Darkening. Toning of a numismatic or medallic item in which dark areas occur on light colored metals. Tin, tin alloys – particularly white metal – are most susceptible to such dark toning or clouding. Darkening is a first stage, discoloration, of tin disease. Should it continue it will result in tin pest, brought on, for the most part, by cold temperatures. Tin pest is a corrosion of the surface metal by sustained exposure to temperatures below 18° centigrade or 64° Fahrenheit. The cold temperature changes unstable betta-tin to a powdery gray form of alpha-tin. Darkening should not be confused with haze, an unnatural toning (as from chemical cleaners or such). Before darkening occurs a light metal alloy in new and shinny state is said to be bright. See tin, white metal.
excerpted with permission from
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor