Patent GB-552230-A (GB)
Title: Production of multi-colour photographic images
IPC: G03C7/26
Filed: 1941-09-25
Abstract: 552,230. Colour photography. TULL, A G. Sept. 25, 1941, No. 12437. [Class 98 (ii)] Multicolour images are produced in multilayer or multistrata sensitive material by forming from the developed silver or the residual silver halide in the layers or strata images of a ferrocyanide other than silver ferrocyanide, chemically toning said images to produce in some of the layers coloured metal ferrocyanide images, and either leaving the original ferrocyanide image in another layer to serve itself as a coloured image, or using it as a medium from which a coloured image is derived. The ferrocyanide images first formed may consist of the insoluble ferrocyanide of a metal such as Pb, Ni, Ba, Cd, or Hg, or of a basic dyestuff such as Auramine, Rhodamine B, or Acronal Yellow. The ferrocyanide in the lastmentioned layer may be chemically toned, e.g. converted into a lead chromate or vanadate image ; or may itself or after conversion into lead chromate serve to mordant a basic dyestuff to form the coloured image. In an example, developed silver images in three-layer material are all bleached to lead ferrocyanide in a bath containing potassium ferrocyanide, lead nitrate and nitric acid ; or, in reversal procedure, the residual silver halide in all the developed layers is first converted into an insoluble ferrocyanide as by treatment described in Specification 540,970 in a bath containing potassium ferrocyanide and thiourea, and then into lead ferrocyanide by treatment in a bath containing lead nitrate, potassium bromide, and nitric acid ; and in either case the developed silver is allowed to remain in the layers. At this stage, the gelatin of the layers may be softened or swelled as by bathing in an oxalic acid bath. The images in the two upper layers are then selectively chemically toned as described in Specifications 540,971 and 541,355 by treatment in a mixed salt bath containing e.g. a ferric salt, a uranium salt, and nitric acid, which produces a Prussian blue image and uranium ferrocynide image in the respective layers. The lead ferrocyanide image in the bottom layer may be toned separately to lead chromate in a bath of potassium chromate and nitric acid ; or the mixed toning bath of ferric and uranium salts may contain a proportion of chromate or bichromate so that all three layers are selectively toned simultaneously. The silver and silver salts are finally removed as with Farmer's Reducer. The uranium ferrocyanide image in the middle layer may be replaced by a dye-toned image as described in Specification 540,979. The lead ferrocyanide image in the bottom layer, or the lead chromate image produced from it may alternatively be used to mordant a yellow basic dyestuff. In a modification in which nickel ferrocyanide original images are used, the ferrocyanide in the bottom layer may be toned to magenta by dimethylglyoxime, the uranium ferrocyanide in the middle layer being converted in this case to a yellow dye image as described in Specification 540,979. If the dimethylglyoxime decolourizes the images in the two upper layers, the colours may be restored in a weakly acid solution of a soluble ferrocyanide. In a further example, silver ferrocyanide images are first formed in all the layers, and then converted into an insoluble ferrocyanide of a dyestuff as by treatment in a bath containing Acronal Yellow TC180, potassium bromide, and sulphuric acid. The two upper layers are then chemically toned to Prussian blue and uranium ferrocyanide as before, leaving the dyestuff-ferrocyanide image in the bottom layer unaffected. The invention may be applied to conventional threelayer material which may have a stripping layer between the bottom emulsion layer and the support as described in Specification 540,977. It is stated that, as applied to negative processing technique, the processing may be carried out so as to yield a negative in false colours, from which a natural colour positive may be produced. Specification 525,549 also is referred to.