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
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