The crude metal obtained by the smelting, whether in the reverberatory or in the blast furnace, has to undergo a refining process. The two chief impurities are iron and lead, the former of which may vary between 1 1/4 and 16 1/2 per cent., while the latter rauges from 1/4 to 10 per cent. The refining may be divided into 2 stages, liquation and tossing.

For the liquation, the reverberatory furnace used in the reducing operation may be employed. It is charged with nigs of the crude metal piled up in such a way as to allow the heat to pass freely through; the fire is urged moderately, and the tin is thus " sweated " out, and flows into one of the pans / (Fig. 176) already heated for its reception. The mass of ore in the furnace gradually crumbles down; as room occurs, more blocks of crude metal are charged in, until 5 or 6 tons of fine metal have collected in the pan. The residual mass is cither removed from the furnace in its crumbled state, or is fused and run into the other pan / for subsequent treatment.

The liquated metal having been accumulated in sufficient quantity in the iron pan, the process of refining is com-menced by forcing under the surface of the metal, a bundle of billets of green wood. The steam and gases evolved from the wood cause a violent ebullition, and the production of a frothy scum containing tin oxide, which is skimmed off, and set aside for further treatment, with the slag produced in the first operations. Instead of boiling with green wood, the same effect is sometimes produced by "tossing," or raising the metal in ladles, and pouring, from some height through the air, back again into the pan. When sufficiently boiled or tossed, and skimmed, which process occupies about 3 hours, the metal is allowed to stand undisturbed-for another hour, during which time it settles into 3 parts or zones, of which the top is the purest, the middle the next in quality, and the lowest the most impure. The heat of the metal is maintained while in the pan by a fire underneath. When it has stood quiet sufficiently long, the metal is carefully skimmed, and then ladled out into iron moulds, which contain about 3 cwt.

The 3 sorts are kept separate, their quality being ascertained by taking a small ladleful, stirring and skimming until sufficiently cold, and then pouring it into a stone ingot-mould, watching the appearance of the metal as it cools. If sufficiently pure to class as " grain " tin, it will remain bright and clear, full, and well rounded on the sides, until quite cold; if only sufficiently good to class as " common " tin, it will remain bright, but not so full and well rounded on the sides, until, at the instant of becoming solid, from the middle of the ingot a frosted crystalline appearance shoots out to the sides. In the third quality, the colour becomes slightly yellowish, and the appearance of the frosted crystalline markings takes place sooner, completely covering the whole of the surface. Examining the ingots when quite cold, by bending them, the remarkable crackling sound characteristic of tin is much greater in proportion as it is purer. The third quality is so impure as to be unfit for commercial purposes, and has to be again refined.

The metal is sent to market in the form of ingots of 3 cwt., 1 cwt., or 1/2 cwt.; or in strips cast in white marble moulds about 2 ft. long, 1 in. wide, and 1/2 in. thick; or it is rendered in the form known as "grain" tin, which is made by heating an ingot to the highest possible temperature short of melting, and letting it fall, or striking it heavily, when it flies into a mass of crystalline, fibrous fragments. It is said that too low a temperature in melting tin, before casting, prejudicially affects its malleability.

Argentine

This product - used for printing on cloths and paper-is a tin moss or sponge, and is obtained by the precipitation of a solution of tin by zinc. The solution, strongly acidulated at first, must be diluted until it contains 3 pints of water to 1 dr. of the tin salt. The sponge having been collected without compression in a sieve, washed with water, and dried by heat, may be brayed with water in a mortar, passed through a hair sieve, and mixed with starch paste for printing. The solution of zinc chloride may be used for soldering, etc, and the grey tin powder for tinning all metals but lead. (Chron. Industr.)