Preserving Carmine of Indigo.

Scientific American 18, 31.10.1863

Carmine of indigo (sulphindigotate of soda), is generally found (in commerce) in a state of semiliquid paste, because in the dry condition it becomes after a time covered with an extremely abundant white efflorescence. This efflorescence is due to the sulphate of soda with which the paste is necessarily impregnated, since the sulphindigotate of soda is insoluble only in a concentrated solution of alkaline salts, and dissolves on the contrary, very readily in pure water. The carmine cannot be freed from the alkaline sulphate of washing. Its pasty condition, more or less watery, presents, however, some serious inconceniences; both as respects its portability, and because this state faclitates the admixture of products of inferior coloring power. If from three to four per cent. of glycerine is added to the carmine of indigo paste, it can be dried and preserved for an indefinite time without becoming covered with saline efflorescence, the glycerine not in the least injuring the brilliancy or purity of the color.

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