the latent image before development may be treated with bromine-iodine tincture, or copper chloride, to which alcohol may be added. To prevent the residual halogen-silver in the image disappearing by a treatment with Farmer's reducing agent a very thin layer of silver iodide (with or without silver bromide or chloride) is deposited on the image by bathing it in a dilute solution of iodine in potassium iodide to which potassium bromide or chloride may be added. Any unreduced silver remaining may alternatively be blackened by developer. To use emulsion on opaque bases, (e.g. bromide paper), if the base is permeable the emulsion is squeezed on to a glass plate after or before exposure, and developed and treated on the glass plate : but if the base is impermeable, the paper is exposed, developed and the unexposed gelatin removed prior to transfer and subsequent treatment.