THE PERMANENCY OF PIGMENTS.
Considering the nature of the image on which the photographic colourist has to work, it is hardly necessary to point out the wisdom of devoting some attention to the permanency of his colours, especially as regards their chemical composition. In some cases certain colours when mixed to gether set up chemical reactions, and even if they do not injuriously affect the photo graphic base of the picture, are almost certain to eventually discolour or blacken it. For example, if two lists be made, one containing all those pigments having a copper base, and the other the sulphides of cadmium, then any colour included in the one list, if mixed with any pigment in the second, will set up some kind of chemi cal decomposition, and can hardly be re garded as permanent, or suitable for the purpose under consideration. In the one group may be placed the cadmium yellows and orange, orient and aurora yellow ; in the opposing list emerald and malachite greens and verdigris. Vermilion and
Kings' yellow are metallic sulphides ; Jaune Brillant, neutral orange, and Naples yellow contain cadmium. Neither of these colours, therefore, can be regarded as safe to mix with any of those in the second list. Chrome orange, red, and yellow, citron and primrose yellows, must not be mixed with Prussian blue or pigments containing it. Organic pigments, such as indigo, In dian yellow, and the lakes, ought not pro perly to be used with the chromates, nor with aureolin or Mars yellow, the latter being rich in oxide of iron. Chinese and flake white should be of good quality, or they are liable to discolour and injuriously affect pigments mixed with them. Indeed, it is practically imperative that all colours used for this description of work should be obtained from a reliable source.