GB1306801A - Production of photographic silver images - Google Patents

Abstract

1306801 Photographic silver halide material AGFA-GEVAERT AG 10 March 1970 [2 April 1969] 17161/69 Heading G2C A photographic material containing a silver halide emulsion layer comprises composite silver halide grains which contain internal sensitivity specks which have been produced by chemically ripening the core grains, and precipitating on the cores a shell of unripened silver halide, and in which the shells have been chemically fogged, the degree of chemical fogging of the shells being such that if the material were imagewise exposed and developed using an exposure E n , which is less than a certain exposure E max , which is the exposure necessary to give a maximum density then a negative image would be obtained, whereas if the exposure E p which is used were greater than E max a direct-positive image would be obtained, the development being carried out under the same conditions. The developer may be a lith-developer i.e. one which contains an addition compound of an aliphatic aldehyde or ketone with a bisulphite and hydroquinone as the sole developing agent. The cores and shells of the grains may comprise silver bromide. Chemical ripening of the cores may be effected with a compound of a noble metal (i.e. Au, Os, Ir, Pt, Ru, Rh or Pd) e.g. gold (III) chloride, a sulphur compound e.g. ammonium thiocyanate, a reducing agent or mixtures thereof. The pAg may be kept constant during the precipitation of the silver bromide cores. Chemical fogging of the shells may be effected with a reducing agent e.g. formamidine sulphinic acid. The degree of chemical fogging of the shells and the particular developer used determine whether or not negative and direct-positive images can be obtained from a particular material.