Chapter 141

THENARD’S BLUE, or COBALT BLUE, is prepared by digesting the oxide of cobalt used in the potteries, with nitric acid, evaporating the nitrate almost to dryness, diluting it with water, and filtering, to separate some arseniate of iron, which usuallyprecipitates. The clear liquor is to be poured into a solution of phosphate of soda, whence an insoluble phosphate of cobalt falls. This being well washed, is to be intimately mixed in its soft state with eight times its weight of well-washed gelatinous alumina, which has been obtained by pouring a solution of alum into water of ammonia in excess. The uniformly coloured paste is to be spread upon plates, dried in a stove, then bruised dry in a mortar, enclosed in a crucible, and subjected to a cherry-red heat for half an hour. On taking out the crucible, and letting it cool, the fine blue pigment is to be removed into a bottle, which is to be stoppered till used.The arseniate of cobalt may be substituted, in the above process, for the phosphate, but it must be mixed with sixteen times its weight of the washed gelatinous alumina. The arseniate is procured by pouring the dilute nitrate of cobalt into a solution of arseniate of potassa. If nitrate of cobalt be mixed with the alumina, and the mixture be treated as above described, a blue pigment will also be obtained, but paler than the preceding, showing that the colour consists essentially of alumina stained with oxide of cobalt.

The arseniate of cobalt may be substituted, in the above process, for the phosphate, but it must be mixed with sixteen times its weight of the washed gelatinous alumina. The arseniate is procured by pouring the dilute nitrate of cobalt into a solution of arseniate of potassa. If nitrate of cobalt be mixed with the alumina, and the mixture be treated as above described, a blue pigment will also be obtained, but paler than the preceding, showing that the colour consists essentially of alumina stained with oxide of cobalt.

THERMOMETER, signifies the measure of heat. Its description belongs to a treatise on chemical physics.

THERMOSTAT, is the name of an apparatus for regulating temperature, in vaporization, distillation, heating baths or hothouses, arid ventilating apartments, &c.; for which I obtained a patent in the year 1831. It operates upon the physical principle, that when two thin metallic bars of different expansibilities are riveted or soldered facewise together, any change of temperature in them will cause a sensible movement of flexure in the compound bar, to one side or other; which movement may be made to operate, by the intervention of levers, &c., in any desired degree, upon valves, stopcocks, stove-registers, air-ventilators, &c.; so as to regulate the temperature of the media in which the said compound bars are placed. Two long rulers, one of steel, and one of hard hammered brass, riveted together, answer very well; the object being not simply toindicate, but tocontrolormodifytemperature. The following diagrams will illustrate a few out of the numerous applications of this instrument:—ThermostatFig.1130.a,b, is a single thermostatic bar, consisting of two or more bars or rulers of differently expansible solids (of which, in certain cases, wood may be one): these bars or rulers are firmly riveted or soldered together, face to face. One end of the compound bar is fixed by bolts ata, to the interior of the containing cistern, boiler, or apartment,a,l,m,b, whereof the temperature has to be regulated, and the other end of the compound bar atb, is left free to move down towardsc, by the flexure which will take place when its temperature is raised.The endb, is connected by a link,b,d, with a leverd,e, which is moved by the flexure into the dotted positionb,g, causing the turning-valve, air-ventilator, or register,o,n, to revolve with a corresponding angular motion, whereby the lever will raise the equipoised slide-damperk,i, which is suspended by a link from the ende, of the levere,d, into the positionk,h. Thus a hothouse or a water-bath may have its temperature regulated by the contemporaneous admission of warm, and discharge of cold air, or water.ThermostatFig.1131.a,b,c, is a thermostatic hoop, immersed horizontally beneath the surface of the water-bath of a still. The hoop is fixed ata, and the two endsb,c, are connected by two linksb d,c d, with a straight sliding rodd,h, to which the hoop will give an endwise motion, when its temperature is altered;e; is an adjusting screw-nut on the rodd,h, for setting the leverf,g, which is fixed on the axis of the turning-valve or cockf; at any desired position, so that the valve may be opened or shut at any desired temperature, corresponding to the widening of the pointsb,c, and the consentaneous retraction of the pointd, towards the circumferencea,b,c, of the hoop.g,h, is an arc graduated by a thermometer, after the screw-pieceehas been adjusted. Through a hole ath, the guide-rod passes.i, is the cold-water cistern;i,f,k, the pipe to admit cold water;l, the overflow pipe, at which the excess of hot water runs off.Thermostatic barsFig.1132.shows a pair of thermostatic bars, bolted fast together at the endsa. The free endsb,c, are of unequal length, so as to act by the cross linksd,f, on the stopcocke. The links are jointed to the handle of the turning plug of the cock, on opposite sidesof its centre; whereby that plug will be turned round in proportion to the widening of the pointsb,c.h,g, is the pipe communicating with the stopcock.ThermostatSuppose that for certain purposes in pharmacy, dyeing, or any other chemical art, a water-bath is required to be maintained steadily at a temperature of 150° F.: let the combined thermostatic bars, hinged together ate,f,fig.1133., be placed in the bath, between the outer and inner vesselsa,b,c,d, being bolted fast to the inner vessel atg; and have their sliding rodk, connected by a link with a lever fixed upon the turning plug of the stopcocki, which introduces cold water from a cisternm, through a pipem,i,n, into the bottom part of the bath. The length of the link must be so adjusted that the flexure of the bars, when they are at a temperature of 150°, will open the said stopcock, and admit cold water to pass into the bottom of the bath through the pipei,n, whereby hot water will be displaced at the top of the bath through an open overflow-pipe atq. An oil bath may be regulated on the same plan; the hot oil overflowing fromq, into a refrigeratory worm, from which it may be restored to the cisternm. When a water bath is heated by the distribution of a tortuous steam pipe through it, asi,n,o,p, it will be necessary to connect the link of the thermostatic bars with the lever of the turning plug of the steam-cock, or of the throttle valvei, in order that the bars, by their flexure, may shut or open the steam passage more or less, according as the temperature of the water in the bath shall tend more or less to deviate from the pitch to which the apparatus has been adjusted. The water of the condensed steam will pass off from the sloping winding-pipei,n,o,p, through the sloping orificep. A saline, acid, or alkaline bath has a boiling temperature proportional to its degree of concentration, and may therefore have its heat regulated by immersing a thermostat in it, and connecting the working part of the instrument with a stopcocki, which will admit water to dilute the bath whenever by evaporation it has become concentrated, and has acquired a higher boiling point. The space for the bath, between the outer and inner pans, should communicate by one pipe with the water-cisternm; and by another pipe, with a safety cisternr, into which the bath may be allowed to overflow during any sudden excess of ebullition.ThermostatFig.1136.is a thermostatic apparatus, composed of three pairs of barsd,d,d, which are represented in a state of flexure by heat; but they become nearly straight and parallel when cold,a,b,c, is a guide rod, fixed at one end by an adjusting screwe, in the strong framef,e, having deep guide grooves at the sides.f,g, is the working-rod, which moves endways when the barsd,d,d, operate by heat or cold. A square register-plateh,g, may be affixed to the rodf,g, so as to be moved backwards and forwards thereby, according to the variations of temperature; or the rodf,g, may cause the circular turning air-registeri, to revolve by rack and wheel-work, or by a chain and pulley. The register-plateh,g, or turning registeri, is situated at the ceiling or upper part of the chamber, and serves to let out hot air.k, is a pulley, over which a cord runs to raise or lower a hot-air registerl, which may be situated near the floor of the apartment or hothouse, to admit hot air into the room.c, is a milled head, for adjusting the thermostat, by means of the screw ate, in order that it may regulate the temperature to any degree.Pyrostat in chimneyFig.1137.represents a chimney, furnished with apyrostata,b,c, acting by the linksb,d,e,c, on a damperf,h,g. The more expansible metal is in the present example supposed to be on the outside. The plane of the damper-plate will, in this case, be turned more directly into the passage of the draught through the chimney by increase of temperature.Turning registerFig.1135.represents a circular turning register, such as is used for a stove, or stove-grate,or for ventilating apartments; it is furnished with a series of spiral thermostatic bars, each bar being fixed fast at the circumference of the circleb,c, of the fixed plate of the air-register; and all the bars act in concert at the centrea, of the twining part of the register, by their ends being inserted between the teeth of a small pinion, or by being jointed to the central part of the turning plate by small pins.ThermostatFig.1134.represents another arrangement of my thermostatic apparatus applied to a circular turning register, like the preceding, for ventilating apartments. Two pairs of compound bars are applied so as to act in concert, by means of the linksa c,b c, on the opposite ends of a short lever, which is fixed on the central part of the turning plate of the air-register. The two pairs of compound barsa,b, are fastened to the circumference of the fixed plate of the turning register, by two sliding rodsa d,b e, which are furnished with adjusting screws. Their motion or flexure is transmitted by the linksa c, andb c, to the turning plate, about its centre, for the purpose of shutting or opening the ventilating sectorial apertures, more or less, according to the temperature of the air which surrounds the thermostatic turning register. By adjusting the screwsa d, andb c, the turning register is made to close all its apertures at any desired degree of temperature; but whenever the air is above that temperature, the flexure of the compound bars will open the apertures.

Thermostat

Fig.1130.a,b, is a single thermostatic bar, consisting of two or more bars or rulers of differently expansible solids (of which, in certain cases, wood may be one): these bars or rulers are firmly riveted or soldered together, face to face. One end of the compound bar is fixed by bolts ata, to the interior of the containing cistern, boiler, or apartment,a,l,m,b, whereof the temperature has to be regulated, and the other end of the compound bar atb, is left free to move down towardsc, by the flexure which will take place when its temperature is raised.

The endb, is connected by a link,b,d, with a leverd,e, which is moved by the flexure into the dotted positionb,g, causing the turning-valve, air-ventilator, or register,o,n, to revolve with a corresponding angular motion, whereby the lever will raise the equipoised slide-damperk,i, which is suspended by a link from the ende, of the levere,d, into the positionk,h. Thus a hothouse or a water-bath may have its temperature regulated by the contemporaneous admission of warm, and discharge of cold air, or water.

Thermostat

Fig.1131.a,b,c, is a thermostatic hoop, immersed horizontally beneath the surface of the water-bath of a still. The hoop is fixed ata, and the two endsb,c, are connected by two linksb d,c d, with a straight sliding rodd,h, to which the hoop will give an endwise motion, when its temperature is altered;e; is an adjusting screw-nut on the rodd,h, for setting the leverf,g, which is fixed on the axis of the turning-valve or cockf; at any desired position, so that the valve may be opened or shut at any desired temperature, corresponding to the widening of the pointsb,c, and the consentaneous retraction of the pointd, towards the circumferencea,b,c, of the hoop.g,h, is an arc graduated by a thermometer, after the screw-pieceehas been adjusted. Through a hole ath, the guide-rod passes.i, is the cold-water cistern;i,f,k, the pipe to admit cold water;l, the overflow pipe, at which the excess of hot water runs off.

Thermostatic bars

Fig.1132.shows a pair of thermostatic bars, bolted fast together at the endsa. The free endsb,c, are of unequal length, so as to act by the cross linksd,f, on the stopcocke. The links are jointed to the handle of the turning plug of the cock, on opposite sidesof its centre; whereby that plug will be turned round in proportion to the widening of the pointsb,c.h,g, is the pipe communicating with the stopcock.

Thermostat

Suppose that for certain purposes in pharmacy, dyeing, or any other chemical art, a water-bath is required to be maintained steadily at a temperature of 150° F.: let the combined thermostatic bars, hinged together ate,f,fig.1133., be placed in the bath, between the outer and inner vesselsa,b,c,d, being bolted fast to the inner vessel atg; and have their sliding rodk, connected by a link with a lever fixed upon the turning plug of the stopcocki, which introduces cold water from a cisternm, through a pipem,i,n, into the bottom part of the bath. The length of the link must be so adjusted that the flexure of the bars, when they are at a temperature of 150°, will open the said stopcock, and admit cold water to pass into the bottom of the bath through the pipei,n, whereby hot water will be displaced at the top of the bath through an open overflow-pipe atq. An oil bath may be regulated on the same plan; the hot oil overflowing fromq, into a refrigeratory worm, from which it may be restored to the cisternm. When a water bath is heated by the distribution of a tortuous steam pipe through it, asi,n,o,p, it will be necessary to connect the link of the thermostatic bars with the lever of the turning plug of the steam-cock, or of the throttle valvei, in order that the bars, by their flexure, may shut or open the steam passage more or less, according as the temperature of the water in the bath shall tend more or less to deviate from the pitch to which the apparatus has been adjusted. The water of the condensed steam will pass off from the sloping winding-pipei,n,o,p, through the sloping orificep. A saline, acid, or alkaline bath has a boiling temperature proportional to its degree of concentration, and may therefore have its heat regulated by immersing a thermostat in it, and connecting the working part of the instrument with a stopcocki, which will admit water to dilute the bath whenever by evaporation it has become concentrated, and has acquired a higher boiling point. The space for the bath, between the outer and inner pans, should communicate by one pipe with the water-cisternm; and by another pipe, with a safety cisternr, into which the bath may be allowed to overflow during any sudden excess of ebullition.

Thermostat

Fig.1136.is a thermostatic apparatus, composed of three pairs of barsd,d,d, which are represented in a state of flexure by heat; but they become nearly straight and parallel when cold,a,b,c, is a guide rod, fixed at one end by an adjusting screwe, in the strong framef,e, having deep guide grooves at the sides.f,g, is the working-rod, which moves endways when the barsd,d,d, operate by heat or cold. A square register-plateh,g, may be affixed to the rodf,g, so as to be moved backwards and forwards thereby, according to the variations of temperature; or the rodf,g, may cause the circular turning air-registeri, to revolve by rack and wheel-work, or by a chain and pulley. The register-plateh,g, or turning registeri, is situated at the ceiling or upper part of the chamber, and serves to let out hot air.k, is a pulley, over which a cord runs to raise or lower a hot-air registerl, which may be situated near the floor of the apartment or hothouse, to admit hot air into the room.c, is a milled head, for adjusting the thermostat, by means of the screw ate, in order that it may regulate the temperature to any degree.

Pyrostat in chimney

Fig.1137.represents a chimney, furnished with apyrostata,b,c, acting by the linksb,d,e,c, on a damperf,h,g. The more expansible metal is in the present example supposed to be on the outside. The plane of the damper-plate will, in this case, be turned more directly into the passage of the draught through the chimney by increase of temperature.

Turning register

Fig.1135.represents a circular turning register, such as is used for a stove, or stove-grate,or for ventilating apartments; it is furnished with a series of spiral thermostatic bars, each bar being fixed fast at the circumference of the circleb,c, of the fixed plate of the air-register; and all the bars act in concert at the centrea, of the twining part of the register, by their ends being inserted between the teeth of a small pinion, or by being jointed to the central part of the turning plate by small pins.

Thermostat

Fig.1134.represents another arrangement of my thermostatic apparatus applied to a circular turning register, like the preceding, for ventilating apartments. Two pairs of compound bars are applied so as to act in concert, by means of the linksa c,b c, on the opposite ends of a short lever, which is fixed on the central part of the turning plate of the air-register. The two pairs of compound barsa,b, are fastened to the circumference of the fixed plate of the turning register, by two sliding rodsa d,b e, which are furnished with adjusting screws. Their motion or flexure is transmitted by the linksa c, andb c, to the turning plate, about its centre, for the purpose of shutting or opening the ventilating sectorial apertures, more or less, according to the temperature of the air which surrounds the thermostatic turning register. By adjusting the screwsa d, andb c, the turning register is made to close all its apertures at any desired degree of temperature; but whenever the air is above that temperature, the flexure of the compound bars will open the apertures.

THIMBLE (Dé à coudre, Fr.;Fingerhut(fingerhat), Germ.); is a small truncated metallic cone, deviating little from a cylinder, smooth within, and symmetrically pitted on the outside with numerous rows of indentations, which is put upon the tip of the middle finger of the right hand, to enable it to push the needle readily and safely through cloth or leather, in the act of sewing. This little instrument is fashioned in two ways; either with a pitted round end, or without one; the latter, called the open thimble, being employed by tailors, upholsterers, and, generally speaking, byneedle-men. The following ingenious process for making this essential implement, the contrivance of M. M. Rouy and Berthier, of Paris, has been much celebrated, and very successful. Sheet-iron, one twenty-fourth of an inch thick, is cut into strips, of dimensions suited to the intended size of the thimbles. These strips are passed under a punch-press, whereby they are cut into discs of about 2 inches diameter, tagged together by a tail. Each strip contains one dozen of these blanks. A child is employed to make them red-hot, and to lay them on a mandril nicely fitted to their size. The workman now strikes the middle of each with a round-faced punch, about the thickness of his finger, and thus sinks it into the concavity of the first mandril. He then transfers it successively to another mandril, which has five hollows of successively increasing depth; and, by striking it into them, brings it to the proper shape.A second workman takes this rude thimble, sticks it in the chuck of his lathe, in order to polish it within, then turns it outside, marks the circles for the gold ornament, and indents the pits most cleverly with a kind of milling tool. The thimbles are next annealed, brightened, and gilt inside, with a very thin cone of gold leaf, which is firmly united to the surface of the iron, simply by the strong pressure of a smooth steel mandril. A gold fillet is applied to the outside, in an annular space turned to receive it, being fixed, by pressure at the edges, into a minute groove formed on the lathe.Thimbles are made in this country by means of moulds in the stamping-machine. SeeStamping of Metals.

A second workman takes this rude thimble, sticks it in the chuck of his lathe, in order to polish it within, then turns it outside, marks the circles for the gold ornament, and indents the pits most cleverly with a kind of milling tool. The thimbles are next annealed, brightened, and gilt inside, with a very thin cone of gold leaf, which is firmly united to the surface of the iron, simply by the strong pressure of a smooth steel mandril. A gold fillet is applied to the outside, in an annular space turned to receive it, being fixed, by pressure at the edges, into a minute groove formed on the lathe.

Thimbles are made in this country by means of moulds in the stamping-machine. SeeStamping of Metals.

THORINA, is a primitive earth, with a metallic basis, discovered in 1828, by Berzelius. It was extracted from the mineralthorite, of which it constitutes 58 per cent., and where it is associated with the oxides of iron, lead, manganese, tin, and uranium, besides earths and alkalis, in all 12 substances. Pure thorina is a white powder, without taste, smell, or alkaline reaction on litmus. When dried and calcined, it is not affected by either the nitric or muriatic acid. It may be fused with borax into a transparent glass, but not with potash or soda. Fresh precipitated thorina is a hydrate, which dissolves readily in the above acids, as well as in solutions of the carbonates of potash, soda, and ammonia, but not in these alkalis in a pure state. This earth consists of 74·5 parts of the metalthorinum, combined with 100 of oxygen. Its hydrate contains one equivalent prime of water. It is hitherto merely a chemical curiosity, remarkable chiefly for a density of 9·402, far greater than that of all the earths, and even of copper.

Thread-frameTHREAD MANUFACTURE. The doubling and twisting of cotton or linen yarn into a compact thread, for weaving bobbin-net, or for sewing garments, is performed by a machine resembling the throstle of the cotton-spinner.Fig.1138.shows the thread-frame in a transverse section, perpendicular to its length.a, is the strong framing of cast iron;b, is thecreel, or shelf, in which the bobbins of yarnl,l, are set loosely upon their respective skewers, along the whole line of the machine, their lower ends turning in oiled steps, and their upper in wire eyes;c, is a glass rod, across which the yarn runs as it is unwound;d,d, are oblong narrow troughs, lined with lead, andfilled with water, for moistening the thread during its torsion; the threads being made to pass through eyes at the bottom of the forke, which has an upright stem for lifting it out, without wetting the fingers, when any thing goes amiss;f,f, are the pressing rollers, the under oneg, being of smooth iron, and the upper oneh, of box-wood; the former extends from end to end of the frame, in lengths comprehending 18 threads, which are joined by square pieces, as in the drawing-rollers of the mule-jenny. The necks of the under rollers are supported, at the ends and the middle, by the standardsi, secured to square basesj, both made of cast iron. The upper cylinder has an iron axis, and is formed of as many rollers as there are threads; each roller being kept in its place upon the lower one by the guidesk, whose vertical slots receive the ends of the axes.The yarn delivered by the bobbinl, glides over the rodc, and descends into the troughd,e, where it gets wetted; on emerging, it goes along the bottom of the rollerg, turns up, so as to pass between it andh, then turns round the top ofh, and finally proceeds obliquely downwards, to be wound upon the bobbinm, after traversing the guide-eyen. These guides are fixed to the end of a plate, which may be turned up by a hinge-joint ato, to make room for the bobbins to be changed.There are three distinct simultaneous movements to be considered in this machine: 1. that of the rollers, or rather of the under roller, for the upper one revolves merely by friction; 2. that of the spindlesm,s′; 3. the up-and-down motion of the bobbins upon the spindles.The first of these motions is produced by means of toothed wheels, upon the right hand of the under set of rollers. The second motion, that of the spindles, is effected by the drumz, which extends the whole length of the frame, turning upon the shaftv, and communicating its rotatory movement (derived from the steam pulley) to the whorlb′, of the spindles, by means of the endless band or corda′. Each of these cords turns four spindles, two upon each side of the frame. They are kept in a proper state of tensionby the weightsc′, which act tangentially upon the circular arcd′, fixed to the extremity of the bell-crank levere′f′g′, and draw in a horizontal direction the tension pulleysh, embraced by the cords. The third movement, or the vertical traverse of the bobbins, along the spindlesm, takes place as follows:—The end of one of the under rollers carries a pinion, which takes into a carrier wheel, that communicates motion to a pinion upon the extremity of the shaftm′, of the heart-shaped pulleyn′. As this eccentric revolves, it gives a reciprocating motion to the leverso′,o′, which oscillate in a vertical plane round the points,p′,p′. The extremities of these levers, on either side, act by means of the linksq′, upon the arms of the sliding socketsr′, and cause the vertical rods′, to slide up and down in guide-holes att′,u′, along with the cast-iron stepv′, which bears the bottom washer of the bobbins. The periphery of the heart-wheeln′, is seen to bear upon friction wheelsx,x′, set in frames adjusted by screws upon the lower end of the bent levers, at such a distance from the pointp′, as that the traverse of the bobbins may be equal to the length of their barrel.By adapting change pinions and their corresponding wheels to the rollers, the delivery of the yarn may be increased or diminished in any degree, so as to vary the degree of twist put into it by the uniform rotation of the drum and spindles. The heart motion being derived from that of the rollers, will necessarily vary with it.Silk thread is commonly twisted in lengths of from 50 to 100 feet, with hand reels, somewhat similar to those employed for making ropes by hand.

Thread-frame

THREAD MANUFACTURE. The doubling and twisting of cotton or linen yarn into a compact thread, for weaving bobbin-net, or for sewing garments, is performed by a machine resembling the throstle of the cotton-spinner.Fig.1138.shows the thread-frame in a transverse section, perpendicular to its length.a, is the strong framing of cast iron;b, is thecreel, or shelf, in which the bobbins of yarnl,l, are set loosely upon their respective skewers, along the whole line of the machine, their lower ends turning in oiled steps, and their upper in wire eyes;c, is a glass rod, across which the yarn runs as it is unwound;d,d, are oblong narrow troughs, lined with lead, andfilled with water, for moistening the thread during its torsion; the threads being made to pass through eyes at the bottom of the forke, which has an upright stem for lifting it out, without wetting the fingers, when any thing goes amiss;f,f, are the pressing rollers, the under oneg, being of smooth iron, and the upper oneh, of box-wood; the former extends from end to end of the frame, in lengths comprehending 18 threads, which are joined by square pieces, as in the drawing-rollers of the mule-jenny. The necks of the under rollers are supported, at the ends and the middle, by the standardsi, secured to square basesj, both made of cast iron. The upper cylinder has an iron axis, and is formed of as many rollers as there are threads; each roller being kept in its place upon the lower one by the guidesk, whose vertical slots receive the ends of the axes.

The yarn delivered by the bobbinl, glides over the rodc, and descends into the troughd,e, where it gets wetted; on emerging, it goes along the bottom of the rollerg, turns up, so as to pass between it andh, then turns round the top ofh, and finally proceeds obliquely downwards, to be wound upon the bobbinm, after traversing the guide-eyen. These guides are fixed to the end of a plate, which may be turned up by a hinge-joint ato, to make room for the bobbins to be changed.

There are three distinct simultaneous movements to be considered in this machine: 1. that of the rollers, or rather of the under roller, for the upper one revolves merely by friction; 2. that of the spindlesm,s′; 3. the up-and-down motion of the bobbins upon the spindles.

The first of these motions is produced by means of toothed wheels, upon the right hand of the under set of rollers. The second motion, that of the spindles, is effected by the drumz, which extends the whole length of the frame, turning upon the shaftv, and communicating its rotatory movement (derived from the steam pulley) to the whorlb′, of the spindles, by means of the endless band or corda′. Each of these cords turns four spindles, two upon each side of the frame. They are kept in a proper state of tensionby the weightsc′, which act tangentially upon the circular arcd′, fixed to the extremity of the bell-crank levere′f′g′, and draw in a horizontal direction the tension pulleysh, embraced by the cords. The third movement, or the vertical traverse of the bobbins, along the spindlesm, takes place as follows:—

The end of one of the under rollers carries a pinion, which takes into a carrier wheel, that communicates motion to a pinion upon the extremity of the shaftm′, of the heart-shaped pulleyn′. As this eccentric revolves, it gives a reciprocating motion to the leverso′,o′, which oscillate in a vertical plane round the points,p′,p′. The extremities of these levers, on either side, act by means of the linksq′, upon the arms of the sliding socketsr′, and cause the vertical rods′, to slide up and down in guide-holes att′,u′, along with the cast-iron stepv′, which bears the bottom washer of the bobbins. The periphery of the heart-wheeln′, is seen to bear upon friction wheelsx,x′, set in frames adjusted by screws upon the lower end of the bent levers, at such a distance from the pointp′, as that the traverse of the bobbins may be equal to the length of their barrel.

By adapting change pinions and their corresponding wheels to the rollers, the delivery of the yarn may be increased or diminished in any degree, so as to vary the degree of twist put into it by the uniform rotation of the drum and spindles. The heart motion being derived from that of the rollers, will necessarily vary with it.

Silk thread is commonly twisted in lengths of from 50 to 100 feet, with hand reels, somewhat similar to those employed for making ropes by hand.

TILES. SeeBricks.

TILTING OF STEEL. SeeSteel. Rees’s Cyclopædia contains an excellent article on this subject.

TIN (Etain, Fr.;Zinn, Germ.); in its pure state, has nearly the colour and lustre of silver. In hardness it is intermediate between gold and lead; it is very malleable, and may be laminated into foil less than the thousandth of an inch in thickness; it has an unpleasant taste, and exhales on friction a peculiar odour; it is flexible in rods or straps of considerable strength, and emits in the act of bending a crackling sound, as if sandy particles were intermixed, called the creaking of tin. A small quantity of lead, or other metal, deprives it of this characteristic quality. Tin melts at 442° Fahr., and is very fixed in the fire at higher heats. Its specific gravity is 7·29. When heated to redness with free access of air, it absorbs oxygen with rapidity, and changes first into a pulverulent gray protoxide, and by longer ignition, into a yellow-white powder, calledputtyof tin. This is the peroxide, consisting of 100 of metal + 27·2 of oxygen.Tin has been known from the most remote antiquity; being mentioned in the books of Moses. The Phœnicians carried on a lucrative trade in it with Spain and Cornwall.There are only two ores of tin; the peroxide, or tin-stone, and tin pyrites; the former of which alone has been found in sufficient abundance for metallurgic purposes. The external aspect of tin-stone has nothing very remarkable. It occurs sometimes in twin crystals; its lustre is adamantine; its colours are very various, as white, gray, yellow, red, brown, black; specific gravity 6·9 at least; which is, perhaps, its most striking feature. It does not melt by itself before the blowpipe; but is reducible in the smoky flame or on charcoal. It is insoluble in acids. It has somewhat of a greasy aspect; and strikes fire with steel.Tin-stone occurs disseminated in the antient rocks, particularly granite; also in beds and veins, in large irregular masses, calledstockwerks; and in pebbles, an assemblage of which is called stream-works, where it occasionally takes a ligneous aspect, and is termedwood-tin.This ore has been found in few countries in a workable quantity. Its principal localities are, Cornwall, Bohemia, Saxony, in Europe; and Malacca and Banca, in Asia. The tin-mines of the Malay peninsula lie between the 10th and 6th degree of south latitude; and are most productive in the island of Junck-Ceylon, where they yield sometimes 800 tons per annum, which are sold at the rate of 48l.each. The ores are found in large caves near the surface; and though actively mined for many centuries, still there is easy access to the unexhausted parts. The mines in the island of Banca, to the east of Sumatra, discovered in 1710, are said to have furnished, in some years, nearly 3500 tons of tin. Small quantities occur in Gallicia in Spain, in the department of Haute Vienne in France, and in the mountain chains of the Fichtel and Riesengebürge in Germany. The columnar pieces of pyramidal tin-ore from Mexico and Chile, are products of stream-works. Small groups of black twin crystals have been lately discovered in the albite rock of Chesterfield in Massachusetts.The Cornish ores occur—1. in small strata or veins, or in masses; 2. in stockwerks, or congeries of small veins; 3. in large veins; 4. disseminated in alluvial deposits.The stanniferous small veins, or thin flat masses, though of small extent, are sometimes very numerous, interposed between certain rocks, parallel to their beds, and are commonly called tin-floors. The same name is occasionally given to stockwerks. Inthe mine of Bottalack, atin-floorhas been found in the killas (primitive schistose rock), thirty-six fathoms below the level of the sea; it is about a foot and a half thick, and occupies the space between a principal vein and its ramification; but there seems to be no connexion between thefloorand the great vein.2. Stockwerks occur in granite and in the felspar porphyry, called in Cornwall,elvan. The most remarkable of these in the granite, is at the tin-mine of Carclase, nearSt. Austle. The works are carried on in the open air, in a friable granite, containing felspar disintegrated intokaolin, or china clay, which is traversed by a great many small veins, composed of tourmaline, quartz, and a little tin-stone, that form black delineations on the face of the light-gray granite. The thickness of these little veins rarely exceeds 6 inches, including the adhering solidified granite, and is occasionally much less. Some of them run nearly east and west, with an almost vertical dip; others, with the same direction, incline to the south at an angle with the horizon of 70 degrees.Stanniferous stockwerks are much more frequent in the elvan (porphyry); of which the mine of Trewidden-ball is a remarkable example. It is worked among flattened masses ofelvan, separated by strata ofkillas, which dip to the east-north-east at a considerable angle. The tin ore occurs in small veins, varying in thickness from half an inch to 8 or 9 inches, which are irregular, and so much interrupted, that it is difficult to determine either their direction or their inclination.3. The large and proper metalliferous veins are not equally distributed over the surface of Cornwall and the adjoining part of Devonshire; but are grouped into three districts; namely, 1. In the south-west of Cornwall, beyond Truro; 2. In the neighbourhood of St. Austle; and 3. In the neighbourhood of Tavistock in Devonshire.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. SeeMines,p. 835.Metal veinsAt 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. Infig.1139.b, marks the first system of tin veins;c, the second; andd, 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 thealluviumwhich 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 ofalluvial tin oreare grouped in the environs of St. Just and St. Austle; where they are calledstream-works; because water is the principal agent employed to separate the metallic oxide from the sand and gravel.Altenberg mineThe 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 workmenzwitter, orambiguous. 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 lineb,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 underB. The parts excavated are shaded black in the figure. There are two masses of ore, one under the shaftB, and another under the shaftC; 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.Ais the descending shaft;Bis the extraction shaft, near the mouth of which there is a water-wheel;Cis another extraction shaft, worked also by means of a water-wheel.AandCare furnished with ladders, but forBthe ladders are placed in an accessory shaftb′; underD, a shaft is sunk for pumping out the water, by means of an hydraulic wheel atD;Eis 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. AtD, 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 calledtopaz pycniteis found in this mine, above 10, between the shaftsCandD.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.Pentowan mineFig.1141.represents a vertical section of the Pentowan mine, taken from thestream-work,Happy Union. A vast excavation,R,T,U,S, has been hollowed out in the open air, in quest of the alluvial tin oreT, which occurs here at an unusual depth, below the level of the strataR,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 bedT, the clay slate occurs, calledkillas(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 effluxk, the waters flow into the bottom of the welll,m, which contains the drainage pumps; and these are put in action by a machinej, moved by a water-wheel. The extraction of the ore is effected by an inclined planei, 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 endh, a horse-gin, for alternately raising and lowering the two baskets of extraction on the pathwayi.Mine tinrequires 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, calledsouth stamps, drives 48 pestles; a second, calledold 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 101⁄2inches. The machine calledsouth stamps, the strongest of the three, gives 171⁄2blows 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.PestleWashing and stamping of tin ores at Polgooth, near St. Austle.—Thestamps or pestlesare of wood, 6 inches by 51⁄2in the square: they carry lifting barsb, secured with a wooden wedge and a bolt of iron, and they terminate below in a lump of cast ironA, called the head, which is fastened to them by a tail, and weighs about 21⁄2cwts. 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 71⁄2inches. 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.Mining apparatusThe openings in the front sides of the troughs are nearly 8 inches by 71⁄2: 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 itsroughin the first basin, and its slimes in the following basins. The rough is washed inbuddles(seeLead,page 751), and intossing tubs; the slimes intrunks, and upon a kind of twin tables, calledracks. Into thetossing-tub, ordolly,fig.1143., the stamped ore is thrown, along with a certain quantity of water, and a workman stirs it aboutwith 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 linesa b,c d,e f g,h i k, in the figure. The portionAis to be washed again in thetrunking-box,figs.1144,1145.;Bis 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 portionD′, which passes through the sieve, andD′′ which remains upon it; the latter is sometimes thrown away, and at others is subjected to the operation called thetie, 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 atM, the workman pushes them back with a shovel fromatowardsb. 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 therack,fig.1146.This is composed of a frameC, 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 planeT. A small boardP, which is attached by a band of leatherL, forms the communication with the lower tableC, 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 uponT, 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 basinB. After working for a few minutes, should the schlich seem tolerably rich, the operative turns the table round its axisK,K, so as to tumble it into the boxes below. The mud is inB; an impure schlich inB′, which must be washed again upon therack; and a schlich fit for roasting inB′′.The slope of the rack-table for washing theroastedtin ore, is 73⁄4inches in the 9 feet.CrushersCrushing 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 infig.1147.A trap being opened in the side of the waggon, the ore falls into the hopperT, whence it passes directly between the twin cylindersC,C, and next upon the sieveD, which receives a seesaw motion horizontally, by means of the rodL, and the crank of the upright turning-shaft. The finer portion of ore, which passes through that sieve, forms the heapS. The coarser portion is tossed over the edge of the sieve, and falls between the cylindersC′C′, upon a lower level, and forms the second heapS′ of sifted, andS′′ of unsifted, ore.The holes of the sievesD,D′, being of the same size, the productsS,S′, are of the same fineness.S′′ is ground again, being mixed, in the uppermost hopperT, along with the lumps from the waggons.Crusher rollThe diameter and length of the under rolls (seefig.1148.) are each 16 inches.a b, is the square end of the gudgeont, 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,chilledor 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 theburning-house; which includes several reverberatory furnaces. At the mine of Poldice, they are 4 or 5 yards long, by from 21⁄2to 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 orkerning, 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.Roasting furnaceFigs.1149,1150.represent the furnace employed at Altenberg, in Saxony, for roasting tin ores.ais 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, calledblack 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 calledmock lead. This is passed once more through the stamps, and washed; when it also is sold asblack 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 useof the rake; 2. by separating the larger conglomerate pebbles from the smaller pure ones; picking, stamping, and washing, on a kind ofsleeping-tables. SeeMetallurgy,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, calledgrain tinin England, andétain en larmesin France; a metal required for certain arts, as dyeing, &c. This method is applied merely to stream tin.In thesmelting-houses, where the tin is worked in reverberatories, two kinds of furnaces are employed; the reduction and the refining furnaces.Smelting furnacesFigs.1151,1152.represent the furnaces for smelting tin at St. Austle, in Cornwall; the former being a longitudinal section, the latter a ground plan,ais the fire-door, through which pitcoal is laid upon the grateb;cis the fire-bridge;d, the door for introducing the ore;e, the door through which the ore is worked upon the hearthf;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.Smelting furnaceFig.1153.represents in a vertical section through the tuyère, andfig.1154.in a horizontal section, in the dotted linex,x, offig.1153., the furnaceemployed 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 tuyèree.f, the sole-stone, of granite, hewn out basin-shaped;g, theeye, through which the tin and slag are drawn off into the fore-hearthh;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.Therefining furnacesare 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 calledblowing-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 atuyère, in which the nozzles of two cylinder single bellows, driven by a water-wheel, are planted. Thetuyèreopens 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 121⁄2to 13 of metallic tin, (621⁄2to 65 per cent.) The treatment consists of two operations,smeltingandrefining.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 ofblind coal, in powder, calledculm; 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 classA, which constitute at least three-fourths of the whole, are as poor as possible, and may be thrown away; the scoriæ of the second classB, which contain some small grains of tin, are sent to the stamps; those of the third classC, 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 thebasin 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, calledtossing, 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 orcoinedby 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 scoriæBandC, 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 scoriæC, are smelted without any preparation; but those markedB, are stamped in the mill, and washed, to concentrate the tin grains; and from this rich mixture, calledprillion, 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 31⁄2tons of coal are consumed in producing 2 of tin.Smelting of tin by the blast furnace.—This mode of reduction employs only woodcharcoal, 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 ortears; whence it is calledgrain tin.On making a comparative estimate of the expense by theblowing-houseprocess, 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 30l., while the purest sold for 60l.One ton of tin, obtained from the reverberatory furnace, cost—11⁄2tons of ore, worth£750013⁄4tons of coals, at 10s.per ton0176Wages of labour, interest on capital, &c.30078176On comparing these results with the former, we perceive that in ablowing-housethe 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 13⁄4tons 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-brownflocks, 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 seeCopper. With iron, it forms tin-plate; with lead, it constitutes pewter, and solder of various kinds (seeLead). Tin-foil coated with quicksilver makes the reflecting surface of glass mirrors. (SeeGlass.) 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. (SeeScarletandTin Mordants.) A compound of tin with gold, gives the fine crimson and purple colours to stained glass and artificial gems. (SeePurple 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.AnAccountofTincoined in Cornwall and Devon, from 1817 to 1829 inclusive:—Years.Blocks.Tons.181725,3794,120181823,0483,7451⁄3181918,8813,065182017,0842,7731⁄2182119,2733,128182218,7323,137182324,0774,031182428,6024,819182524,9024,170182626,2994,406182731,7445,316182828,1794,696182926,3444,396Tinimported.Duty, 50s.per cwt.Tinexported.Cwts.Cwts.18272,2172,93818283,3863,25818292,6742,581183015,53910,42618318,09912,226183229,20321,720183335,12439,850183446,76946,685183517,70523,796183623,23617,231The 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 tin and pewter wares and tin-plates exported in-1827.1829.1831.1833.1835.302,255l.235,178l.239,143l.282,176l.381,076l.1828.1830.1832.1834.1836.266,651l.249,657l.243,259l.337,056l.387,951l.Of these goods, from two-fifths to three-fifths go to the United States of America.AbstractofTincoined in Cornwall and Devon, in the year ending June 30, 1835; from theMining Review, vol. iii.Smelters.Blocks ofGrain Tin.Blocks ofCommon Tin.Totals.1834.1835.1834.1835.1834.1835.Daubuz and Co.7288756114449468425369Grenfell and Boase3441963776309741203293Bolitho and Sons2291533829309940583252R. and J. Michell10175709575810650Wheal Vor Adventurers——3925406939254069Taylor, Sons and Co.—112—1250—1362John Batten and Son28492352235123802400Joseph Carne——896851896851William Cornish——622574622574Gill and Co.(at Morwelham)——758—758—Ditto(at Calstock)60—605—665—Rundle, Paul and Co.—12—1545—1557Total1490147223586219052507623377Total, in 1834, 4180 tons; in 1835, 3899 tons. (6 blocks = 1 ton.)

Tin has been known from the most remote antiquity; being mentioned in the books of Moses. The Phœnicians carried on a lucrative trade in it with Spain and Cornwall.

There are only two ores of tin; the peroxide, or tin-stone, and tin pyrites; the former of which alone has been found in sufficient abundance for metallurgic purposes. The external aspect of tin-stone has nothing very remarkable. It occurs sometimes in twin crystals; its lustre is adamantine; its colours are very various, as white, gray, yellow, red, brown, black; specific gravity 6·9 at least; which is, perhaps, its most striking feature. It does not melt by itself before the blowpipe; but is reducible in the smoky flame or on charcoal. It is insoluble in acids. It has somewhat of a greasy aspect; and strikes fire with steel.

Tin-stone occurs disseminated in the antient rocks, particularly granite; also in beds and veins, in large irregular masses, calledstockwerks; and in pebbles, an assemblage of which is called stream-works, where it occasionally takes a ligneous aspect, and is termedwood-tin.

This ore has been found in few countries in a workable quantity. Its principal localities are, Cornwall, Bohemia, Saxony, in Europe; and Malacca and Banca, in Asia. The tin-mines of the Malay peninsula lie between the 10th and 6th degree of south latitude; and are most productive in the island of Junck-Ceylon, where they yield sometimes 800 tons per annum, which are sold at the rate of 48l.each. The ores are found in large caves near the surface; and though actively mined for many centuries, still there is easy access to the unexhausted parts. The mines in the island of Banca, to the east of Sumatra, discovered in 1710, are said to have furnished, in some years, nearly 3500 tons of tin. Small quantities occur in Gallicia in Spain, in the department of Haute Vienne in France, and in the mountain chains of the Fichtel and Riesengebürge in Germany. The columnar pieces of pyramidal tin-ore from Mexico and Chile, are products of stream-works. Small groups of black twin crystals have been lately discovered in the albite rock of Chesterfield in Massachusetts.

The Cornish ores occur—1. in small strata or veins, or in masses; 2. in stockwerks, or congeries of small veins; 3. in large veins; 4. disseminated in alluvial deposits.

The stanniferous small veins, or thin flat masses, though of small extent, are sometimes very numerous, interposed between certain rocks, parallel to their beds, and are commonly called tin-floors. The same name is occasionally given to stockwerks. Inthe mine of Bottalack, atin-floorhas been found in the killas (primitive schistose rock), thirty-six fathoms below the level of the sea; it is about a foot and a half thick, and occupies the space between a principal vein and its ramification; but there seems to be no connexion between thefloorand the great vein.

2. Stockwerks occur in granite and in the felspar porphyry, called in Cornwall,elvan. The most remarkable of these in the granite, is at the tin-mine of Carclase, nearSt. Austle. The works are carried on in the open air, in a friable granite, containing felspar disintegrated intokaolin, or china clay, which is traversed by a great many small veins, composed of tourmaline, quartz, and a little tin-stone, that form black delineations on the face of the light-gray granite. The thickness of these little veins rarely exceeds 6 inches, including the adhering solidified granite, and is occasionally much less. Some of them run nearly east and west, with an almost vertical dip; others, with the same direction, incline to the south at an angle with the horizon of 70 degrees.

Stanniferous stockwerks are much more frequent in the elvan (porphyry); of which the mine of Trewidden-ball is a remarkable example. It is worked among flattened masses ofelvan, separated by strata ofkillas, which dip to the east-north-east at a considerable angle. The tin ore occurs in small veins, varying in thickness from half an inch to 8 or 9 inches, which are irregular, and so much interrupted, that it is difficult to determine either their direction or their inclination.

3. The large and proper metalliferous veins are not equally distributed over the surface of Cornwall and the adjoining part of Devonshire; but are grouped into three districts; namely, 1. In the south-west of Cornwall, beyond Truro; 2. In the neighbourhood of St. Austle; and 3. In the neighbourhood of Tavistock in Devonshire.

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. SeeMines,p. 835.

Metal veins

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. Infig.1139.b, marks the first system of tin veins;c, the second; andd, 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 thealluviumwhich 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 ofalluvial tin oreare grouped in the environs of St. Just and St. Austle; where they are calledstream-works; because water is the principal agent employed to separate the metallic oxide from the sand and gravel.

Altenberg mine

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 workmenzwitter, orambiguous. 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 lineb,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 underB. The parts excavated are shaded black in the figure. There are two masses of ore, one under the shaftB, and another under the shaftC; 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.Ais the descending shaft;Bis the extraction shaft, near the mouth of which there is a water-wheel;Cis another extraction shaft, worked also by means of a water-wheel.AandCare furnished with ladders, but forBthe ladders are placed in an accessory shaftb′; underD, a shaft is sunk for pumping out the water, by means of an hydraulic wheel atD;Eis 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. AtD, 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 calledtopaz pycniteis found in this mine, above 10, between the shaftsCandD.

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.

Pentowan mine

Fig.1141.represents a vertical section of the Pentowan mine, taken from thestream-work,Happy Union. A vast excavation,R,T,U,S, has been hollowed out in the open air, in quest of the alluvial tin oreT, which occurs here at an unusual depth, below the level of the strataR,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 bedT, the clay slate occurs, calledkillas(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 effluxk, the waters flow into the bottom of the welll,m, which contains the drainage pumps; and these are put in action by a machinej, moved by a water-wheel. The extraction of the ore is effected by an inclined planei, 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 endh, a horse-gin, for alternately raising and lowering the two baskets of extraction on the pathwayi.

Mine tinrequires 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, calledsouth stamps, drives 48 pestles; a second, calledold 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 101⁄2inches. The machine calledsouth stamps, the strongest of the three, gives 171⁄2blows 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.

Pestle

Washing and stamping of tin ores at Polgooth, near St. Austle.—Thestamps or pestlesare of wood, 6 inches by 51⁄2in the square: they carry lifting barsb, secured with a wooden wedge and a bolt of iron, and they terminate below in a lump of cast ironA, called the head, which is fastened to them by a tail, and weighs about 21⁄2cwts. 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 71⁄2inches. 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.

Mining apparatus

The openings in the front sides of the troughs are nearly 8 inches by 71⁄2: 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 itsroughin the first basin, and its slimes in the following basins. The rough is washed inbuddles(seeLead,page 751), and intossing tubs; the slimes intrunks, and upon a kind of twin tables, calledracks. Into thetossing-tub, ordolly,fig.1143., the stamped ore is thrown, along with a certain quantity of water, and a workman stirs it aboutwith 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 linesa b,c d,e f g,h i k, in the figure. The portionAis to be washed again in thetrunking-box,figs.1144,1145.;Bis 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 portionD′, which passes through the sieve, andD′′ which remains upon it; the latter is sometimes thrown away, and at others is subjected to the operation called thetie, 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 atM, the workman pushes them back with a shovel fromatowardsb. 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 therack,fig.1146.This is composed of a frameC, 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 planeT. A small boardP, which is attached by a band of leatherL, forms the communication with the lower tableC, 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 uponT, 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 basinB. After working for a few minutes, should the schlich seem tolerably rich, the operative turns the table round its axisK,K, so as to tumble it into the boxes below. The mud is inB; an impure schlich inB′, which must be washed again upon therack; and a schlich fit for roasting inB′′.

The slope of the rack-table for washing theroastedtin ore, is 73⁄4inches in the 9 feet.

Crushers

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 infig.1147.A trap being opened in the side of the waggon, the ore falls into the hopperT, whence it passes directly between the twin cylindersC,C, and next upon the sieveD, which receives a seesaw motion horizontally, by means of the rodL, and the crank of the upright turning-shaft. The finer portion of ore, which passes through that sieve, forms the heapS. The coarser portion is tossed over the edge of the sieve, and falls between the cylindersC′C′, upon a lower level, and forms the second heapS′ of sifted, andS′′ of unsifted, ore.

The holes of the sievesD,D′, being of the same size, the productsS,S′, are of the same fineness.S′′ is ground again, being mixed, in the uppermost hopperT, along with the lumps from the waggons.

Crusher roll

The diameter and length of the under rolls (seefig.1148.) are each 16 inches.a b, is the square end of the gudgeont, 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,chilledor 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 theburning-house; which includes several reverberatory furnaces. At the mine of Poldice, they are 4 or 5 yards long, by from 21⁄2to 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 orkerning, 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.

Roasting furnace

Figs.1149,1150.represent the furnace employed at Altenberg, in Saxony, for roasting tin ores.ais 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, calledblack 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 calledmock lead. This is passed once more through the stamps, and washed; when it also is sold asblack 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 useof the rake; 2. by separating the larger conglomerate pebbles from the smaller pure ones; picking, stamping, and washing, on a kind ofsleeping-tables. SeeMetallurgy,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, calledgrain tinin England, andétain en larmesin France; a metal required for certain arts, as dyeing, &c. This method is applied merely to stream tin.

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

Smelting 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,ais the fire-door, through which pitcoal is laid upon the grateb;cis the fire-bridge;d, the door for introducing the ore;e, the door through which the ore is worked upon the hearthf;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.

Smelting furnace

Fig.1153.represents in a vertical section through the tuyère, andfig.1154.in a horizontal section, in the dotted linex,x, offig.1153., the furnaceemployed 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 tuyèree.f, the sole-stone, of granite, hewn out basin-shaped;g, theeye, through which the tin and slag are drawn off into the fore-hearthh;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.

Therefining furnacesare 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 calledblowing-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 atuyère, in which the nozzles of two cylinder single bellows, driven by a water-wheel, are planted. Thetuyèreopens 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 121⁄2to 13 of metallic tin, (621⁄2to 65 per cent.) The treatment consists of two operations,smeltingandrefining.

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 ofblind coal, in powder, calledculm; 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 classA, which constitute at least three-fourths of the whole, are as poor as possible, and may be thrown away; the scoriæ of the second classB, which contain some small grains of tin, are sent to the stamps; those of the third classC, 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 thebasin 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, calledtossing, 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 orcoinedby 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 scoriæBandC, 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 scoriæC, are smelted without any preparation; but those markedB, are stamped in the mill, and washed, to concentrate the tin grains; and from this rich mixture, calledprillion, 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 31⁄2tons of coal are consumed in producing 2 of tin.

Smelting of tin by the blast furnace.—This mode of reduction employs only woodcharcoal, 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 ortears; whence it is calledgrain tin.

On making a comparative estimate of the expense by theblowing-houseprocess, 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 30l., while the purest sold for 60l.One ton of tin, obtained from the reverberatory furnace, cost—

On comparing these results with the former, we perceive that in ablowing-housethe 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 13⁄4tons 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-brownflocks, 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 seeCopper. With iron, it forms tin-plate; with lead, it constitutes pewter, and solder of various kinds (seeLead). Tin-foil coated with quicksilver makes the reflecting surface of glass mirrors. (SeeGlass.) 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. (SeeScarletandTin Mordants.) A compound of tin with gold, gives the fine crimson and purple colours to stained glass and artificial gems. (SeePurple 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.

AnAccountofTincoined in Cornwall and Devon, from 1817 to 1829 inclusive:—

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.

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

AbstractofTincoined in Cornwall and Devon, in the year ending June 30, 1835; from theMining Review, vol. iii.

Back to Index Next