Patent GB-690997-A (GB)
Title: Process and photographic material for the direct production of positive photographic images
IPC: G03C1/485
Filed: 1949-10-21
Abstract: 690,997. Photographic process and material for direct positives, colour photography, photomechanical printing. FARBENFABRIKEN BAYER. Oct. 21, 1949, [July 4, 1949], No. 27154/49. Class 98 (ii). A material comprising two light-sensitive silver salts, one of higher light sensitivity than the other, is exposed to an object to affect only the higher sensitive salt, and treated with a developer containing a solvent agent of higher dissolving power for the lower than the higher sensitive salt, the developer containing no salts whose anions are the same as those of the higher light-sensitive salt or containing such salts in such a small concentration that development of the lower sensitive salt is not inhibited, whereby the light affected higher sensitive salt is developed to a negative, and the lower sensitive salt is developed at non-light-affected parts, to form a positive. Silver bromide is preferred as higher light-sensitive salt and silver chloride as lower, in this case advantage being taken of the property of silver chloride of blackening on treatment with a developer containing no bromide without exposure; the blackening being inhibited at exposed parts by soluble bromide released from the developed silver bromide. Use of other silver salts such as oxalate, stearate and ferrocyanide is suggested. The solvent agent in the developer may be sodium sulphite or thiosulphate or organic amines including colour developing substances. The higher light-sensitive silver salt does not exceed ten per cent of the lower, resulting in weak negative blackening only, and this may be removed, after fixing and washing, by gentle action of a reducer to leave a positive with good whites. Development nuclei such as colloidal silver or gold and silver sulphide may be included in the material to accelerate blackening in non-exposed parts. The silver salts of different light sensitivity may be contained in the same or in separate layers. Coloured images may be made as by use of colour formers, and multicoloured images by multilayer material having layers sensitive to different regions of the spectrum. Hardening by way of silver images may be effected. Colloids other than gelatin may be used in the emulsions. The invention comprises also the light-sensitive materials themselves but only when development nuclei are present therein. Examples 1-3 describe the process using mixed chloride and bromide emulsions in a single layer which contains also colloidal silver or gold. Developers and other details are specified. In example 4 the mixed emulsion is made by adding potassium bromide or iodide to a chloride emulsion to convert a little of the silver chloride to bromide or iodide. Example 5 uses silver ferrocyanide as the salt of lower light sensitivity and silver chloride as that of the higher. Example 6 specifies a chloride-bromide emulsion mixture, the bromide being ortho-chromatically sensitized and the sodium salt of 2- stearoylamino-5-7-disulphonic acid is present to increase solubility of the silver chloride in the developer, the resulting layers being suitable for microfilms. Example 7 indicates production of a finely-grained chloride emulsion in an acetone-water solution of partly hydrolysed acetyl cellulose, precipitation of this with water, washing to remove soluble salts, re-solution in acetone-water, addition of colloidal silver and then a high-sensitive silver bromide emulsion prepared with gelatin but diluted with acetone, casting of this mixture on to paper or film, exposure and development. Example 8, directed to production of a printing relief, indicates coating of a silver chloride emulsion containing silver sulphide as nuclei and mixed with a higher-sensitive silver bromide emulsion, on to a support, exposure, treatment with a metol developer containing sodium sulphite, and then after fixing and washing with a bichromate, bromide and copper sulphate mixture in water solution to produce the printing relief. Example 9 describes exposure and colour development of a multilayer material having, on paper, glass or film, red, green and blue sensitive mixed silver bromide and silver chloride emulsions containing colloidal silver nuclei and subtractive colour formers, the material being provided with the usual yellow filter.