Rammelsberg mineA mine so ancient as that of Rammelsberg, and which was formerly divided among several adventurous companies, cannot fail to present a great many shafts and excavations; but out of the 15 pits, only two are employed for the present workings; namely, those markedA BandE F, infig.736., by which the whole extraction and drainage are executed.—The general system of exploitation by fire, as practised in this mine, consists of the following operations:—1. An advance is made towards the deposits of ore, successively at different levels, by transverse galleries which proceed from the shaft of extraction, and terminate at the wall of the stratiform mass.2. There is formed in the level to be worked, large vaults in the heart of the ore, by means of fire, as we shall presently describe.3. The floor of these vaults is raised up by means of terraces formed from the rubbish, in proportion as the roof is scooped out.4. The ores detached by the fire from their bed, are picked and gathered; sometimes the larger blocks are blasted with gunpowder.5. Lastly, the ores thus obtained are wheeled towards the shaft of extraction, and turned out to the day.Let us now see how the excavation by fire is practised; and in that view, let us consider the state of the workings in the mines of Rammelsberg in 1809. We may remark infig.736.the regularity of the vaults previously scooped out above the levelB C, and the other vaults which are in full activity of operation. It is, therefore, towards the lower levels that the new workings must be directed. For this purpose, the transverse gallery being already completed, there is prepared on the first of these floors a vault of exploitation atb, which eventually is to become similar to those of the superior levels. At the same time, there is commenced at the starting point below it, reached by a small well dug in the line of the mineral deposit, a transverse gallery in the rock, by means of blasting with gunpowder. The rock is also attacked at the starting-point by a similarcut, which advances to meet the first perforation. In this way, whenever the vaults of the levelCare exhausted of ore and terraced up with rubbish, those of the level beneath it will be in full activity.Others will then be prepared at a lower level; and the exploitation may afterwards be driven below this level by pursuing the same plan, by which the actual depth of excavation has been gained.In workings by fire we must distinguish, 1. The case where it is necessary to open a vault immediately from the floor; 2. The case where the vault having already a certain elevation, it is necessary to heighten its roof. In the former case, the wall or floor of the mineral deposit is first penetrated by blasting with gunpowder. As soon as this penetration is effected over a certain length, parallel to the direction of the future vault, as happens atb, there is arranged on the bottom a horizontal layer of billets of firwood, over which other billets are piled in nearly a vertical position, which rest upon the ore, so that the flame in its expansion comes to play against the mineral mass to bedetached. When after some similar operations, the flame of the pile can no longer reach the ore of the roof on account of its height, a small terrace of rubbish must be raised on the floor of the deposit; and over this terrace, a new pile of faggots is to be heaped up as above described. The ancient miners committed the fault of constantly placing such terraces close to the roof, and consequently arranging the faggots against this portion of the ore, so that the flame circulated from the roof down to the floor. The result of such procedure was the weakening of the roof, and the loss of much of the ore which could not be extracted from so unstable a fabric; and besides, much more wood was burned than at the present day, because the action of the flame was dissipated in part against the whole mass of the roof, instead of being concentred on the portion of the ore which it was desired to dislodge. Now, the flame is usually made to circulate from the floor to the roof, in commencing a new vault.When the vault has already a certain height, care is always taken that between the roof of the vault and the rubbish on which the pile is arranged, no more than two yards of space should intervene, in order that the flame may embrace equally the whole concavity of the vault, and produce an uniform effect on all its parts. Here, the pile is formed of horizontal beds, disposed crosswise above one another, and presents four free vertical faces, whence it has been called achestby the miners.It is usually on Saturday that the fire is applied to all the piles of faggots distributed through the course of the week. Those in the upper floors of exploitation are first burned, in order that the inferior piles may not obstruct by their vitiated air, the combustion of the former. Thus, at 4 o’clock in the morning, the fires are kindled in the upper ranges; from pile to pile, the fireman and his assistant descend towards the lower floors, which occupies them till 3 o’clock in the afternoon. Vainly should we endeavour to describe the majestic and terrific spectacle which the fire presents, as it unfolds its wings under its metallic vaults, soon filled with vast volumes of smoke and flame. Let us mark the useful effect which it produces.When the flame has beat for a few instants on the beds of ore, a strong odour of sulphur, and sometimes of arsenic is perceived; and soon thereafter loud detonations are heard in the vaults. Suddenly the flame is seen to assume a blue colour, or even a white; and at this period, after a slight explosion, flakes of the ore, of greater or less magnitude, usually fall down on the fire, but the chief portion of the heated mineral still remains fixed to the vault. The ores pass now into a shattered and divided condition, which allows them afterwards to be detached by long forks of iron. In this manner the fire, volatilizing entirely some principles, such as sulphur, zinc, arsenic, and water, changing the aggregation of the constituent parts of the ore, and causing fissures by their unequal expansibilities, facilitates the excavation of such materials as resist by their tenacity the action of gunpowder.The combustion goes on without any person entering the mine from Saturday evening till Monday morning, on which day, the fireman and his assistants proceed to extinguish the remains of the bonfires. On Monday also some piles are constructed in the parts where the effect of the former ones has been incomplete; and they are kindled after the workmen have quitted the mine. On Tuesday all hands are employed in detaching the ores, in sorting them, taking them out, and preparing new piles against the next Saturday.The labour of a week consists for every man of five posts during the day, each of 8 hours, and of one post of four hours for Saturday. Moreover, an extra allowance is made to such workmen as employ themselves some posts during the night.The labour of one compartment oratelierof the mine consists therefore in arranging the faggots, in detaching the ore which has already experienced the action of the fire, in breaking the blocks obtained, in separating the ore from thedébrisof the pile, and whenever it may be practicable or useful, in boring holes for blasting with gunpowder. The heat is so great in this kind of mine, that the men are obliged to work in it without clothing.We have already remarked, that besides the working by fire, which is chiefly used here, recourse is sometimes had to blasting by gunpowder. This is done in order either to recover the bottom part or ground of the vaults on which the fire can act but imperfectly, to clear away some projections which would interfere with the effect of the pile, or lastly to strip the surrounding rock from the mass of the ore, and thence to obtain schist proper for the construction of the rubbish-terraces.The blasting process is employed when the foremen of the workshop or mine-chamber judge that a hole well placed may separate enough of ore to pay the time, the repair of tools, and the gunpowder expended. But this indemnification is rarely obtained. The following statement will give an idea of the tenacity which the mineral deposit often presents.In 1808, in a portion of the Rammelsberg mine, the ore, consisting of extremely compact iron and copper pyrites, was attacked by a single man, who bored a mining hole.After 11 posts of obstinate labour, occupying altogether 88 hours, the workman, being vigilantly superintended, had been able to advance the hole to a depth of no more than 4 inches; in doing which he had rendered entirely unserviceable 126 punches or borers, besides 26 others which had been re-tipped with steel, and 201 which had been sharpened; 61⁄4pounds of oil had been consumed in giving him light; and half a pound of gunpowder was required for blasting the bore. It was found from a calculation made upon these facts by the administration of mines, that every inch deep of this hole cost, at their low price of labour, nearly a florin, value two shillings and sixpence.It is therefore evident that though the timber, of which the consumption is prodigiously great, were much less abundant and dearer than it still is at Rammelsberg, mining by fire would be preferable to every other mode of exploitation. It is even certain, that on any supposition, the employment of gunpowder would not be practicable for every part of the mine; and if fuel came to fail, it would be requisite to renounce the workings at Rammelsberg, although this mountain still contains a large quantity of metals.If in all mines the free circulation of air be an object of the highest importance, we must perceive how indispensable it must be in every part of a mine where the mode of exploitation maintains the temperature of the air at 112° Fahr., when the workmen return into it after the combustion of the piles, and in which besides it is necessary that this combustion be effected with activity in their absence. But in consequence of the extent and mutual ramifications of the workings, the number of the shafts, galleries, and their differences of level, the ventilation of the mine is in a manner spontaneously maintained. The high temperature is peculiarly favourable to it. The aid of art consists merely in placing some doors judiciously, which may be opened or shut at pleasure, to carry on the circulation of the air.In considering the Rammelsberg from its summit, which rises about 400 yards above the town of Goslar, we observe, first, beds of slaty sandstone, which become the more horizontal the nearer they approach to the surface. At about 160 yards below the top level there occurs, in the bosom of the slaty graywacke, a powerful stratum of shells impasted in a ferruginous sandstone. SeeD,fig.730.In descending towards the face of the ore, the parallel stratification of the clay-slate which forms its walls and roof grows more and more manifest. Here the slate is black, compact, and thinly foliated. The inclination of the different beds of rock is indicated atB. The substance of the workable mass is copper and iron pyrites, along with sulphuret of lead, accompanied by quartz, carbonate of lime, compact sulphate of baryta, and occasionally gray copper (fahlerz), sulphuret of zinc, and arsenical pyrites.The ores are argentiferous and auriferous, but very slightly so, especially as to the gold. It is the ores of lead and copper which contain the silver, and in the latter the gold is found, but without its being well ascertained in what mineral it is deposited. Sometimes the copper occurs in the native state, or as copper of cementation. Beautiful crystals of sulphate of lime are found in the old workings.Kahnenkuhler shaftFig. 737 enlarged(150 kB)Infigs.736.737.,A Bis the shaft of extraction, called theKahnenkuhler;Nis the ventilation shaft, calledBreitlingerwetterschacht;Pis the extraction shaft, calledInnier-schacht.E F, is a new extraction-shaft, calledNeuer treibschacht, by which also the water is pumped up; byA B, andE F, the whole extraction and draining are carried on. The ores are raised in these shafts to the level of the waggon-gallery (galerie de roulage)i, by the whimsl,q, provided with ropes and buckets. 1, 2, 3, 4,fig.736., represent the positions of four water-wheels for working the whims; the first two being employed in extracting the ores, the last two in draining. The driving stream is led to the wheel 1, along the driftl; whence it falls in succession upon the wheels 2, 3, 4. The general system of working consists of the following operation;—1. The bed of ore is got at by the transverse galleries,m,n,o,q,r,s, which branch off from the extraction shaft, and terminate at the wall of the main bed;2. Great vaults are scooped out at the level of the workings, by means of fire;3. The roofs of these vaults are progressively propped with mounds of rubbish;4. The ores thus detached, or by blasting with gunpowder, are then collected;5. Lastly, they are wheeled out to the day; and washed nearZ.Comparative Tableof celebratedMinesinEuropeandAmerica. By F. Burr, Esq.(Quarterly Mining Review for July, 1835, p. 60.)Consolidated and United Mines.Veta Grande Mines.Mine of Valenciana.Mine of Himmelsfürst.(At present the richest mines in Cornwall.)(At present the richest mines in Mexico.)(Richest of the Mexican mines at the beginning of the present century.)(Richest of the Saxon mines at the beginning of the present century.)SituationTwo miles east of Redruth.Four miles north of Zacatecas.One mile north of Guanaxuato.Two miles south-east of Freyberg.ElevationElevation of the surface above the level of the sea, from 200 to 300 ft.; depth of the bottom of the mine below the level of the sea, about 1,370 feet.Elevation of the surface above the level of the sea, supposed to be about 6000 feet. Elevation of the bottom of the mine above the level of the sea, probably near 5,000 feet.Elevation of the surface above the level of the sea, 7,617 feet. Elevation of the bottom of the mine above the level of the sea, 5,730 feet.Elevation of the surface above the level of the sea, 1,346 feet. Elevation of the bottom of the mine above the level of the sea, 263 feet.Nature of the rockTheVeta Madreof Guanaxuato, upon which this mine is worked, traverses both clay slate and porphyry, but it is most productive in the former rock. The clay slate is considered by Humboldt to belong to the transition class, but situate near the limits of primary formations. This rock in depth, passes into chlorite slate, and talc slate. It contains subordinate beds of syenite, hornblende slate, and serpentine. The porphyry rests upon the clay slate, and is conformable to it, both in direction and stratification.The rock prevailing in the neighbourhood of Freyberg, in which this and most of the other mines are situate, is a formation of primary gneiss.Primary clay slate resting immediately on granite, a short distance westward of the mines. The clay slate is intersected by numerous channels of porphyry, which have nearly the same direction as the mineral veins, and are often of considerable width. The porphyry sometimes appears also to form large irregular masses in the clay slate. Both rocks are traversed by veins of quartz and clay intersecting the metalliferous veins.Transition clay slate, alternating with dolomite, and occasionally with greywacke. This clay slate is sometimes decomposed; it rests on syenitic rocks, and is in some places covered with porphyry.Nature of the metalliferous depositsIn the consolidated mines, the eight following lodes are extensively worked:—Wheal Fortune lode, Cusvea lode, Deeble’s lode, Old lode, Taylor’s lode, Tregonning’s lode, Martin’s lode, and Glover’s lode. In the united mines, the principal workings are upon the Old lode, and about five or six others are more or less productive. Numerous smaller lodes or “branches” occur also in both mines. The principal lodes are from 2 or 3, to 7 or 8 feet wide; the “branches” are generally 12 or 18 inches wide. The direction of the lodes varies from nearly east and west to about 20 degrees north of east and south of west. The underlie of the principal lodes, is from 2 to 3 feet per fathom north, that of the smaller ones about the same south.One principal vein (theVeta Grande) which is generally separated into three branches, and sometimes into four. When ramified, the width extends to 60 or 70 feet; when united, it varies from 8 or 10 to 20 or 30 feet. The branches are generally about 10 or 12 feet wide, and the upper one is most productive. The direction of the Veta Grande, is from 30 to 40 degrees south of east, and north of west, and its underlie, from two to three feet per fathom south. Other veins of less size, occur in the neighbourhood of the Veta Grande, which cross it at an acute angle. One of these appears to heave the vein for about 700 feet, being the most remarkable derangement of the kind on record.One Veta (theVetaMadre) which is often separated into three branches, extending from 130 to 160 feet in width. When not ramified, its width varies from 20 or 30 to 60 or 70 feet, but is more commonly from 40 to 50 feet. The direction of the vein, is north-west and south-east; its underlie is south, and about five or six feet per fathom. There are five veins worked in this mine.The principal vein (Teichflache) is from one foot six inches, to three feet in width, the others are from six to 12 inches wide. The direction of this vein, is nearly north and south, its underlie is west, and about three feet per fathom. Some of the other veins intersect it.OresChiefly copper ore, occasionally native copper, blue and green carbonate of copper. Tin, or oxide of tin, also occurs, but not in very great abundance.Chiefly red silver, native silver, sulphuret of silver, and argentiferous pyrites.Sulphuret of silver, native silver, prismatic black silver, red silver, native gold, argentiferous galena.Argentiferous sulphuret of lead, native silver, sulphuret of silver, red silver.Produce of the ores91⁄4per cent. of fine copper; average produce in 100 parts of ore.31⁄2oz. per quintal.Four ounces of silver per quintal of 100 lbs., equivalent to 21⁄2parts of metal in 1,000 of ore, or1⁄4per cent.Six to seven ounces of silver per quintal of 100 lbs. Equivalent to from 33⁄4to 41⁄2parts of metal in 1,000 of ore, or from 3-8ths to nearly1⁄2per cent.VeinstoneChiefly quartz, of which many varieties occur.Chiefly quartz, occasionally amethyst, carbonate of lime, and sulphate of barytes.Quartz, amethyst, carbonate of lime, pearlspar, and hornstone.Quartz, pearlspar, and calcareous spar.Mineral substancesThe ores are generally accompanied by “gossan”[34]in the backs of the lodes, by blende, and by iron, and arsenical pyrites in depth.The ores are generally accompanied by blende, sulphuret of antimony, and iron pyrites.The ores are accompanied by blende, spathose iron, copper and iron pyrites.The ores are accompanied by blende, spathose iron, and a little iron and arsenical pyrites.Depth of the principal shaftsWoolf’s engine-shaft, 248 fathoms; Pearce’sengine-shaft, 275 fathoms. Some of the other engine shafts are scarcely inferior in depth.Tiro General, 182 fathoms;Gallegashaft, 138 fathoms.Tiro General, 310 fathoms.Frankenschacht, 180 fathoms.[855]Depth of adit at the principle shaftsAt Woolf’s engine-shaft, 13 fathoms. The average depth of the adit at the other engine-shafts is about 30 or 40 fathoms.There is no adit to this mine.There is no adit to this mine.The adit at the shaft called Frankenschacht is 47 fathoms in depth.Quantity of waterVaries from 2,000 to 3,000 gallons per minute.About 80 gallons per minute.The Valenciana was a dry mine from its commencement in 1760 to 1780, when it first became troubled with water, in consequence of some of the workings being inadvertently communicated with the adjoining mine of Tepeyac; which, although upon the same vein, was extremely wet. The quantity of water raised during the late working appears to have been about 110 gallons per minute, but the regular influx was much less.50 gallons per minute.Height to which the water is raisedAbout 230 fathoms at the consolidated mines, at the united mines, about 110 fathoms.On an average about 150 fathoms.310 fathoms.133 fathoms.Power employed in drainage9 steam-engines; 3 of 90-inch cylinder, 3 of 85, 1 of 80, and 2 of 65. A water wheel, 48 feet in diameter.Usually 10 malacates.[b]A steam-engine of 30-inch cylinder, and 7 malacates.Two water-wheels, each 42 feet in diameter.Probable equivalent in actual horsepower1,500 constantly at work, or a total number of above 4,500.32 horses constantly working, or a total number of about 100 horses.[c]65 horses constantly at work, or a total number of about 200.16 horses constantly at work or a total number of about 50.[d]Average annual expense in drainage12,700l.taking the average of the last ten years.[a]20,000l.per annum.[c]About 40,000l., per annum.[d]Cannot be ascertained, but evidently very small.[d]Quantity of ore annually produced16,400 tons of copper ore, a few tons of tin ore.[a]21,380 tons of silver ore.[c]32,500 tons of silver ore.[d]630 tons of silver ore.[d]Produce in metal1,517 tons of fine copper, a little tin.[a]153,000 lbs. troy of silver.[c]221,900 lbs. troy silver.[d]6,160 lbs. troy of silver.[d]Total returns, or value of the above119,800l.[a]423,400l.per annum.[c]About 600,000l.[d]About 18,000l.[d]Total costs of the mine93,500l.exclusive of lord’s dues; 98,600l.including lord’s dues.[a]252,170l.per annum.[c]197,900l.per annum.[d]9,500l.per annum.[d]Clear profit to the proprietors21,000l.per annum.[a]171,240l.per annum.[c]118,750l.per annum.[d]3,560l.per annum.[d]Amount of capital invested75,000l.[a]130,000l.[c]Cannot be ascertained, but known to have been very small.[d]Cannot be ascertained, but probably very small.[d]Interest on capital invested280 per cent. after paying back the original capital.[a]Nearly 700 per cent. after paying back the original capital.[c]Not known, but certainly many hundred per cent.[d]Not known, but probably very high.[d]Proportion of costs to returnsCosts exclusive of lord’s dues, 78 per cent.[a]About 591⁄2per cent.Costs 60 per cent. In the nine years following, the proportion was 80 per cent., at the end of that time the working of the mine was stopped by the revolution, in the year 1809.[d]Costs 73 per cent.[d]Number of men employedAbout 2,500 persons, of whom about 1,450 are employed under ground.About 900, of whom nearly 600 are employed under ground.3,100 Indians and Mestizoes, of whom 1,800 are employed under ground.700 miners of whom 550 are employed under ground.Wages of the mines per dayProbably about 3 shillings on an average.About 8 or 9 shillings per day.From 4 to 5 shillings.About 1s.6d.per day.Quantity and expense of powder1,420 cwt.; value 15,830l.240 cwt.; value 1,070l.Manner in which the ores are disposed ofSold to the smelting companies, and smelted by them at Swansea, in South Wales.Chiefly reduced by the company at the hacienda of Sanceda, by smelting and amalgamation.Sold to the Rescatadores, and reduced by smelting and amalgamation at haciendas, in the neighbourhood of Guanaxuato.Delivered to the government reduction works in the neighbourhood of Freyberg, where they are partly smelted, and partly amalgamated.[a]Average of the last Ten Years.[b]Malacate; a horse whim.[c]Average of the last Six Years.[d]Average year at the end of the Eighteenth Century.[34]Gossan, or Gozzan; oxide of iron and quartz.VENTILATION OF MINES.When men penetrate by narrow passages into the interior of the earth, their respiration, joined to the combustion of candle and gunpowder, are not long of vitiating the air. The decomposition of wood contributes to the same effect, as also the mineral bed itself, especially in coal mines, by the carburetted hydrogen and carbonic acid evolved, and from the absorption of oxygen by pyrites. In many cases, arsenical and mercurial vapours are disengaged. Hence the necessity of maintaining in subterraneancavities a continual circulation of air, which may renew the atmosphere round the miners. The whole of the means employed to produce this effect, constitutes what is called theventilation of mines.These means are divided intonaturalandartificial. Thenatural meansare the currents produced by the difference of density between the air of mines and the external air; theartificialare air-exhausters or condensers, fires, &c.The temperature of the air of the subterranean workings surpasses the mean temperature of the place in which the mine is opened. Hence it is lighter in winter, but in summer often heavier than the air of the atmosphere. For this reason, when the mine presents two openings at different levels, the air naturally flows out by the most elevated in winter, and by the lowest in summer. We may take advantage of this circumstance, to lead the air into the bottom of even a very long gallery, opening into the side of the mountain, by piercing a shaft into its roof at some distance from the entrance, and dividing the gallery by a horizontal floor into two parts, which have no mutual communication, except at the furthest extremity—the upper part communicating with the shaft, and the under with the mouth of the gallery. If the two compartments have different dimensions, the air in the smaller sooner comes into an equilibrium of temperature with the rock; and the difference of temperature of the two compartments is sufficient to produce a current. If a streamlet of water flows through this gallery, it facilitates the flow of the air along the lower compartment. If a mine has several openings situated on the same level, it rarely happens but some peculiar circumstance destroys, during the colds of winter and the heats of summer, the equilibrium of the air. But in spring and autumn, when the external air is nearly of the same temperature with that of the mines, the above-named causes are almost always too feeble to excite an issuing current. This effect is, however, frequently obtained by raising over one of the shafts a chimney 20 or 30 yards high, which alone produces the effect of an opening at a different level. It has been remarked that stormy weather usually deranges every system of ventilation. SeePitcoalandVentilation.
Rammelsberg mine
A mine so ancient as that of Rammelsberg, and which was formerly divided among several adventurous companies, cannot fail to present a great many shafts and excavations; but out of the 15 pits, only two are employed for the present workings; namely, those markedA BandE F, infig.736., by which the whole extraction and drainage are executed.—The general system of exploitation by fire, as practised in this mine, consists of the following operations:—
1. An advance is made towards the deposits of ore, successively at different levels, by transverse galleries which proceed from the shaft of extraction, and terminate at the wall of the stratiform mass.
2. There is formed in the level to be worked, large vaults in the heart of the ore, by means of fire, as we shall presently describe.
3. The floor of these vaults is raised up by means of terraces formed from the rubbish, in proportion as the roof is scooped out.
4. The ores detached by the fire from their bed, are picked and gathered; sometimes the larger blocks are blasted with gunpowder.
5. Lastly, the ores thus obtained are wheeled towards the shaft of extraction, and turned out to the day.
Let us now see how the excavation by fire is practised; and in that view, let us consider the state of the workings in the mines of Rammelsberg in 1809. We may remark infig.736.the regularity of the vaults previously scooped out above the levelB C, and the other vaults which are in full activity of operation. It is, therefore, towards the lower levels that the new workings must be directed. For this purpose, the transverse gallery being already completed, there is prepared on the first of these floors a vault of exploitation atb, which eventually is to become similar to those of the superior levels. At the same time, there is commenced at the starting point below it, reached by a small well dug in the line of the mineral deposit, a transverse gallery in the rock, by means of blasting with gunpowder. The rock is also attacked at the starting-point by a similarcut, which advances to meet the first perforation. In this way, whenever the vaults of the levelCare exhausted of ore and terraced up with rubbish, those of the level beneath it will be in full activity.
Others will then be prepared at a lower level; and the exploitation may afterwards be driven below this level by pursuing the same plan, by which the actual depth of excavation has been gained.
In workings by fire we must distinguish, 1. The case where it is necessary to open a vault immediately from the floor; 2. The case where the vault having already a certain elevation, it is necessary to heighten its roof. In the former case, the wall or floor of the mineral deposit is first penetrated by blasting with gunpowder. As soon as this penetration is effected over a certain length, parallel to the direction of the future vault, as happens atb, there is arranged on the bottom a horizontal layer of billets of firwood, over which other billets are piled in nearly a vertical position, which rest upon the ore, so that the flame in its expansion comes to play against the mineral mass to bedetached. When after some similar operations, the flame of the pile can no longer reach the ore of the roof on account of its height, a small terrace of rubbish must be raised on the floor of the deposit; and over this terrace, a new pile of faggots is to be heaped up as above described. The ancient miners committed the fault of constantly placing such terraces close to the roof, and consequently arranging the faggots against this portion of the ore, so that the flame circulated from the roof down to the floor. The result of such procedure was the weakening of the roof, and the loss of much of the ore which could not be extracted from so unstable a fabric; and besides, much more wood was burned than at the present day, because the action of the flame was dissipated in part against the whole mass of the roof, instead of being concentred on the portion of the ore which it was desired to dislodge. Now, the flame is usually made to circulate from the floor to the roof, in commencing a new vault.
When the vault has already a certain height, care is always taken that between the roof of the vault and the rubbish on which the pile is arranged, no more than two yards of space should intervene, in order that the flame may embrace equally the whole concavity of the vault, and produce an uniform effect on all its parts. Here, the pile is formed of horizontal beds, disposed crosswise above one another, and presents four free vertical faces, whence it has been called achestby the miners.
It is usually on Saturday that the fire is applied to all the piles of faggots distributed through the course of the week. Those in the upper floors of exploitation are first burned, in order that the inferior piles may not obstruct by their vitiated air, the combustion of the former. Thus, at 4 o’clock in the morning, the fires are kindled in the upper ranges; from pile to pile, the fireman and his assistant descend towards the lower floors, which occupies them till 3 o’clock in the afternoon. Vainly should we endeavour to describe the majestic and terrific spectacle which the fire presents, as it unfolds its wings under its metallic vaults, soon filled with vast volumes of smoke and flame. Let us mark the useful effect which it produces.
When the flame has beat for a few instants on the beds of ore, a strong odour of sulphur, and sometimes of arsenic is perceived; and soon thereafter loud detonations are heard in the vaults. Suddenly the flame is seen to assume a blue colour, or even a white; and at this period, after a slight explosion, flakes of the ore, of greater or less magnitude, usually fall down on the fire, but the chief portion of the heated mineral still remains fixed to the vault. The ores pass now into a shattered and divided condition, which allows them afterwards to be detached by long forks of iron. In this manner the fire, volatilizing entirely some principles, such as sulphur, zinc, arsenic, and water, changing the aggregation of the constituent parts of the ore, and causing fissures by their unequal expansibilities, facilitates the excavation of such materials as resist by their tenacity the action of gunpowder.
The combustion goes on without any person entering the mine from Saturday evening till Monday morning, on which day, the fireman and his assistants proceed to extinguish the remains of the bonfires. On Monday also some piles are constructed in the parts where the effect of the former ones has been incomplete; and they are kindled after the workmen have quitted the mine. On Tuesday all hands are employed in detaching the ores, in sorting them, taking them out, and preparing new piles against the next Saturday.
The labour of a week consists for every man of five posts during the day, each of 8 hours, and of one post of four hours for Saturday. Moreover, an extra allowance is made to such workmen as employ themselves some posts during the night.
The labour of one compartment oratelierof the mine consists therefore in arranging the faggots, in detaching the ore which has already experienced the action of the fire, in breaking the blocks obtained, in separating the ore from thedébrisof the pile, and whenever it may be practicable or useful, in boring holes for blasting with gunpowder. The heat is so great in this kind of mine, that the men are obliged to work in it without clothing.
We have already remarked, that besides the working by fire, which is chiefly used here, recourse is sometimes had to blasting by gunpowder. This is done in order either to recover the bottom part or ground of the vaults on which the fire can act but imperfectly, to clear away some projections which would interfere with the effect of the pile, or lastly to strip the surrounding rock from the mass of the ore, and thence to obtain schist proper for the construction of the rubbish-terraces.
The blasting process is employed when the foremen of the workshop or mine-chamber judge that a hole well placed may separate enough of ore to pay the time, the repair of tools, and the gunpowder expended. But this indemnification is rarely obtained. The following statement will give an idea of the tenacity which the mineral deposit often presents.
In 1808, in a portion of the Rammelsberg mine, the ore, consisting of extremely compact iron and copper pyrites, was attacked by a single man, who bored a mining hole.After 11 posts of obstinate labour, occupying altogether 88 hours, the workman, being vigilantly superintended, had been able to advance the hole to a depth of no more than 4 inches; in doing which he had rendered entirely unserviceable 126 punches or borers, besides 26 others which had been re-tipped with steel, and 201 which had been sharpened; 61⁄4pounds of oil had been consumed in giving him light; and half a pound of gunpowder was required for blasting the bore. It was found from a calculation made upon these facts by the administration of mines, that every inch deep of this hole cost, at their low price of labour, nearly a florin, value two shillings and sixpence.
It is therefore evident that though the timber, of which the consumption is prodigiously great, were much less abundant and dearer than it still is at Rammelsberg, mining by fire would be preferable to every other mode of exploitation. It is even certain, that on any supposition, the employment of gunpowder would not be practicable for every part of the mine; and if fuel came to fail, it would be requisite to renounce the workings at Rammelsberg, although this mountain still contains a large quantity of metals.
If in all mines the free circulation of air be an object of the highest importance, we must perceive how indispensable it must be in every part of a mine where the mode of exploitation maintains the temperature of the air at 112° Fahr., when the workmen return into it after the combustion of the piles, and in which besides it is necessary that this combustion be effected with activity in their absence. But in consequence of the extent and mutual ramifications of the workings, the number of the shafts, galleries, and their differences of level, the ventilation of the mine is in a manner spontaneously maintained. The high temperature is peculiarly favourable to it. The aid of art consists merely in placing some doors judiciously, which may be opened or shut at pleasure, to carry on the circulation of the air.
In considering the Rammelsberg from its summit, which rises about 400 yards above the town of Goslar, we observe, first, beds of slaty sandstone, which become the more horizontal the nearer they approach to the surface. At about 160 yards below the top level there occurs, in the bosom of the slaty graywacke, a powerful stratum of shells impasted in a ferruginous sandstone. SeeD,fig.730.In descending towards the face of the ore, the parallel stratification of the clay-slate which forms its walls and roof grows more and more manifest. Here the slate is black, compact, and thinly foliated. The inclination of the different beds of rock is indicated atB. The substance of the workable mass is copper and iron pyrites, along with sulphuret of lead, accompanied by quartz, carbonate of lime, compact sulphate of baryta, and occasionally gray copper (fahlerz), sulphuret of zinc, and arsenical pyrites.
The ores are argentiferous and auriferous, but very slightly so, especially as to the gold. It is the ores of lead and copper which contain the silver, and in the latter the gold is found, but without its being well ascertained in what mineral it is deposited. Sometimes the copper occurs in the native state, or as copper of cementation. Beautiful crystals of sulphate of lime are found in the old workings.
Kahnenkuhler shaftFig. 737 enlarged(150 kB)
Fig. 737 enlarged(150 kB)
Infigs.736.737.,A Bis the shaft of extraction, called theKahnenkuhler;Nis the ventilation shaft, calledBreitlingerwetterschacht;Pis the extraction shaft, calledInnier-schacht.
E F, is a new extraction-shaft, calledNeuer treibschacht, by which also the water is pumped up; byA B, andE F, the whole extraction and draining are carried on. The ores are raised in these shafts to the level of the waggon-gallery (galerie de roulage)i, by the whimsl,q, provided with ropes and buckets. 1, 2, 3, 4,fig.736., represent the positions of four water-wheels for working the whims; the first two being employed in extracting the ores, the last two in draining. The driving stream is led to the wheel 1, along the driftl; whence it falls in succession upon the wheels 2, 3, 4. The general system of working consists of the following operation;—
1. The bed of ore is got at by the transverse galleries,m,n,o,q,r,s, which branch off from the extraction shaft, and terminate at the wall of the main bed;
2. Great vaults are scooped out at the level of the workings, by means of fire;
3. The roofs of these vaults are progressively propped with mounds of rubbish;
4. The ores thus detached, or by blasting with gunpowder, are then collected;
5. Lastly, they are wheeled out to the day; and washed nearZ.
Comparative Tableof celebratedMinesinEuropeandAmerica. By F. Burr, Esq.(Quarterly Mining Review for July, 1835, p. 60.)
[34]Gossan, or Gozzan; oxide of iron and quartz.
[34]Gossan, or Gozzan; oxide of iron and quartz.
VENTILATION OF MINES.
When men penetrate by narrow passages into the interior of the earth, their respiration, joined to the combustion of candle and gunpowder, are not long of vitiating the air. The decomposition of wood contributes to the same effect, as also the mineral bed itself, especially in coal mines, by the carburetted hydrogen and carbonic acid evolved, and from the absorption of oxygen by pyrites. In many cases, arsenical and mercurial vapours are disengaged. Hence the necessity of maintaining in subterraneancavities a continual circulation of air, which may renew the atmosphere round the miners. The whole of the means employed to produce this effect, constitutes what is called theventilation of mines.
These means are divided intonaturalandartificial. Thenatural meansare the currents produced by the difference of density between the air of mines and the external air; theartificialare air-exhausters or condensers, fires, &c.
The temperature of the air of the subterranean workings surpasses the mean temperature of the place in which the mine is opened. Hence it is lighter in winter, but in summer often heavier than the air of the atmosphere. For this reason, when the mine presents two openings at different levels, the air naturally flows out by the most elevated in winter, and by the lowest in summer. We may take advantage of this circumstance, to lead the air into the bottom of even a very long gallery, opening into the side of the mountain, by piercing a shaft into its roof at some distance from the entrance, and dividing the gallery by a horizontal floor into two parts, which have no mutual communication, except at the furthest extremity—the upper part communicating with the shaft, and the under with the mouth of the gallery. If the two compartments have different dimensions, the air in the smaller sooner comes into an equilibrium of temperature with the rock; and the difference of temperature of the two compartments is sufficient to produce a current. If a streamlet of water flows through this gallery, it facilitates the flow of the air along the lower compartment. If a mine has several openings situated on the same level, it rarely happens but some peculiar circumstance destroys, during the colds of winter and the heats of summer, the equilibrium of the air. But in spring and autumn, when the external air is nearly of the same temperature with that of the mines, the above-named causes are almost always too feeble to excite an issuing current. This effect is, however, frequently obtained by raising over one of the shafts a chimney 20 or 30 yards high, which alone produces the effect of an opening at a different level. It has been remarked that stormy weather usually deranges every system of ventilation. SeePitcoalandVentilation.