Tin Pest
Tin pest (tin disease, tin blight, tin plaque or tin leprosy) is a non-scientific term describing the allotropic transformation of white, metallic tin to grey non-metallic tin. The effected tin decomposes, disintegrates when touched, expands in volume, and looses specific weight. Tin pest is slow to initiate, but cold and humidity aids it, and the presence of tin pest will catalyse the transformation. The photos show Games Workshop miniatures strikken by tin pest.
Silvery tin is stable at temperatures between 16 and 181 degrees celsius. At 13.2 degrees celsius and below, silvery tin transforms to brittle grey tin, which decomposes to powder. Since the molar volume of grey tin is 30 percent greater than that of silvery tin, the material expands, cracks and disintegrates. The swelling will crack any coat of paint which may have been applied to the miniature.
Infected tin can be cured by remelting. If tin pest appears on works of art, it is recommended to boil the object in soft water with a pinch of soda, and clean it with a soft humid Chamois leather cloth dipped in polishing chalk. Tin pest is probably the reason why few antique tin objects have been preserved for posterity. And, tin pest may be a recurring problem in our times, as the use of lead in alloys is banned. Alloying with antimony or bismuth helps avoid tin pest, whereas alloying with zinc and aluminium makes tin more susceptible to decomposition. See plastic corrosion for similar damage to soft plastic miniatures.
Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909