Q
uestions as to the methods of smelting ores and of obtaining metals I discussed in Book IX. Following this, I should explain in what manner the precious metals are parted from the base metals, or on the other hand the base metals from the precious[1]. Frequently two metals, occasionally more than two, are melted out of one ore, because in nature generally there is some amount of gold in silver and in copper, and some silver in gold, copper, lead, and iron; likewise some copper in gold, silver, lead, and iron, and some lead in silver; and lastly, some iron in copper[2]. But I will begin with gold.
Gold is parted from silver, or likewise the latter from the former, whether it be mixed by nature or by art, by means ofaqua valens[3], and by powders which consist of almost the same things as thisaqua. In order to preserve the sequence, I will first speak of the ingredients of which thisaquais made, then of the method of making it, then of the manner in which gold is parted from silver or silver from gold. Almost all these ingredients contain vitriol or alum, which, by themselves, but much more when joined with saltpetre, are powerful to part silver from gold. As to the other things that are added to them, they cannot individually by their own strength and nature separate those metals, but joined they are very powerful. Since there are many combinations, I will set out a few. In the first, the use of which is common and general, there is onelibraof vitriol and as much salt, added to a third of alibraof spring water. The second contains twolibraeof vitriol, one of saltpetre, and as much spring or river water by weight as will pass away whilst the vitriol is being reduced to powder by the fire. The third consists of fourlibraeof vitriol, two and a halflibraeof saltpetre, half alibraof alum, and one and a halflibraeof spring water. The fourth consists of twolibraeof vitriol, as manylibraeof saltpetre, one quarter of alibraof alum, and three-quarters of alibraof spring water. The fifth is composed of onelibraof saltpetre,threelibraeof alum, half alibraof brick dust, and three-quarters of alibraof spring water. The sixth consists of fourlibraeof vitriol, threelibraeof saltpetre, one of alum, onelibralikewise of stones which when thrown into a fierce furnace are easily liquefied by fire of the third order, and one and a halflibraeof spring water. The seventh is made of twolibraeof vitriol, one and a halflibraeof saltpetre, half alibraof alum, and onelibraof stones which when thrown into a glowing furnace are easily liquefied by fire of the third order, and five-sixths of alibraof spring water. The eighth is made of twolibraeof vitriol, the same number oflibraeof saltpetre, one and a halflibraeof alum, onelibraof the lees of theaquawhich parts gold from silver; and to each separatelibraa sixth of urine is poured over it. The ninth contains twolibraeof powder of baked bricks, onelibraof vitriol, likewise onelibraof saltpetre, a handful of salt, and three-quarters of alibraof spring water. Only the tenth lacks vitriol and alum, but it contains threelibraeof saltpetre, twolibraeof stones which when thrown into a hot furnace are easily liquefied by fire of the third order, half alibraeach of verdigris[4], ofstibium, of iron scales and filings, and of asbestos[5], and one and one-sixthlibraeof spring water.
All the vitriol from which theaquais usually made is first reduced to powder in the following way. It is thrown into an earthen crucible lined on the inside with litharge, and heated until it melts; then it is stirred with a copper wire, and after it has cooled it is pounded to powder. In the same manner saltpetre melted by the fire is pounded to powder when it has cooled. Some indeed place alum upon an iron plate, roast it, and make it into powder.
Although all theseaquaecleanse gold concentrates or dust from impurities, yet there are certain compositions which possess singular power.The first of these consists of onelibraof verdigris and three-quarters of alibraof vitriol. For eachlibrathere is poured over it one-sixth of alibraof spring or river water, as to which, since this pertains to all these compounds, it is sufficient to have mentioned once for all. The second composition is made from onelibraof each of the following, artificial orpiment, vitriol, lime, alum, ash which the dyers of wool use, one quarter of alibraof verdigris, and one and a halfunciaeofstibium. The third consists of threelibraeof vitriol, one of saltpetre, half alibraof asbestos, and half alibraof baked bricks. The fourth consists of onelibraof saltpetre, onelibraof alum, and half alibraof sal-ammoniac.[6]
Nitric Acid MakingA—Furnace. B—Its round hole. C—Air-holes. D—Mouth of the furnace. E—Draught opening under it. F—Earthenware crucible. G—Ampulla. H—Operculum. I—Its spout. K—Other ampulla. L—Basket in which this is usually placed lest it be broken.[Pg 442]The furnace in whichaqua valensis made[7]is built of bricks, rectangular, two feet long and wide, and as many feet high and a half besides. It is covered with iron plates supported with iron rods; these plates are smeared on the top with lute, and they have in the centre a round hole, large enough to hold the earthen vessel in which the glass ampulla is placed, and on each side of the centre hole are two small round air-holes. The lower part of the furnace, in order to hold the burning charcoal, has iron plates at the height of a palm, likewise supported by iron rods. In the middle of the front there is the mouth, made for the purpose of putting the fire into the furnace; this mouth is half a foot high and wide, and rounded at the top, and under it is the draught opening. Into the earthen vessel set over the hole is placed clean sand a digit deep, and in it the glass ampulla is set as deeply as it is smeared with lute. The lower quarter is smeared eight or ten times with nearly liquid lute, each time to the thickness of a blade, and each time it is dried again, until it has become as thick as the thumb; this kind of lute is well beaten with an iron rod, and is thoroughly mixed with hair or cotton thread, or with wool and salt, that it should not crackle. The many things of which the compounds are made must not fill the ampulla completely, lest when boiling they rise into the operculum. The operculum is likewise made of glass, and is closely joined to the ampulla with linen, cemented with wheat flour and white of egg moistened with water, and then lute free from salt is spread over that part of it. In a similar way the spout of the operculum is joined by linen covered with lute to another glass ampulla which receives the distilledaqua. A kind of thin iron nail or small wooden peg, a little thicker than a needle, is fixed in this joint, in order that when air seems necessary to the artificer distilling by this process he can pull it out; this is necessary when too much of the vapour has been driven into the upper part. The four air-holes which, as I have said, are on the top of the furnace beside the large hole on which the ampulla is placed, are likewise covered with lute.
All this preparation having been accomplished in order, and the ingredients placed in the ampulla, they are gradually heated over burning charcoal until they begin to exhale vapour and the ampulla is seen to trickle with moisture. But when this, on account of the rising of the vapour, turns red, and theaquadistils through the spout of the operculum, then one must work with the utmost care, lest the drops should fall at a quicker rate than one for every five movements of the clock or the striking of its bell, and not slower than one for every ten; for if it falls faster the glasses will be broken, and if it drops more slowly the work begun cannot be completed within the definite time, that is within the space of twenty-four hours. To prevent the first accident, part of the coals are extracted by means of an iron implement similar to pincers; and in order to prevent the second happening, small dry pieces of oak are placed upon the coals, and the substances in the ampulla are heated with a sharper fire, and the air-holes on the furnace are re-opened if need arise. As soon as the drops are being distilled, the glass ampulla which receives them is covered with a piece of linenmoistened with water, in order that the powerful vapour which arises may be repelled. When the ingredients have been heated and the ampulla in which they were placed is whitened with moisture, it is heated by a fiercer fire until all the drops have been distilled[8]. After the furnace has cooled, theaquais filtered and poured into a small glass ampulla, and into the same is put half adrachmaof silver[9], which when dissolved makes the turbidaquaclear. This is poured into the ampulla containing all the rest of theaqua, and as soon as the lees have sunk to the bottom theaquais poured off, removed, and reserved for use.
Gold is parted from silver by the following method[10]. The alloy, with lead added to it, is first heated in a cupel until all the lead is exhaled, and eightounces of the alloy contain only fivedrachmaeof copper or at most six, for if there is more copper in it, the silver separated from the gold soon unites with it again. Such molten silver containing gold is formed into granules, being stirred by means of a rod split at the lower end, or else is poured into an iron mould, and when cooled is made into thin leaves. As the process of making granules from argentiferous gold demands greater care and diligence than making them from any other metals, I will now explain the method briefly. The alloy is first placed in a crucible, which is then covered with a lid and placed in another earthen crucible containing a few ashes. Then they are placed in the furnace, and after they are surrounded by charcoal, the fire is blown by the blast of a bellows, and lest the charcoal fall away it is surrounded by stones or bricks. Soon afterward charcoal is thrown over the upper crucible and covered with live coals; these again are covered with charcoal, so that the crucible is surrounded and covered on all sides with it. It is necessary to heat the crucibles with charcoal for the space of half an hour or a little longer, and to provide that there is no deficiency of charcoal, lest the alloy become chilled; after this the air is blown in through the nozzle of the bellows, that the gold may begin to melt. Soon afterward it is turned round, and a test is quickly taken to see whether it be melted, and if it is melted, fluxes are thrown into it; it is advisable to cover up the crucible again closely that the contents may not be exhaled. The contents are heated together for as long as it would take to walk fifteen paces, and then the crucible is seized with tongs and the gold is emptied into an oblong vessel containing very cold water, by pouring it slowly from a height so that the granules will not be too big; in proportion as they are lighter, more fine and more irregular, the better they are, therefore the water is frequently stirred with a rod split into four parts from the lower end to the middle.
The leaves are cut into small pieces, and they or the silver granules are put into a glass ampulla, and theaquais poured over them to a height of a digit above the silver. The ampulla is covered with a bladder or with waxed linen, lest the contents exhale. Then it is heated until the silver is dissolved, the indication of which is the bubbling of theaqua. The gold remains in the bottom, of a blackish colour, and the silver mixed with theaquafloats above. Some pour the latter into a copper bowl and pour into it cold water, which immediately congeals the silver; this they take out and dry, having poured off theaqua[11]. They heat the dried silver in an earthenware crucible until it melts, and when it is melted they pour it into an iron mould.
The gold which remains in the ampulla they wash with warm water, filter, dry, and heat in a crucible with a littlechrysocollawhich is called borax, and when it is melted they likewise pour it into an iron mould.
Some workers, into an ampulla which contains gold and silver and theaquawhich separates them, pour two or three times as much of thisaqua valenswarmed, and into the same ampulla or into a dish into which all is poured, throw fine leaves of black lead and copper; by this means the gold adheres to the lead and the silver to the copper, and separately the lead from the gold, and separately the copper from the silver, are parted in a cupel. But no method is approved by us which loses theaquaused to part gold from silver, for it might be used again[12].
Parting precious metals with nitric acidA—Ampullae arranged in the vessels. B—An ampulla standing upright between iron rods. C—Ampullae placed in the sand which is contained in a box, the spouts of which reach from the opercula into ampullae placed under them. D—Ampullae likewise placed in sand which is contained in a box, of which the spouts from the opercula extend crosswise into ampullae placed under them. E—Other ampullae receiving the distilledaquaand likewise arranged in sand contained in the lower boxes. F—Iron tripod, in which the ampulla is usually placed when there are not many particles of gold to be parted from the silver. G—Vessel.[Pg 446]A glass ampulla, which bulges up inside at the bottom like a cone, is covered on the lower part of the outside with lute in the way explained above, and into it is put silver bullion weighing three and a half Romanlibrae. Theaquawhich parts the one from the other is poured into it, and the ampulla is placed in sand contained in an earthen vessel, or in a box, that it may be warmed with a gentle fire. Lest theaquashould be exhaled, the top of the ampulla is plastered on all sides with lute, and it is covered with a glass operculum, under whose spout is placed another ampulla which receives the distilled drops; this receiver is likewise arranged in a box containing sand. When the contents are heated it reddens, but when the redness no longer appears to increase, it is taken out of the vessel or box and shaken; by this motion theaquabecomes heated again and grows red; if this is done two or three times before otheraquais added to it, the operation is sooner concluded, and much lessaquais consumed. When the first charge has all been distilled, as much silver as at first is again put into the ampulla, for if too much were put in at once, the gold would be parted from it with difficulty. Then the secondaquais poured in, but it is warmed in order that it and the ampulla may be of equal temperature, so that the latter may not be cracked by the cold; also if a cold wind blows on it, it is apt to crack. Then the thirdaquais poured in, and also if circumstances require it, the fourth, that is to say moreaquaand again more is poured in until the gold assumes the colour of burned brick. The artificer keeps in hand twoaquae, one of which is stronger than the other; the stronger is used at first, then the less strong, then at the last again the stronger. When the gold becomes of a reddish yellow colour, spring water is poured in and heated until it boils. The gold is washed four times and then heated in the crucible until it melts. The water with which it was washed is put back, for there is a little silver in it; for this reason it is poured into an ampulla and heated, and the drops first distilled are received by one ampulla, while those which come later, that is to say when the operculum begins to get red, fall into another. This latteraquais useful for testing the gold, the former for washing it; the former may also be poured over the ingredients from which theaqua valensis made.
Theaquathat was first distilled, which contains the silver, is poured into an ampulla wide at the base, the top of which is also smeared with lute and covered by an operculum, and is then boiled as before in order that it may be separated from the silver. If there be so muchaquathat (when boiled) itrises into the operculum, there is put into the ampulla one lozenge or two; these are made of soap, cut into small pieces and mixed together with powdered argol, and then heated in a pot over a gentle fire; or else the contents are stirred with a hazel twig split at the bottom, and in both cases theaquaeffervesces, and soon after again settles. When the powerful vapour appears, theaquagives off a kind of oil, and the operculum becomes red. But, lest the vapours should escape from the ampulla and the operculum in that part where their mouths communicate, they are entirely sealed all round. Theaquais boiled continually over a fiercer fire, and enough charcoal must be put into the furnace so that the live coals touch the vessel. The ampulla is taken out as soon as all theaquahas been distilled, and the silver, which is dried by the heat of the fire, alone remains in it; the silver is shaken out and put in an earthenware crucible, and heated until it melts. The molten glass is extracted with an iron rod curved at the lower end, and the silver is madeinto cakes. The glass extracted from the crucible is ground to powder, and to this are added litharge, argol, glass-galls, and saltpetre, and they are melted in an earthen crucible. The button that settles is transferred to the cupel and re-melted.
If the silver was not sufficiently dried by the heat of the fire, that which is contained in the upper part of the ampulla will appear black; this when melted will be consumed. When the lute, which was smeared round the lower part of the ampulla, has been removed, it is placed in the crucible and is re-melted, until at last there is no more appearance of black[13].
If to the firstaquathe other which contains silver is to be added, it must be poured in before the powerful vapours appear, and theaquagives off the oily substance, and the operculum becomes red; for he who pours in theaquaafter the vapour appears causes a loss, because theaquagenerally spurts out and the glass breaks. If the ampulla breaks when the gold is being parted from the silver or the silver from theaqua, theaquawill be absorbed by the sand or the lute or the bricks, whereupon, without any delay, the red hot coals should be taken out of the furnace and the fire extinguished. The sand and bricks after being crushed should be thrown into a copper vessel, warm water should be poured over them, and they should be put aside for the space of twelve hours; afterward the water should be strained through a canvas, and the canvas, since it contains silver, should be dried by the heat of the sun or the fire, and then placed in an earthen crucible and heated until the silver melts, this being poured out into an iron mould. The strained water should be poured into an ampulla and separated from the silver, of which it contains a minute portion; the sand should be mixed with litharge, glass-galls, argol, saltpetre, and salt, and heated in an earthen crucible. The button which settles at the bottom should be transferred to a cupel, and should be re-melted, in order that the lead may be separated from the silver. The lute, with lead added, should be heated in an earthen crucible, then re-melted in a cupel.
We also separate silver from gold by the same method when we assay them. For this purpose the alloy is first rubbed against a touchstone, in order to learn what proportion of silver there is in it; then as much silver as is necessary is added to the argentiferous gold, in abesof which there must be less than asemi-unciaor asemi-unciaand asicilicus[14]of copper. After lead has been added, it is melted in a cupel until the lead and the copper have exhaled, then the alloy of gold with silver is flattened out, and little tubes are made of the leaves; these are put into a glass ampulla, and strongaquais poured over them two or three times. The tubes after this are absolutely pure, with the exception of only a quarter of asiliqua, which is silver; for only this much silver remains in eightunciaeof gold[15].
As great expense is incurred in parting the metals by the methods that I have explained, as night vigils are necessary whenaqua valensis made, and as generally much labour and great pains have to be expended on this matter, other methods for parting have been invented by clever men, which are less costly, less laborious, and in which there is less loss if through carelessness an error is made. There are three methods, the first performed with sulphur, the second with antimony, the third by means of some compound which consists of these or other ingredients.
Parting precious metals with sulphurA—Pot. B—Circular fire. C—Crucibles. D—Their lids. E—Lid of the pot. F—Furnace. G—Iron rod.[Pg 449]In the first method,[16]the silver containing some gold is melted in a crucible and made into granules. For everylibraof granules, there is taken a sixth of alibraand asicilicusof sulphur (not exposed to the fire); this, when crushed, is sprinkled over the moistened granules, and then they are put into a new earthen pot of the capacity of foursextarii, or into several of them if there is an abundance of granules. The pot, having been filled, is covered with an earthen lid and smeared over, and placed within a circle of fire set one and a half feet distant from the pot on all sides, in order that the sulphur added to the silver should not be distilled when melted. The pot is opened,the black-coloured granules are taken out, and afterward thirty-threelibraeof these granules are placed in an earthen crucible, if it has such capacity. For everylibraof silver granules, weighed before they were sprinkled withsulphur, there is weighed out also a sixth of alibraand asicilicusof copper, if eachlibraconsists either of three-quarters of alibraof silver and a quarter of alibraof copper, or of three-quarters of alibraand asemi-unciaof silver and a sixth of alibraand asemi-unciaof copper. If, however, the silver contains five-sixths of alibraof silver and a sixth of alibraof copper, or five-sixths of alibraand asemi-unciaof silver and anunciaand a half of copper, then there are weighed out a quarter of alibraof copper granules. If alibracontains eleven-twelfths of alibraof silver and oneunciaof copper, or eleven-twelfths and asemi-unciaof silver and asemi-unciaof copper, then are weighed out a quarter of alibraand asemi-unciaand asicilicusof copper granules. Lastly, if there is only pure silver, then as much as a third of alibraand asemi-unciaof copper granules are added. Half of these copper granules are added soon afterward to the black-coloured silver granules. The crucible should be tightly covered and smeared over with lute, and placed in a furnace, into which the air is drawn through the draught-holes. As soon as the silver is melted, the crucible is opened, and there is placed in it a heaped ladleful more of granulated copper, and also a heaped ladleful of a powder which consists of equal parts of litharge, of granulated lead, of salt, and of glass-galls; then the crucible is again covered with the lid. When the copper granules are melted, more are put in, together with the powder, until all have been put in.
A little of the regulus is taken from the crucible, but not from the gold lump which has settled at the bottom, and adrachmaof it is put into each of the cupels, which contain anunciaof molten lead; there should be many of these cupels. In this way half adrachmaof silver is made. As soon as the lead and copper have been separated from the silver, a third of it is thrown into a glass ampulla, andaqua valensis poured over it. By this method is shown whether the sulphur has parted all the gold from the silver, or not. If one wishes to know the size of the gold lump which has settled at the bottom of the crucible, an iron rod moistened with water is covered with chalk, and when the rod is dry it is pushed down straight into the crucible, and the rod remains bright to the height of the gold lump; the remaining part of the rod is coloured black by the regulus, which adheres to the rod if it is not quickly removed.
If when the rod has been extracted the gold is observed to be satisfactorily parted from the silver, the regulus is poured out, the gold button is taken out of the crucible, and in some clean place the regulus is chipped off from it, although it usually flies apart. The lump itself is reduced to granules, and for everylibraof this gold they weigh out a quarter of alibraeach of crushed sulphur and of granular copper, and all are placed together in an earthen crucible, not into a pot. When they are melted, in order that the gold may more quickly settle at the bottom, the powder which I have mentioned is added.
Although minute particles of gold appear to scintillate in the regulus of copper and silver, yet if all that are in alibrado not weigh as much as a single sesterce, then the sulphur has satisfactorily parted the gold from thesilver; but if it should weigh a sesterce or more, then the regulus is thrown back again into the earthen crucible, and it is not advantageous to add sulphur, but only a little copper and powder, by which method a gold lump is again made to settle at the bottom; and this one is added to the other button which is not rich in gold.
When gold is parted from sixty-sixlibraeof silver, the silver, copper, and sulphur regulus weighs one hundred and thirty-twolibrae. To separate the copper from the silver we require five hundredlibraeof lead, more or less, with which the regulus is melted in the second furnace. In this manner litharge and hearth-lead are made, which are re-smelted in the first furnace. The cakes that are made from these are placed in the third furnace, so that the lead may be separated from the copper and used again, for it contains very little silver. The crucibles and their covers are crushed, washed, and the sediment is melted together with litharge and hearth-lead.
Those who wish to separate all the silver from the gold by this method leave one part of gold to three of silver, and then reduce the alloy to granules. Then they place it in an ampulla, and by pouringaqua valensover it, part the gold from the silver, which process I explained inBook VII.
If sulphur from the lye with whichsal artificiosusis made, is strong enough to float an egg thrown into it, and is boiled until it no longer emits fumes, and melts when placed upon glowing coals, then, if such sulphur is thrown into the melted silver, it parts the gold from it.
Parting precious metals with antimonyA—Furnace in which the air is drawn in through holes. B—Goldsmith's forge. C—Earthen crucibles. D—Iron pots. E—Block.[Pg 453]Silver is also parted from gold by means ofstibium[17]. If in abes ofgold there are seven, or six, or five doublesextulaeof silver, then three parts ofstibiumare added to one part of gold; but in order that thestibiumshould not consume the gold, it is melted with copper in a red hot earthen crucible. If the gold contains some portion of copper, then to eightunciaeofstibiumasicilicusof copper is added; and if it contains no copper, then half anuncia, because copper must be added tostibiumin order to part gold from silver. The gold is first placed in a red hot earthen crucible, and when melted it swells, and a littlestibiumis added to it lest it run over; in a short space of time, when this has melted, it likewise again swells, and when this occurs it is advisable to put in all the remainder of thestibium, and to cover the crucible with a lid, and then to heat the mixture for the time required to walk thirty-five paces. Then it is at once poured out into an iron pot, wide at the top and narrow at the bottom, which was first heated and smeared over with tallow or wax, and set on an iron or wooden block. It is shaken violently, and by this agitation the gold lump settles to the bottom, and when the pot has cooled it is tapped loose, and is again melted four times in the same way. But each time a less weight ofstibiumis added to the gold, until finally only twice as muchstibiumis added as there is gold, or a little more; then the gold lump is melted in a cupel. Thestibiumis melted again three or four times in an earthen crucible, and each time a gold lump settles, so that there are three or four gold lumps, and these are all melted together in a cupel.
To twolibraeand a half of suchstibiumare added twolibraeof argol and onelibraof glass-galls, and they are melted in an earthen crucible, where a lump likewise settles at the bottom; this lump is melted in the cupel. Finally, thestibiumwith a little lead added, is melted in the cupel, in which, after all the rest has been consumed by the fire, the silver alone remains. If thestibiumis not first melted in an earthen crucible with argol and glass-galls, before it is melted in the cupel, part of the silver is consumed, and is absorbed by the ash and powder of which the cupel is made.
The crucible in which the gold and silver alloy are melted withstibium, and also the cupel, are placed in a furnace, which is usually of the kindin which the air is drawn in through holes; or else they are placed in a goldsmith's forge.
Just asaqua valenspoured over silver, from which the sulphur has parted the gold, shows us whether all has been separated or whether particles of gold remain in the silver; so do certain ingredients, if placed in the pot or crucible "alternately" with the gold, from which the silver has been parted bystibium, and heated, show us whether all have been separated or not.
We use cements[18]when, withoutstibium, we part silver or copper or both so ingeniously and admirably from gold. There are various cements. Someconsist of half alibraof brick dust, a quarter of alibraof salt, anunciaof saltpetre, half anunciaof sal-ammoniac, and half anunciaof rock salt. The bricks or tiles from which the dust is made must be composed of fatty clays, free from sand, grit, and small stones, and must be moderately burnt and very old.
Another cement is made of abesof brick dust, a third of rock salt, anunciaof saltpetre, and half anunciaof refined salt. Another cement is made of abesof brick dust, a quarter of refined salt, one and a halfunciaeof saltpetre, anunciaof sal-ammoniac, and half anunciaof rock salt. Another has onelibraof brick dust, and half alibraof rock salt, to which some add a sixth of alibraand asicilicusof vitriol. Another is made of half alibraof brick dust, a third of alibraof rock salt, anunciaand a half of vitriol, and oneunciaof saltpetre. Another consists of abesof brick dust, a third of refined salt, a sixth of white vitriol[19], half anunciaof verdigris, and likewise half anunciaof saltpetre. Another is made of one and a thirdlibraeof brick dust, abesof rock salt, a sixth of alibraand half anunciaof sal-ammoniac, a sixth and half anunciaof vitriol, and a sixth of saltpetre. Another contains alibraof brick dust, a third of refined salt, and one and a halfunciaeof vitriol.
Those ingredients above are peculiar to each cement, but what follows is common to all. Each of the ingredients is first separately crushed to powder; the bricks are placed on a hard rock or marble, and crushed with an iron implement; the other things are crushed in a mortar with a pestle; each is separately passed through a sieve. Then they are all mixed together, and are moistened with vinegar in which a little sal-ammoniac has been dissolved, if the cement does not contain any. But some workers, however, prefer to moisten the gold granules or gold-leaf instead.
The cement should be placed, alternately with the gold, in new and clean pots in which no water has ever been poured. In the bottom the cement is levelled with an iron implement, and afterward the gold granules or leaves are placed one against the other, so that they may touch it on all sides; then, again, a handful of the cement, or more if the pots are large, is thrown in and levelled with an iron implement; the granules and leaves are laid over this in the same manner, and this is repeated until the pot is filled. Then it is covered with a lid, and the place where they join is smeared over with artificial lute, and when this is dry the pots are placed in the furnace.
Parting precious metals by cementationA—Furnace. B—Pot. C—Lid. D—Air-holes.[Pg 455]The furnace has three chambers, the lowest of which is a foot high; into this lowest chamber the air penetrates through an opening, and into it theashes fall from the burnt wood, which is supported by iron rods, arranged to form a grating. The middle chamber is two feet high, and the wood is pushed in through its mouth. The wood ought to be oak, holmoak, or turkey-oak, for from these the slow and lasting fire is made which is necessary for this operation. The upper chamber is open at the top so that the pots, for which it has the depth, may be put into it; the floor of this chamber consists of iron rods, so strong that they may bear the weight of the pots and the heat of the fire; they are sufficiently far apart that the fire may penetrate well and may heat the pots. The pots are narrow at the bottom, so that the fire entering into the space between them may heat them; at the top the pots are wide, so that they may touch and hold back the heat of the fire. The upper part of the furnace is closed in with bricks not very thick, or with tiles and lute, and two or three air-holes are left, through which the fumes and flames may escape.
The gold granules or leaves and the cement, alternately placed in the pots, are heated by a gentle fire, gradually increasing for twenty-four hours, if the furnace was heated for two hours before the full pots were stood in it, and if this was not done, then for twenty-six hours. The fire should be increased in such a manner that the pieces of gold and the cement, in which is the potency to separate the silver and copper from the gold, may not melt, for in this case the labour and cost will be spent in vain; therefore, it is ample to have the fire hot enough that the pots always remain red. After so many hours all the burning wood should be drawn out of the furnace. Then the refractory bricks or tiles are removed from the top of the furnace, and the glowing pots are taken out with the tongs. The lids are removed, and if there is time it is well to allow the gold to cool by itself, for then there is less loss; but if time cannot be spared for that operation, the pieces of gold are immediately placed separately into a wooden or bronze vessel of water and gradually quenched, lest the cement which absorbs the silver should exhale it. The pieces of gold, and the cement adhering to them, when cooled or quenched, are rolled with a little mallet so as to crush the lumps and free the gold from the cement. Then they are sifted by a fine sieve, which is placed over a bronze vessel; in this manner the cement containing the silver or the copper or both, falls from the sieve into the bronze vessel, and the gold granules or leaves remain on it. The gold is placed in a vessel and again rolled with the little mallet, so that it may be cleansed from the cement which absorbs silver and copper.
The particles of cement, which have dropped through the holes of the sieve into the bronze vessel, are washed in a bowl, over a wooden tub, being shaken about with the hands, so that the minute particles of gold which have fallen through the sieve may be separated. These are again washed in a little vessel, with warm water, and scrubbed with a piece of wood or a twig broom, that the moistened cement may be detached. Afterward all the gold is again washed with warm water, and collected with a bristle brush, and should be washed in a copper full of holes, under which is placed a little vessel. Then it is necessary to put the gold on an iron plate, under which is a vessel,and to wash it with warm water. Finally, it is placed in a bowl, and, when dry, the granules or leaves are rubbed against a touchstone at the same time as a touch-needle, and considered carefully as to whether they be pure or alloyed. If they are not pure enough, the granules or the leaves, together with the cement which attracts silver and copper, are arranged alternately in layers in the same manner, and again heated; this is done as often as is necessary, but the last time it is heated as many hours as are required to cleanse the gold.
Some people add another cement to the granules or leaves. This cement lacks the ingredients of metalliferous origin, such as verdigris and vitriol, for if these are in the cement, the gold usually takes up a little of the base metal; or if it does not do this, it is stained by them. For this reason some very rightly never make use of cements containing these things, because brick dust and salt alone, especially rock salt, are able to extract all the silver and copper from the gold and to attract it to themselves.
It is not necessary for coiners to make absolutely pure gold, but to heat it only until such a fineness is obtained as is needed for the gold money which they are coining.
The gold is heated, and when it shows the necessary golden yellow colour and is wholly pure, it is melted and made into bars, in which case they are either prepared by the coiners withchrysocolla, which is called by the Moors borax, or are prepared with salt of lye made from the ashes of ivy or of other salty herbs.
The cement which has absorbed silver or copper, after water has been poured over it, is dried and crushed, and when mixed with hearth-lead and de-silverized lead, is smelted in the blast furnace. The alloy of silver and lead, or of silver and copper and lead, which flows out, is again melted in the cupellation furnace, in order that the lead and copper may be separated from the silver. The silver is finally thoroughly purified in the refining furnace, and in this practical manner there is no silver lost, or only a minute quantity.
There are besides this, certain other cements[20]which part gold from silver, composed of sulphur,stibiumand other ingredients. One of these compounds consists of half anunciaof vitriol dried by the heat of the fire and reduced to powder, a sixth of refined salt, a third ofstibium, half alibraof prepared sulphur (not exposed to the fire), onesicilicusof glass, likewise onesicilicusof saltpetre, and adrachmaof sal-ammoniac.[21]The sulphur is prepared as follows: it is first crushed to powder, then it is heated for six hours in sharp vinegar, and finally poured into a vessel and washed with warm water; then that which settles at the bottom of the vessel is dried. To refine the salt it is placed in river water and boiled, and again evaporated. The second compound contains onelibraof sulphur (not exposed to fire) and twolibraeof refined salt. The third compound is made from onelibraof sulphur (not exposed to the fire), half alibraof refined salt, a quarter of alibraof sal-ammoniac, and oneunciaof red-lead. The fourth compound consists of onelibraeach of refined salt, sulphur (not exposed to the fire) and argol, and half alibraofchrysocollawhich the Moors call borax. The fifth compound has equal proportions of sulphur (not exposed to the fire), sal-ammoniac, saltpetre, and verdigris.
The silver which contains some portion of gold is first melted with lead in an earthen crucible, and they are heated together until the silver exhales the lead. If there was alibraof silver, there must be sixdrachmaeof lead. Then the silver is sprinkled with twounciaeof that powdered compoundand is stirred; afterward it is poured into another crucible, first warmed and lined with tallow, and then violently shaken. The rest is performed according to the process I have already explained.
Gold may be parted without injury from silver goblets and from other gilt vessels and articles[22], by means of a powder, which consists of one part of sal-ammoniac and half a part of sulphur. The gilt goblet or other article is smeared with oil, and the powder is dusted on; the article is seized in the hand, or with tongs, and is carried to the fire and sharply tapped, and by this means the gold falls into water in vessels placed underneath, while the goblet remains uninjured.
Gold is also parted from silver on gilt articles by means of quicksilver. This is poured into an earthen crucible, and so warmed by the fire that the finger can bear the heat when dipped into it; the silver-gilt objects are placed in it, and when the quicksilver adheres to them they are taken out and placed on a dish, into which, when cooled, the gold falls, together with the quicksilver. Again and frequently the same silver-gilt object is placed in heated quicksilver, and the same process is continued until at last no more gold is visible on the object; then the object is placed in the fire, and the quicksilver which adheres to it is exhaled. Then the artificer takes a hare's foot, and brushes up into a dish the quicksilver and the gold which havefallen together from the silver article, and puts them into a cloth made of woven cotton or into a soft leather; the quicksilver is squeezed through one or the other into another dish.[23]The gold remains in the cloth or the leather, and when collected is placed in a piece of charcoal hollowed out, and is heated until it melts, and a little button is made from it. This button is heated with a littlestibiumin an earthen crucible and poured out into another little vessel, by which method the gold settles at the bottom, and thestibiumis seen to be on the top; then the work is completed. Finally, the gold button is put in a hollowed-out brick and placed in the fire, and by this method the gold is made pure. By means of the above methods gold is parted from silver and also silver from gold.
Now I will explain the methods used to separate copper from gold[24].The salt which we callsal-artificiosus,[25]is made from alibraeach of vitriol, alum, saltpetre, and sulphur not exposed to the fire, and half alibraof sal-ammoniac; these ingredients when crushed are heated with one part of lye made from the ashes used by wool dyers, one part of unslaked lime, and four parts of beech ashes. The ingredients are boiled in the lye until the whole has been dissolved. Then it is immediately dried and kept in a hot place, lest it turn into oil; and afterward when crushed, alibraof lead-ash is mixed with it. With eachlibraof this powdered compound one and a halfunciaeof the copper is gradually sprinkled into a hot crucible, and it is stirred rapidly and frequently with an iron rod. When the crucible has cooled and been broken up, the button of gold is found.
The second method for parting is the following. Twolibraeof sulphur not exposed to the fire, and fourlibraeof refined salt are crushed and mixed; a sixth of alibraand half anunciaof this powder is added to abesof granules made of lead, and twice as much copper containing gold; they are heated together in an earthen crucible until they melt. When cooled, the button is taken out and purged of slag. From this button they again make granules, to a third of alibraof which is added half alibraof that powder of which I have spoken, and they are placed in alternate layers in the crucible; it is well to cover the crucible and to seal it up, and afterward it is heated over a gentle fire until the granules melt. Soon afterward, the crucible is taken off the fire, and when it is cool the button is extracted. From this, when purified and again melted down, the third granules are made, to which, if they weigh a sixth of alibra, is added one half anunciaand asicilicusof the powder, and they are heated in the same manner, and the button of gold settles at the bottom of the crucible.
The third method is as follows. From time to time small pieces of sulphur, enveloped in or mixed with wax, are dropped into sixlibraeof the molten copper, and consumed; the sulphur weighs half anunciaand asicilicus. Then one and a halfsiciliciof powdered saltpetre are dropped into the same copper and likewise consumed; then again half anunciaand asicilicusof sulphur enveloped in wax; afterward one and a halfsiciliciof lead-ash enveloped in wax, or of minium made from red-lead. Then immediately the copper is taken out, and to the gold button, which is now mixed with only a little copper, they addstibiumto double the amount of the button; these are heated together until thestibiumis driven off; then the button, together with lead of half the weight of the button, are heated in a cupel.Finally, the gold is taken out of this and quenched, and if there is a blackish colour settled in it, it is melted with a little of thechrysocollawhich the Moors call borax; if too pale, it is melted withstibium, and acquires its own golden-yellow colour. There are some who take out the molten copper with an iron ladle and pour it into another crucible, whose aperture is sealed up with lute, and they place it over glowing charcoal, and when they have thrown in the powders of which I have spoken, they stir the whole mass rapidly with an iron rod, and thus separate the gold from the copper; the former settles at the bottom of the crucible, the latter floats on the top. Then the aperture of the crucible is opened with the red-hot tongs, and the copper runs out. The gold which remains is re-heated withstibium, and when this is exhaled the gold is heated for the third time in a cupel with a fourth part of lead, and then quenched.
The fourth method is to melt one and a thirdlibraeof the copper with a sixth of alibraof lead, and to pour it into another crucible smeared on the inside with tallow or gypsum; and to this is added a powder consisting of half anunciaeach of prepared sulphur, verdigris, and saltpetre, and anunciaand a half ofsal coctus. The fifth method consists of placing in a crucible onelibraof the copper and twolibraeof granulated lead, with one and a halfunciaeofsal-artificiosus; they are at first heated over a gentle fire and then over a fiercer one. The sixth method consists in heating together abesof the copper and one-sixth of alibraeach of sulphur, salt, andstibium. The seventh method consists of heating together abesof the copper and one-sixth each of iron scales and filings, salt,stibium, and glass-galls. The eighth method consists of heating together onelibraof the copper, one and a halflibraeof sulphur, half alibraof verdigris, and alibraof refined salt. The ninth method consists of placing in onelibraof the molten copper as much pounded sulphur, not exposed to the fire, and of stirring it rapidly with an iron rod; the lump is ground to powder, into which quicksilver is poured, and this attracts to itself the gold.
Gilded copper articles are moistened with water and placed on the fire, and when they are glowing they are quenched with cold water, and the gold is scraped off with a brass rod. By these practical methods gold is separated from copper.
Either copper or lead is separated from silver by the methods which I will now explain.[26]This is carried on in a building near by the works, or in the works in which the gold or silver ores or alloys are smelted. The middle wall of such a building is twenty-one feet long and fifteen feet high, and from this a front wall is distant fifteen feet toward the river; the rear wallis nineteen feet distant, and both these walls are thirty-six feet long and fourteen feet high; a transverse wall extends from the end of the front wall to the end of the rear wall; then fifteen feet back a second transverse wall is built out from the front wall to the end of the middle wall. In that space which is between those two transverse walls are set up the stamps, by means of which the ores and the necessary ingredients for smelting are broken up. From the further end of the front wall, a third transverse wall leads to the other end of the middle wall, and from the same to the end of the rear wall. The space between the second and third transverse walls, and between the rear and middle long walls, contains the cupellation furnace, in which leadis separated from gold or silver. The vertical wall of its chimney is erected upon the middle wall, and the sloping chimney-wall rests on the beams which extend from the second transverse wall to the third; these are so located that they are at a distance of thirteen feet from the middle long wall and four from the rear wall, and they are two feet wide and thick. From the ground up to the roof-beams is twelve feet, and lest the sloping chimney-wall should fall down, it is partly supported by means of many iron rods, and partly by means of a few tie-beams covered with lute, which extend from the small beams of the sloping chimney-wall to the beams of the vertical chimney-wall. The rear roof is arranged in the same way as the roofof the works in which ore is smelted. In the space between the middle and the front long walls and between the second[27]and the third transverse walls are the bellows, the machinery for depressing and the instrument for raising them. A drum on the axle of a water-wheel has rundles which turn the toothed drum of an axle, whose long cams depress the levers of the bellows, and also another toothed drum on an axle, whose cams raise the tappets of the stamps, but in the opposite direction. So that if the cams which depress the levers of the bellows turn from north to south, the cams of the stamps turn from south to north.