In regard to the art of refining, I propose to shew the progress of the ore, from the mine till it comes to this spongy mass or cake. After breaking the stone or ore taken out of the veins, it is grinded in mills between grindstones, or pounded in the ingenious reales, or royal engines, by means of hammers or beetles, like the mills for Paris plaster. These generally have a wheel of twenty-five or thirty feet diameter, with a long axle or lying shaft, set round with smooth triangular projections, which, as the axle turns, lay hold of the iron hammers, of about two hundred-weight each, lifting them to a certain height, whence they drop down with such violence that they crush and reduce the hardest stones to powder. The pounded ore is afterwards sifted through iron or copper sieves, which allow the finest powder to go through, the coarse being returned to the mill. When the one happens to be mixed with copper or other metals which prevent its reduction to powder, it is roasted or calcined in an oven or reverberatory furnace, and pounded over again.
At the smaller mines, where they only use grindstones, they, for the most part, grind the ore along with water, forming it into a liquid paste, which runs out into receivers. When grinded dry, it has to be afterwards mixed with water, and well moulded up with the feet for a long time. For this purpose, they make a court or floor, on which that mud, or paste of pounded ore and water, is disposed in square parcels of about a foot thick, each parcel containing half a caxon, or chest, which is twenty-five quintals or hundred-weights of ore, and these parcels are called cuerpos, or bodies. On each of these they throw about two hundred-weights of sea-salt, more or less, according to the nature of the ore, which they mould or incorporate with the moistened ore for two or three days. They then add a certain quantity of quicksilver, squeezing it from a skin bag, to make it fall in drops equally on the mass or cuerpo, allowing to each mass ten, fifteen, or twenty pounds of quicksilver, according to the nature or quality of the ore, as the richer it is, it requires the more mercury to draw it to the silver contained in the mass, so that they know the quantity by long experience. An Indian is employed to mould or trample one of these square cuerpos eight times a-day, that the mercury may thoroughly incorporate with the silver. To expedite this incorporation, they often mix lime with the mass, when the ore happens to be what they call greasy, and in this great caution is required, as they say the mass sometimes grows so hot that they neither find mercury nor silver in it, which seems quite incredible. Sometimes also they strew in some lead or tin ore, to facilitate the operation of the mercury, which is slower in very cold weather; wherefore, at Potosi and Lipes, they are often obliged to mould or work up their cuerpos during a month or six weeks; but, in more temperate climates, the amalgama is completed in eight or ten days. To facilitate the action of the mercury, they, in some places, as at Puno and elsewhere, construct their buiterons or floors on arches, under which they keep fires for twenty-four hours, to heat the masses or cuerpos, which are in that case placed as a pavement of bricks.
When it is thought that the mercury has attracted all the silver, the assayer takes a small quantity of ore from each cuerpo, which he washes separately in a small earthen plate or wooden bowl; and, by the colour and appearance of the amalgama found at the bottom, when the earthy matters are washed away, he knows whether the mercury has produced its proper effect. When blackish, the ore is said to have been too much heated, and they add more salt, or some other temper. In this case they say that mercury is dispara, that is, shoots or flees away. If the mercury remains white, they put a drop under the thumb, and pressing it hastily, the silver in the amalgam sticks to the thumb, and the mercury slips away in little drops. When they conceive that all the silver has incorporated with the mercury, the mixed mass, or cuerpo, is carried to a basin or pond, into which a small stream of water is introduced to wash it, much in the same way as I shall afterwards describe the manner in which they wash gold, only that as the silver-ore is reduced to a fine mud without stones, it is stirred by an Indian with his feet, to dissolve it thoroughly, and loosen the silver. From the first basin it falls into a second, and thence into a third, where the stirring and washing is repeated, that any amalgam which has not subsided in the first and second may not escape the third.
The whole being thoroughly washed in these basins, which are lined with leather, till the water runs clear off, the amalgam of mercury and silver is found at the bottom, and is termed la pella. This is put into a woollen bag and hung up, from whence some of the mercury runs out. The bag is then beaten and pressed as much as they can, laying upon it a flat piece of wood loaded with a heavy weight, to get out as much of the mercury as they can. The paste is then put into a mould of wooden planks bound together, generally in the form of an octagon pyramid cut short, its bottoms being a plate of copper, full of small holes, into which the paste is stirred and pressed down, in order to fasten it. When they design to make many pinnas, or spongy lumps of various weights, these are divided from each other by thin beds or layers of earth, which hinder them from uniting. For this purpose, the pella, or mass of amalgam, must be weighed out in separate portions, deducting two-thirds for the contained mercury, by which they know to a small matter the quantity of silver contained in each. They then take off the mould, and place the pella or mass with its copper base on a trivet, or such like instrument, standing over a great earthen vessel full of water, and cover it with an earthen cap, which again is covered by lighted coals. This fire is fed and kept up for some hours, by which the mass of pella below becomes violently heated, the contained mercury being thereby raised into vapour: But, having no means of escape through the cap or cover, it is forced down to the water underneath, where it condenses into quicksilver and sinks to the bottom. By this contrivance, little of the mercury is lost, and the same serves over again. But the quantity must be increased, because it grows weak.[2] At Potosi, as Acosta relates, they formerly consumed six or seven thousand quintals of mercury every year, by which Some idea may be formed of the silver there procured.
On the evaporation of the mercury, nothing remains but a spongy lump of contiguous grains of silver, very light and almost mouldering, called la pinna by the Spaniards. These masses must be carried to the king's receipt or mint, to pay the royal fifth; and are there cast into ingots, on which are stamped the arms of the crown, the place where cast, and their weight and fineness. All these ingots, having paid the fifth, are sure to be without fraud or deceit; but it is not so with the pinnas, as these have often iron, sand, or some other matter contained within them, to increase their weight; Hence, prudence requires that these should be opened, and made red hot in a fire; for, if falsified, the fire will turn them black or yellow, or melt them more easily. This trial by fire is also necessary to extract moisture, which they contract in places where they are purposely laid to render them heavier, as also for separating the mercury with which the bottom of the mass is always more or less impregnated. The weight of these pinnas may be increased nearly a third, by dipping them while red hot into water. It also sometimes happens that the same mass of pinna may be of different fineness in different parts.
The ore, or stones taken from the mines, or the mineray, as it is called in Peru, from which the silver is extracted, is not always of the same nature, consistence, and colour. Some are white and grey, mixed with red or bluish spots, called plata blanca or white silver; of which sort the one in the Lipes mines mostly consists. For the most part, some little grains of silver are to be discerned, and very often small branches are seen, ramifying along the layers of the stone. Some ores are as black as the dross of iron, and in which no silver is to be seen, which is called negrillo or blackish ore. Sometimes the ore is rendered black by admixture of lead, and is called plombo ronco, or coarse lead, in which the silver appears as if scratched by something harsh. This ore is generally the richest in silver, and from it also the silver is got at the smallest charge; as instead of having to be moulded or kneaded with quicksilver, it has only to be melted in furnaces, where the lead evaporates by the force of fire, and the silver remains pure behind. From this sort. of mines, the Indians drew their silver before the coming of the Spaniards, having no knowledge of the use of mercury, and they accordingly only wrought those mines of which the ore would melt; and, having but little wood, they heated their furnaces with ylo, the dung of the Llamas or Peruvian sheep, placing their furnaces on the sides of mountains, that the wind might render their fires fierce.
There is another sort of black ore, in which the silver does not at all appear; and which, when wetted and rubbed against iron, becomes red. This ore is called rosicler, signifying that ruddiness which appears at the dawn of day. This is very rich, and affords the finest silver. Another kind, called zoroche, glitters like talc, and is generally very poor, yielding little silver: Its outer coat is very soft and of a yellowish red, but seldom rich; and the mines of this sort are wrought on account of the easiness of extracting the ore, being very easily dug. Another kind, not much harder than the last, is of a green colour, called cobrissa or copperish, and is very rare. Although the silver usually appears in this kind, and it is almost mouldering, it is the most difficult of all to manage, as it parts very difficultly with the silver. Sometimes, after being stamped or reduced to powder, it has to be burnt in the fire, and several other expedients must be used to separate the silver, doubtless because mixed with copper. There is another very rare sort of ore, which has only been found in the mine of Cotamiso at Potosi, being threads of pure silver entangled, or wound up together, like burnt lace, and so fine that it is called arana, or spider ore, from its resemblance to a cobweb.
The veins of mineray, of whatever sort they may be, are generally richer in the middle than towards the edges; and where two veins happen to cross each other, the place where they meet is always very rich. It is also observed that those which lie north and south are richer than those which lie in any other direction. Those also which are near to places where mills can be erected, and can consequently be more commodiously wrought, are often preferable to others that are richer, but require more expense in working. For this reason, at Lipes and Potosi, a chest of ore must yield ten marks or eighty ounces of silver, to pay the charges of working; while those in the province of Tarama only require five merks or forty ounces to defray the expences. When even very rich, and they happen to sink down so as to be liable to be flooded, the adventurers must have recourse to pumps and machines in order to drain them; or to cocabones or levels dug through the sides of the mountain, which often ruin the owners by the enormous expence they are insensibly drawn into. At some of the mines, where the methods of separation already described fail, they use other means of extracting the silver from the ore, and from other metals which may be combined with it; as by fire, or strong separating waters; and there the silver is cast into a sort of ingots, called bollos. But the most general and useful method is that already described.
It may naturally be supposed that mines, as well as other things, are subject to variation in their productiveness. The mines which, till very lately, yielded most silver, were those of Oroura, a small town about eight leagues from Arica. In the year 1712, one was discovered at Ollachea near Cusco, so rich that it yielded 2500 marks of silver of eight ounces each, or 20,000 ounces, out of each caxon or chest, being almost a fifth part of the ore; but it has since declined much, and is now [1720] only reckoned among the ordinary sort. Those of Lipes have had a similar fate. Those at Potosi now yield but little, and are worked at a very heavy expence, owing to their excessive depth. Although the mines here are far diminished in their productiveness, yet the quantity of ore which has been formerly wrought, and has lain many years on the surface, is now thought capable of yielding a second crop; and when I was at Lima, they were actually turning it up, and milling it over again with great success. This is a proof that these minerals generate in the earth like all other inanimate things;[3] and it likewise appears, from all the accounts of the Spaniards, that gold, silver, and other metals are continually growing and forming in the earth. This opinion is verified by experience in the mountain of Potosi, where several mines had fallen in, burying the workmen and their tools; and these being again opened up after some years, many boxes and pieces of wood were discovered, having veins of silver actually running through them.[4]
All these mines become the property of their first discoverer, who immediately presents a petition to the magistrates, desiring to have such a piece of ground for his own. This is accordingly granted, and a spot of ground eighty Spanish yards in length by forty in breadth[5] is measured out and appropriated to the discoverer, who chuses what spot he pleases within these bounds, and does with it as he thinks fit. The exact same quantity is then measured off as belonging to the king, and is sold to the best bidder, there being always many who are willing to purchase, what may turn out an inestimable treasure. After this, if any person may incline to work a part of this mine on his own account, he bargains with the proprietor for a particular vein. All that is dug out by any one is his own, subject however to payment of the royal duties; being one-twentieth part for gold, and a fifth for silver; and some proprietors find a good account in letting out their grounds and mills to others.