The tin ore thus far purified has next to be deprived of its sulphur and arsenic; this is done in a reverberatory furnace (q.v.), the flues of which are connected with large •ondensino. chambers, in which the arsenic is deposited in a crystalline form (see AR sExic), and is afterward resublimed, to form the white arsenic of commerce. The sulphur which is present in the state of sulphide of iron is decomposed by the heat into sulphurous acid gas, and the remaining oxide of iron is removed by a subsequent washing. Sulphide of copper, when present, is converted by roasting, and afterward -exposing it to the air, hits° sulphate of copper, and is then easily dissolved out by lixivia tion.
After this final washing the ore is ready for smelting in a reverberatory furnace. The charge consists of from 20 to 25 cwts. of ore mixed with one-sixth of its weight of powdered anthracite or charcoal, and a small quantity of lime or fluor-spar, to serve as a flux for the siliceous impurities. Before being put into the furnace, the mixture is moistened with water, to prevent the finely-powdered ore being carried away by the -draught. When the charge is placed on the hearth of the furnace, the doors are closed, and the heat gradually raised for about six hours; the oxide is then reduced by the car bon of the coal. At this stage the furnace-door is opened, and the mass worked with a paddle, to separate the slag, which is raked off, and the richer portion of it melted over again. The reduced tin subsides to the bottom, and is run off into a cast-iron pan, from which it is ladled into molds, to produce blocks or ingots of a convenient size.
The tin has still to be purified, first by a process of liquation, and afterward by that of boiling. " Liguation" consists in moderately heating the blocks iu a reverberatory furnace till the tin, owing to its comparatively easy fusibility, melts and flows into the refining basin, leaving on the hearth of the furnace a residuary alloy of tin with iron and other metals. Afore blocks are added and heated in the same way, till the refining basin contains about five tons. The tin is then ready for "boiling." In this operation billets of green wood are plunged into the melted metal, the disengagement of gas from which produces a constant ebullition, and so causes a scum (chiefly oxide of tin) to rise to the surface, which is then easily removed; at the same time, impure and dense parts fall to the bottom. When the agitation has gone on long enough, the bath is allowed to settle and cool. The tin then separates into zones—the upper consisting of the purest portion; the middle being slightly mixed with other metals; and the lower so much so that it requires to go through the refining process again. The residuary alloy of the
Equation process has also its tin extracted and refined again.
In former times in Cornwall tin was smelted in a blast furnace (q.v.) instead of a reverberatory one; and this is still the case on the continent. By this method a pure tin is obtained, but the loss of metal in the process is greater. It suits best where coal is scarce and wood abundant.
Tin ores which contain the mineral wolfram (tungstate of iron and manganese) are treated by a special process, patented by Mr. R. Oxland, of Plymouth. This mineral and tin ore are so nearly the same iu specific gravity, that no mechanical process of washing will separate them. Mr. Oxland's process consists in roasting the dressed tin ore with sulphate of soda, for the purpose of converting the insoluble tungstate of iron and manganese into the soluble tungstate of soda, which is easily removed by lixiviation. The oxides of iron and manganese, which are left in a finely-divided state, can then, from their lower density, be readily got rid of by washing. Since the invention of this process, some of the Cornish tin ores which used to sell at the lowest, now bring the highest price. The tungstate of soda procured in the operation has lately been found to be one of the most valuable substances for rendering cotton cloths noninflammable.
Tin when heated up to nearly its melting-point becomes brittle, and can then be broken into prismatic fragments called dropped or grain tin. The metal which is sus ceptible of this change may be considered to be of fine quality, as impure tin does not become brittle when so treated. The peculiar properties of tin, especially its malle ability, its brilliancy, and the slowness with which it oxidizes at common temperature in the atmosphere, render it of great service in the arts. Utensils coated with silver require six cleanings for one that would suffice with "tinned" vessels. Tin is conse quently very largely used to coat the surface of other metals, as iron and copper, espe cially thin sheet-iron, to form tin-plate (q.v.). Tin-plate goods are now manufactured on a gigantic scale in Birmingham, Wolverhampton, and Dudley. The increase has been brought about mainly by the introduction of stamping machinery, which does away with much of the labor formerly required to solder the pieces together.
With other metals tin forms some valuable alloys. See ALLOY. An amalgam of tin and mercury forms the metallic coating of mirrors. The applications of tin-foil, which is not more than of an inch in thickness, are well known. From 60 to 80 tons of tin are annually used in Birmingham in the manufacture of coffin-lace.