Chapter M: D’Arcet states the analysis of Marseilles soap at (10)

The first group is by far the richest, and the best explored. The formation most abundant in tin mines is principally granitic; whilst that of the copper mines is most frequently schistose or killas; though with numerous exceptions. The great tin veins are the most antient metalliferous veins in Cornwall; yet they are not all of one formation, but belong to two different systems. Their direction is, however, nearly the same, but some of them dip towards the north, and others towards the south. The first are older than the second; for in all the mines where these two sets of veins are associated, the one which dips to the north, cuts across and throws out the one which dips to the south. See MINES, p. 835.

At Trevannance mines, the two systems of tin veins are both intersected by the oldest of the copper veins; indicating the prior existence of the tin veins. In _fig._ 1139. _b_, marks the first system of tin veins; _c_, the second; and _d_, the east and west copper veins. Some of these tin veins, as at Poldice, have been traced over an extent of two miles; and they vary in thickness from a small fraction of an inch to several feet, the average width being from 2 to 4 feet; though this does not continue uniform for any length, as these veins are subject to continual narrowings and expansions. The gangue is quartz, chlorite, tourmaline, and sometimes decomposed granite and fluor spar.

4. _Alluvial tin ore, stream tin._--Peroxide of tin occurs disseminated both in the _alluvium_ which covers the gentle slopes of the hills adjoining the rich tin-mines, and also in the alluvium which fills the valleys that wind round their base; but in these numerous deposits the tin-stone is rarely distributed in sufficient quantities to make it worth the working. The most important explorations of _alluvial tin ore_ are grouped in the environs of St. Just and St. Austle; where they are called _stream-works_; because water is the principal agent employed to separate the metallic oxide from the sand and gravel.

The tin mine of Altenberg, in Saxony (_fig._ 1140., which is a vertical projection in a plane passing from west to east,) is remarkable for a stockwerke, or interlaced mass of ramifying veins, which has been worked ever since the year 1458. The including rock is a primitive porphyry, superposed upon gneiss; becoming very quartzose as it approaches the lode. This is usually disseminated in minute particles, and accompanied with wolfram, copper and arsenical pyrites, _fer oligiste_, sulphuret of molybdenum, and bismuth, having gangues of lithomarge, fluor spar, mica, and felspar. The space which the ore occupies in the heart of the quartz, is a kind of dædalus, the former being often so dispersed among the latter as to seem to merge into it; whence it is called by the workmen _zwitter_, or _ambiguous_. In 1620, the mine was worked by 21 independent companies, in a most irregular manner, whereby it was damaged to a depth of 170 fathoms by a dreadful downfall of the roofs. This happened on a Sunday, providentially, when the pious miners were all at church. The depth of this abyss, marked by the curved line _b_, _b_, _b_, is 66 fathoms; but the devastation is manifest to a depth of 95 fathoms below that curve, and 35 fathoms below the actual workings, represented at the bottom of the shaft under B. The parts excavated are shaded black in the figure. There are two masses of ore, one under the shaft B, and another under the shaft C; which at the levels 5 and 10 are in communication, but not at 6, 7. There is a direct descent from 8 to 9. The deposits are by no means in one vertical plane, but at a considerable horizontal distance from each other. A is the descending shaft; B is the extraction shaft, near the mouth of which there is a water-wheel; C is another extraction shaft, worked also by means of a water-wheel. A and C are furnished with ladders, but for B the ladders are placed in an accessory shaft _b´_; under D, a shaft is sunk for pumping out the water, by means of an hydraulic wheel at D; E is the gallery or drift for admitting the water which drives the wheels. This falls 300 feet, and ought to be applied to a water-pressure engine, instead of the paddles of a wheel. At D, is the gallery of discharge for the waters, which serves also to ventilate the mine, being cut to the day, through 936 toises of syenitic porphyry and gneiss. J, is a great vaulted excavation. The mine has 13 stages of galleries, of which 11 serve for extracting the ore; 1 is the mill-course; the rest are marked with the numbers 2, 3, 4, &c.; each having besides a characteristic German name. The rare mineral called _topaz pycnite_ is found in this mine, above 10, between the shafts C and D.

The only rule observed in taking ore from this mine, has been to work as much out of each of these levels as is possible, without endangering the superincumbent or collateral galleries; on which account many pillars are constructed to support the roofs. The mine yields annually 1600 quintals (Leipzick) of tin, being four-fifths of the whole furnished by the district of Altenberg; to produce which, 400,000 quintals of ore are raised. 1000 parts of the rock yield 8 of concentrated schlich, equivalent to only 4 of metal; being only 1 in 250 parts.

But the most extensive and productive stream-works, are those of Pentowan, near St. Austle.

_Fig._ 1141. represents a vertical section of the Pentowan mine, taken from the _stream-work_, _Happy Union_. A vast excavation, R, T, U, S, has been hollowed out in the open air, in quest of the alluvial tin ore T, which occurs here at an unusual depth, below the level of the strata R, S. Before getting at this deposit, several successive layers had to be sunk through; namely, 1, 2, 3; the gravel, containing in its middle a band of ochreous earth 2, or ferruginous clay; 4, a black peat, perfectly combustible, of a coarse texture, composed of reeds and woody fibres, cemented into a mass by a fine loam; 5, coarse sea-sand, mingled with marine shells; 6, a blackish marine mud, filled with shells. Below these the deposit of tin-stone occurs, including fragments of various size, of clay slate, flinty slate, quartz, iron ore, jasper; in a word, of all the rocks and gangues to be met with in the surrounding territory, with the exception of granite. Among these fragments there occur, in rounded particles, a coarse quartzose sand, and the tin-stone, commonly in small grains and crystals. Beneath the bed T, the clay slate occurs, called _killas_ (A, X, Y), which supports all the deposits of more recent formation.

The system of mining is very simple. The successive beds, whose thickness is shown in the figure, are visibly cut out into steps or platforms. By a level or gallery of efflux _k_, the waters flow into the bottom of the well _l_, _m_, which contains the drainage pumps; and these are put in action by a machine _j_, moved by a water-wheel. The extraction of the ore is effected by an inclined plane _i_, cut out of one of the sides of the excavation, at an angle of about 45 degrees. At the lower end of this sloping pathway there is a place of loading; and at its upper end _h_, a horse-gin, for alternately raising and lowering the two baskets of extraction on the pathway _i_.

_Mine tin_ requires peculiar care in its mechanical preparation or dressing, on account of the presence of foreign metals, from which, as we have stated, the stream tin is free.

1. As the mine tin is for the most part extremely dispersed through the gangue, it must be all stamped and reduced to a very fine powder, to allow the metallic particles to be separated from the stony matters.

2. As the density of tin-stone is much greater than that of most other metallic ores, it is less apt to run off in the washing; and may, therefore, be dressed so as to be completely stripped of every matter not chemically combined.

3. As the peroxide of tin is not affected by a moderate heat, it may be exposed to calcination; whereby the specific gravity of the associated sulphurets and arseniurets is so diminished as to facilitate their separation.

We may therefore conclude, that tin ore should be first of all pounded very fine in the stamp-mill, then subjected to reiterated washings, and afterwards calcined. The order of proceeding in Cornwall is as follows:--

1. _Cleaning the ore._--This is usually done at the mouth of the gallery of efflux, by agitating the ore in the stream of water as it runs out. Sometimes the ore is laid on a grating, under a fall of water.

2. _Sorting._--The ore thus cleaned, is sorted on the grate, into four heaps: 1. stones rich in tin; 2. stones containing both tin and copper ore; 3. copper ore; 4. sterile pieces, composed in a great measure of stony gangue, with iron and arsenical pyrites. In those veins where there is no copper ore, the second and third heaps are obviously absent. When present, the compound ore is broken into smaller pieces with a mallet, and the fragments are sorted anew.

3. _Stamping._--The stanniferous fragments (No. 1.) are stamped into a sand, of greater or less fineness, according to the dissemination of the tin-stone in the gangue. The determination of the size of the sand, is an object of great importance. It is regulated by a copper plate pierced with small holes, through which every thing from the stamping-mill must run off with the rapid stream introduced for this purpose. This plate forms the front of the stamp cistern.

Several years ago, all the stamp mills were driven by water-wheels, which limited the quantity of ore that could be worked to the hydraulic power of the stream or waterfall; but since the steam engine has been applied to this purpose, the annual product of tin has been greatly increased. On the mine of Huel Vor, there are three steam engines appropriated to the stamping-mills. Their force is 25 horses at least. One of these machines, called _south stamps_, drives 48 pestles; a second, called _old stamps_, drives 36; and a third, 24. The weight of these pestles varies from 370 to 387 pounds; and they generally rise through a space of 10-1/2 inches. The machine called _south stamps_, the strongest of the three, gives 17-1/2 blows in the minute, each pestle being lifted twice for every stroke of the piston. The steam engine of this mill has a power of 25 horses, and it consumes 1062 bushels of coals in the month. Three pestles constitute a battery, or stamp-box.

_Washing and stamping of tin ores at Polgooth, near St. Austle._--The stamps or pestles are of wood, 6 inches by 5-1/2 in the square: they carry lifting bars _b_, secured with a wooden wedge and a bolt of iron, and they terminate below in a lump of cast iron A, called the head, which is fastened to them by a tail, and weighs about 2-1/2 cwts. The shank of the pestle is strengthened with iron hoops. A turning-shaft communicates motion to the stamps by cams stuck round its circumference, so arranged that the second falls while the first and third of each set are uplifted. There are 4 cams on one periphery, and the shaft makes 7 turns in the minute. Each stamp, therefore, gives 28 strokes per minute, and falls through a space of 7-1/2 inches. The stamp chest is open behind, so that the ore slips away under the pestles, by its weight, along the inclined plane with the stream of water. The bottom of the troughs consists of stamped ores. With 6 batteries of 6 pestles each, at Poldice, near Redruth, 120 bags of ore are stamped in 12 hours; each bag containing 18 gallons of 282 cubic inches; measuring altogether 352 cubic feet, and 864 cubic inches.

The openings in the front sides of the troughs are nearly 8 inches by 7-1/2: they are fitted with an iron frame, which is closed with sheet iron, pierced with about 160 holes in the square inch, bored conically, being narrower within. The ore, on issuing, deposits its _rough_ in the first basin, and its slimes in the following basins. The rough is washed in _buddles_ (see LEAD, page 751), and in _tossing tubs_; the slimes in _trunks_, and upon a kind of twin tables, called _racks_. Into the _tossing-tub_, or _dolly_, _fig._ 1143., the stamped ore is thrown, along with a certain quantity of water, and a workman stirs it about with an iron shovel for three or four minutes. He then removes a little of the water with a handled pitcher, and strikes the sides of the tub for 8 or 10 minutes with a hammer, which hastens the subsidence of the denser parts. The water is next poured off by inclining the tub to one side. In one operation of this kind, four distinct strata of the ores may be procured, as indicated by the lines _a b_, _c d_, _e f g_, _h i k_, in the figure. The portion A is to be washed again in the _trunking-box_, _figs._ 1144, 1145.; B is to be washed upon the German chests or racks, _fig._ 1146.; C, the most considerable, is put aside, as schlich fit for the market; D, forming a nucleus the centre of the tub, is to be passed through sieves of copper wire, having 18 meshes in the square inch. This product thus affords a portion D´, which passes through the sieve, and D´´ which remains upon it; the latter is sometimes thrown away, and at others is subjected to the operation called the _tie_, viz., a washing upon the sloping bottom of a long trough.

The slimes are freed from the lighter mud in the trunking-box, _figs._ 1144, 1145.; which is from 7 to 8 feet long. Being accumulated at M, the workman pushes them back with a shovel from _a_ towards _b_. The metallic portion is carried off, and deposited by the stream of water upon the table; but the earthy matters are floated along into a basin beyond it. The product collected in the chest is divided into two portions; the one of which is washed once, and the other twice, upon the _rack_, _fig._ 1146. This is composed of a frame C, which carries a sloping board or table, susceptible of turning round to the right or left upon two pivots, K, K. The head of the table is the inclined plane T. A small board P, which is attached by a band of leather L, forms the communication with the lower table C, whose slope is generally 5 inches in its whole length of 9 feet; but this may vary with the nature of the ore, being somewhat less when it is finely pulverized. The ore is thrown upon T, in small portions of 20 or 25 lbs. A woman spreads it with a rake, while a stream of water sweeps a part of it upon the table, where it gets washed. The fine mud falls through a cross slit near the lower end, into a basin B. After working for a few minutes, should the schlich seem tolerably rich, the operative turns the table round its axis K, K, so as to tumble it into the boxes below. The mud is in B; an impure schlich in B´, which must be washed again upon the _rack_; and a schlich fit for roasting in B´´.

The slope of the rack-table for washing the _roasted_ tin ore, is 7-3/4 inches in the 9 feet.

_Crushing rolls at the Pembroke mines._--Waggons, moved on a railway by an endless rope, bring the ore to be crushed, immediately over the rolls, as shown in _fig._ 1147. A trap being opened in the side of the waggon, the ore falls into the hopper T, whence it passes directly between the twin cylinders C, C, and next upon the sieve D, which receives a seesaw motion horizontally, by means of the rod L, and the crank of the upright turning-shaft. The finer portion of ore, which passes through that sieve, forms the heap S. The coarser portion is tossed over the edge of the sieve, and falls between the cylinders C´ C´, upon a lower level, and forms the second heap S´ of sifted, and S´´ of unsifted, ore.

The holes of the sieves D, D´, being of the same size, the products S, S´, are of the same fineness. S´´ is ground again, being mixed, in the uppermost hopper T, along with the lumps from the waggons.

The diameter and length of the under rolls (see _fig._ 1148.) are each 16 inches. _a b_, is the square end of the gudgeon _t_, which prevents the shaft shifting laterally out of its place. The diameter of the upper rolls is 18 inches, but their length is the same. Both are made of white cast iron, _chilled_ or case-hardened by being cast in iron moulds instead of sand; and they last a month, at least, when of good quality. They make from 10 to 15 turns in a minute, according to the hardness of the ores of tin or copper; and can grind about 50 tons of rich copper ore in 12 hours; but less of the poorer sort.

The next process is the calcination in the _burning-house_; which includes several reverberatory furnaces. At the mine of Poldice, they are 4 or 5 yards long, by from 2-1/2 to 3 yards wide. Their hearth is horizontal; the elevation, about 26 inches high near the fireplace, sinks slightly towards the chimney. There is but one opening, which is in the front; it is closed by a plate-iron door, turning on hinges. Above the door there is a chimney, to let the sulphureous and arsenical vapours fly off, which escape out of the hearth, without annoying the workmen. This chimney leads to horizontal flues, in which the arsenious acid is condensed.

Six hundred weight of ore are introduced; the calcination of which takes from 12 to 18 hours, according to the quantity of pyrites contained in the ore. At the beginning of the operation, a moderate heat is applied, after which it is pushed to a dull red, and kept so during several hours. The door is shut; the materials are stirred from time to time with an iron rake, to expose new surfaces, and prevent them from agglutinating or _kerning_, as the workmen say. The more pyrites is present, the more turning is necessary. Should the ore contain black oxide of iron, it becomes peroxidized, and is then easily removed by a subsequent washing.

_Figs._ 1149, 1150. represent the furnace employed at Altenberg, in Saxony, for roasting tin ores. _a_ is the grate; _b_, the sole of the roasting hearth; _c_, an opening in the arched roof for introducing the dried schlich (the ground and elutriated ore); _d_, is the smoke-mantle or chimney-hood, at the end of the furnace, under which the workmen turn over the spread schlich, with long iron rods bent at their ends; _e_, is the poison vent, which conducts the arsenical vapours to the poison chamber (_gifthaus_) of condensation.

When the ore is sufficiently calcined, as is shown by its ceasing to exhale vapours, it is taken out, and exposed for some days to the action of the air, which decomposes the sulphurets, or changes them into sulphates. The ore is next put into a tub filled with water, stirred up with a wooden rake, and left to settle; by which means the sulphate of copper that may have been formed, is dissolved out. After some time, this water is drawn off into a large tank, and its copper recovered by precipitation with pieces of old iron. In this way, almost all the copper contained in the tin ore is extracted.

The calcined ore is sifted, and treated again on the racks, as above described. The pure schlich, called _black tin_, is sold under this name to the smelters; and that which collects on the middle part of the inclined wash-tables, being much mixed with wolfram, is called _mock lead_. This is passed once more through the stamps, and washed; when it also is sold as _black tin_.

Stream tin is dressed by similar methods: 1. by washing in a trunking-box, of such dimensions that the workman stands upon it in thick boots, and makes a skilful use of the rake; 2. by separating the larger conglomerate pebbles from the smaller pure ones; picking, stamping, and washing, on a kind of _sleeping-tables_. See METALLURGY, _figs._ 677, 678.

The tin ores of Cornwall and Devonshire are all reduced within the counties where they are mined, as the laws prohibit their exportation out of them. Private interests suffer no injury from this prohibition; because the vessels which bring the fuel from Wales, for smelting these ores, return to Swansea and Neath loaded with copper ores.

The smelting-works belong in general to individuals who possess no tin mines, but who purchase at the cheapest rate the ores from the mining proprietors. The ores are appraised according to their contents in metal, and its fineness; conditions which they determine by the following mode of assay. When a certain number of bags of ore, of nearly the same quality, are brought to the works, a small sample is taken from each bag, and the whole are well blended. Two ounces of this average ore are mixed with about 4 per cent. of ground coal, put into an open earthen crucible, and heated in an air furnace (in area about 10 inches square) till reduction takes place. As the furnace is very hot when the crucible is introduced, the assay is finished in about a quarter of an hour. The metal thus revived, is poured into a mould, and what remains in the crucible is pounded in a mortar, that the grains of tin may be added to the ingot.

This method, though imperfect in a chemical point of view, serves the smelter’s purpose, as it affords him a similar result to what he would get on the great scale. A more exact assay would be obtained by fusing, in a crucible lined with hard-rammed charcoal, the ore mixed with 5 per cent. of ground glass of borax. To the crucible a gentle heat should be applied during the first hour, then a strong heat during the second hour, and, lastly, an intense heat for a quarter of an hour. This process brings out from 4 to 5 per cent. more tin than the other; but it has the inconvenience of reducing the iron, should any be present; which by subsequent solution in nitric acid will be readily shown. This assay would be too tedious for the smelter, who may have occasion to try a great many samples in one day.

The smelting of tin ores is effected by two different methods:--

In the first, a mixture of the ore with charcoal is exposed to heat on the hearth of a reverberatory furnace fired with coal.

In the second, the tin ore is fused in a blast furnace, called a blowing-house, supplied with wood charcoal. This method is practised in only a few works, in order to obtain a very pure quality of tin, called _grain tin_ in England, and _étain en larmes_ in France; a metal required for certain arts, as dyeing, &c. This method is applied merely to stream tin.

In the _smelting-houses_, where the tin is worked in reverberatories, two kinds of furnaces are employed; the reduction and the refining furnaces.

_Figs._ 1151, 1152. represent the furnaces for smelting tin at St. Austle, in Cornwall; the former being a longitudinal section, the latter a ground plan, _a_ is the fire-door, through which pitcoal is laid upon the grate _b_; _c_ is the fire-bridge; _d_, the door for introducing the ore; _e_, the door through which the ore is worked upon the hearth _f_; _g_, the stoke-hole; _h_, an aperture in the vault or roof, which is opened at the discharge of the waste schlich, to secure the free escape of the fumes up the chimney; _i_, _i_, air channels, for admitting cold air under the fire-bridge and the sole of the hearth, with the view of protecting them from injury by the intensity of the heat above. _k_, _k_, are basins into which the melted tin is drawn off; _l_, the flue; _m_, the chimney, from 35 to 50 feet high. The roasted and washed schlich is mixed with small coal or culm, along with a little slaked lime, or fluor spar, as a flux; each charge of ore amounts to from 15 to 24 cwt., and contains from 60 to 70 per cent. of metal.

_Fig._ 1153. represents in a vertical section through the tuyère, and _fig._ 1154. in a horizontal section, in the dotted line _x_, _x_, of _fig._ 1153., the furnace employed for smelting tin at the Erzgebirge mines, in Saxony. _a_, are the furnace pillars, of gneiss; _b_, _b_, are shrouding or casing walls; _c_, the tuyère wall; _d_, front wall, both of granite; as also the tuyère _e_. _f_, the sole-stone, of granite, hewn out basin-shaped; _g_, the _eye_, through which the tin and slag are drawn off into the fore-hearth _h_; _i_, the stoke-hearth; _k_, _k_, the light ash chambers; _l_, the arch of the tuyère; _m_, _m_, the common flue, which is placed under the furnace and the hearths, and has its outlet under the vault of the tuyère.

In the smelting furnaces at Geyer the following dimensions are preferred:--Length of the tuyère wall, 11 inches; of the breast wall, 11 inches; depth of the furnace, 17 inches. High chimney-stalks are advantageous where a great quantity of ores is to be reduced, but not otherwise.

The _refining furnaces_ are similar to those which serve for reducing the ore; only, instead of a basin of reception, they have a refining basin placed alongside, into which the tin is run. This basin is about 4 feet in diameter, and 32 inches deep; it consists of an iron pan, placed over a grate, in which a fire may be kindled. Above this pan there is a turning gib, by means of which a billet of wood may be thrust down into the bath of metal, and kept there by wheeling the gibbet over it, lowering a rod, and fixing it in that position.

The works in which the blast furnaces are employed, are called _blowing-houses_. The smelting furnaces are 6 feet high, from the bottom of the crucible (concave hearth) to the throat, which is placed at the origin of a long and narrow chimney, interrupted by a chamber, where the metallic dust, carried off by the blast, is deposited. This chamber is not placed vertically over the furnace; but the lower portion of the chimney has an oblique direction from it. The furnace is lined with an upright cylinder of cast iron, coated internally with loam, with an opening in it for the blast. This opening, which corresponds to the lateral face opposite to the charging side, receives a _tuyère_, in which the nozzles of two cylinder single bellows, driven by a water-wheel, are planted. The _tuyère_ opens at a small height above the sole of the furnace. On a level with the sole, the iron cylinder presents a slope, below which is the hemispherical basin of reception, set partly beneath the interior space of the furnace, and partly without. Near the corner of the building there is a second basin of reception, larger than the first, which can discharge itself into the former by a sloping gutter. Near this basin there is another, for the refining operation. These are all made either of brick or cast iron.

The quality of the average ground-tin ore prepared for smelting is such, that 20 parts of it yield from 12-1/2 to 13 of metallic tin, (62-1/2 to 65 per cent.) The treatment consists of two operations, _smelting_ and _refining_.

_First operation; deoxidization of the ore, and fusion of the tin._--Before throwing the ore into the smelting furnace, it is mixed with from one-fifth to one-eighth of its weight of _blind coal_, in powder, called _culm_; and a little slaked lime is sometimes added, to render the ore more fusible. These matters are carefully blended, and damped with water, to render the charging easier, and to prevent the blast from sweeping any of it away at the commencement. From 12 to 16 cwt. are introduced at a charge; and the doors are immediately closed and luted, while the heat is progressively raised. Were the fire too strong at first, the tin oxide would unite with the quartz of the gangue, and form an enamel. The heat is applied for 6 or 8 hours, during which the doors are not opened; of course the materials are not stirred. By this time the reduction is, in general, finished; the door of the furnace is removed, and the melted mass is worked up to complete the separation of the tin from the scoriæ, and to ascertain if the operation be in sufficient forwardness. When the reduction seems to be finished, the scoriæ are taken out at the same door, with an iron rake, and divided into three sorts; those of the first class A, which constitute at least three-fourths of the whole, are as poor as possible, and may be thrown away; the scoriæ of the second class B, which contain some small grains of tin, are sent to the stamps; those of the third class C, which are last removed from the surface of the bath of tin, are set apart, and re-smelted, as containing a considerable quantity of metal in the form of grain tin. These scoriæ are in small quantity. The stamp slag contains fully 5 per cent. of metallic tin.

As soon as the scoriæ are cleared away, the channel is opened which leads to the basin of reception, into which the tin consequently flows out. Here it is left for some time, that the scoriæ which may be still mixed with the metal, may separate, in virtue of the difference of their specific gravities. When the tin has sufficiently settled, it is lifted out with ladles, and poured into cast-iron moulds, in each of which a bit of wood is fixed, to form a hole in the ingot, for the purpose of drawing it out when it becomes cold.

_Refining of tin._--The object of this operation is to separate from the tin, as completely as possible, the metals reduced and alloyed along with it. These are, principally, iron, copper, arsenic, and tungsten; to which are joined, in small quantities, some sulphurets and arseniurets that have escaped decomposition, a little unreduced oxide of tin, and also some earthy matters which have not passed off with the scoriæ.

_Liquation._--The refining of tin consists of two operations; the first being a liquation, which, in the interior, is effected in a reverberatory furnace, similar to that employed in smelting the ore. (_figs._ 1151, 1152.) The blocks being arranged on the hearth of the furnace, near the bridge, are moderately heated; the tin melts, and flows away into the refining-basin; but, after a certain time, the blocks cease to afford tin, and leave on the hearth a residuum, consisting of a very ferruginous alloy.

Fresh tin blocks are now arranged on the remains of the first; and thus the liquation is continued till the refining-basin be sufficiently full, when it contains about 5 tons. The residuums are set aside, to be treated as shall be presently pointed out.

_Refining proper._--Now begins the second part of the process. Into the tin-bath, billets of green wood are plunged, by aid of the gibbet above described. The disengagement of gas from the green wood produces a constant ebullition in the tin; bringing up to its surface a species of froth, and causing the impurest and densest parts to fall to the bottom. That froth, composed almost wholly of the oxides of tin and foreign metals, is successively skimmed off, and thrown back into the furnace. When it is judged that the tin has boiled long enough, the green wood is lifted out, and the bath is allowed to settle. It separates into different zones, the upper being the purest; those of the middle are charged with a little of the foreign metals; and the lower are much contaminated with them. When the tin begins to cool, and when a more complete separation of its different qualities cannot be looked for, it is lifted out in ladles, and poured into cast-iron moulds. It is obvious, that the order in which the successive blocks are obtained, is that of their purity; those formed from the bottom of the basin being usually so impure, that they must be subjected anew to the refining process, as if they had been directly smelted from the ore.

The refining operation takes 5 or 6 hours; namely, an hour to fill the basin, three hours to boil the tin with the green wood, and from one to two hours for the subsidence.

Sometimes a simpler operation, called _tossing_, is substituted for the above artificial ebullition. To effect it, a workman lifts some tin in a ladle, and lets it fall back into the boiler, from a considerable height, so as to agitate the whole mass. He continues this manipulation for a certain time; after which, he skims with care the surface of the bath. The tin is afterwards poured into moulds, unless it be still impure. In this case, the separation of the metals is completed by keeping the tin in a fused state in the boiler for a certain period, without agitation; whereby the upper portion of the bath (at least one-half) is pure enough for the market.

The moulds into which the tin blocks are cast, are usually made of granite. Their capacity is such, that each block shall weigh a little more than three hundred weight. This metal is called block tin. The law requires them to be stamped or _coined_ by public officers, before being exposed to sale. The purest block tin is called refined tin.

The treatment just detailed gives rise to two stanniferous residuums, which have to be smelted again. These are--

1. The scoriæ B and C, which contain some granulated particles of tin.

2. The dross found on the bottom of the reverberatory furnace, after re-melting the tin to refine it.

The scoriæ C, are smelted without any preparation; but those marked B, are stamped in the mill, and washed, to concentrate the tin grains; and from this rich mixture, called _prillion_, smelted by itself, a tin is procured of very inferior quality. This may be readily imagined, since the metal which forms these granulations is what, being less fusible than the pure tin, solidified quickly, and could not flow off into the metallic bath.

Whenever all the tin blocks have thoroughly undergone the process of liquation, the fire is increased, to melt the less fusible residuary alloy of tin with iron and some other metals, and this is run out into a small basin, totally distinct from the refining basin. After this alloy has reposed for some time, the upper portion is lifted out into block moulds, as impure tin, which needs to be refined anew. On the bottom and sides of the basin there is deposited a white, brittle alloy, with a crystalline fracture, which contains so great a proportion of foreign metals, that no use can be made of it. About 3-1/2 tons of coal are consumed in producing 2 of tin.

_Smelting of tin by the blast furnace._--This mode of reduction employs only wood charcoal, and its object is to obtain tin of the maximum purity to which it can be brought by manufacturing processes. The better ores of the stream-works, and the finer tin sands, are selected for this operation. The washings being always well performed, the oxide of tin is exempt from every arsenical or sulphureous impurity, and is associated with nothing but a little hematite. It is therefore never calcined.

The smelting is effected without addition; only, in a few cases, some of the residuary matters of a former operation are added to the ore. About a ton and six-tenths of wood charcoal are burned for one ton of fine smelted tin. The only rule is, to keep the furnace always full of charcoal and ore. The revived tin is received immediately in the first basin; then run off into the second, where it is allowed to settle for some time. The scoriæ that run off into the first basin, are removed as soon as they fix. These scoriæ are divided into two classes; namely, such as still retain tin oxide, and such as hold none of the metal in that state, but only in granulations. The metallic bath is divided, by repose, into horizontal zones, of different degrees of purity; the more compound and denser matters falling naturally to the bottom of the basin. The tin which forms the superior zones, being judged to be pure enough, is transvased by ladles into the refining basin, previously heated, and under which, if it is of cast-iron, a moderate fire is applied. The tin near the bottom of the receiving basin is always laded out apart, to be again smelted; sometimes, indeed, when the furnace is turning out very impure tin, none of it is transvased into the second basin; but the whole is cast into moulds, to be again treated in the blast furnace.

In general they receive no other preparation, but the green wood ebullition, before passing into the market. Sometimes, however, the block of metal is heated till it becomes brittle, when it is lifted to a considerable height, and let fall, by which it is broken to pieces, and presents an agglomeration of elongated grains or _tears_; whence it is called _grain tin_.

On making a comparative estimate of the expense by the _blowing-house_ process, and by the reverberatory furnace, it has been found that the former yields about 66 per cent. of tin, in smelting the stream or alluvial ore, whose absolute contents are from 75 to 78 parts of metal in the hundred. One ton of tin consumes a ton and six-tenths of wood charcoal, and suffers a loss of 15 per cent. In working with the reverberatory furnace, it is calculated that ore whose mean contents by an exact analysis are 70 per cent., yields 65 per cent. on the great scale. The average value of tin ore, as sold to the smelter, is 50 pounds sterling per ton; but it fluctuates, of course, with the market prices. In 1824, the ore of inferior quality cost 30_l._, while the purest sold for 60_l._ One ton of tin, obtained from the reverberatory furnace, cost--

1-1/2 tons of ore, worth _£_75 0 0
1-3/4 tons of coals, at 10_s._ per ton 0 17 6
Wages of labour, interest on capital, &c. 3 0 0
---------
78 17 6

On comparing these results with the former, we perceive that in a _blowing-house_ the loss of tin is 15 per cent., whereas it is only 5 in the reverberatory furnace. The expense in fuel is likewise much less relatively in the latter process; for only 1-3/4 tons of coals are consumed for one ton of tin; while a ton and six-tenths of wood charcoal are burned to obtain the same quantity of tin in the blowing-house; and it is admitted that one ton of wood charcoal is equivalent to two tons of coal, in calorific effect. Hence every thing conspires to turn the balance in favour of the reverberatory plan. The operation is also, in this way, much simpler, and may be carried on by itself. The scoriæ, besides, from the reverberatory hearth, contain less tin than those derived from the same ores treated with charcoal by the blast, as is done at Altenberg. It must be remembered, however, that the grain tin procured by the charcoal process is reckoned to be finer, and fetches a higher price; a superiority partly due to the purity of the ore reduced, and partly to the purity of the fuel.

To test the quality of tin, dissolve a certain weight of it with heat in muriatic acid; should it contain arsenic, brown-black flocks will be separated during the solution, and arseniuretted hydrogen gas will be disengaged, which, on being burned at a jet, will deposit the usual gray film of metallic arsenic upon a white saucer held a little way above the flame. Other metals present in the tin, are to be sought for, by treating the above solution with nitric acid of spec. grav. 1·16, first in the cold, and at last with heat and a small excess of acid. When the action is over, the supernatant liquid is to be decanted off the peroxidized tin, which is to be washed with very dilute nitric acid, and both liquors are to be evaporated to dissipate the acid excess. If, on the addition of water to the concentrated liquor, a white powder falls, it is a proof that the tin contains bismuth; if on adding sulphate of ammonia, a white precipitate appears, the tin contains lead; water of ammonia added to supersaturation, will occasion reddish-brown flocks, if iron is present; and on evaporating the supernatant liquid to dryness, the copper will be obtained.

The uses of tin are very numerous. Combined with copper, in different proportions, it forms bronze, and a series of other useful alloys; for an account of which see COPPER. With iron, it forms tin-plate; with lead, it constitutes pewter, and solder of various kinds (see LEAD). Tin-foil coated with quicksilver makes the reflecting surface of glass mirrors. (See GLASS.) Nitrate of tin affords the basis of the scarlet dye on wool, and of many bright colours to the calico-printer and the cotton-dyer. (See SCARLET and TIN MORDANTS.) A compound of tin with gold, gives the fine crimson and purple colours to stained glass and artificial gems. (See PURPLE OF CASSIUS.) Enamel is made by fusing oxide of tin with the materials of flint glass. This oxide is also an ingredient in the white and yellow glazes of pottery-ware.

An ACCOUNT of TIN coined in Cornwall and Devon, from 1817 to 1829 inclusive:--

+------+-------+---------+
|Years.|Blocks.|Tons. |
+------+-------+---------+
| 1817 | 25,379|4,120 |
| 1818 | 23,048|3,745-1/3|
| 1819 | 18,881|3,065 |
| 1820 | 17,084|2,773-1/2|
| 1821 | 19,273|3,128 |
| 1822 | 18,732|3,137 |
| 1823 | 24,077|4,031 |
| 1824 | 28,602|4,819 |
| 1825 | 24,902|4,170 |
| 1826 | 26,299|4,406 |
| 1827 | 31,744|5,316 |
| 1828 | 28,179|4,696 |
| 1829 | 26,344|4,396 |
+------+-------+---------+

+---------------------+-------------+
| Tin imported. |Tin exported.|
|Duty, 50_s._ per cwt.| |
+-------+-------------+-------------+
| | _Cwts._ | _Cwts._ |
| 1827 | 2,217 | 2,938 |
| 1828 | 3,386 | 3,258 |
| 1829 | 2,674 | 2,581 |
| 1830 | 15,539 | 10,426 |
| 1831 | 8,099 | 12,226 |
| 1832 | 29,203 | 21,720 |
| 1833 | 35,124 | 39,850 |
| 1834 | 46,769 | 46,685 |
| 1835 | 17,705 | 23,796 |
| 1836 | 23,236 | 17,231 |
+-------+-------------+-------------+

The principal importations are from the East India Company’s territories and Ceylon:--they amounted in 1832 to 24,585 cwts.; in 1833 to 27,928; in 1834 to 33,611; in 1835 to 10,104; and in 1836 to 17,729. From Sumatra and Java 1961 cwts. were imported in 1832, and 1145 in 1834, but in the other years greatly less.

Declared } | | | |
value of } 1827. | 1829. | 1831. | 1833. | 1835.
tin and }302,255_l._|235,178_l._|239,143_l._|282,176_l._|381,076_l._
pewter } | | | |
wares and } | | | |
tin-plates} 1828. | 1830. | 1832. | 1834. | 1836.
exported }266,651_l._|249,657_l._|243,259_l._|337,056_l._|387,951_l._
in } | | | |

Of these goods, from two-fifths to three-fifths go to the United States of America.

ABSTRACT of TIN coined in Cornwall and Devon, in the year ending June 30, 1835; from the _Mining Review_, vol. iii.

+---------------------------+-----------+-------------+------------+
| |Blocks of | Blocks of | |
| |Grain Tin. | Common Tin. | Totals. |
| Smelters. +-----+-----+------+------+------+-----+
| |1834.|1835.|1834. |1835. |1834. |1835.|
+---------------------------+-----+-----+------+------+------+-----+
|Daubuz and Co. | 728 | 875 | 6114 | 4494 | 6842 | 5369|
|Grenfell and Boase | 344 | 196 | 3776 | 3097 | 4120 | 3293|
|Bolitho and Sons | 229 | 153 | 3829 | 3099 | 4058 | 3252|
|R. and J. Michell | 101 | 75 | 709 | 575 | 810 | 650|
|Wheal Vor Adventurers | -- | -- | 3925 | 4069 | 3925 | 4069|
|Taylor, Sons and Co. | -- | 112 | -- | 1250 | -- | 1362|
|John Batten and Son | 28 | 49 | 2352 | 2351 | 2380 | 2400|
|Joseph Carne | -- | -- | 896 | 851 | 896 | 851|
|William Cornish | -- | -- | 622 | 574 | 622 | 574|
|Gill and Co. (at Morwelham)| -- | -- | 758 | -- | 758 | --|
| Ditto (at Calstock) | 60 | -- | 605 | -- | 665 | --|
|Rundle, Paul and Co. | -- | 12 | -- | 1545 | -- | 1557|
| +-----+-----+------+------+------+-----+
| Total |1490 |1472 |23586 |21905 |25076 |23377|
+---------------------------+-----+-----+------+------+------+-----+

Total, in 1834, 4180 tons; in 1835, 3899 tons. (6 blocks = 1 ton.)

TINCAL, crude borax.

TINCTORIAL MATTER. One of the most curious and valuable facts ascertained upon this subject, is, that madder kept in casks, in a warm place, undergoes a species of fermentation, which, by ripening or rather deoxidizing the colouring-matter, increases its dyeing power by no less than from 20 to 50 per cent. See M. H. Schlumberger’s memoir read to the _Société Industrielle de Mulhausen_, 24 November, 1837.

TINCTURE is a title used by apothecaries to designate alcohol, in a somewhat dilute state, impregnated with the active principles of either vegetable or animal substances.

TIN-GLASS, is a name of bismuth.

TIN MORDANTS, for dyeing scarlet:--

_Mordant_ A, as commonly made by the dyers, is composed of 8 parts of aquafortis, 1 part of common salt or sal ammoniac, and 1 of granulated tin. This preparation is very uncertain.

_Mordant_ B.--Pour into a glass globe with a long neck, 3 parts of pure nitric acid at 30° B.; and 1 part of muriatic acid at 17°; shake the globe gently, avoiding the corrosive vapours, and put a loose stopper in its mouth. Throw into this nitro-muriatic acid, one-eighth of its weight of pure tin, in small bits at a time. When the solution is complete, and settled, decant it into bottles, and close them with ground stoppers. It should be diluted only when about to be used.

_Mordant_ C, by Dambourney.--In two drams Fr. (144 grs.) of pure muriatic acid, dissolve 18 grains of Malacca tin. This is reckoned a good mordant for brightening or fixing the colour of peachwood.

_Mordant_ D, by Hellot.--Take 8 ounces of nitric acid, diluted with as much water; dissolve in it half an ounce of sal ammoniac, and 2 drams of nitre. In this acid solution dissolve one ounce of granulated tin of Cornwall, observing not to put in a fresh piece till the preceding be dissolved.

_Mordant_ E, by Scheffer.--Dissolve one part of tin in four of a nitro-muriatic acid, prepared with nitric acid diluted with its own weight of water, and one thirty-secondth of sal ammoniac.

_Mordant_ F, by Poërner.--Mix one pound of nitric acid with one pound of water, and dissolve in it an ounce and a half of sal ammoniac. Stir it well, and add, by very slow degrees, two ounces of tin turned into thin ribbons upon the lathe.

_Mordant_ G, by Berthollet.--Dissolve in nitric acid of 30° B., one-eighth of its weight of sal ammoniac, then add by degrees one-eighth of its weight of tin, and dilute the solution with one-fourth of its weight of water.

_Mordant_ K, by Dambourney.--In one dram (72 grs.) of muriatic acid at 17°, one of nitric acid at 30°, and 18 grains of water, dissolve, slowly and with some heat, 18 grains of fine Malacca tin.

_Mordant_ L, is the birch bark prescribed by Dambourney.--This bark, dried and ground, is said to be a very valuable substance for fixing the otherwise fugitive colours produced by woods, roots, archil, &c.

TIN-PLATE. The only alloy of iron interesting to the arts, is that with tin, in the formation of _tin-plate_, or _white-iron_.

The sheet iron intended for this manufacture is refined with charcoal instead of coke, subsequently rolled to various degrees of thinness, and cut into rectangles of different sizes, by means of a shearing-machine driven by a water-wheel, which will turn out 100 boxes a day, or four times the number cut by hand labour. The first step towards tinning, is to free the metallic surface from every particle of oxide or impurity, for any such would inevitably prevent the iron from alloying with the tin. The plates are next bent separately by hand into a saddle or Λ shape, and ranged in a reverberatory oven, so that the flame may play freely among them, and heat them to redness. They are then plunged into a bath, composed of four pounds of muriatic acid diluted with three gallons of water, for a few minutes, taken out and drained on the floor, and once more exposed to ignition in a furnace, whereby they are _scaled_, that is to say, cast their scales. The above bath will suffice for scaling 1800 plates. When taken out, they are beat level and smooth on a cast-iron block, after which they appear mottled blue and white, if the _scaling_ has been thoroughly done. They are next passed through _chilled_ rolls or cast-iron cylinders, rendered very hard by being cast in thick iron moulds, as has been long practised by the Scotch founders in casting bushes for cart-wheels. After this process of _cold rolling_, the plates are immersed, for ten or twelve hours, in an acidulous lye, made by fermenting bran-water, taking care to set them separately on edge, and to turn them at least once, so that each may receive a due share of the operation. From this lye-steep they are transferred into a leaden trough, divided by partitions, and charged with dilute sulphuric acid. Each compartment is called a _hole_ by the workmen, and is calculated to receive about 225 plates, the number afterwards packed up together in a _box_. In this liquid they are agitated about an hour, till they become perfectly bright, and free from such black spots as might stain their surface at the time of immersion. This process, called pickling, is both delicate and disagreeable, requiring a good workman, at high wages. The temperature of the two last steeps should be at least 90° or 100° F., which is kept up by stoves in the apartments. The plates are finally scoured with hemp and sand in a body of water, and then put aside for use in a vessel of pure water, under which they remain bright and free from rust for many months, a very remarkable circumstance.

The _tinning_ follows these preparatory steps. A range of rectangular cast-iron pots is set over a fire-flue in an apartment called the _stow_, the workmen stationing themselves opposite to the narrow ends. The first rectangle in the range is the tin-pot; the second is the wash-pot, with a partition in it; the third is the grease-pot; the fourth is the pan, grated at bottom; the fifth is the list-pot, and is greatly narrower than any of the rest: they are all of the same length.

The prepared plates, dried by rubbing bran upon them, are first immersed one by one in a pot filled with melted tallow alone, and are left there for nearly an hour. They are thence removed, with the adhering grease, into pot No. 1., filled with a melted mixture of block and grain tin, covered with about four inches of tallow, slightly carbonized. This pot is heated by a fire, playing under its bottom and round its sides, till the metal becomes so hot as nearly to inflame the grease. Here about 340 plates are exposed, upright, to the action of the tin for an hour and a half, or more, according to their thickness. They are next lifted out, and placed upon an iron grating, to let the superfluous metal drain off; but this is more completely removed in the next process, called _washing_.