Patent GB-547722-A (GB)
Title: Production of coloured photographic images
IPC: G03C5/48
Filed: 1941-04-04
Abstract: 547,722. Colour photography and kinematography. TULL, A. G. April 4, 1941, Nos. 13088/41 [divided out of 540,979] and 13201/41. [Class 98 (ii)] A method of producing a reversed coloured image in a silver halide emulsion layer after developing a silver image therein consists in treating the layer in a bath containing thiourea or a derivative thereof such as thiosinamine to form from the residual halide a complex image, dyeing with an acid dye which is precipitated by the complex image, and washing out the unaffected dye. The image may be a separate monochrome, or a component of a multicolour picture. The dye should be one that does not stain gelatin, and the dye bath may contain a small proportion of the complex-forming reagent. To prevent formation of silver sulphide, the dye bath and wash-water may be slightly acid, or a special acid rinse bath may be used. The residual halide may be chemically toned or bleached (e.g. to ferrocyanide of zinc or cadmium or lead) before the thiourea and dye treatment; and such toned or bleached images are preferably treated before fixing. To simplify procedure the various baths used may be combined. The strength of the thiourea bath may normally be from 0.1 to 2 per cent, but may be up to 5 per cent with a time of treatment of 5 minutes at room temperature ; and the upper limit may be raised as far as 10 per cent if a soluble salt, e.g. potassium ferrocyanide or a halide is also present in the bath. The transparency of the resulting dye-lake image increases with the concentration of the thiourea, and depends also on the nature of any soluble salt used. The precipitated dye-radical can be fixed with an agent such as barium chloride, or an organic base such as a quaternary ammonium compound, or with a basic dye of the same colour, whereafter the silver salt complex may be removed by hypo, or with Farmer's Reducer if the original silver image is also to be removed. In reversal colour processing of a multilayer material, some or all of the layers, after development of silver images therein, may be treated with a bath of thiourea and a ferrocyanide; whereafter some of the layers may be chemically toned with metal salts, e.g. as described in Specifications 540,970 and 540,971, and another layer may be treated with an acid dye bath, preferably containing thiourea, to precipitate a dye image therein. The method may be applied to images made on material having two superimposed emulsion layers on one side and one emulsion layer on the other side of a transparent support, the two sides being separately treated either by flotation methods, or by the aid of a protective varnish layer applied to one side, or, in the case of single pictures, by the use of a developing dish comprising a double hinged frame having packing rings round its abutting edges to grip the film to be processed so that this, when gripped, forms the base of a dish whichever side is uppermost. In the first step, all the layers are treated in a bath of thiourea of ferrocyanide to form in each layer a complex silver ferrocyanide image ; thereafter the twolayer side is processed by selective chemical toning with ferric and uranium salts as described in Specification 540,971, and the single-layer side is bathed in an acid dye bath to form a dye-lake image. This dye image may be fixed as described above, and silver and silver salts removed as with Farmer's Reducer or the silver may be removed from all three layers as with acid permanganate or bichromate, and silver salts removed from the two-layer side only as by acid hypo. A particular procedure making use of a varnish layer on the single-layer side is also described. The chemical toning step of Specification 540,971 may be modified as described in Specification 540,979 to convert the uranium-toned image into a dyetone image. The invention may be applied to the production of colour component prints made separately on stripping material to permit them to be superimposed by transfer. Specifications 335,930, [Class 98 (ii)], 466,710 and 540,975 also are referred to.