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Printing with Cobalt Salts

image, salt and obtained

COBALT SALTS, PRINTING WITH Cobalt belongs to the same group of metals as iron and manganese, and, like these, many of its salts are sensitive to light. Although no practical process has so far been founded on this fact, it is as well to record briefly the researches of A. and L. Lumiere on the subject. The most promising salt is obtained by dis solving cobaltic oxide, to saturation in oxalic acid solution, or by precipitating cobaltic oxyhydrate from a cobaltous salt solution by means of sodium peroxide and dissolving the precipitate, after careful washing, to saturation in oxalic acid solution, the cobaltic salt being kept all the time in excess. This operation must be performed in the cold, and takes some hours. A green solution is obtained which can be used to sensitise gelatinised paper, and, after drying, on exposure to light under an ordinary negative, a pale rose-coloured image of a cobaltous salt is obtained. The action is extremely rapid, taking but 6 fraction of the time necessary to print under similar conditions with a silver salt. The print, when ready, should next be immersed in a 5 per cent. solution of potassium ferricyanide

and washed. The image thus obtained is a pale rose colour and not very intense, consisting of cobalt ferrocyanide. This may be toned with an alkaline sulphide, which produces dark brown cobalt sulphide. By treatment with an iron salt, a blue image is obtained ; a nickel salt gives a red image. Attempts to develop the cobaltous image with organic compounds (see "Manganese, Printing with") were not satisfactory, in all cases it being found much more difficult, and the only substances proved to be of any value were hmatoxyline, which gave a violet blue image that was changed to reddish by hydro chloric acid, and benzidine, toluidine, and their hydrochlorate salts. These produced on the places not affected by light, so that they would give a negative print from a negative, an intense blue image, which was turned brown by ammonia and pale yellow by hydrochloric acid.

Further researches with the citrate, stannate, nitrite, tartrate, gallate, and sulphocyanide of cobalt have been made, but the results were still less promising.