Encyclopædia Britannica, Ninth Edition/Tin

TIN (Lat. stannum, whence the chemical symbol "Sn"; atomic weight = 117 6, O = 16), being a component of bronze, was used as a metal thousands of years prior to the dawn of history. But it does not follow that pre historic bronzes were made of metallic tin. When the un alloyed metal was first introduced cannot be ascertained with certainty. All we know is that about the 1st century the Greek word Kacro-iVe/Dos designated tin, and that tin was imported from Cornwall into Italy after, if not before, the invasion of Britain by Julius Caesar. From Pliny's writings it appears that the Romans in his time did not realize the distinction between tin and lead: the former was called plumbum album or candidum to distinguish it from plum bum nigrum (lead proper). The word stannum definitely assumed its present meaning in the 4th century (H. Kopp). Grains of metallic tin occur as a subordinate admixture in the gold ores of Siberia, Guiana, and Bolivia. Of tin mineral compounds (which are not numerous) tinstone, SnOn, is the most important; besides it only tin pyrites, which, according to Rammelsberg, exists in two varieties, FeCu 2 SnS 4 and ZnFeCu 4 Sn 2 S 8, need be named here.

Tinstone or Cassiterite. This native oxide of tin, SnO.,, forms very hard quadratic crystals of specific gravity 6 8. The pure mineral is colourless, and it is very scarce; most specimens are brown owing to the presence of ferric or manganic oxide. The faces of the crystals exhibit diamond lustre. There is also another native form, known as " wood tin," occurring in roundish masses with a fibrous radiating fracture. The ore is found in veins or layers within the older crystalline rocks and slates. Being much more highly proof against the action of water and carbonic acid than its matrix, the ore often presents itself in loose crystals as part of the sand of rivers (stream tin). The oldest known deposit of tinstone is that of Cornwall, where it occurs in granite and in the " killas " (a kind of metamorphic clayish slate), associated with wolframite, apatite, topaz, mica, tourmaline, arsenide of iron, and other minerals. Cornish tin ore is characteristically rich in arsenic. Minor Euro: pean deposits occur in the Erzgebirge, in Brittany, and in Galicia (Spain). A very considerable deposit of pure ore (chiefly stream tin) exists in the island of Banca; and in Malacca tinstone is found. Other relatively abundant deposits occur in Bolivia and Peru, and in Queensland and New South Wales (lately discovered).

Metallurgy. In the extraction of tin from tinstone ore the first step is to pound the crude ore and wash away the lighter gangue with water (see METALLURGY, vol. xvi. p. 59). The washed ore is "roasted" to burn away the arsenic and sulphur and to convert the iron, originally present in the heavy and compact form of pyrites or arsenide, into light friable oxide, which is removed by a second washing process. If much oxide of copper is con tained in the product, it is extracted with dilute sulphuric acid, and from the solution is recovered by precipitation with metallic iron (see COPPER, vol. vi. p. 347). The puri fied ore, known as "black tin," goes to the smelting furnace. During the roasting process the ore must be constantly agitated to prevent caking, and to bring the arseniferous, &c., parts to the surface. To save manual labour, Oxland and Hocking have constructed a mechanical roaster. It consists of a slanting tube of boiler-iron, coated inside with fire-brick. The lower end opens into the fire-place; the upper communicates with a set of chambers for the con densation of the white arsenic produced. The washed ore, after being dried on the top of the chamber, is run thence by a funnel into the pipe, which is made to rotate about its axis from three to eight times per minute. Before the ore has travelled far down the arsenic and sulphur catch fire, and by the time it reaches the bottom it is fully roasted. It falls into a receptacle below the level of the fire. Of the impurities of the ore the wolframite (tungstate of iron and manganese) is the most troublesome, because on account of its high specific gravity it cannot be Avashed away as gangue. To remove it, Oxland fuses the ore with a certain proportion of carbonate of soda, which suffices to convert the tungsten into soluble alkaline tungstate, with out producing noteworthy quantities of soluble stannate from the oxide of tin ; the tungstate is easily removed by treatment with water.

Smelting. The purified ore is mixed with about one- fifth of its weight of anthracite smalls, the mixture being moistened to prevent it from being blown off by the draught, and is then fused on the sole of a reverberatory furnace for five or six hours. The slag and metal pro duced are then run off and the latter is cast into bars ; these are in general contaminated with iron, arsenic, copper, and other impurities. To refine them, the bars are heated cautiously on an inclined hearth, when relatively pure tin runs off, while a skeleton of impure metal remains. The metal run off is further purified by poling, i.e., by stirring it with the branch of a tree, the apple tree being preferred traditionally. This operation is no doubt in tended to remove the oxygen diffused throughout the metal as oxide, part of it perhaps chemically by reduction of the oxide to metal, the rest by conveying the finely diffused oxide to the surface and causing it to unite there with the oxide scum. After this the metal is allowed to rest for a time in the pot at a temperature above its freezing point and is then ladled out into ingot forms, care being taken at each stage to ladle off the top stratum. The original top stratum is the purest, and each succeeding lower stratum has a greater proportion of impurities ; the lowest consists largely of a solid or semi-solid alloy of tin and iron.

To test the purity of the metal, the tin-smelter heats the bars to a certain temperature just below the fusing point, and then strikes them with a hammer or lets them fall on a stone floor from a given height. If the tin is pure it splits into a mass of granular strings. Tin which has been thus manipulated and proved incidentally to be very pure is sold as grain tin. A lower quality goes by the name of block tin. Of the several commercial varieties Banca tin is the purest; it is indeed almost chemically pure. Next comes English grain tin. For the preparation of chemically pure tin two methods are employed. (1) Commercially pure tin is treated with nitric acid, which converts the tin proper into an insoluble hydrate of SnO 2 , while the copper, iron, &c., become nitrates ; the oxide is washed first with dilute nitric acid, then with water, and is lastly dried and reduced by fusion with black flux or cyanide of potassium. (2) A solution of pure stannous chloride in very dilute hydrochloric acid is reduced with a galvanic current. According to Stolba, beautiful crystals of pure tin can be obtained as follows. A platinum basin, coated over with wax or paraffin outside, except a small circle at the very lowest point, is placed on a plate of amal gamated zinc, lying on the bottom of a beaker, and is filled with a solution of pure stannous chloride. The beaker also is cautiously filled with acidulated water up to a point beyond the edge of the platinum basin. The whole is then left to itself, when crystals of tin gradually separate out on the bottom of the basin.

Properties of Pure Tin.—An ingot of pure tin is pure white (ex cept for a slight tinge of blue) ; it exhibits considerable lustre and is not subject to tarnishing on exposure to normal air. The metal is pretty soft and easily flattened out under the hammer, but almost devoid of tenacity. That it is elastic, within narrow limits, is proved by its clear ring when struck with a hard body under circumstances permitting of free vibration. The specific gravity of ingot tin is 7 293 at 13 C. (Matthiessen). A tin ingot, though seemingly amor phous, has a crystalline structure, consisting of an aggregate of (quadratic octahedra ; hence the characteristic crackling noise which a bar of tin gives out when being bent. This structure can be rendered visible by superficial etching with dilute acid. As the minuter crystals dissolve more quickly than the larger ones, the surface assumes a frosted appearance (moirde metallique), not unlike that of a frozen window-pane in winter time. Its crystalline struc ture must account for the striking fact that the ingot, when exposed for a sufficient time to very low temperatures (to -39 C. for 14 hours), becomes so brittle that it falls into powder under a pestle or hammer ; it indeed sometimes crumbles into powder spontaneously. At ordinaiy temperatures tin proves fairly ductile under the hammer, and its ductility seems to increase as the temperature rises up to about 100 C. At some temperature near its fusing point it be comes brittle (vide supra), and still more brittle from - 14 C. downwards. This behaviour of the metal may probably be explained by assuming that in any tin crystal the coefficient of thermic ex pansion has one value in the direction of the principal axis and another in that of either of the subsidiary axes. From to 100 the two coefficients are practically identical ; below -14 and from somewhere above 100 C. upwards they assume different values ; and, as the several crystals are oriented in a lawless fashion, this must tend to disintegrate the mass. Tin fuses at 232 7 (Persoz); at a red heat it begins to volatilize slowly ; at 1600 to 1808 C. (Carnelley and Williams) it boils. The hot vapour produced com bines with the oxygen of the air into white oxide, Sn0 2 .

Industrial Applications. Commercially pure tin is used (princi pally in Germany) for the making of pharmaceutical apparatus, such as evaporating basins for extracts, infusion pots, stills, &c. It is also employed for making two varieties of tin-foil, one for the silvering of mirrors (see MIRROR, vol. xvi. p. 500), the other for wrapping up chocolate, toilet soap, tobacco, &c. The mirror foil must contain some copper to prevent it from being too readily amalgamated by the mercury. For making tin-foil the metal is rolled into thin sheets, pieces of which are beaten out with a wooden mallet. As pure tin does not tarnish in the air and is proof against acid liquids, such as vinegar, lime juice, &c., it is utilized for culinary and domestic vessels. But it is expensive, and tin vessels have to be made very heavy to give them sufficient stability of form ; hence it is generally employed merely as a protecting coating for utensils made essentially of copper or iron. The tinning of a copper basin is an easy operation. The basin, made scrupulously clean, is heated over a charcoal fire to beyond the fusing point of tin. Molten tin is then poured in, a little powdered sal-ammoniac added, and the tin spread over the inside with a bunch of tow. The sal- ammoniac removes the last unavoidable film of oxide, leaving a purely metallic surface, to which the tin adheres firmly. For tinning small objects of copper or brass (i.e., pins, hooks, &c. ) a wet-way process is followed. One part of cream of tartar, two of alum, and two of common salt are dissolved in boiling water, and the solution is boiled with granulated metallic tin (or, better, mixed with a little stannous chloride) to produce a tin solution ; and into this the articles are put at a boiling heat. In the absence of metallic tin there is no visible change ; but, as soon as the metal is introduced, a galvanic action sets in and the articles get coated over with a firmly adhering film of tin. Tinning wrought iron is effected by immersion. The most important form of the operation is mak ing tinned from ordinary sheet iron (making what is called "sheet tin"). The iron plates, having been carefully cleaned with sand and muriatic or sulphuric acid, and lastly with water, are plunged into heated tallow to drive away the water without oxidation of the metal. They are next steeped in a bath, first of molten ferrugin ous, then of pure tin. They are then taken out and kept suspended in hot tallow to enable the surplus tin to run off. The tin of the second bath dissolves iron gradually and becomes fit for the first bath. To tin cast-iron articles they must be decarburetted superficially by ignition within a bath of ferric oxide (powdered hsematite or similar material), then cleaned with acid, and tinned by immersion, as explained above. By far the greater part of the tin produced metallnrgically is used for making tin alloys, the majority of which have been treated of in preceding articles ; see LEAD, vol. xiv. p. 378 ; PEWTER, vol. xviii. p. 725 ; BRONZE, vol. iv. p. 366 ; PHOSPHORUS, vol. xviii. p. 817.

Tin Compounds. The most important of these may be arranged into two classes, namely, stannous compounds, SnX 2 , and stannic compounds, SnX 4 , where X stands for Cl, Br, 0, &c. Stannous compounds are, in general at least, characteristically prone to pass into the stannic form by taking up additional X 2 in the form of oxygen, chlorine, &c.

Stannous Chloride, SnCl 2 . This can be obtained pure only by heating pure tin in a current of pure dry hydrochloric acid gas. It is a white solid, fusing at 250 C. and volatilizing at a red heat in nitrogen, a vacuum, or hydrochloric acid, without decomposition. The vapour density below 700 C. corresponds to Sn 2 Cl 4 , above 800 C. to nearly SnCl 2 (Von Meyer and Ziibliu). The chloride readily com bines with water into an easily soluble crystallizable hydrate ("tin crystals "). This is made without difficulty by dissolving tin in strong hydrochloric acid and allowing it to crystallize. For its industrial preparation Nollner passes sufficiently hydrated hydrohydrochloric acid gas over granulated tin contained in stoneware bottles and evaporates the concentrated solution produced in tin basins over granulated tin. The basin itself is not attacked. The crystals contain one H 2 according to Berzelius, while Marignac finds two; probably both are right. The crystals are very soluble in cold water, and if the salt is really pure a small proportion of water forms a clear solution; but on adding much water most of the salt is decomposed, with the formation of a precipitate of oxy-chloride

2SnCl 2 + 3H 2 = 2HC1 + Sn 2 OCl 2 .2H 2 O.

According to Michel and Kraft, one litre of cold saturated solution of tin crystals weighs 1827 grammes and contains 1333 grammes of SnCl 2 . The same oxy-chloride is produced when the moist crystals, or their solution, are exposed to the air; by the action of the atmo spheric oxygen

+ 3SnCl 2 = Sn 2 CLjO + SnCl.

Hence all tin crystals as kept in the laboratory give with water a turbid solution, which contains stannic in addition to stannous chloride. The complete conversion of stannous into stannic chloride may be effected by a great many reagents, for instance, by chlorine (bromine, iodine) readily; by mercuric chloride, HgCl-j, in the heat, with precipitation of calomel, HgCl, or metallic mercury; by ferric chloride in the heat, with formation of ferrous salt, FeCl 2; by arsenious chloride in strongly hydrochloric solutions, with precipita tion of chocolate-brown metallic arsenic. All these reactions are available as tests for stannosum or the respective agents. In opposi tion to stannous chloride, even sulphurous acid (solution) behaves as an oxidizing agent. If the two reagents are mixed, a precipitate of yellow stannic sulphide is produced. By first intention

SO 3 H 2 + 3Sn"Cl a = 3Sn IV Cl 2 + H 2 S.

The stannic oxy-chloride readily exchanges its for C1 2 at the ex pense of the hydrochloric acid, which is always present, and the H 2 S decomposes one-half of a molecule of SnCl 4 with formation of 81182. A strip of metallic zinc when placed in a solution of stanuous chloride precipitates the tin in crystals and takes its place in the solution. Stannous chloride is largely used in the laboratory as a reducing agent, in dyeing as a mordant.

Stannaus Oxide. This as a hydrate is obtained from a solution of stannous chloride by addition of carbonate of soda; it forms a white precipitate, which can be washed with air-free water and dried at 80C. without much change by oxidation. If the hydrate is heated in carbonic acid, the black anhydride SnO remains (Otto). Precipi tated stannous hydrate dissolves readily in caustic potash ley; if the solution is evaporated quickly, it suffers decomposition, with formation of metal and staunate,

2SnO + K 2 = SnO 2 K 2 + Sn.

If it is evaporated slowly, anhydrous stannous oxide crystallizes out at a certain stage (Otto). Dry stannous oxide, if touched with a glowing body, catches fire and burns into binoxide, SnO^ Stannous oxalate when heated by itself in a tube leaves stannous oxide (Liebig).

Stannic chloride, SnCl 4, is obtained by passing dry chlorine over granulated tin contained in a retort; the tetrachloride distils over as a heavy liquid, from which the excess of chlorine is easily re moved by shaking with a small quantity of tin filings and re-dis tilling. It is a colourless fuming liquid of specific gravity 2 269 at (Pierre) and 2 234 at 15 C. (Gerlach), is fluid at - 29 C., and boils at 115 4 C. under 753 1 mm. pressure (Pierre). The chloride unites energetically with water into crystalline hydrates (ex. SnCl 4,3H 2 O), easily soluble in water. It combines readily with alkaline and other chlorides into double salts: thus SnC14 + 2KCl = SnCl 6 K,, analogous to the chloro-platinate; another example is the salt SnCl 6 (NH 4 ) 2, known industrially as " pink salt," because it is used as a mordant to produce a pink colour. The plain chloride solution is similarly used. It is usually prepared by dissolving the metal in aqua regia.

Stannic Oxide, SnO,. This, if the term is taken to include the hydrates, exists in a variety of forms. (1) Tinstone (see above) is proof against all acids. Its disintegration for analytical purposes can be effected by fusion with caustic alkali in silver, with the formation of soluble stannate, or by fusion with sulphur and car bonate of soda, with the formation of a soluble thio- stannate, SnS 2 + xNa 2 S. (2) A similar oxide is produced by burning tin in air at high temperatures or exposing any of the hydrates to a strong red heat. Such tin-ash, as it is called, is used for the polishing of optical glasses. (3) Meta-stannic acid (H 2 OSnO 2, generally written Hi Sn 5 15, to account for the complicated composition of metastan nates, e.g., the soda salt H 8 Na.,Sn 5 pj 5 ) is the white hydrate produced from the metal by means of nitric acid. It is insoluble in water and in nitric acid and apparently so in hydrochloric acid; but if heated with this last for some time it passes into a hydrochlorate, which, after the acid mother liquor has been decanted off, dissolves in water. The solution when subjected to distillation behaves pretty much like a physical solution of the oxide in hydro chloric acid, while a solution of ortho-stannic acid in hydrochloric acid behaves like a solution of SnCl 4 in water, i.e., gives off no hydrochloric acid and no precipitate of hydrated SnO 2 . (4) Orthostannic acid is obtained as a white precipitate on the addition of carbonate of soda or the exact quantity of precipitated carbonate of lime to a solution of the chloride. This hydrate, SnO 2 H2O, is readily soluble in acids forming stannic salts, and in caustic potash and soda, with the formation of ortho-stanuates. Of these stauuate of sodium, Na 2 SuO 2, is produced industrially by heating tin with Chili saltpetre and caustic soda, or by fusing very finely powdered tinstone with caustic soda in iron vessels. A solution of the pure salt yields fine prisms of the composition Na-jSiiOj + lOHgO, which effloresce in the air. The salt is much used as a mordant in dyeing and calico-printing. Alkaline and other stannates when treated with aqueous hydrofluoric acid are converted into fluo-stannates (e.g., K2SnO 3 into K 2 SnF 6 ), which are closely analogous to, and isomorphous with, fluo-silicates.

Sulphides. If tin is heated with sulphur the two unite very readily into stanuous sulphide, SnS, a lead-grey mass, which under the circumstances refuses to take up more sulphur. But, if a mixture of tin (or, better, tin amalgam), sulphur, and sal-ammoniac in proper proportions be heated, stannic sulphide, SnS 2, is produced in the beautiful form of aurum musivum (mosaic gold), a solid consisting of golden yellow, metallic lustrous scales. It is used chiefly as a yellow "bronze" for plaster- of- Paris statuettes, &c.

Analysis. Tin compounds when heated on charcoal with car bonate of soda in the reducing blowpipe flame yield metal and a scanty ring of white SnO^ The reduction, however, succeeds better with cyanide of potassium as a flux. Stannous salt solutions yield a brown precipitate of SnS with sulphuretted hydrogen, which is insoluble in cold dilute acids and in real sulphide of ammonium, (NH 4 ) 2 S; but the yellow, or the colourless reagent on addition of sulphur, dissolves the precipitate as SnS 2 salt. The solution on acidification yields a yellow precipitate of this sulphide. Stannic salt, SnCl 4, solutions give a yellow precipitate of SnS 2 with sulphur etted hydrogen, which is insoluble in cold dilute acids but readily soluble in sulphide of ammonium, and is re-precipitated therefrom as SnS 2 on acidification. Only stannous salts (not stannic) give a precipitate of calomel in mercuric chloride solution. A mixture of stannous and stannic chloride when added to a sufficient quantity of solution of chloride of gold, gives an intensely purple precipitate of gold purple (purple of Cassius), a compound which, although known for centuries, is to this day little understood chemically. It behaves on the whole like a compound of Sn 2 3 with Au 2 O. The test is very delicate, although the colour is not in all cases a pure purple.(W. D.)