Do Not Expose These Plates to the Light for a Longer Time Than Is Necessary.

The pigments used in the preparation of these Plates are the most durable known, those which have been proven unstable having been, as far as possible, discarded. The latter include carmine and other cochineal lakes, colors of vegetable origin (as gamboge, violet carmine, indigo, etc.), and most of the aniline or coal tar dyes, though among the last are a considerable number which are really more permanent than several colors habitually used by artists. Certain colors in this work could not, however, possibly be reproduced except by the employment of pigments which are more or less sensitive to prolonged exposure to light, and hence this caution not to expose the plates unnecessarily.

(See Church: "The Chemistry of Paints and Painting," third edition, pages 257-263.)