tin, metallic chemical element; symbol Sn [Lat. stannum]; at. no. 50; at. wt. 118.710; m.p. 231.9681°C; b.p. 2,270°C; sp. gr. 5.75 (gray), 7.3 (white); valence +2 or +4. Tin exhibits allotropy; above 13.2°C it is a lustrous, silver-white, highly crystalline metal with tetragonal structure. A brittle form with orthorhombic structure may exist above 161°C. Below 13.2°C pure tin tends to become a gray powder, a change commonly designated "tin pest" or "tin disease." Tin is very soft (only slightly harder than lead) and malleable; it can be rolled, pressed, or hammered into extremely thin sheets (tin foil). When iron or steel is dipped into molten tin, a layer of tin is deposited on the surface. A tin coating may also be applied by electroplating, which uses less tin. The tin serves to prevent rusting, since it is barely affected by moisture. The tin plate used in tin cans is an iron or steel sheet coated with tin. A tin coating is used to protect copper and other metals. Tin is a component of antifriction metal, bell metal, britannia metal, bronze, gunmetal, pewter, solder, and other alloys. Tin forms stannous compounds, in which it has valence +2, and stannic compounds, in which it has valence +4, as well as stannites, stannates, and other complex salts. Industrially useful compounds of tin include stannous chloride, important as a reducing agent, as a mordant in dyeing, and for weighting silk; stannic chloride, for the last two purposes and to stabilize perfume and color in soap; stannic oxide, for the preparation of white porcelain enamelware; and sodium stannite, a reducing agent. Stannous fluoride is added to toothpastes and water supplies to prevent tooth decay. Tin forms a number of toxic organometallic compounds that are used as fungicides, catalysts, and for other uses. Tin very rarely occurs uncombined in nature; the dioxide, which occurs as cassiterite, or tinstone, is the only ore of commercial importance. It is obtained chiefly from Bolivia, Indonesia, the Malay Peninsula, Congo (Kinshasa), and Nigeria. The tin mines of Cornwall, England, were formerly the principal source. The metal is prepared from cassiterite by heating in the reverberatory furnace. The ore from the mines is first given special treatment, and the "concentrates" thus obtained are mixed with coal in the furnaces. Tin was known and used by humans at least as early as the Bronze Age. The metal and its compounds were known and used by the alchemists. In 1673, Robert Boyle published a description of experiments on the oxidation (calcination) of tin. The metal was recognized as an element by Lavoisier.