A variant of the wet collodion process to produce a direct-positive camera original. The collodion is coated on glass, the image underexposed and then underdeveloped, the glass backed with black lacquer or black cloth, and the whole mounted in a case or frame. Occasionally, the image was made on ruby glass.

Source: Art & Architecture Thesaurus

http://vocab.getty.edu/page/aat/300137324