TIN MANUFACTURE, Tim ores of tin aised in Cornwall and Devonshire are mostly reduced or smelted within those counties. The smelting-works do not generally belong to the proprietors of the mines, but to other parties who purchase the ore from them, their value being determined by a kind of assay. The smelting is effected by two different methods, according as tin-stone or stream-tin is to be acted on.
In the former process the prepared ore, which is called sehlich, is mixed with from one • fifth to one-eighth of its weight of powdered anthracite, or culm, to which a little slaked lime or fluor-spar is sometimes added as a flux. The charge (from 12 to 24 cwts.) is spread upon the concave hearth of the furnace, and then the apertures by which it is inserted are closed and luted, and the furnace is gra dually heated, and kept hot for six or eight hours, by which time the reduction of the ore is complete. 'Men the fusion or reduction of the ore is considered to be finished, one of the apertures of the furnace is opened, and the scorn removed ; after which a channel is opened, by which the melted tin flows from the hearth into a large vessel, where it is allowed to rest for some time, in order that the impurities yet remaining with the metal may separate, by their different specific gravi ties. When it has settled, the tin is ladled into moulds, so as to form it into large blocks or ingots.
The ingots produced by the above process frequently contain portions of other sub stances, to remove which the tin is exposed to the process of refining. The tin is again melted in another furnace; and into the molten metal billets of green wood are plunged. This occasions the disengagement of consi derable volumes of gas from the wood, and thus a kind of ebullition is produced in the tin, which causes the lighter impurities to rise to the surface in a frothy form, and the heavier to fall to the bottom. The scum is removed, and the rest is allowed to settle, whereby all the purest tin rises to the top, and the quality deteriorates thence to the bottom ; by which means the tin, poured into moulds, presents many different qualities.
The moulds are made of granite, and yield blocks of tin weighing about 3 cwts. each : granite having been found well fitted for this purpose. An inferior kind of tin is pro duced by remelting the scorn. The average quality of the tin ore, as prepared for the smelting-furnaces, is such as to yield 62 to 63 per cent. of pure tin ; and the quantity of coal required for producing one ton Of tin is about a ton and three-quarters.
The smelting of tin by the blast-furnace, with wood-charcoal, is practised on a limited scale for the production of tin of the greatest possible purity, and from stream-tin instead of tin-stone. No substance is added to the ore and charcoal, unless it be the residuary matter of a previous smelting ; and the proportion of charcoal consumed is about one ton and six-tenths for every ton of tin produced. The melted tin runs from the furnace into an open basin, whence it is run off into a large vessel in which it is allowed to settle. The scorke which run with the metal are skimmed off, and separated into two portions, one consisting of such as retain tin oxide, and the other of such as have no oxide, but contain tin in a granulated state. In order to convert the blocks of tin produced by tho blast-furnace process into the form known as grain-tin, they are heated until they become brittle, and made to fall from a considerable height in a semi-fluid state, thus producing an agglome rated mass of elongated grains.
Tin is rarely employed alone in our metal line manufactures, but when laid in a thin coat upon the surface of sheet iron by the process of Dnning, it produces a material of extensive use in the manufacture of culinary and other articles. Most of the tin used in the manufacture of articles composed exclu sively of that metal is that which is expanded by rolling and hammering into leaves of tin foil; this is the substance which is laid upon the back of glass mirrors, and there amalga mated with mercury, so as to form what is called the Silvering.