Patent GB-469133-A (GB)
Title: Improvements in or relating to the toning of photographic prints
IPC: G03C5/46
Filed: 1936-02-05
Abstract: 469,133. Photographic toning processes. SNYDER, Inc., F. H. Feb. 5, 1936, Nos. 3556 and 3557. Convention dates, Feb. 26, 1935 and Jan. 18, 1936. [Class 98 (ii)] A photographic silver image is converted into a metallic salt image formed at least in part of a metal ferrocyanide by immersion in an acidified bath including an acid stronger than ferricyanic acid, a soluble nickel or cadmium salt, an alkali-metal ferricyanide, and a reagent to prevent the precipitation of a ferrocyanide, and toned to a coloured image by a reagent forming a coloured pigment with the metal ferrocyanide. The reagent preventing the precipitation of a ferricyanide may be a soluble citrate tartrate, malate, succinate, oxalate or benzoate. In one example, the photographic silver print is immersed in a bath containing nickel chloride, potassium citrate, citric acid, formaldehyde, and potassium ferricyanide. After the image is bleached the print is washed, and immersed in a solution of dimethylglyoxime dissolved in a mixture of monoethanolamine, triethanolamine, and water. The silver salt is removed in a bath containing sodium thiosulphate or ammonia or both, or potassium, ammonium or other thiocyanate may be used. Alpha-benzildioxime, methylpropylglyoxime, or alpha-furildioxime may replace dimethylglyoxime. The dioximes may be dissolved as above, or in monoethanolamine, triethanolamine, or either mixed with diethanolamine, in tertiary butylamine, alcohol or acetone. In a second example, the first bath contains sodium thiosulphate and the nickel salt may be replaced by a cadmium salt. In a third example, a cadmium salt and sodium sulphide or ammonium polysulphide are used, sodium bicarbonate or bisulphate being used in the sulphide bath to act as a buffer. A yellow image is produced by oxidizing a nickel ferrocyanide with chlorine. In other examples the nickel or cadmium bath is followed by treatment with a ferric or ferrous salt to produce a green colour, which may be converted to a blue colour by treatment with alkali-metal thiosulphate. A blue-green image is produced by partial conversion of a nickel ferrocyanide image to iron ferrocyanide. Flesh colour is produced by treatment with nickel and alpha-benzildioxime baths. Brown is produced by nickel and sulphide baths, violet by a nickel bath followed by successive treatments with ferric and dioxime baths; orange by a combined nickel-cadmium bath followed by successive treatments with dioxime and sulphide baths, yellow-green by a combined nickel-cadmium bath followed by successive treatments with ferric and sulphide baths, and maroon by a combined nickel-cobalt bath, followed by a dioxime bath. Toned prints may be treated locally by a brush with a colour modifying agent such as an acidified solution of a ferric salt or chlorine water. Positive prints from bipack negatives or from three colour-component negatives may be toned in minus colours and superimposed on gelatin-coated white paper. Positive prints from colour-component negatives obtained from a multicolour screen negative by three separate enlargements using colour filters are similarly treated. Triethanolamine may be added to a sodium sulphide toning bath to minimize odour.