GB857632A - Improvements in or relating to photochemical stencils - Google Patents

Abstract

857,632. Stencils; etching. DICK CO. A. B. Feb. 13, 1959 [Feb. 19, 1958], No. 5111/59. Drawings to Specification. Class 100(2) A nhotochemical stencil blank comprises a stencil base tissue having a continuous layer of an ink-impervious material comprising a proteinaceous colloid containing an agent which can react with a silver balide complex to form a dispersed silver image in the colloid laver such that subsequent treatment with an acid etch causes degradation and removal of the colloid in the imaged areas. The colloid layer is hardened and may be gelatin, alginate, casein or albumen. The agent may be ammonium sulphide or a metal sulphide such as sodium or potassium sulphide, to which may be added silver nitrate or ammonium thiosulphate, or a water-soluble thiocyanate, and is present 0À1-0À5% by weight of the coating. An example of a coating composition is 3À6% gelatin, 86À8% water, 9À0% methanol, 0À05% sodium sulphide, 0À10% ammonium thiosulphate, 0À3% resorcinol, 0À05% 2, 4, dihydroxy benyaldehyde, 0À1 % silver nitrate. To form the stencil from the blank, a photonegative containing a light-sensitive silver halide in a gelatin emulsion is exposed to an original and developed with the coated surface in contact with the stencil blank, the soluble silver halide formed of the silver halide in the unexposed portions of the negative coating transferring by diffusion into the stencil blank, where it is reduced by the reacting agent to a silver image. Alternatively, the negative coating may be provided as an overcoating on the stencil blank. Etching. The imaged blank is etched to degrade the gelatin in the imaged areas and remove the colloid, and the negative coating if present, with a liquid comprising 750 c.c. water, 120 grams copper sulphate, 150 grams citric acid and 7À5 grams potassium bromide dissolved in water to make up 1 litre, and then mixed with a 3% solution of hydrogen peroxide.