The process of roasting, whether carried on in furnaces such as these or not, results in the formation of oxide instead of sulphide; in other words, the sulphur is turned out and oxygen takes its place. The dislodged sulphur then joins up with some more oxygen and forms sulphur dioxide, which can be led away to the sulphuric acid plant and there, by union with water, turned into that extremely valuable substance, sulphuric acid.
We cannot, however, treat zinc oxide as we would iron oxide or copper oxide, for zinc is volatile, and so, instead of accumulating in the bottom of a blast furnace as the iron and copper do, would pass off up the chimney.
The oxide is therefore mixed with coal or some other form of carbon and placed in retorts made of fireclay. These retorts are fixed in rows one above the other like the retorts at a gasworks, and hot gases from a gas-producer down below pass around and among them. To the mouth of each retort is fitted a condenser, also made of fireclay.
Now what happens in the retorts is this: the heat loosens the bonds between the zinc and the oxide, the latter passing into union with some carbon from the coal. The zinc at the same time becomes vapour and passes into the condenser, the lower temperature of which turns it into a liquid which the workmen remove at intervals in ladles. On being poured into moulds and allowed to solidify this metal is called by the name of "spelter," which bears to zinc the same relation that pig-iron does to the more highly developed forms of iron. Spelter is simply zinc in its crudest form.
Tin, although less important in war than copper and zinc, plays a not unimportant part. It has been found for centuries in Cornwall. The Romans used to trade with the natives of Britain for tin. Although considerable quantities of it is still obtained from there, the greatest tin-producing country of all at present is the Federated Malay States. Australia also furnishes ore, as does Bolivia and Nigeria.
In Cornwall the ore occurs as rock in veins or lodes filling up what must once have been fissures in granite rocks. That near the surface has long been taken, so that to-day the mines are very deep and costly to work. Some can only afford to operate
when the market price of tin is above a certain limit. Much of the ore from the newer districts—the Malay States, for example—is in small fragments mixed with gravel in beds near the surface. Such is called alluvial or stream tin, since the deposits were undoubtedly put in their present position by streams or rivers. So long as they last these easily accessible alluvial deposits will always be cheaper to work than the deep mines. On the other hand, they may give out, and recent explorations underground seem to indicate that there is still much valuable ore not only of tin but of other metals too, to be obtained from the old mines of Cornwall.
The ore of tin, like so many other ores, is generally oxide. It is first roasted to expel sulphur and arsenic which are often present as impurities, and then it is melted in a reverberatory furnace such as that described for the manufacture of wrought iron. As usual, the oxygen combines with carbon, the impurities form slag which floats on the top, and the pure metal falls to the bottom of the furnace from whence it can be drawn off.
Mixed with or in the neighbourhood of tin ore there is sometimes found another mineral called wolfram, which plays an extremely important part in modern warfare, so much so that the British and other Governments engaged in the war were at times hard put to it to find enough. Its value resides in the fact that it contains tungsten, an element which has wonderful powers in hardening steel.