De Re Metallica Part 34

In other respects this method of washing does not differ from the last.

[Ill.u.s.tration 337 (Ground Sluicing): A--Stream. B--Ditch. C--Mattock.

D--Pieces of turf. E--Seven-p.r.o.nged fork. F--Iron shovel. G--Trough.

H--Another trough below it. I--Small wooden trowel.]

I have spoken of the various methods of washing sand which contains grains of gold; I will now speak of the methods of washing the material in which are mixed the small black stones from which tin is made[20].

Eight such methods are in use, and of these two have been invented lately. Such metalliferous material is usually found torn away from veins and stringers and scattered far and wide by the impetus of water, although sometimes _venae dilatatae_ are composed of it. The miners dig out the latter material with a broad mattock, while they dig the former with a pick. But they dig out the little stones, which are not rare in this kind of ore, with an instrument like the bill of a duck. In districts which contain this material, if there is an abundant supply of water, and if there are valleys or gentle slopes and hollows, so that rivers can be diverted into them, the washers in summer-time first of all dig a long ditch sloping so that the water will run through it rapidly. Into the ditch is thrown the metallic material, together with the surface material, which is six feet thick, more or less, and often contains moss, roots of plants, shrubs, trees, and earth; they are all thrown in with a broad mattock, and the water flows through the ditch.

The sand and tin-stone, as they are heavy, sink to the bottom of the ditch, while the moss and roots, as they are light, are carried away by the water which flows through the ditch. The bottom of the ditch is obstructed with turf and stones in order to prevent the water from carrying away the tin-stone at the same time. The washers, whose feet are covered with high boots made of hide, though not of rawhide, themselves stand in the ditch and throw out of it the roots of the trees, shrubs, and gra.s.s with seven-p.r.o.nged wooden forks, and push back the tin-stone toward the head of the ditch. After four weeks, in which they have devoted much work and labour, they raise the tin-stone in the following way; the sand with which it is mixed is repeatedly lifted from the ditch with an iron shovel and agitated hither and thither in the water, until the sand flows away and only the tin-stone remains on the shovel. The tin-stone is all collected together and washed again in a trough by pushing it up and turning it over with a wooden trowel, in order that the remaining sand may separate from it. Afterward they return to their task, which they continue until the metalliferous material is exhausted, or until the water can no longer be diverted into the ditches.

[Ill.u.s.tration 338 (Sluicing Tin): A--Trough. B--Wooden shovel. C--Tub.

D--Launder. E--Wooden trowel. F--Transverse trough. G--Plug. H--Falling water. I--Ditch. K--Barrow conveying material to be washed. L--Pick like the beak of a duck with which the miner digs out the material from which the small stones are obtained.]

The trough which I mentioned is hewn out of the trunk of a tree and the interior is five feet long, three-quarters of a foot deep, and six digits wide. It is placed on an incline and under it is put a tub which contains interwoven fir twigs, or else another trough is put under it, the interior of which is three feet long and one foot wide and deep; the fine tin-stone, which has run out with the water, settles in the bottom.

Some people, in place of a trough, put a square launder underneath, and in like manner they wash the tin-stone in this by agitating it up and down and turning it over with a small wooden trowel. A transverse trough is put under the launder, which is either open on one end and drains off into a tub or settling-pit, or else is closed and perforated through the bottom; in this case, it drains into a ditch beneath, where the water falls when the plug has been partly removed. The nature of this ditch I will now describe.

[Ill.u.s.tration 340 (Sluicing Tin): A--Launder. B--Interlacing fir twigs.

C--Logs; three on one side, for the fourth cannot be seen because the ditch is so full with material now being washed. D--Logs at the head of the ditch. E--Barrow. F--Seven-p.r.o.nged fork. G--Hoe.]

If the locality does not supply an abundance of water, the washers dig a ditch thirty or thirty-six feet long, and cover the bottom, the full length, with logs joined together and hewn on the side which lies flat on the ground. On each side of the ditch, and at its head also, they place four logs, one above the other, all hewn smooth on the inside. But since the logs are laid obliquely along the sides, the upper end of the ditch is made four feet wide and the tail end, two feet. The water has a high drop from a launder and first of all it falls into interlaced fir twigs, in order that it shall fall straight down for the most part in an unbroken stream and thus break up the lumps by its weight. Some do not place these twigs under the end of the launder, but put a plug in its mouth, which, since it does not entirely close the launder, nor altogether prevent the discharge from it, nor yet allow the water to spout far afield, makes it drop straight down. The workman brings in a wheelbarrow the material to be washed, and throws it into the ditch. The washer standing in the upper end of the ditch breaks the lumps with a seven-p.r.o.nged fork, and throws out the roots of trees, shrubs, and gra.s.s with the same instrument, and thereby the small black stones settle down. When a large quant.i.ty of the tin-stone has acc.u.mulated, which generally happens when the washer has spent a day at this work, to prevent it from being washed away he places it upon the bank, and other material having been again thrown into the upper end of the ditch, he continues the task of washing. A boy stands at the lower end of the ditch, and with a thin pointed hoe stirs up the sediment which has settled at the lower end, to prevent the washed tin-stone from being carried further, which occurs when the sediment has acc.u.mulated to such an extent that the fir branches at the outlet of the ditch are covered.

[Ill.u.s.tration 341 (Sifting Ore): A--Strakes. B--Tank. C--Launder.

D--Plug. E--Wooden shovel. F--Wooden mallet. G--Wooden shovel with short handle. H--The plug in the strake. I--Tank placed under the plug.]

The third method of washing materials of this kind follows. Two strakes are made, each of which is twelve feet long and a foot and a half wide and deep. A tank is set at their head, into which the water flows through a little launder. A boy throws the ore into one strake; if it is of poor quality he puts in a large amount of it, if it is rich he puts in less. The water is let in by removing the plug, the ore is stirred with a wooden shovel, and in this way the tin-stone, mixed with the heavier material, settles in the bottom of the strake, and the water carries the light material into the launder, through which it flows on to a canvas strake. The very fine tin-stone, carried by the water, settles on to the canvas and is cleansed. A low cross-board is placed in the strake near the head, in order that the largest sized tin-stone may settle there. As soon as the strake is filled with the material which has been washed, he closes the mouth of the tank and continues washing in the other strake, and then the plug is withdrawn and the water and tin-stone flow down into a tank below. Then he pounds the sides of the loaded strake with a wooden mallet, in order that the tin-stone clinging to the sides may fall off; all that has settled in it, he throws out with a wooden shovel which has a short handle. Silver slags which have been crushed under the stamps, also fragments of silver-lead alloy and of cakes melted from pyrites, are washed in a strake of this kind.

[Ill.u.s.tration 342 (Sifting Ore): A--Sieve. B--Tub. C--Water flowing out of the bottom of it. D--Strake. E--Three-toothed rake. F--Wooden scrubber.]

Material of this kind is also washed while wet, in a sieve whose bottom is made of woven iron wire, and this is the fourth method of washing.

The sieve is immersed in the water which is contained in a tub, and is violently shaken. The bottom of this tub has an opening of such size that as much water, together with tailings from the sieve, can flow continuously out of it as water flows into it. The material which settles in the strake, a boy either digs over with a three-toothed iron rake or sweeps with a wooden scrubber; in this way the water carries off a great part of both sand and mud. The tin-stone or metalliferous concentrates settle in the strake and are afterward washed in another strake.

[Ill.u.s.tration 343 (Sluicing Tin): A--Box. B--Perforated plate.

C--Trough. D--Cross-boards. E--Pool. F--Launder. G--Shovel. H--Rake.]

These are ancient methods of washing material which contains tin-stone; there follow two modern methods. If the tin-stone mixed with earth or sand is found on the slopes of mountains or hills, or in the level fields which are either devoid of streams or into which a stream cannot be diverted, miners have lately begun to employ the following method of washing, even in the winter months. An open box is constructed of planks, about six feet long, three feet wide, and two feet and one palm deep. At the upper end on the inside, an iron plate three feet long and wide is fixed, at a depth of one foot and a half from the top; this plate is very full of holes, through which tin-stone about the size of a pea can fall. A trough hewn from a tree is placed under the box, and this trough is about twenty-four feet long and three-quarters of a foot wide and deep; very often three cross-boards are placed in it, dividing it off into compartments, each one of which is lower than the next. The turbid waters discharge into a settling-pit.

The metalliferous material is sometimes found not very deep beneath the surface of the earth, but sometimes so deep that it is necessary to drive tunnels and sink shafts. It is transported to the washing-box in wheelbarrows, and when the washers are about to begin they lay a small launder, through which there flows on to the iron plate so much water as is necessary for this washing. Next, a boy throws the metalliferous material on to the iron plate with an iron shovel and breaks the small lumps, stirring them this way and that with the same implement. Then the water and sand penetrating the holes of the plate, fall into the box, while all the coa.r.s.e gravel remains on the plate, and this he throws into a wheelbarrow with the same shovel. Meantime, a younger boy continually stirs the sand under the plate with a wooden scrubber nearly as wide as the box, and drives it to the upper end of the box; the lighter material, as well as a small amount of tin-stone, is carried by the water down into the underlying trough. The boys carry on this labour without intermission until they have filled four wheelbarrows with the coa.r.s.e and worthless residues, which they carry off and throw away, or three wheelbarrows if the material is rich in black tin. Then the foreman has the plank removed which was in front of the iron plate, and on which the boy stood. The sand, mixed with the tin-stone, is frequently pushed backward and forward with a scrubber, and the same sand, because it is lighter, takes the upper place, and is removed as soon as it appears; that which takes the lower place is turned over with a spade, in order that any that is light can flow away; when all the tin-stone is heaped together, he shovels it out of the box and carries it away. While the foreman does this, one boy with an iron hoe stirs the sand mixed with fine tin-stone, which has run out of the box and has settled in the trough and pushes it back to the uppermost part of the trough, and this material, since it contains a very great amount of tin-stone, is thrown on to the plate and washed again. The material which has settled in the lowest part of the trough is taken out separately and piled in a heap, and is washed on the ordinary strake; that which has settled in the pool is washed on the canvas strake. In the summer-time this fruitful labour is repeated more often, in fact ten or eleven times. The tin-stone which the foreman removes from the box, is afterward washed in a jigging sieve, and lastly in a tub, where at length all the sand is separated out. Finally, any material in which are mixed particles of other metals, can be washed by all these methods, whether it has been disintegrated from veins or stringers, or whether it originated from _venae dilatatae_, or from streams and rivers.

[Ill.u.s.tration 345 (Ground Sluicing): A--Launder. B--Cross trough. C--Two spouts. D--Boxes. E--Plate. F--Grating. G--Shovels. H--Second cross trough. I--Strake. K--Wooden scrubber. L--Third cross trough.

M--Launder. N--Three-toothed rake.]

The sixth method of washing material of this kind is even more modern and more useful than the last. Two boxes are constructed, into each of which water flows through spouts from a cross trough into which it has been discharged through a pipe or launder. When the material has been agitated and broken up with iron shovels by two boys, part of it runs down and falls through the iron plates full of holes, or through the iron grating, and flows out of the box over a sloping surface into another cross trough, and from this into a strake seven feet long and two and a half feet wide. Then the foreman again stirs it with a wooden scrubber that it may become clean. As for the material which has flowed down with the water and settled in the third cross trough, or in the launder which leads from it, a third boy rakes it with a two-toothed rake; in this way the fine tin-stone settles down and the water carries off the valueless sand into the creek. This method of washing is most advantageous, for four men can do the work of washing in two boxes, while the last method, if doubled, requires six men, for it requires two boys to throw the material to be washed on to the plate and to stir it with iron shovels; two more are required with wooden scrubbers to keep stirring the sand, mixed with the tin-stone, under the plate, and to push it toward the upper end of the box; further, two foremen are required to clean the tin-stone in the way I have described. In the place of a plate full of holes, they now fix in the boxes a grating made of iron wire as thick as the stalks of rye; that these may not be depressed by the weight and become bent, three iron bars support them, being laid crosswise underneath. To prevent the grating from being broken by the iron shovels with which the material is stirred in washing, five or six iron rods are placed on top in cross lines, and are fixed to the box so that the shovels may rub them instead of the grating; for this reason the grating lasts longer than the plates, because it remains intact, while the rods, when worn by rubbing, can easily be replaced by others.

[Ill.u.s.tration 346 (Ground Sluicing): A--Pits. B--Torrent.

C--Seven-p.r.o.nged fork. D--Shovel.]

Miners use the seventh method of washing when there is no stream of water in the part of the mountain which contains the black tin, or particles of gold, or of other metals. In this case they frequently dig more than fifty ditches on the slope below, or make the same number of pits, six feet long, three feet wide, and three-quarters of a foot deep, not any great distance from each other. At the season when a torrent rises from storms of great violence or long duration, and rushes down the mountain, some of the miners dig the metalliferous material in the woods with broad hoes and drag it to the torrent. Other miners divert the torrent into the ditches or pits, and others throw the roots of trees, shrubs, and gra.s.s out of the ditches or pits with seven-p.r.o.nged wooden forks. When the torrent has run down, they remove with shovels the uncleansed tin-stone or particles of metal which have settled in the ditches or pits, and cleanse it.

[Ill.u.s.tration 347 (Ground Sluicing): A--Gully. B--Ditch. C--Torrent.

D--Sluice box employed by the Lusitanians.]

The eighth method is also employed in the regions which the Lusitanians hold in their power and sway, and is not dissimilar to the last. They drive a great number of deep ditches in rows in the gullies, slopes, and hollows of the mountains. Into these ditches the water, whether flowing down from snow melted by the heat of the sun or from rain, collects and carries together with earth and sand, sometimes tin-stone, or, in the case of the Lusitanians, the particles of gold loosened from veins and stringers. As soon as the waters of the torrent have all run away, the miners throw the material out of the ditches with iron shovels, and wash it in a common sluice box.

[Ill.u.s.tration 348 (Trough for washing alluvial): A--Trough. B--Launder.

C--Hoe. D--Sieve.]

The Poles wash the impure lead from _venae dilatatae_ in a trough ten feet long, three feet wide, and one and one-quarter feet deep. It is mixed with moist earth and is covered by a wet and sandy clay, and so first of all the clay, and afterward the ore, is dug out. The ore is carried to a stream or river, and thrown into a trough into which water is admitted by a little launder, and the washer standing at the lower end of the trough drags the ore out with a narrow and nearly pointed hoe, whose wooden handle is nearly ten feet long. It is washed over again once or twice in the same way and thus made pure. Afterward when it has been dried in the sun they throw it into a copper sieve, and separate the very small pieces which pa.s.s through the sieve from the larger ones; of these the former are smelted in a f.a.ggot pile and the latter in the furnace. Of such a number then are the methods of washing.

[Ill.u.s.tration 349 (Tin burning Furnace): A--Furnace. B--Its mouth.

C--Poker. D--Rake with two teeth. E--Hoe.]

One method of burning is princ.i.p.ally employed, and two of roasting. The black tin is burned by a hot fire in a furnace similar to an oven[21]; it is burned if it is a dark-blue colour, or if pyrites and the stone from which iron is made are mixed with it, for the dark blue colour if not burnt, consumes the tin. If pyrites and the other stone are not volatilised into fumes in a furnace of this kind, the tin which is made from the tin-stone is impure. The tin-stone is thrown either into the back part of the furnace, or into one side of it; but in the former case the wood is placed in front, in the latter case alongside, in such a manner, however, that neither firebrands nor coals may fall upon the tin-stone itself or touch it. The fuel is manipulated by a poker made of wood. The tin-stone is now stirred with a rake with two teeth, and now again levelled down with a hoe, both of which are made of iron. The very fine tin-stone requires to be burned less than that of moderate size, and this again less than that of the largest size. While the tin-stone is being thus burned, it frequently happens that some of the material runs together.

The burned tin-stone should then be washed again on the strake, for in this way the material which has been run together is carried away by the water into the cross-trough, where it is gathered up and worked over, and again washed on the strake. By this method the metal is separated from that which is devoid of metal.

[Ill.u.s.tration 350 (Stall Roasting Matte): A--Pits. B--Wood. C--Cakes.

D--Launder.]

Cakes from pyrites, or _cadmia_, or cupriferous stones, are roasted in quadrangular pits, of which the front and top are open, and these pits are generally twelve feet long, eight feet wide, and three feet deep.

The cakes of melted pyrites are usually roasted twice over, and those of _cadmia_ once. These latter are first rolled in mud moistened with vinegar, to prevent the fire from consuming too much of the copper with the bitumen, or sulphur, or orpiment, or realgar. The cakes of pyrites are first roasted in a slow fire and afterward in a fierce one, and in both cases, during the whole following night, water is let in, in order that, if there is in the cakes any alum or vitriol or saltpetre capable of injuring the metals, although it rarely does injure them, the water may remove it and make the cakes soft. The solidified juices are nearly all harmful to the metal, when cakes or ore of this kind are smelted.

The cakes which are to be roasted are placed on wood piled up in the form of a crate, and this pile is fired[22].

[Ill.u.s.tration 351 (Matte Roasting): A--Cakes. B--Bundles of f.a.ggots.

C--Furnaces.]

The cakes which are made of copper smelted from schist are first thrown upon the ground and broken, and then placed in the furnace on bundles of f.a.ggots, and these are lighted. These cakes are generally roasted seven times and occasionally nine times. While this is being done, if they are bituminous, then the bitumen burns and can be smelled. These furnaces have a structure like the structure of the furnaces in which ore is smelted, except that they are open in front; they are six feet high and four feet wide. As for this kind of furnace, three of them are required for one of those in which the cakes are melted. First of all they are roasted in the first furnace, then when they are cooled, they are transferred into the second furnace and again roasted; later they are carried to the third, and afterward back to the first, and this order is preserved until they have been roasted seven or nine times.

END OF BOOK VIII.

FOOTNOTES:

[1] As would be expected, practically all the technical terms used by Agricola in this chapter are adaptations. The Latin terms, _ca.n.a.lis_, _area_, _lacus_, _vasa_, _cribrum_, and _fossa_, have had to be pressed into service for many different devices, largely by extemporised combinations. Where the devices described have become obsolete, we have adopted the nomenclature of the old works on Cornish methods. The following examples may be of interest:--

Simple buddle = _Ca.n.a.lis simplex_ Divided buddle = _Ca.n.a.lis tabellis distinctus_ Ordinary strake = _Ca.n.a.lis devexus_ Short strake = _Area curta_ Canvas strake = _Area linteis extensis contecta_ Limp = _Radius_.

The strake (or streke) when applied to alluvial tin, would have been termed a "tye" in some parts of Cornwall, and the "short strake" a "gounce." In the case of the stamp mill, inasmuch as almost every mechanical part has its counterpart in a modern mill, we have considered the reader will have less difficulty if the modern designations are used instead of the old Cornish. The following are the essential terms in modern, old Cornish, and Latin:--

Stamp Stamper _Pilum_ Stamp-stem Lifter _Pilum_ Shoes Stamp-heads _Capita_ Mortar-box Box _Capsa_ Cam-shaft Barrell _Axis_ Cams Caps _Dentes_ Tappets Tongues _Pili dentes_ Screen Crate _Laminae foraminum plenae_ Settling pit Catchers _Lacus_ Jigging sieve Dilleugher _Cribrum angustum_

[2] Agricola uses four Latin verbs in connection with heat operations at temperatures under the melting point: _Calefacio_, _uro_, _torreo_, and _cremo_. The first he always uses in the sense of "to warm" or "to heat," but the last three he uses indiscriminately in much the same way as the English verbs burn, roast, and calcine are used; but in general he uses the Latin verbs in the order given to indicate degrees of heat.

We have used the English verbs in their technical sense as indicated by the context.

It is very difficult to say when roasting began as a distinct and separate metallurgical step in sulphide ore treatment. The Greeks and Romans worked both lead and copper sulphides (see note on p. 391, and note on p. 403), but neither in the remains of old works nor in their literature is there anything from which satisfactory details of such a step can be obtained. The Ancients, of course, understood lime-burning, and calcined several salts to purify them or to render them more caustic. Practically the only specific mention is by Pliny regarding lead ores (see p. 391). Even the statement of Theophilus (1050-1100, A.D.), may refer simply to rendering ore more fragile, for he says (p.

305) in regard to copper ore: "This stone dug up in abundance is placed upon a pile and burned (_comburitur_) after the manner of lime. Nor does it change colour, but loses its hardness and can be broken up, and afterward it is smelted." The _Probierbuchlein_ casually mentions roasting prior to a.s.saying, and Biringuccio (III, 2) mentions incidentally that "dry and ill-disposed ores before everything must be roasted in an open oven so that the air can get in." He gives no further information; and therefore this account of Agricola's becomes practically the first. Apparently roasting, as a preliminary to the treatment of copper sulphides, did not come into use in England until some time later than Agricola, for in Col. Grant Francis' "Smelting of Copper in the Swansea District" (London, 1881, p. 29), a report is set of the "Doeinges of Jochim Ganse"--an imported German--at the "Mynes by Keswicke in c.u.mberland, A.D., 1581," wherein the delinquencies of the then current practice are described: "Thei never coulde, nether yet can make (copper) under XXII. tymes pa.s.singe thro the fire, and XXII. weekes doeing thereof ane sometyme more. But now the nature of these IX.

hurtfull humors abovesaid being discovered and opened by Jochim's way of doeing, we can, by his order of workeinge, so correct theim, that parte of theim beinge by nature hurtfull to the copper in wasteinge of it, ar by arte maide freindes, and be not onely an encrease to the copper, but further it in smeltinge; and the rest of the other evill humors shalbe so corrected, and their humors so taken from them, that by once rosteinge and once smeltinge the ure (which shalbe done in the s.p.a.ce of three dayes), the same copper ure shall yeeld us black copper." Jochim proposed by 'rostynge' to be rid of "sulphur, arsineque, and antimony."

[3] _Orpiment_ and _realgar_ are the red and yellow a.r.s.enical sulphides.

(See note on p. 111).