For the manufacture of tin plates, the best soft charcoal iron is obliged to be used. After it has been rolled and cut to the requisite size, its surface is made chemically clean by immersion for a few minutes in dilute sulphuric acid. The sheets are then heated to a red heat in a reverberatory furnace, withdrawn, al lowed to cool, hammered flat, and passed between polished rollers, and then washed in dilute acid. This preparation is needed to free the surface of the iron from the slightest portion of oxide, to which the tin would not adhere. In order to tin them, they are plunged one by one into a vessel of tallow, from which they are transferred to a bath of tin. From this they are taken after a certain time, al lowed to drain, and dipped again. The superfluous tin at the edge of the plate is removed by dipping it in the melted tin once more, and detaching it by giving the plate a sharp blow. The tin and iron form a perfect alloy with each other.
The appearance known as moire mgtal lique is given by sponging the surface of the tin with dilute nitro-hydrochloric acid, washed with water, and afterward varnished with plain or colored varnish. Copper is tinned in the same way, but with greater ease, it being so much less difficult to clean the surface of that metal than iron. Tinfoil is made by beating pure tin to the requisite thinness. The alloys of tin are numerous and important. The principal of these are Britannia metal, consisting of equal parts of brass, tin, an timony, and bismuth; pewter, 4 tin, and 1 lead; Queen's metal, 9 tin, and 1 each of antimony, bismuth, and lead; the vari ous solders; bell-metal, 78 copper, 22 tin; bronze, copper, with 4 to 6 per cent. of tin; gun metal, 78 copper and 22 tin; and several others. Speculum metal, used for the mirrors of reflecting telescopes, is a steel-white, hard, brittle alloy, of 1 part of tin and 2 of copper. An amal gam of tin and mercury is used for sil vering looking glasses. A sheet of tin foil is laid on an edged slab of stone carefully levelled, and mercury is poured on it till it forms a layer Ys inch thick. Glass is laid down on this, and the table is tilted to let the superfluous mercury run off, weights being gradually placed on the glass to facilitate the operation. Tin ores are met with but in few lo calities.
Cornwall, Banca, Mexico, and some parts of Australia, are the only impor tant tin yielding districts. The most celebrated tin mines are those of Corn wall, England, which have been worked uninterruptedly from the earliest historic periods. In the United States, crystals of the oxide of tin have been found in localities of Massachusetts, New Hamp shire, New York, New Jersey, Virginia, California, Missouri, and perhaps in other States. Some of the mines, as those of Missouri and California, seem promising. Our product in 1916 was 140 short tons, most of which came from Alaska. In 1919, the world produced 125,760 metric tons of tin, of which the United States imported about 50 per cent. The only ore of importance is tin stone, a hard dark-brown crystalline body con sisting of the binoxide in a crystalline condition. To extract the metal, the ore is first stamped and washed, to get rid of the lighter particles of sand or earth adhering to it. It is then roasted, to free it from arsenic and sulphur, and again washed to carry off the sulphate of copper and oxide of iron. The washed ore is mixed with from one-fifth to one eighth its weight of powdered anthracite or charcoal, and with a small portion of lime to form a fusible slag with any of the remaining gangue. The charge is placed on the hearth of a low-crowned reverberatory furnace, and the doors are closed up. Heat is applied very gradu
ally for five or six hours, care being taken to raise the temperature high enough to cause the carbon to reduce the tin without melting the siliceous gangue, which would form with the binoxide an enamel troublesome to remove. When nearly all the tin is reduced, the heat is raised considerably, the slags being thus rendered fluid and capable of floating on the top of the melted metal. The tin is then run off into cast iron pans, from which it is ladled off into molds to form ingots. The tin thus procured is far from being pure; it is therefore submit ted to the process of lignation, which consists in heating the ingots to incipient fusion. By this means the purer tin which fuses at a comparatively low heat separates, running down, and leaving the impure portions behind. The less fusible portion, when remelted, forms block tin and the part which has run out is again melted and agitated with wet stakes. The steam thus formed bubbles up to the surface, carrying with it all other mechanical impurities contained in the tin. The mass is then skimmed and al lowed to cool. When just about to set, the upper half is ladled out, the other metals and impurities having sunk into the bottom half, from the tendency that this metal has to separate from its al loys. The finest quality of tin is fre quently heated to a temperature just short of its melting point. At this heat it becomes brittle, and is broken up into masses, showing the crystals of the metal, and forming what is known as grain tin. The formation of crystals is to some ex tent a guarantee of its purity, since im pure tin does not become brittle in this way. English tin generally contains small quantities of arsenic, copper, iron and lead.
Chlorides of Tin.—There are two chlo rides of tin, the protochloride and the per or bichloride. The protochloride, SnCI, may be prepared in the anhydrous state by the action of dry hydrochloric acid on tin at a gentle heat. The hydrat ed chloride is obtained by dissolving the metal in hydrochloric acid diluted with an equal bulk of water. It crystallizes in transparent needles, containing two equivalents of water. It is a powerful reducing agent, and is much used by dyers for altering reducible coloring matters, such as sesquioxide of iron and peroxide of manganese. It is also used as an antichlore. It forms crystallizable double salts with the alkaline chlorides. The bichloride, perchloride, or fuming liquor of Libavius, SnCl2, is made by passing chlorine over an inclined tube, fitted to a receiver, and containing pieces of tin-foil rolled up. It is used to a con siderable extent in solution in dyeing. It absorbs sulphuretted and phosphuretted hydrogen, and forms a compound of am monia. In some respects it plays the part of an acid—chlorostannic acid.
Oxides of Tin.—There are two oxides of tin—the protoxide and the binoxide. The protoxide is prepared in a variety of ways, too prolix for description here, and is only interesting in a chemical point of view. When heated in air, it burns like tinder, and is converted into the binoxide. The only ore of tin, tin-stone, is a form of the binoxide.
Sulphides of Tin.—There are two sul phides of tin—the protosulphide, formed by fusing together metallic tin and sulphur. The bisulphide, when prepared in the dry way, is known by the name of "aurum musivum," or mosaic gold, and is used as bronze powder in coarse dec orative works. It is prepared by fusing together seven parts of flowers of sulphur and six of salammoniac with an amalgam of 12 parts of tin and 6 of mer cury.