764 TIN Uses of Tin. Tin foil is used for coating the backs of mirrors, wrapping articles requiring to be kept from the air, lining boxes, covering Leyden flasks, &c. The latter uses require less copper in the composition, and the material is sometimes called stanniol. Of the following four analyses by Stotzel, the first two are of foil for large mirrors, the third for small mir- rors, and the fourth for wrappers and linings : CONSTITUENTS. l. 2. 3. 4. Tin 97*6 97-8 98-47 96-21 2'16 1-28 0-88 0'95 L,ead 0-04 0-76 0-84 2-41 Iron . O'll o-io 0-12 0-09 Nickel 0'80 Tin foil is prepared by rolling cast tin into plates, and beating and doubling as with gold foil, though by a simpler process. (See GOLD- BEATING). Tin foil consisting of a surface of tin, with an interior of lead or tin-lead alloy, is prepared by placing a plate of lead or alloy in a mould slightly larger, casting tin around it, and rolling and hammering. Tin-lined lead pipe for plumbers' use is made by setting a core of block tin in the centre of a mass of melted lead, so that the more fusible tin is melted, but does not mix with the remainder of the bath, and then proceeding as in the ordinary manufacture of lead pipe. (See LEAD, vol. x., p. 262.) Tin plating is performed either by covering the metallic articles to be plated with melted tin, or by humid processes. The former method is chiefly confined to copper, iron, and zinc. Copper may be heated, cleaned with sal ammoniac, sprinkled with resin to prevent oxidation, and then plated by pouring melted tin upon it, and spreading the tin with tow, a high temperature being maintained. The plating of sheet iron, to form so-called " tin plate" or sheet tin, for domestic utensils, &c., is conducted as follows : The thin sheets of iron are cleaned by immersion in dilute sul- phuric acid and subsequent rubbing with sand and water and washing, after which they are annealed by exposure to cherry heat for 12 hours in cast-iron boxes, tightly closed and luted. Imperfect or seriously oxidized plates are rejected. The accepted ones, which are purplish from a thin external film of oxide, are polished by being passed cold through rolls, then subjected to a second and less prolonged annealing, then sorted and cleansed again, and finally taken to the tinning apparatus. After cleansing they will quickly rust on exposure to air, but may be kept indefinitely without injury if immersed in pure water. The tin- ning apparatus comprises a series of long rec- tangular pots or tanks, with a fire under each. These tanks contain the liquid baths into which the plates are to be plunged. The operation comprises a series of immersions: first into melted grease, in which the plates are left till all moisture has evaporated ; then successively into several baths of tin, each of which is purer than the preceding, so that the sheets acquire a coating first of alloy and finally of pure tin ; then into melted grease again, in which the superfluous tin runs off, while the liquid grease prevents a too rapid cooling and consequent cracking of the surface. As the tin in the final tin bath becomes fouled by alloyed iron, it is removed to the preceding tin bath, and from this in turn to the first bath. After the final grease bath (tallow and palm oil), which an- neals the plates, the edging of tin which usu- ally forms around them is removed by dip- ping into melted cast iron, which melts it, so that a quick blow on the plate causes it to drop off. The plates are at last rubbed with bran and then with sheepskin to remove grease and dirt, sorted, packed in boxes, and marked to in- dicate size and quality. The sheet iron for tin plates is rolled from the best charcoal or coke bar. Terne plates have, instead of tin, a coat- ing of tin-lead alloy, containing from one third to two thirds lead. Iron may be coated with zinc first, and then very readily tinned by dip- ping into the fused metal, since tin and zinc unite with ease. Sheet zinc is tinned in the same way, but should not be left in the bath so long as to become alloyed with tin beyond the surface. Lead and its alloys may be tinned in like manner. The process above given for tinning iron is not applicable to cast iron, un- less it has been decarbonized on the surface by heating in iron oxide, after the manner of the "annealing" practised in the manufacture of malleable castings. The humid methods of plating tin upon various metals are numerous. Pins, which are made of brass wire, and other objects of brass or copper, are dipped into an aqueous solution, containing 1 part argal, 2 parts alum, and 2 parts salt, in which tin has been dissolved, or to which stannous chloride has been added. In this liquid they remain unaffected until brought into contact with me- tallic tin, whereby an electro-chemical action is caused, and all the objects connected directly or through one another with the metallic tin are immediately coated with tin reduced and precipitated from the solution. Boiling brass or copper objects, in contact with tin filings, in a solution of stannic oxide in caustic potash, is also an excellent way. Iron objects (nails, hooks and eyes, &c.) may be tinned, after suit- able cleansing, in a bath of argal and stannous chloride, with the addition of zinc filings ; or the bath may be composed of equal parts of the tin salt and common salt, dissolved in water, or of 1 part tin salt, part sal ammoniac, and 1 part common salt, dissolved in 2 parts nitric and 4 parts muriatic acid, diluted with water. In the latter liquid most metallic objects may be tinned by sufficiently prolonged immersion, copper or iron being kept in contact with a zinc wire during {he process. Zinc is most easily tinned. For galvanic tinning a weak battery may be employed, and a solution of stannic chloride in caustic potash. But the use of the battery in this and similar opera-
