652 4 2 TIN-PLATE. case to be dissolved as stannous sulphate, which is oxidized to stannic sulphate at the expense of the sulphurous acid, and sulphur is deposited; Sn+2 H2SO, yielding Sn"SO+SO2+2H2O; and 2 Sn" SO,+SO, +2 H2SO, furnish 2 (Sniv 2 SO)+S+2H2O. The hydrates of potash and soda act upon tin at high temperatures, hydrogen being evolved, whilst a soluble metastannate of the alkali-metal is formed. Tin combines readily with sulphur, phosphorus, chlorine, and bromine, if heated with them. Owing to its brilliancy, and its power of resisting ordinary atmospheric changes, tin is largely employed as a coating upon other more abundant but more oxidizable metals, to protect them during use. Iron and copper are especially adapted to the operation of tinning. In India, tin is applied instead of silver to steel and iron articles by way of ornament; the tin is melted, and while still liquid is agitated in a box till it has become solid; the fine powder thus procured is separated, by suspension in water, from the coarser particles, and is made into a thin paste with glue; it is then applied in the desired pattern; when perfectly dry it is burnished, and afterwards varnished; its brilliancy is thus preserved unchanged. (811) Tin-Plate.-The ordinary process of tinning iron differs from the foregoing one, and is far more important in its economical results. In tin-plate an actual alloy of the two metals is formed upon the surface of the iron, the external surface being pure tin. For the manufacture of tin-plate, the best charcoal iron is required. After the iron has been rolled and cut into sheets of suitable thickness and size, its surface is made chemically clean. For this purpose the sheets are immersed for four or five minutes in a mixture of sulphuric acid and water; after which they are raised to a red heat in a reverberatory furnace; they are then withdrawn, allowed to cool, and hammered flat. In order to detach from them all the scales of oxide, they are passed between polished rollers, and as they emerge they are plunged one by one into a mixture of bran and water which has become sour by exposure to the air; here they remain for some hours, and are thence transferred to a vessel containing a mixture of diluted sulphuric and hydrochloric acids; lastly, they are scoured with bran, and plunged into pure water or lime-water, in which last, if the surface be clean on immersion, they may remain for any length of time without rusting: these preliminary steps are necessary in order to secure a clean surface, as the tin will not adhere to an oxidated or even a dusty plate. In some works, the plates, after they have been scoured, are further cleaned with TIN-PLATE-PROCESSES OF TINNING. 653 hydrochloric acid holding zinc in solution, and then dipped into the melted tin in the manner about to be described. : The plates having been prepared by either of the foregoing processes are next plunged one by one into a large vessel of melted tallow free from salt, and after remaining there for an hour they are immersed in the bath of melted tin, which is preserved from oxidation by a stratum of grease three or four inches thick. Here they remain for about an hour and a half; they are then withdrawn and allowed to drain. After this they are plunged into a second bath of pure tin, and the excess of tin is removed by again heating them in a bath of tallow the tin melts and runs down to the lower edge of the plate; when cool, this thickened margin is finally reduced by dipping the edge of the plate once more into tin kept at a temperature much above its melting-point; the heat quickly fuses the superfluous metal, which is then detached by giving the plate a sharp blow. Tinplate is sometimes made to exhibit a beautiful crystalline appearance, known under the term moirée métallique. A mixture of 2 parts of nitric acid, with 2 of hydrochloric acid, is made with 4 of water the tin-plate is gently heated, and the liquid spread evenly over with a sponge; the crystals gradually appear. The plate is then plunged into water, dried quickly, and varnished. Different coloured varnishes are used to vary the effects. Tinning of copper is the same in principle, but is a simpler operation than the tinning of iron: the surface of the metal is rendered clean by rubbing it, while heated, with sal ammoniac ; when quite bright the copper is sprinkled with a little rosin to prevent oxidation, and melted tin is then poured on and spread over the surface with tow by the workman, who keeps the article constantly at a high temperature; the superfluous tin is wiped off with the tow. The addition to the tin of one-fourth of its weight of lead renders the operation more easy, as the alloy is more perfectly liquefied. Pins, which are made of brass wire, are tinned by boiling them for a few minutes with a solution containing I part of cream of tartar, 2 parts of alum, and 2 of common salt in 12 parts of water, with a quantity of granulated tin: in the course of a few minutes a brilliant, white, closely adhering coat of tin is deposited upon the surface of the pins. (812) The alloys of tin which are employed in the arts are numerous. Britannia metal is one which is a good deal used for making teapots and spoons of a low price; it consists of equal parts of brass, tin, antimony, and bismuth. Pewter is another alloy of this description; both of these possess considerable mal 654 ALLOYS OF TIN WITH LEAD AND COPPER. leability, pewter being intermediate in hardness between lead and Britannia metal. The best pewter consists of 4 parts of tin, and I of lead. Another alloy, which is intermediate in properties between pewter and Britannia metal, is called Queen's metal; it is used for the manufacture of teapots and common spoons. It consists of 9 parts of tin, I part of antimony, 1 of bismuth, and 1 of lead. Plumber's solder is an alloy of tin and lead which is more fusible than pure lead: fine solder consists of 2 parts tin and I of lead; common solder of equal parts of lead and tin; and coarse solder is composed of 2 of lead to 1 of tin. Lead and tin may be melted together in all proportions, and notwithstanding their difference in density, they do not separate when the fused mixture is allowed to cool slowly (Matthiessen). The same is true also of the alloy of tin and zinc; if the two metals be fused together in equal proportions, the result forms a hard, white alloy nearly as tough as brass. Gun metal contains Tin forms several important alloys with copper. Speculum metal, used for the mirrors of reflecting telescopes, consists of I part tin and 2 copper, or (Єu,Sn): it is of a steel-white colour, extremely hard, brittle, and susceptible of a high polish. The proportions of the constituents of speculum metal recommended by different authorities vary, and sometimes a small quantity of arsenic is added to the alloy. Bell metal consists of about 78 of copper and 22 of tin, or (Eu Sn); sometimes a mixture of zinc and lead is substituted for a part of the tin. only 9 or 10 per cent. of tin. Bronze contains less tin than bellmetal, with usually an addition of 3 or 4 per cent. of zinc. The bronze used for coin consists of 95 parts of copper, 4 of tin, and 1 of zinc. Bronze admits of a peculiar kind of tempering. If it be annealed, and allowed to cool slowly, it becomes hard, brittle, and elastic; but if cooled suddenly, it may be hammered, and worked at the lathe; this property is taken advantage of in the manufacture of articles with this alloy; they are wrought in the soft state, and are afterwards hardened by annealing. The effect of sudden cooling upon bronze is therefore just the reverse of that which is produced by it upon steel. These alloys of copper and tin are much harder than copper itself, and considerably more fusible. The melting-point of copper, according to Daniell, is 1996°; but an alloy of tin and copper containing 6.6 per cent. of tin, fused at 1690°; and 1 with 123 per cent. of tin, at 1534° F. These alloys have a specific gravity greater than the mean of that of the metals which enter into their composition. They resist oxidation in the air more completely than copper. AMALGAM OF TIN AND MERCURY. 655 An inconvenience in the use of the alloys of copper and tin arises from the circumstance, that, when melted, the two metals, owing to their difference in density, have a tendency to separate from each other, even after they have been well incorporated: the tin accumulates in the upper portions of the melted mass, where it forms a more fusible alloy. It is therefore very difficult in large castings to obtain a mass of metal the composition of which is uniform throughout. The amalgam of tin and mercury is employed for the silvering of mirrors. In order to apply it to the glass, a sheet of tinfoil is spread evenly upon a smooth slab of stone, which forms the top of a table carefully levelled, and surrounded by a groove, for the reception of the superfluous mercury. Clean mercury is poured upon the tinfoil, and spread uniformly over it with a roll of flannel; more mercury is then poured on till it forms a fluid layer of the thickness of about half-a-crown; the surface is cleared of impurities by passing a linen cloth lightly over it; the plate of glass is carefully dried, and its edge being made to dip below the surface of the mercury, is pushed forward cautiously; all bubbles of air are thus excluded as it glides over and adheres to the surface of the amalgam. The plate is then covered with flannel, weights are placed upon the glass, and the stone is gently inclined so as to allow the excess of mercury to drain off; at the end of 24 hours it is placed upon a wooden table, the inclination of which is increased from day to day until the mirror assumes a vertical position: in about a month it is sufficiently drained to allow the mirror to be framed. The amalgam usually contains about 4 parts of tin to I part of mercury. Several of the compounds of tin are employed in the arts. The binoxide is used to some extent in the preparation of enamels, and both the chlorides of tin are substances of great importance to the dyer and the calico-printer. (813) OXIDES OF TIN.-With oxygen, tin forms two principal compounds, the protoxide and the binoxide, besides some intermediate oxides of minor importance. Stannous oxide or protoxide of tin (Sn0=134, or SnO=67) is obtained as a white hydrate (2 SnO,H,O), by pouring a solution of stannous chloride into one of carbonate of sodium or of potassium in excess; the carbonic anhydride escapes with effervescence. When moist, this hydrate absorbs oxygen from the air, but not when dry. By ignition in closed vessels filled with nitrogen or with carbonic anhydride it becomes anhydrous. The anhydrous protoxide may also be obtained by decomposing If heated in the open stannous oxalate by heat in closed vessels. air it glows, and is converted into the binoxide. If the hydrated oxide be boiled with a solution of potash in excess, it is dissolved, and in a few days metallic tin is separated, peroxide of the metal remaining in solution. If boiled with a weak solution of potash, in quantity insufficient to dissolve the oxide, it becomes anhydrous, and is converted into a mass of black crystalline needles; these needles when heated decrepitate powerfully, increase in bulk, and are converted into an olive-brown powder. By evaporating down a solution of sal ammoniac containing hydrated oxide of tin in suspension until the sal ammoniac begins to crystallize, the oxide of tin becomes anhydrous and assumes a brilliant scarlet colour, which, however, by friction disappears, and becomes brown. The hydrated oxide is readily dissolved by acids, but the anhydrous oxide is more slowly acted upon by them. (814) Binoxide of Tin (Sn0,=150, or SnO,=75); Sp. Gr.6·95; Comp. in 100 parts, Sn, 78.66; →, 2134.-This oxide occurs native in the anhydrous form as tin-stone, and constitutes the only ore of tin that is worked. It is met with crystallized in square prisms, which are hard enough to scratch glass; they have usually a brown colour, owing to the presence of peroxide of iron or of manganese. It is insoluble in acids, but if heated with an alkali, it enters into combination with it, and forms a soluble compound. In its hydrated condition binoxide of tin has the characters of an acid, and forms two remarkable varieties, which have been termed respectively metastannic and stannic acids (Fremy, Ann. de Chimie, III. xxiii. 393). Like the metaphosphoric and phosphoric acids, they require each a different amount of base for saturation, the stannic acid combining with the greatest proportion of base. 5 Metastannic Acid (HSn.11.4 H,O) is readily procured by treating metallic tin with nitric acid; violent action, attended with extrication of nitrous fumes, occurs, and the tin is converted into a white, crystalline, insoluble mass, which is hydrated metastannic acid; after washing it with cold water, the acid, when dried in air, consists of Sn,010, 10 H2O (Fremy). In this state it reddens litmus-paper; when dried at 212° it loses half its water, and by ignition becomes anhydrous, and of a pale buff colour: in this form it possesses the properties of the native oxide, and constitutes the putty powder employed for polishing plate; it is also largely used for giving whiteness and opacity to enamels. In its hydrated condition, metastannic acid is insoluble in nitric acid; concentrated sulphuric acid, when heated with it, dis |