The vast heaps of scrap tin found about tinware works, and the quantities of refuse tin cans that form such an item in city waste, have often been made the subject of experiment to separate the tin coating from the sheet iron. Melting the scrap gives only a spongy iron, and the extraction of the tin by the action of acids or chlorine gas is too expensive, so that hundreds of tons of this material are wasted every year. Reid's experiments, however, seem to promise a cheap method of recovering both the tin and iron in a pure and useful shape. The tin scraps are placed in a furnace where the temperature and the supply of air can be carefully adjusted. This gives a roasting in free air that causes the film of tin on the iron to oxidize. The alloy of tin and iron under the film of tin is next oxidized, and then the scrap is taken from the furnace, and the coating of oxides on the iron is shaken off by simple machinery. This leaves the iron in a comparatively pure state, while the powdered oxides may be smelted with other tin ores, or, preferably, they may be submitted to the action of hot sulphuric acid, which dissolves the iron oxide, leaving the tin untouched.

The tin may then be separated from the solution of iron sulphate, and melted, while the solution may be evaporated to dryness and then placed in retorts to recover the sulphuric acid, the residue in the retorts being valuable in making paints. The waste heat from the retorts is used to assist in roasting the scrap, and in evaporating the solution of iron sulphate. Waste fruit-tins are first roasted to remove the solder that may cling to them, and are then treated by the same process.

Properties And Uses

Refined tin possesses colour and lustre resembling those of silver, but it is slowly tarnished by exposure to the air. Its hardness exceeds that of lead, and its malleability is such that it may be laminated into foil or leaves of exceeding thinness, but it cannot be drawn into wire. Under friction it exhales a peculiar odour, and communicates an unpleasant flavour to the tongue. Strips of pure tin are flexible but not elastic, and thick bars emit a creaking sound when bent. The metal melts at about 455° F. (235° C), boils at a white heat, and bears high temperatures in close vessels without volatilization or loss. Heated to redness in contact with air or oxygen, it rapidly passes into the grey protoxide, and later into the peroxide, a yellowish-white powder called "putty powder," much used for polishing. Its sp. gr. is 7.291. Pure tin is little affected by dilute acids and reagents, but is converted into a white powder by nitric acid of medium strength; its resistance to corrosion renders it valuable for plating the surfaces of other more easily destructible metals.

It is much more largely used as an accessory to other metals than alone: its importance in plating has already been mentioned, and amongst the alloys in common use it forms a principal ingredient • in bronze, gun-metal, bell-metal, Britannia metal, pewter, and solders (see Alloys).