CHRYSOTYPE An obsolete process analogous to the blue print process, invented by Sir John Herschel in 1842 ; known also as " chripotype." It can be best described in the inventor's own words : " Paper is washed with a moderately strong solution of ammonia-citrate of iron, and dried. The strength of the solution should be such as to dry into a good yellow colour and not at all brown. In this state it is ready to receive a photographic image, which may be impressed on it either from nature in the camera or from an engraving (or positive) in a printing frame in sunlight. The image so impressed, however, is very faint and sometimes hardly perceptible. The moment it is removed from the frame or the camera, it must be washed over with a neutral solution of gold of such strength as to have about the colour of sherry wine. Instantly the picture appears, not, indeed, at once of its full intensity, but darkening with great rapidity up to a certain point, depending upon exposure and strength of solutions. At this point nothing
can surpass the sharpness and perfection of detail of the resulting picture. To arrest this process and to fix the picture (so far at least as the further agency of light is concerned) it is to be thrown into water very slightly acidified with sulphuric acid, and well soaked, dried, washed with hydrobromate of potash, rinsed, and dried again." Herschel later recommended developing with a solution of nitrate of silver instead of gold, and fixing in sodium hyposulphite ; and other workers advocated the use of a solution of potassium iodide after developing with gold. The finished chrysotype pictures were of a purplish colour.