II.

Properties and Occurrence

Tin is highly ductile and malleable at a temperature of 100° C (212° F). It is attacked by strong acids. Ordinarily a silver-white metal, at temperatures below 13° C (55° F) it often changes into an allotropic (distinctly different) form known as grey tin, which is an amorphous, greyish powder with a relative density of 5.75. Because of the mottled appearance of tin objects undergoing this decomposition, the action is commonly referred to as tin disease or tin pest. Ordinary bar tin, when bent, issues a crackling sound called tin cry, caused by the friction of the tin crystals.

Tin ranks 49th in abundance of the elements in the Earth's crust. Ordinary tin melts at about 232° C (about 450° F), boils at about 2260° C (about 4100° F), and has a relative density of 7.28. The atomic weight of tin is 118.69.

The principal ore of tin is the mineral cassiterite (or tinstone), SnO₂, found abundantly in Cornwall, England and in Germany, the Malay Peninsula, Bolivia, Brazil, and Australia. In the extraction of tin, the ore is first ground and washed to remove all impurities and then roasted to oxidize the sulphides of iron and copper. After a second washing, the ore is reduced by carbon in a reverberatory furnace; the molten tin that collects on the bottom is drawn off and moulded into blocks known as block tin. In this form, the tin is resmelted at low temperatures; the impurities form an insoluble mass. Tin may also be purified by electrolysis.