De Re Metallica Part 23

[Ill.u.s.tration 229 (Containers): A--Scorifier. B--Triangular crucible.

C--Cupel.]

The crucibles differ in the materials from which they are made, because they are made of either clay or ashes; and those of clay, which we also call "earthen," differ in shape and size. Some are made in the shape of a moderately thick salver (scorifiers), three digits wide, and of a capacity of an _uncia_ measure; in these the ore mixed with fluxes is melted, and they are used by those who a.s.say gold or silver ore. Some are triangular and much thicker and more capacious, holding five, or six, or even more _unciae_; in these copper is melted, so that it can be poured out, expanded, and tested with fire, and in these copper ore is usually melted.

The cupels are made of ashes; like the preceding scorifiers they are tray-shaped, and their lower part is very thick but their capacity is less. In these lead is separated from silver, and by them a.s.says are concluded. Inasmuch as the a.s.sayers themselves make the cupels, something must be said about the material from which they are made, and the method of making them. Some make them out of all kinds of ordinary ashes; these are not good, because ashes of this kind contain a certain amount of fat, whereby such cupels are easily broken when they are hot.

Others make them likewise out of any kind of ashes which have been previously leached; of this kind are the ashes into which warm water has been infused for the purpose of making lye. These ashes, after being dried in the sun or a furnace, are sifted in a hair sieve; and although warm water washes away the fat from the ashes, still the cupels which are made from such ashes are not very good because they often contain charcoal dust, sand, and pebbles. Some make them in the same way out of any kind of ashes, but first of all pour water into the ashes and remove the sc.u.m which floats thereon; then, after it has become clear, they pour away the water, and dry the ashes; they then sift them and make the cupels from them. These, indeed, are good, but not of the best quality, because ashes of this kind are also not devoid of small pebbles and sand. To enable cupels of the best quality to be made, all the impurities must be removed from the ashes. These impurities are of two kinds; the one sort light, to which cla.s.s belong charcoal dust and fatty material and other things which float in water, the other sort heavy, such as small stones, fine sand, and any other materials which settle in the bottom of a vessel. Therefore, first of all, water should be poured into the ashes and the light impurities removed; then the ashes should be kneaded with the hands, so that they will become properly mixed with the water. When the water has become muddy and turbid, it should be poured into a second vessel. In this way the small stones and fine sand, or any other heavy substance which may be there, remain in the first vessel, and should be thrown away. When all the ashes have settled in this second vessel, which will be shown if the water has become clear and does not taste of the flavour of lye, the water should be thrown away, and the ashes which have settled in the vessel should be dried in the sun or in a furnace. This material is suitable for the cupels, especially if it is the ash of beech wood or other wood which has a small annual growth; those ashes made from twigs and limbs of vines, which have rapid annual growth, are not so good, for the cupels made from them, since they are not sufficiently dry, frequently crack and break in the fire and absorb the metals. If ashes of beech or similar wood are not to be had, the a.s.sayer makes little b.a.l.l.s of such ashes as he can get, after they have been cleared of impurities in the manner before described, and puts them in a baker's or potter's oven to burn, and from these the cupels are made, because the fire consumes whatever fat or damp there may be. As to all kinds of ashes, the older they are the better, for it is necessary that they should have the greatest possible dryness. For this reason ashes obtained from burned bones, especially from the bones of the heads of animals, are the most suitable for cupels, as are also those ashes obtained from the horns of deer and the spines of fishes. Lastly, some take the ashes which are obtained from burnt sc.r.a.pings of leather, when the tanners sc.r.a.pe the hides to clear them from hair. Some prefer to use compounds, that one being recommended which has one and a half parts of ashes from the bones of animals or the spines of fishes, and one part of beech ashes, and half a part of ashes of burnt hide sc.r.a.pings. From this mixture good cupels are made, though far better ones are obtained from equal portions of ashes of burnt hide sc.r.a.pings, ashes of the bones of heads of sheep and calves, and ashes of deer horns. But the best of all are produced from deer horns alone, burnt to powder; this kind, by reason of its extreme dryness, absorbs metals least of all. a.s.sayers of our own day, however, generally make the cupels from beech ashes. These ashes, after being prepared in the manner just described, are first of all sprinkled with beer or water, to make them stick together, and are then ground in a small mortar. They are ground again after being mixed with the ashes obtained from the skulls of beasts or from the spines of fishes; the more the ashes are ground the better they are. Some rub bricks and sprinkle the dust so obtained, after sifting it, into the beech ashes, for dust of this kind does not allow the hearth-lead to absorb the gold or silver by eating away the cupels. Others, to guard against the same thing, moisten the cupels with white of egg after they have been made, and when they have been dried in the sun, again crush them; especially if they want to a.s.say in it an ore of copper which contains iron. Some moisten the ashes again and again with cow's milk, and dry them, and grind them in a small mortar, and then mould the cupels. In the works in which silver is separated from copper, they make cupels from two parts of the ashes of the crucible of the cupellation furnace, for these ashes are very dry, and from one part of bone-ash. Cupels which have been made in these ways also need to be placed in the sun or in a furnace; afterward, in whatever way they have been made, they must be kept a long time in dry places, for the older they are, the dryer and better they are.

[Ill.u.s.tration 231 (Cupel Moulds and Pestles): A--Little mould.

B--Inverted mould. C--Pestle. D--Its k.n.o.b. E--Second pestle.]

Not only potters, but also the a.s.sayers themselves, make scorifiers and triangular crucibles. They make them out of fatty clay, which is dry[5], and neither hard nor soft. With this clay they mix the dust of old broken crucibles, or of burnt and worn bricks; then they knead with a pestle the clay thus mixed with dust, and then dry it. As to these crucibles, the older they are, the dryer and better they are. The moulds in which the cupels are moulded are of two kinds, that is, a smaller size and a larger size. In the smaller ones are made the cupels in which silver or gold is purged from the lead which has absorbed it; in the larger ones are made cupels in which silver is separated from copper and lead. Both moulds are made out of bra.s.s and have no bottom, in order that the cupels can be taken out of them whole. The pestles also are of two kinds, smaller and larger, each likewise of bra.s.s, and from the lower end of them there projects a round k.n.o.b, and this alone is pressed into the mould and makes the hollow part of the cupel. The part which is next to the k.n.o.b corresponds to the upper part of the mould.

So much for these matters. I will now speak of the preparation of the ore for a.s.saying. It is prepared by roasting, burning, crushing, and washing. It is necessary to take a fixed weight of ore in order that one may determine how great a portion of it these preparations consume. The hard stone containing the metal is burned in order that, when its hardness has been overcome, it can be crushed and washed; indeed, the very hardest kind, before it is burned, is sprinkled with vinegar, in order that it may more rapidly soften in the fire. The soft stone should be broken with a hammer, crushed in a mortar and reduced to powder; then it should be washed and then dried again. If earth is mixed with the mineral, it is washed in a basin, and that which settles is a.s.sayed in the fire after it is dried. All mining products which are washed must again be dried. But ore which is rich in metal is neither burned nor crushed nor washed, but is roasted, lest that method of preparation should lose some of the metal. When the fires have been kindled, this kind of ore is roasted in an enclosed pot, which is stopped up with lute. A less valuable ore is even burned on a hearth, being placed upon the charcoal; for we do not make a great expenditure upon metals, if they are not worth it. However, I will go into fuller details as to all these methods of preparing ore, both a little later, and in the following Book.

For the present, I have decided to explain those things which mining people usually call fluxes[6] because they are added to ores, not only for a.s.saying, but also for smelting. Great power is discovered in all these fluxes, but we do not see the same effects produced in every case; and some are of a very complicated nature. For when they have been mixed with the ore and are melted in either the a.s.say or the smelting furnace, some of them, because they melt easily, to some extent melt the ore; others, because they either make the ore very hot or penetrate into it, greatly a.s.sist the fire in separating the impurities from the metals, and they also mix the fused part with the lead, or they partly protect from the fire the ore whose metal contents would be either consumed in the fire, or carried up with the fumes and fly out of the furnace; some fluxes absorb the metals. To the first order belongs lead, whether it be reduced to little granules or resolved into ash by fire, or red-lead[7], or ochre made from lead[8], or litharge, or hearth-lead, or galena; also copper, the same either roasted or in leaves or filings[9]; also the slags of gold, silver, copper, and lead; also soda[10], its slags, saltpetre, burned alum, vitriol, _sal tostus_, and melted salt[11]; stones which easily melt in hot furnaces, the sand which is made from them[12]; soft _tophus_[13], and a certain white schist[14]. But lead, its ashes, red-lead, ochre, and litharge, are more efficacious for ores which melt easily; hearth-lead for those which melt with difficulty; and galena for those which melt with greater difficulty. To the second order belong iron filings, their slag, _sal artificiosus_, argol, dried lees of vinegar[15], and the lees of the _aqua_ which separates gold from silver[16]; these lees and _sal artificiosus_ have the power of penetrating into ore, the argol to a considerable degree, the lees of vinegar to a greater degree, but most of all those of the _aqua_ which separates gold from silver; filings and slags of iron, since they melt more slowly, have the power of heating the ore. To the third order belong pyrites, the cakes which are melted from them, soda, its slags, salt, iron, iron scales, iron filings, iron slags, vitriol, the sand which is resolved from stones which easily melt in the fire, and _tophus_; but first of all are pyrites and the cakes which are melted from it, for they absorb the metals of the ore and guard them from the fire which consumes them. To the fourth order belong lead and copper, and their relations. And so with regard to fluxes, it is manifest that some are natural, others fall in the category of slags, and the rest are purged from slag. When we a.s.say ores, we can without great expense add to them a small portion of any sort of flux, but when we smelt them we cannot add a large portion without great expense. We must, therefore, consider how great the cost is, to avoid incurring a greater expense on smelting an ore than the profit we make out of the metals which it yields.

The colour of the fumes which the ore emits after being placed on a hot shovel or an iron plate, indicates what flux is needed in addition to the lead, for the purpose of either a.s.saying or smelting. If the fumes have a purple tint, it is best of all, and the ore does not generally require any flux whatever. If the fumes are blue, there should be added cakes melted out of pyrites or other cupriferous rock; if yellow, litharge and sulphur should be added; if red, gla.s.s-galls[17] and salt; if green, then cakes melted from cupriferous stones, litharge, and gla.s.s-galls; if the fumes are black, melted salt or iron slag, litharge and white lime rock. If they are white, sulphur and iron which is eaten with rust; if they are white with green patches, iron slag and sand obtained from stones which easily melt; if the middle part of the fumes are yellow and thick, but the outer parts green, the same sand and iron slag. The colour of the fumes not only gives us information as to the proper remedies which should be applied to each ore, but also more or less indication as to the solidified juices which are mixed with it, and which give forth such fumes. Generally, blue fumes signify that the ore contains azure yellow, orpiment; red, realgar; green, chrysocolla; black, black bitumen; white, tin[18]; white with green patches, the same mixed with chrysocolla; the middle part yellow and other parts green show that it contains sulphur. Earth, however, and other things dug up which contain metals, sometimes emit similarly coloured fumes.

If the ore contains any _stibium_, then iron slag is added to it; if pyrites, then are added cakes melted from a cupriferous stone and sand made from stones which easily melt. If the ore contains iron, then pyrites and sulphur are added; for just as iron slag is the flux for an ore mixed with sulphur, so on the contrary, to a gold or silver ore containing iron, from which they are not easily separated, is added sulphur and sand made from stones which easily melt.

_Sal artificiosus_[19] suitable for use in a.s.saying ore is made in many ways. By the first method, equal portions of argol, lees of vinegar, and urine, are all boiled down together till turned into salt. The second method is from equal portions of the ashes which wool-dyers use, of lime, of argol purified, and of melted salt; one _libra_ of each of these ingredients is thrown into twenty _librae_ of urine; then all are boiled down to one-third and strained, and afterward there is added to what remains one _libra_ and four _unciae_ of unmelted salt, eight pounds of lye being at the same time poured into the pots, with litharge smeared around on the inside, and the whole is boiled till the salt becomes thoroughly dry. The third method follows. Unmelted salt, and iron which is eaten with rust, are put into a vessel, and after urine has been poured in, it is covered with a lid and put in a warm place for thirty days; then the iron is washed in the urine and taken out, and the residue is boiled until it is turned into salt. In the fourth method by which _sal artificiosus_ is prepared, the lye made from equal portions of lime and the ashes which wool-dyers use, together with equal portions of salt, soap, white argol, and saltpetre, are boiled until in the end the mixture evaporates and becomes salt. This salt is mixed with the concentrates from washing, to melt them.

Saltpetre is prepared in the following manner, in order that it may be suitable for use in a.s.saying ore. It is placed in a pot which is smeared on the inside with litharge, and lye made of quicklime is repeatedly poured over it, and it is heated until the fire consumes it. Wherefore the saltpetre does not kindle with the fire, since it has absorbed the lime which preserves it, and thus it is prepared[20].

The following compositions[21] are recommended to smelt all ores which the heat of fire breaks up or melts only with difficulty. Of these, one is made from stones of the third order, which easily melt when thrown into hot furnaces. They are crushed into pure white powder, and with half an _uncia_ of this powder there are mixed two _unciae_ of yellow litharge, likewise crushed. This mixture is put into a scorifier large enough to hold it, and placed under the m.u.f.fle of a hot furnace; when the charge flows like water, which occurs after half an hour, it is taken out of the furnace and poured on to a stone, and when it has hardened it has the appearance of gla.s.s, and this is likewise crushed.

This powder is sprinkled over any metalliferous ore which does not easily melt when we are a.s.saying it, and it causes the slag to exude.

Others, in place of litharge, subst.i.tute lead ash,[22] which is made in the following way: sulphur is thrown into lead which has been melted in a crucible, and it soon becomes covered with a sort of sc.u.m; when this is removed, sulphur is again thrown in, and the skin which forms is again taken off; this is frequently repeated, in fact until all the lead is turned into powder. There is a powerful flux compound which is made from one _uncia_ each of prepared saltpetre, melted salt, gla.s.s-gall, and argol, and one-third of an _uncia_ of litharge and a _bes_ of gla.s.s ground to powder; this flux, being added to an equal weight of ore, liquefies it. A more powerful flux is made by placing together in a pot, smeared on the inside with litharge, equal portions of white argol, common salt, and prepared saltpetre, and these are heated until a white powder is obtained from them, and this is mixed with as much litharge; one part of this compound is mixed with two parts of the ore which is to be a.s.sayed. A still more powerful flux than this is made out of ashes of black lead, saltpetre, orpiment, _stibium_, and dried lees of the _aqua_ with which gold workers separate gold from silver. The ashes of lead[23]

are made from one pound of lead and one pound of sulphur; the lead is flattened out into sheets by pounding with a hammer, and placed alternately with sulphur in a crucible or pot, and they are heated together until the fire consumes the sulphur and the lead turns to ashes. One _libra_ of crushed saltpetre is mixed with one _libra_ of orpiment similarly ground to powder, and the two are cooked in an iron pan until they liquefy; they are then poured out, and after cooling are again ground to powder. A _libra_ of _stibium_ and a _bes_ of the dried lees (_of what?_) are placed alternately in a crucible and heated to the point at which they form a b.u.t.ton, which is similarly reduced to powder.

A _bes_ of this powder and one _libra_ of the ashes of lead, as well as a _libra_ of powder made out of the saltpetre and orpiment, are mixed together and a powder is made from them, one part of which added to two parts of ore liquefies it and cleanses it of dross. But the most powerful flux is one which has two _drachmae_ of sulphur and as much gla.s.s-galls, and half an _uncia_ of each of the following,--_stibium_, salt obtained from boiled urine, melted common salt, prepared saltpetre, litharge, vitriol, argol, salt obtained from ashes of musk ivy, dried lees of the _aqua_ by which gold-workers separate gold from silver, alum reduced by fire to powder, and one _uncia_ of camphor[24] combined with sulphur and ground into powder. A half or whole portion of this mixture, as the necessity of the case requires, is mixed with one portion of the ore and two portions of lead, and put in a scorifier; it is sprinkled with powder of crushed Venetian gla.s.s, and when the mixture has been heated for an hour and a half or two hours, a b.u.t.ton will settle in the bottom of the scorifier, and from it the lead is soon separated.

There is also a flux which separates sulphur, orpiment and realgar from metalliferous ore. This flux is composed of equal portions of iron slag, white _tophus_, and salt. After these juices have been secreted, the ores themselves are melted, with argol added to them. There is one flux which preserves _stibium_ from the fire, that the fire may not consume it, and which preserves the metals from the _stibium_; and this is composed of equal portions of sulphur, prepared saltpetre, melted salt, and vitriol, heated together in lye until no odour emanates from the sulphur, which occurs after a s.p.a.ce of three or four hours.[25]

It is also worth while to subst.i.tute certain other mixtures. Take two portions of ore properly prepared, one portion of iron filings, and likewise one portion of salt, and mix; then put them into a scorifier and place them in a m.u.f.fle furnace; when they are reduced by the fire and run together, a b.u.t.ton will settle in the bottom of the scorifier.

Or else take equal portions of ore and of lead ochre, and mix with them a small quant.i.ty of iron filings, and put them into a scorifier, then scatter iron filings over the mixture. Or else take ore which has been ground to powder and sprinkle it in a crucible, and then sprinkle over it an equal quant.i.ty of salt that has been three or four times moistened with urine and dried; then, again and again alternately, powdered ore and salt; next, after the crucible has been covered with a lid and sealed, it is placed upon burning charcoal. Or else take one portion of ore, one portion of minute lead granules, half a portion of Venetian gla.s.s, and the same quant.i.ty of gla.s.s-galls. Or else take one portion of ore, one portion of lead granules, half a portion of salt, one-fourth of a portion of argol, and the same quant.i.ty of lees of the _aqua_ which separates gold from silver. Or else take equal portions of prepared ore and a powder in which there are equal portions of very minute lead granules, melted salt, _stibium_ and iron slag. Or else take equal portions of gold ore, vitriol, argol, and of salt. So much for the fluxes.

In the a.s.say furnace, when it has been prepared in the way in which I have described, is first placed a m.u.f.fle. Then selected pieces of live charcoals are laid on it, for, from pieces of inferior quality, a great quant.i.ty of ash collects around the m.u.f.fle and hinders the action of the fire. Then the scorifiers are placed under the m.u.f.fle with tongs, and glowing coals are placed under the fore part of the m.u.f.fle to warm the scorifiers more quickly; and when the lead or ore is to be placed in the scorifiers, they are taken out again with the tongs. When the scorifiers glow in the heat, first of all the ash or small charcoals, if any have fallen into them, should be blown away with an iron pipe two feet long and a digit in diameter; this same thing must be done if ash or small coal has fallen into the cupels. Next, put in a small ball of lead with the tongs, and when this lead has begun to be turned into fumes and consumed, add to it the prepared ore wrapped in paper. It is preferable that the a.s.sayer should wrap it in paper, and in this way put it in the scorifier, than that he should drop it in with a copper ladle; for when the scorifiers are small, if he uses a ladle he frequently spills some part of the ore. When the paper is burnt, he stirs the ore with a small charcoal held in the tongs, so that the lead may absorb the metal which is mixed in the ore; when this mixture has taken place, the slag partly adheres by its circ.u.mference to the scorifier and makes a kind of black ring, and partly floats on the lead in which is mixed the gold or silver; then the slag must be removed from it.

The lead used must be entirely free from every trace of silver, as is that which is known as _Villacense_.[26] But if this kind is not obtainable, the lead must be a.s.sayed separately, to determine with certainty that proportion of silver it contains, so that it may be deducted from the calculation of the ore, and the result be exact; for unless such lead be used, the a.s.say will be false and misleading. The lead b.a.l.l.s are made with a pair of iron tongs, about one foot long; its iron claws are so formed that when pressed together they are egg-shaped; each claw contains a hollow cup, and when the claws are closed there extends upward from the cup a pa.s.sage, so there are two openings, one of which leads to each hollow cup. And so when the molten lead is poured in through the openings, it flows down into the hollow cup, and two b.a.l.l.s are formed by one pouring.

In this place I ought not to omit mention of another method of a.s.saying employed by some a.s.sayers. They first of all place prepared ore in the scorifiers and heat it, and afterward they add the lead. Of this method I cannot approve, for in this way the ore frequently becomes cemented, and for this reason it does not stir easily afterward, and is very slow in mixing with the lead.

[Ill.u.s.tration 240a (Tongs): A--Claws of the tongs. B--Iron, giving form of an egg. C--Opening.]

If the whole s.p.a.ce of the furnace covered by the m.u.f.fle is not filled with scorifiers, cupels are put in the empty s.p.a.ce, in order that they may become warmed in the meantime. Sometimes, however, it is filled with scorifiers, when we are a.s.saying many different ores, or many portions of one ore at the same time. Although the cupels are usually dried in one hour, yet smaller ones are done more quickly, and the larger ones more slowly. Unless the cupels are heated before the metal mixed with lead is placed in them, they frequently break, and the lead always sputters and sometimes leaps out of them; if the cupel is broken or the lead leaps out of it, it is necessary to a.s.say another portion of ore; but if the lead only sputters, then the cupels should be covered with broad thin pieces of glowing charcoal, and when the lead strikes these, it falls back again, and thus the mixture is slowly exhaled. Further, if in the cupellation the lead which is in the mixture is not consumed, but remains fixed and set, and is covered by a kind of skin, this is a sign that it has not been heated by a sufficiently hot fire; put into the mixture, therefore, a dry pine stick, or a twig of a similar tree, and hold it in the hand in order that it can be drawn away when it has been heated. Then take care that the heat is sufficient and equal; if the heat has not pa.s.sed all round the charge, as it should when everything is done rightly, but causes it to have a lengthened shape, so that it appears to have a tail, this is a sign that the heat is deficient where the tail lies. Then in order that the cupel may be equally heated by the fire, turn it around with a small iron hook, whose handle is likewise made of iron and is a foot and a half long.

[Ill.u.s.tration 240b (Hook): Small iron hook.]

Next, if the mixture has not enough lead, add as much of it as is required with the iron tongs, or with the bra.s.s ladle to which is fastened a very long handle. In order that the charge may not be cooled, warm the lead beforehand. But it is better at first to add as much lead as is required to the ore which needs melting, rather than afterward when the melting has been half finished, that the whole quant.i.ty may not vanish in fumes, but part of it remain fast. When the heat of the fire has nearly consumed the lead, then is the time when the gold and silver gleam in their varied colours, and when all the lead has been consumed the gold or silver settles in the cupel. Then as soon as possible remove the cupel out of the furnace, and take the b.u.t.ton out of it while it is still warm, in order that it does not adhere to the ashes. This generally happens if the b.u.t.ton is already cold when it is taken out. If the ashes do adhere to it, do not sc.r.a.pe it with a knife, lest some of it be lost and the a.s.say be erroneous, but squeeze it with the iron tongs, so that the ashes drop off through the pressure. Finally, it is of advantage to make two or three a.s.says of the same ore at the same time, in order that if by chance one is not successful, the second, or in any event the third, may be certain.

[Ill.u.s.tration 241 (Shield for m.u.f.fle Furnace): A--Handle of tablet.

B--Its crack.]

While the a.s.sayer is a.s.saying the ore, in order to prevent the great heat of the fire from injuring his eyes, it will be useful for him always to have ready a thin wooden tablet, two palms wide, with a handle by which it may be held, and with a slit down the middle in order that he may look through it as through a crack, since it is necessary for him to look frequently within and carefully to consider everything.

Now the lead which has absorbed the silver from a metallic ore is consumed in the cupel by the heat in the s.p.a.ce of three quarters of an hour. When the a.s.says are completed the m.u.f.fle is taken out of the furnace, and the ashes removed with an iron shovel, not only from the brick and iron furnaces, but also from the earthen one, so that the furnace need not be removed from its foundation.

From ore placed in the triangular crucible a b.u.t.ton is melted out, from which metal is afterward made. First of all, glowing charcoal is put into the iron hoop, then is put in the triangular crucible, which contains the ore together with those things which can liquefy it and purge it of its dross; then the fire is blown with the double bellows, and the ore is heated until the b.u.t.ton settles in the bottom of the crucible. We have explained that there are two methods of a.s.saying ore,--one, by which the lead is mixed with ore in the scorifier and afterward again separated from it in the cupel; the other, by which it is first melted in the triangular earthen crucible and afterward mixed with lead in the scorifier, and later separated from it in the cupel.

Now let us consider which is more suitable for each ore, or, if neither is suitable, by what other method in one way or another we can a.s.say it.

We justly begin with a gold ore, which we a.s.say by both methods, for if it is rich and seems not to be strongly resistant to fire, but to liquefy easily, one _centumpondium_ of it (known to us as the lesser weights),[27] together with one and a half, or two _unciae_ of lead of the larger weights, are mixed together and placed in the scorifier, and the two are heated in the fire until they are well mixed. But since such an ore sometimes resists melting, add a little salt to it, either _sal torrefactus_ or _sal artificiosus_, for this will subdue it, and prevent the alloy from collecting much dross; stir it frequently with an iron rod, in order that the lead may flow around the gold on every side, and absorb it and cast out the waste. When this has been done, take out the alloy and cleanse it of slag; then place it in the cupel and heat it until it exhales all the lead, and a bead of gold settles in the bottom.

If the gold ore is seen not to be easily melted in the fire, roast it and extinguish it with brine. Do this again and again, for the more often you roast it and extinguish it, the more easily the ore can be crushed fine, and the more quickly does it melt in the fire and give up whatever dross it possesses. Mix one part of this ore, when it has been roasted, crushed, and washed, with three parts of some powder compound which melts ore, and six parts of lead. Put the charge into the triangular crucible, place it in the iron hoop to which the double bellows reaches, and heat first in a slow fire, and afterward gradually in a fiercer fire, till it melts and flows like water. If the ore does not melt, add to it a little more of these fluxes, mixed with an equal portion of yellow litharge, and stir it with a hot iron rod until it all melts. Then take the crucible out of the hoop, shake off the b.u.t.ton when it has cooled, and when it has been cleansed, melt first in the scorifier and afterward in the cupel. Finally, rub the gold which has settled in the bottom of the cupel, after it has been taken out and cooled, on the touchstone, in order to find out what proportion of silver it contains. Another method is to put a _centumpondium_ (of the lesser weights) of gold ore into the triangular crucible, and add to it a _drachma_ (of the larger weights) of gla.s.s-galls. If it resists melting, add half a _drachma_ of roasted argol, and if even then it resists, add the same quant.i.ty of roasted lees of vinegar, or lees of the _aqua_ which separates gold from silver, and the b.u.t.ton will settle in the bottom of the crucible. Melt this b.u.t.ton again in the scorifier and a third time in the cupel.

We determine in the following way, before it is melted in the m.u.f.fle furnace, whether pyrites contains gold in it or not: if, after being three times roasted and three times quenched in sharp vinegar, it has not broken nor changed its colour, there is gold in it. The vinegar by which it is quenched should be mixed with salt that is put in it, and frequently stirred and dissolved for three days. Nor is pyrites devoid of gold, when, after being roasted and then rubbed on the touchstone, it colours the touchstone in the same way that it coloured it when rubbed in its crude state. Nor is gold lacking in that, whose concentrates from washing, when heated in the fire, easily melt, giving forth little smell and remaining bright; such concentrates are heated in the fire in a hollowed piece of charcoal covered over with another charcoal.

We also a.s.say gold ore without fire, but more often its sand or the concentrates which have been made by washing, or the dust gathered up by some other means. A little of it is slightly moistened with water and heated until it begins to exhale an odour, and then to one portion of ore are placed two portions of quicksilver[28] in a wooden dish as deep as a basin. They are mixed together with a little brine, and are then ground with a wooden pestle for the s.p.a.ce of two hours, until the mixture becomes of the thickness of dough, and the quicksilver can no longer be distinguished from the concentrates made by the washing, nor the concentrates from the quicksilver. Warm, or at least tepid, water is poured into the dish and the material is washed until the water runs out clear. Afterward cold water is poured into the same dish, and soon the quicksilver, which has absorbed all the gold, runs together into a separate place away from the rest of the concentrates made by washing.

The quicksilver is afterward separated from the gold by means of a pot covered with soft leather, or with canvas made of woven threads of cotton; the amalgam is poured into the middle of the cloth or leather, which sags about one hand's breadth; next, the leather is folded over and tied with a waxed string, and the dish catches the quicksilver which is squeezed through it. As for the gold which remains in the leather, it is placed in a scorifier and purified by being placed near glowing coals. Others do not wash away the dirt with warm water, but with strong lye and vinegar, for they pour these liquids into the pot, and also throw into it the quicksilver mixed with the concentrates made by washing. Then they set the pot in a warm place, and after twenty-four hours pour out the liquids with the dirt, and separate the quicksilver from the gold in the manner which I have described. Then they pour urine into a jar set in the ground, and in the jar place a pot with holes in the bottom, and in the pot they place the gold; then the lid is put on and cemented, and it is joined with the jar; they afterward heat it till the pot glows red. After it has cooled, if there is copper in the gold they melt it with lead in a cupel, that the copper may be separated from it; but if there is silver in the gold they separate them by means of the _aqua_ which has the power of parting these two metals. There are some who, when they separate gold from quicksilver, do not pour the amalgam into a leather, but put it into a gourd-shaped earthen vessel, which they place in the furnace and heat gradually over burning charcoal; next, with an iron plate, they cover the opening of the operculum, which exudes vapour, and as soon as it has ceased to exude, they smear it with lute and heat it for a short time; then they remove the operculum from the pot, and wipe off the quicksilver which adheres to it with a hare's foot, and preserve it for future use. By the latter method, a greater quant.i.ty of quicksilver is lost, and by the former method, a smaller quant.i.ty.

If an ore is rich in silver, as is _rudis_ silver[29], frequently silver glance, or rarely ruby silver, gray silver, black silver, brown silver, or yellow silver, as soon as it is cleansed and heated, a _centumpondium_ (of the lesser weights) of it is placed in an _uncia_ of molten lead in a cupel, and is heated until the lead exhales. But if the ore is of poor or moderate quality, it must first be dried, then crushed, and then to a _centumpondium_ (of the lesser weights) an _uncia_ of lead is added, and it is heated in the scorifier until it melts. If it is not soon melted by the fire, it should be sprinkled with a little powder of the first order of fluxes, and if then it does not melt, more is added little by little until it melts and exudes its slag; that this result may be reached sooner, the powder which has been sprinkled over it should be stirred in with an iron rod. When the scorifier has been taken out of the a.s.say furnace, the alloy should be poured into a hole in a baked brick; and when it has cooled and been cleansed of the slag, it should be placed in a cupel and heated until it exhales all its lead; the weight of silver which remains in the cupel indicates what proportion of silver is contained in the ore.

We a.s.say copper ore without lead, for if it is melted with it, the copper usually exhales and is lost. Therefore, a certain weight of such an ore is first roasted in a hot fire for about six or eight hours; next, when it has cooled, it is crushed and washed; then the concentrates made by washing are again roasted, crushed, washed, dried, and weighed. The portion which it has lost whilst it is being roasted and washed is taken into account, and these concentrates by washing represent the cake which will be melted out of the copper ore. Place three _centumpondia_ (lesser weights) of this, mixed with three _centumpondia_ (lesser weights) each of copper scales[30], saltpetre, and Venetian gla.s.s, mixed, into the triangular crucible, and place it in the iron hoop which is set on the hearth in front of the double bellows.

Cover the crucible with charcoal in such a way that nothing may fall into the ore which is to be melted, and so that it may melt more quickly. At first blow a gentle blast with the bellows in order that the ore may be heated gradually in the fire; then blow strongly till it melts, and the fire consumes that which has been added to it, and the ore itself exudes whatever slag it possesses. Next, cool the crucible which has been taken out, and when this is broken you will find the copper; weigh this, in order to ascertain how great a portion of the ore the fire has consumed. Some ore is only once roasted, crushed, and washed; and of this kind of concentrates, three _centumpondia_ (lesser weights) are taken with one _centumpondium_ each of common salt, argol and gla.s.s-galls. Heat them in the triangular crucible, and when the mixture has cooled a b.u.t.ton of pure copper will be found, if the ore is rich in this metal. If, however, it is less rich, a stony lump results, with which the copper is intermixed; this lump is again roasted, crushed, and, after adding stones which easily melt and saltpetre, it is again melted in another crucible, and there settles in the bottom of the crucible a b.u.t.ton of pure copper. If you wish to know what proportion of silver is in this copper b.u.t.ton, melt it in a cupel after adding lead.

With regard to this test I will speak later.

Those who wish to know quickly what portion of silver the copper ore contains, roast the ore, crush and wash it, then mix a little yellow litharge with one _centumpondium_ (lesser weights) of the concentrates, and put the mixture into a scorifier, which they place under the m.u.f.fle in a hot furnace for the s.p.a.ce of half an hour. When the slag exudes, by reason of the melting force which is in the litharge, they take the scorifier out; when it has cooled, they cleanse it of slag and again crush it, and with one _centumpondium_ of it they mix one and a half _unciae_ of lead granules. They then put it into another scorifier, which they place under the m.u.f.fle in a hot furnace, adding to the mixture a little of the powder of some one of the fluxes which cause ore to melt; when it has melted they take it out, and after it has cooled, cleanse it of slag; lastly, they heat it in the cupel till it has exhaled all of the lead, and only silver remains.

Lead ore may be a.s.sayed by this method: crush half an _uncia_ of pure lead-stone and the same quant.i.ty of the _chrysocolla_ which they call borax, mix them together, place them in a crucible, and put a glowing coal in the middle of it. As soon as the borax crackles and the lead-stone melts, which soon occurs, remove the coal from the crucible, and the lead will settle to the bottom of it; weigh it out, and take account of that portion of it which the fire has consumed. If you also wish to know what portion of silver is contained in the lead, melt the lead in the cupel until all of it exhales.

Another way is to roast the lead ore, of whatsoever quality it be, wash it, and put into the crucible one _centumpondium_ of the concentrates, together with three _centumpondia_ of the powdered compound which melts ore, mixed together, and place it in the iron hoop that it may melt; when it has cooled, cleanse it of its slag, and complete the test as I have already said. Another way is to take two _unciae_ of prepared ore, five _drachmae_ of roasted copper, one _uncia_ of gla.s.s, or gla.s.s-galls reduced to powder, a _semi-uncia_ of salt, and mix them. Put the mixture into the triangular crucible, and heat it over a gentle fire to prevent it from breaking; when the mixture has melted, blow the fire vigorously with the bellows; then take the crucible off the live coals and let it cool in the open air; do not pour water on it, lest the lead b.u.t.ton being acted upon by the excessive cold should become mixed with the slag, and the a.s.say in this way be erroneous. When the crucible has cooled, you will find in the bottom of it the lead b.u.t.ton. Another way is to take two _unciae_ of ore, a _semi-uncia_ of litharge, two _drachmae_ of Venetian gla.s.s and a _semi-uncia_ of saltpetre. If there is difficulty in melting the ore, add to it iron filings, which, since they increase the heat, easily separate the waste from lead and other metals. By the last way, lead ore properly prepared is placed in the crucible, and there is added to it only the sand made from stones which easily melt, or iron filings, and then the a.s.say is completed as formerly.

You can a.s.say tin ore by the following method. First roast it, then crush, and afterward wash it; the concentrates are again roasted, crushed, and washed. Mix one and a half _centumpondia_ of this with one _centumpondium_ of the _chrysocolla_ which they call borax; from the mixture, when it has been moistened with water, make a lump. Afterwards, perforate a large round piece of charcoal, making this opening a palm deep, three digits wide on the upper side and narrower on the lower side; when the charcoal is put in its place the latter should be on the bottom and the former uppermost. Let it be placed in a crucible, and let glowing coal be put round it on all sides; when the perforated piece of coal begins to burn, the lump is placed in the upper part of the opening, and it is covered with a wide piece of glowing coal, and after many pieces of coal have been put round it, a hot fire is blown up with the bellows, until all the tin has run out of the lower opening of the charcoal into the crucible. Another way is to take a large piece of charcoal, hollow it out, and smear it with lute, that the ore may not leap out when white hot. Next, make a small hole through the middle of it, then fill up the large opening with small charcoal, and put the ore upon this; put fire in the small hole and blow the fire with the nozzle of a hand bellows; place the piece of charcoal in a small crucible, smeared with lute, in which, when the melting is finished, you will find a b.u.t.ton of tin.

In a.s.saying bis.m.u.th ore, place pieces of ore in the scorifier, and put it under the m.u.f.fle in a hot furnace; as soon as they are heated, they drip with bis.m.u.th, which runs together into a b.u.t.ton.

Quicksilver ore is usually tested by mixing one part of broken ore with three-parts of charcoal dust and a handful of salt. Put the mixture into a crucible or a pot or a jar, cover it with a lid, seal it with lute, place it on glowing charcoal, and as soon as a burnt cinnabar colour shows in it, take out the vessel; for if you continue the heat too long the mixture exhales the quicksilver with the fumes. The quicksilver itself, when it has become cool, is found in the bottom of the crucible or other vessel. Another way is to place broken ore in a gourd-shaped earthen vessel, put it in the a.s.say furnace, and cover with an operculum which has a long spout; under the spout, put an ampulla to receive the quicksilver which distills. Cold water should be poured into the ampulla, so that the quicksilver which has been heated by the fire may be continuously cooled and gathered together, for the quicksilver is borne over by the force of the fire, and flows down through the spout of the operculum into the ampulla. We also a.s.say quicksilver ore in the very same way in which we smelt it. This I will explain in its proper place.

Lastly, we a.s.say iron ore in the forge of a blacksmith. Such ore is burned, crushed, washed, and dried; a magnet is laid over the concentrates, and the particles of iron are attracted to it; these are wiped off with a brush, and are caught in a crucible, the magnet being continually pa.s.sed over the concentrates and the particles wiped off, so long as there remain any particles which the magnet can attract to it.

These particles are heated in the crucible with saltpetre until they melt, and an iron b.u.t.ton is melted out of them. If the magnet easily and quickly attracts the particles to it, we infer that the ore is rich in iron; if slowly, that it is poor; if it appears actually to repel the ore, then it contains little or no iron. This is enough for the a.s.saying of ores.

I will now speak of the a.s.saying of the metal alloys. This is done both by coiners and merchants who buy and sell metal, and by miners, but most of all by the owners and mine masters, and by the owners and masters of the works in which the metals are smelted, or in which one metal is parted from another.

First I will describe the way a.s.says are usually made to ascertain what portion of precious metal is contained in base metal. Gold and silver are now reckoned as precious metals and all the others as base metals.

Once upon a time the base metals were burned up, in order that the precious metals should be left pure; the Ancients even discovered by such burning what portion of gold was contained in silver, and in this way all the silver was consumed, which was no small loss. However, the famous mathematician, Archimedes[31], to gratify King Hiero, invented a method of testing the silver, which was not very rapid, and was more accurate for testing a large ma.s.s than a small one. This I will explain in my commentaries. The alchemists have shown us a way of separating silver from gold by which neither of them is lost[32].

Gold which contains silver,[33] or silver which contains gold, is first rubbed on the touchstone. Then a needle in which there is a similar amount of gold or silver is rubbed on the same touchstone, and from the lines which are produced in this way, is perceived what portion of silver there is in the gold, or what portion of gold there is in the silver. Next there is added to the silver which is in the gold, enough silver to make it three times as much as the gold. Then lead is placed in a cupel and melted; a little later, a small amount of copper is put in it, in fact, half an _uncia_ of it, or half an _uncia_ and a _sicilicus_ (of the smaller weights) if the gold or silver does not contain any copper. The cupel, when the lead and copper are wanting, attracts the particles of gold and silver, and absorbs them. Finally, one-third of a _libra_ of the gold, and one _libra_[34] of the silver must be placed together in the same cupel and melted; for if the gold and silver were first placed in the cupel and melted, as I have already said, it absorbs particles of them, and the gold, when separated from the silver, will not be found pure. These metals are heated until the lead and the copper are consumed, and again, the same weight of each is melted in the same manner in another cupel. The b.u.t.tons are pounded with a hammer and flattened out, and each little leaf is shaped in the form of a tube, and each is put into a small gla.s.s ampulla. Over these there is poured one _uncia_ and one _drachma_ (of the large weight) of the third quality _aqua valens_, which I will describe in the Tenth Book.

This is heated over a slow fire, and small bubbles, resembling pearls in shape, will be seen to adhere to the tubes. The redder the _aqua_ appears, the better it is judged to be; when the redness has vanished, small white bubbles are seen to be resting on the tubes, resembling pearls not only in shape, but also in colour. After a short time the _aqua_ is poured off and other is poured on; when this has again raised six or eight small white bubbles, it is poured off and the tubes are taken out and washed four or five times with spring water; or if they are heated with the same water, when it is boiling, they will shine more brilliantly. Then they are placed in a saucer, which is held in the hand and gradually dried by the gentle heat of the fire; afterward the saucer is placed over glowing charcoal and covered with a charcoal, and a moderate blast is blown upon it with the mouth and then a blue flame will be emitted. In the end the tubes are weighed, and if their weights prove equal, he who has undertaken this work has not laboured in vain.

Lastly, both are placed in another balance-pan and weighed; of each tube four grains must not be counted, on account of the silver which remains in the gold and cannot be separated from it. From the weight of the tubes we learn the weight both of the gold and of the silver which is in the b.u.t.ton. If some a.s.sayer has omitted to add so much silver to the gold as to make it three times the quant.i.ty, but only double, or two and a half times as much, he will require the stronger quality of _aqua_ which separates gold from silver, such as the fourth quality. Whether the _aqua_ which he employs for gold and silver is suitable for the purpose, or whether it is more or less strong than is right, is recognised by its effect. That of medium strength raises the little bubbles on the tubes and is found to colour the ampulla and the operculum a strong red; the weaker one is found to colour them a light red, and the stronger one to break the tubes. To pure silver in which there is some portion of gold, nothing should be added when they are being heated in the cupel prior to their being parted, except a _bes_ of lead and one-fourth or one-third its amount of copper of the lesser weights. If the silver contains in itself a certain amount of copper, let it be weighed, both after it has been melted with the lead, and after the gold has been parted from it; by the former we learn how much copper is in it, by the latter how much gold. Base metals are burnt up even to-day for the purpose of a.s.say, because to lose so little of the metal is small loss, but from a large ma.s.s of base metal, the precious metal is always extracted, as I will explain in Books X. and XI.