and crystoleum. Tinted photographs are made with dyed printing papers produced by commercial manufacturers. A single overall colour underlies the image and is most apparent in the highlights and mid-tones. From the 1870s albumen printing papers were available in pale pink or blue, and from the 1890s gelatin-silver printing-out papers in pale mauve or pink were available. There were other kinds of tinted papers as well. Over time such colouration often becomes very faded. Toning refers to a variety of methods for altering the overall colour of the photographic image itself. Compounds of gold, platinum or other metals are used in combination with variations in development time, temperature and other factors to produce a range of tones, including warm browns, purples, sepias, blues, olives, red-browns and blue-blacks. A well-known type of toning is sepia tone. Besides adding colour to a monochromatic print, toning often improves image stability and increases contrast. Retouching uses many of the same tools and techniques as hand-colouring, but with the intent of covering damage, hiding unwanted features, accentuating details, or adding missing elements in a photographic print. In a portrait retouching could be used to improve a sitter's appearance, for instance, by removing facial blemishes, and in a landscape with an overexposed sky, clouds could be painted into the image. Water-colours, inks, dyes, and chemical reducers are used with such tools as scalpels, pointed brushes, airbrushes, and retouching pencils. The crystoleum, from "crystal" + "oleum" (oil), process was yet another method of applying colour to albumen prints. The 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. The image was then coloured by hand. 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. See also Film colorization Film tinting Handschiegl colour process Photochrom(e) Photograph conservation Photo manipulation Preservation (library and archival science) Selective colour References Further reading Baldwin, G. (1991). Looking at photographs: A guide to technical terms. Malibu, Calif: J. Paul Getty Museum in association with British Museum Press, p. 7, 35, 55, 58, 74, 80-82. Jones, B. E. (1974). Encyclopedia of photography: With a new picture portfolio. New York: Arno Press,