The crystoleum, from "crystal" + "oleum" (oil), process was a method of applying colour to an albumen print, popular from c. 1880 – c. 1910.
[1] An albumen print was pasted face down to the inside of a concave piece of glass.
Once the adhesive (usually starch paste or gelatin) was dry, the paper backing of the print was rubbed away, leaving only the transparent emulsion on the glass.
Another piece of glass was added to the back and this could also be coloured by hand.
Both pieces of glass were bound together creating a detailed, albeit fragile, image.