Section II
Of Operations on Metals.
Chap. I Of Gold
Process I.
To separate Gold, by Amalgamation with Mercury, from the Earth and Stones with which it is found mixed.
Pulverize the earths and stones containing Gold. Put the powder into a little wooden tray; dip this tray in water, gently shaking it and its contents. The water will grow muddy, by taking up the earthy parts of the ore. Continue washing it in this manner till the water cease to appear turbid. Upon the ore thus washed pour strong vinegar, having first dissolved therein, by the help of heat, about a tenth part of its weight of alum. The powder must be quite drenched and covered with this liquor, and so left to stand for twice twenty-four hours.
Decant the vinegar, and wash your powder with warm water, till the last that comes off hath no taste: then dry it, and put it into an iron mortar, with four times its weight of Quick-silver: triturate the whole with a heavy wooden pestle, till all the powder be of a blackish colour: then pour in a little water, and continue rubbing for some time longer. More earthy and heterogeneous particles will be separated from the metalline parts by means of this water, which will look dirty: it must then be decanted, and more fair water added. Repeat this several times; then dry what remains in the mortar with a sponge, and by the help of a gentle heat: you will find it an Amalgam of the Mercury with the Gold.
Put this Amalgam into a chamoy bag: tie a knot on its neck, and squeeze it hard between your fingers, over some wide-mouthed vessel; there will issue through the pores of the leather numberless little jets of Mercury, forming a sort of shower, that will collect into large globules in the vessel placed underneath. When you can force out no more Mercury by this means, open the bag, and in it you will find the Amalgam freed from the superfluous Mercury; the Gold retaining only about as much thereof as nearly equals itself in weight.
Put this Amalgam into a glass retort; set this retort in the sand-bath of a reverberating furnace; cover it quite over with sand; apply a glass receiver half full of water, so that the nose of the retort may be under the water. The receiver need not be luted to the retort. Give a gradual heat, and raise the fire till drops of the sublimed Mercury appear in the neck of the retort, and fall into the water with a hissing noise. If you hear any noise in the retort slacken your fire a little. Lastly, when you observe, that, though you raise the fire still higher than before, nothing more will come over, take out your retort, break it, and there you will find the Gold, which must be melted in a crucible with Borax.
OBSERVATIONS.
Gold is a perfect metal, which can by no means be deprived of its Phlogiston, and on which few, even of the most powerful chymical solvents, have any effect: and therefore it almost always hath its metalline form when found in the earth; from which it may sometimes be separated by simple lotion. The Gold dust found in the sands of certain rivers is of this kind. When it resides in stones, or tenacious earths, it may be extracted by the process here delivered; to wit, by Amalgamation, or combination of Mercury with Gold. Mercury is incapable of uniting with any earthy substances, not even with the metallic earths, when they are deprived of their Phlogiston, and consequently have not the metalline form.
Hence it follows, that when Mercury is triturated with particles of Gold, of earth, and of stone, mingled together, it unites with the Gold, and separates it from those heterogeneous matters. Yet, if there be along with the Gold any other metal, in its metalline form, except Iron, the Mercury will amalgamate with that also. This often happens to Silver, which being a perfect metal as well as Gold, is for that reason sometimes dug up in its metalline form, and even incorporated with Gold. When this is the case, the mass that remains in the retort, after abstracting the Mercury of the Amalgama, is a compound of Gold and Silver, which are to be separated from each other by the methods we shall give for that purpose. The present process is therefore applicable to Silver as well as Gold.
Sometimes Gold is intimately combined with such mineral matters as hinder the Mercury from acting upon it. In that case the mixed mass must be roasted before you proceed to Amalgamation: for if the matters be volatile, such, for instance, as antimony or arsenic, the fire will carry them off; so that, after roasting, the Amalgamation will succeed. But sometimes these matters are fixed, and require fusion; if so, recourse must be had to some particular methods, which we shall describe when we come to treat of Silver, as these two perfect metals are to be treated in the same manner.
Ores containing Gold must be washed before an Amalgam is attempted; that the metalline parts, being freed from the numerous particles of earth with which they are encompassed, may the more readily incorporate with the Mercury. Besides, it is the property of Mercury to take the form of a dark unmetallic powder, after being long rubbed with other matters, so that it cannot be easily distinguished from the particles of earth. And hence, if you still continue to grind the matters together, after the Amalgamation is completed, and wash them again and again, the water that comes off will always look turbid, being impregnated with some particles of the Amalgam. This is easily proved: for if you let the turbid water settle, and distil the sediment, you will obtain Quick-silver from it.
The ore is to be steeped in vinegar charged with alum, in order to cleanse the surface of the Gold, which is often covered with a thin coat of earth that obstructs the Amalgamation.
Great care must be taken that the Mercury employed in this operation be very pure. If it be adulterated with any metallic substance, it must be freed therefrom by the methods which we shall propose in their proper place.
The way of separating Mercury from Gold is founded on the different properties of these two metallic substances; the one being exceedingly fixed, and the other very volatile. The union which Mercury contracts with the metals is not intimate enough, to give the new compound which results therefrom all the properties of either of the two united substances; at least so far as concerns their degrees of fixity and volatility. Hence it comes, that, in our Amalgam, the Gold communicates but very little of its fixity to the Mercury, and the Mercury communicates to the Gold but very little of its volatility. Yet if the Mercury be distilled off with a much greater degree of heat than is necessary to elevate it, a pretty considerable quantity of Gold will most certainly be carried up along with it.
It is also of consequence, on another account, that the fire be duly governed on this occasion. For if too great a degree of heat be applied, and the fire afterwards lowered, the water in the receiver, which covers the nose of the retort, will rise into its body, break it to pieces, and spoil the operation.
The cause of this phenomenon depends on the property which air possesses of rarefying with heat and condensing with cold, joined to its weight. As soon as the retort is acted on by a less degree of heat than acted on it the instant before, the air contained therein is condensed, and leaves a vacuum, which the external air, by virtue of its weight, tends to occupy; but, the orifice of the retort being under water, the external air can no way gain admittance, but by pushing in before it the water which intercepts its passage. This caution, as we observed above, must be applied to all distillations, where the vessels are disposed as they are in this.
Care must also be taken that the nose of the retort be not placed too deep under water: for as the neck grows very warm during the operation, because the degree of heat required to raise Mercury is about three times greater than that which raises water, it may easily be broken by the contact of the cold water in the receiver.
This method of extracting Gold and Silver from their ores, by Amalgamation with Mercury, is not to be absolutely depended on as a sure proof of the quantity of those metals that may be contained in the earth assayed by this means: for some small part of the Amalgam is always lost in washing it; and, moreover, the Mercury, when squeezed through chamoy, always carries with it a small portion of Gold. So that if you desire to know more exactly, by this method, the quantity of Gold or Silver contained in any earth, the Amalgam must not be squeezed through chamoy, but distilled altogether. Much the surest method of making an accurate assay is that by fusion and scorification, which we shall describe under the head of Silver.
In some countries, and especially in America, the method of Amalgamation is used for extracting Gold and Silver in large quantities, from the matrices which contain them in their metalline form. Agricola and other metallurgists have described the machines by means whereof such Amalgamations are managed.
Process II
To dissolve Gold in Aqua regis, and by that means to separate it from Silver. Aurum Fulminans. Aurum Fulminans reduced.
Take Gold that is perfectly pure, or alloyed with Silver only. Reduce it to little thin plates, by hammering it on an anvil. If it be not sufficiently tough, neal it till it be red in a moderate, clear fire, quite free from smoking coals, and then let it cool gradually, which will restore its ductility.
When the plates are thin enough, make them red hot once more, and cut them into small bits with a pair of sheers. Put these bits into a tall, narrow-mouthed cucurbit, and pour on them twice their weight of good Aqua regis, made of one part Sal Ammoniac, or Spirit of Salt, and four parts Spirit of Nitre. Set the cucurbit in a sand-bath moderately heated, stopping its orifice slightly with a paper coffin, to prevent any dirt from falling in. The Aqua regis will presently begin to smoke. Round the little bits of Gold will be formed an infinite number of small bubbles, which will rise to the surface of the liquor. The Gold will totally dissolve, if it be pure, and the solution will be of a beautiful yellow colour: if the Gold be alloyed with a small quantity of Silver, the latter will remain at the bottom of the vessel in the form of a white powder. If the Gold be alloyed with much Silver, when the Gold is dissolved the Silver will retain the form of the little metalline plates put into the vessel.
When the dissolution is completed, gently pour off the liquor into another low, wide-mouthed, glass cucurbit, taking care that none of the Silver, which lies at the bottom in the form of a powder, escape with the liquor. On this powder of Silver pour as much fresh Aqua regis as will cover it entirely; and repeat this till you are sure that nothing more can be taken up by it. Lastly, having decanted the Aqua regis from the Silver, wash the Silver with a little Spirit of Salt weakened with water, and add this Spirit of Salt to the Aqua regis in which your Gold is dissolved. Then to the body containing these liquors fit a head and a receiver, and distil with a gentle heat, till the matter contained in the cucurbit become dry.
OBSERVATIONS.
It is certain that aqua regis is the true solvent of Gold, and that it does not touch Silver: so that if the Gold dissolved in it were alloyed with Silver, which is often the case, the two metals would by this means be pretty accurately separated from each other. But if you desire to obtain from this solution a Gold absolutely pure, you must free it, before you dissolve it, from every other metallic substance but Silver; because aqua regis acts upon most of the other metals and the semi-metals. We shall shew under the head of Silver, as we promised before, how to purify a mass of Gold and Silver from every other metallic alloy. Thither also we refer the common parting assay performed by means of aqua fortis: because in that operation the Silver is dissolved, and not the Gold.
If the Gold put to dissolve in aqua regis be pure, the dissolution is easily and readily effected. But if, on the contrary, it be alloyed with Silver, the aqua regis finds more difficulty in dissolving it. Nay, if the Silver exceed the Gold in quantity, the dissolution will not take place at all, for the reasons adduced in our Theoretical Elements; of which we shall speak more fully when we come to treat of the Parting Assay.
In the process we directed the Gold to be dissolved in a tall body. This precaution is necessary to prevent the loss of some part thereof: for it is the property of aqua regis to carry off along with it some of the Gold, especially when there is any Sal Ammoniac in its composition, if the vessel be heated while the dissolution is going on, or if the aqua regis be very strong. Yet it is proper to make use of aqua regis that is too strong rather than too weak: for if it prove too strong, and be observed not to act upon the metal for that reason, it is easy to weaken it, by gradually adding small quantities of pure water, till you perceive it begin to act with vigour. This is a general rule regarding all metallic dissolutions in Acids.
When the solution of Gold is evaporated to dryness, if you desire to reduce into a mass the Gold dust left at the bottom of the cucurbit, you must put it into a crucible, and cover it with pulverized borax, mixed with a little nitre and calcined wine-lees; then cover the crucible close, heat it with a moderate fire, which must be afterwards increased so as to melt the contents. At the bottom of the crucible you will find a lump of Gold, over which the salts you added will be as it were vitrified. These salts are added chiefly to promote the fusion of the metal.
The Gold may, if you will, be separated from its solvent without evaporating the solution as above directed. You need only mix with the solution a fixed or volatile Alkali by little and little, till you see no more precipitate fall, and then let the liquor stand to settle, at the bottom of which you will find a sediment: filter the whole, and dry what is left on the filter.
Both fixed and volatile Alkalis possessing, as hath been frequently repeated, a greater affinity with Acids than metallic substances have, they precipitate the Gold, and separate it from the Acids in which it is dissolved: but it is of great consequence to take notice, that, if you attempt to melt this precipitated Gold in a crucible, it will fulminate as soon as it feels the heat, with such a terrible explosion, that, if the quantity be at all considerable, it may prove fatal to the operator: even rubbing it a little hard will make it blow up. This preparation is therefore called Aurum Fulminans.
Hitherto no satisfactory explanation hath been given of this phenomenon. Some Chymists considering, that, in the precipitation of the Gold, a Nitre is regenerated by the union of the Alkali with the Nitrous Acid which enters into the composition of the aqua regis, imagine that some of this regenerated Nitre, combining with the precipitated Gold, takes fire and detonates, either by means of some small portion of Phlogiston that may be contained in the Alkali, or by means of that which constitutes the Gold itself. But, in the first place, it is well known that Fixed Alkalis do not contain Phlogiston enough to make Nitre detonate. Indeed, if a Volatile Alkali be employed in the precipitation, a Nitrous Ammoniacal Salt will be formed, containing Phlogiston enough to be capable of detonating without the concourse of any additional Phlogiston: but this detonation of the Nitrous Ammoniacal Salt is not to be compared, as to the violence of its effects, with the fulmination of Gold. Besides, we do not find that Gold precipitated by a Volatile Alkali explodes with greater force than that precipitated by a Fixed Alkali. As for the Gold, it is certain that it suffers no decomposition at all by fulminating. When fulminated under a glass bell, in such small quantities as not to endanger the operator, the Gold is found scattered about under the bell in very fine particles, without having undergone any alteration.
Others have fancied this fulmination of the Gold to be nothing but the decrepitation of the Sea-salt that is regenerated, in the precipitation of the metal, by the Fixed Alkali uniting with the Acid of Sea-salt which makes part of the aqua regis. But to this it may be said, that Gold precipitated by a Volatile Alkali fulminates as violently as that precipitated by a Fixed Alkali; and yet no sea-salt can be formed in the liquor by the addition of a Volatile Alkali, but only a Sal Ammoniac which has not the property of decrepitating. Moreover, there is no comparison, as to the effects, between the decrepitation of Sea-salt and the fulmination of Gold.
Nor, lastly, can this fulmination be attributed, as it is by some, to the effort made by the Salts to escape from amidst the particles of Gold, in which they are supposed by them to be imprisoned: for then we might deprive this Gold entirely of its fulminating quality by only boiling it in water, and so washing off all the saline particles, which probably adhere to its surface only. It is plain there is great room for very beautiful discoveries on this subject. In Walerius’s Mineralogy we find some observations that may throw a little light on the point before us.
“The quantity,” says he, “of fulminating Gold precipitated exceeds that of the Gold dissolved: if the aqua regis be made with Sal Ammoniac the explosion will be stronger; it will also be more violent if the solution be precipitated with a Volatile Alkali, than if a Fixed Alkali be used for that purpose.”
One of the speediest and easiest methods to deprive this Gold of its fulminating quality, is to grind in a mortar twice as much flowers of Sulphur as you have Gold to reduce, mixing your fulminating Gold therewith by little and little, as you grind them together; then to put the mixture into a crucible, and heat it just enough to melt the Sulphur. Part of the Sulphur will be dissipated in vapours, and the rest will burn away. When it is quite consumed, increase the fire so as to make the crucible red-hot. When you perceive no more smell of Sulphur, pour on the Gold a little Borax, previously melted in another crucible with a Fixed Alkali, as calcined Wine-lees, or Nitre fixed with Tartar; and then raise the fire sufficiently to make the whole flow. After the fusion is completed, you will find a button of Gold at the bottom of the crucible under the Salts.
Fulminating Gold may also be reduced by pouring on it a sufficient quantity of Fixed Alkali reduced to a liquor, or of oil of Vitriol, evaporating all the moisture, and gradually throwing what remains, mixed up with some pinguinous matter, into a crucible kept red-hot in a furnace. The reason why these substances deprive the Gold of its fulminating quality, depends on the causes that produce the fulmination.
Gold may also be separated from aqua regis, and precipitated by the means of several metallic substances that have a greater affinity, either with aqua regis, or with one of the two Acids that compose it. Mercury is one of the fittest for this purpose. On dropping a solution of Mercury in the Nitrous Acid by little and little into a solution of Gold, the mixture becomes turbid, and a precipitate is formed. Continue dropping in more of the solution of Mercury till no more precipitate falls; then let the liquor stand to settle, and at the bottom of it you will find a sediment, which is the precipitated Gold: pour off the liquor by inclination, and wash the precipitate with fair water.
Mercury hath a greater affinity with the Marine than with the Nitrous Acid. The affinity which Mercury hath with the Marine Acid is also greater than that of Gold with the Marine Acid; for unless this Acid be associated either with the Nitrous Acid, or at least with a certain proportion of Phlogiston, it will not dissolve Gold. Hence it comes, that when a solution of Mercury in the Nitrous Acid is dropped into a solution of Gold in aqua regis, the Mercury unites with the Acid of Sea-salt, which is an ingredient in the aqua regis: but the Marine Acid cannot on this occasion join the Mercury, without deserting the Gold and the Nitrous Acid with which it was united; and then the Gold, which cannot be kept in solution by the Nitrous Acid alone, is forced to quit its solvent and precipitate. The liquor, therefore, that now floats over the Gold thus precipitated, must contain Mercury united with the Acid of Sea-salt: and in fact it yields a true Corrosive Sublimate, which is known to be a combination of Mercury with the Marine Acid.
Mercury dissolved in Spirit of Nitre is employed to procure the precipitation we are speaking of; because metallic substances, when so comminuted by an Acid, are much fitter for such experiments than when they are in a concrete form.
Gold precipitated in this manner by a metallic substance doth not fulminate.
Process III
To dissolve Gold by Liver of Sulphur.
Mix together equal parts of common Brimstone, and a very strong Fixed Alkali; for instance, Nitre fixed by Charcoal. Put them in a crucible, and melt the mixture, stirring it from time to time with a small rod. There is no occasion to make the fire very brisk; because the Sulphur facilitates the fusion of the Fixed Alkali. Some sulphureous vapours will rise from the crucible; the two substances will mix intimately together, and form a reddish compound. Then throw into the crucible some little pieces of Gold beat into thin plates, so that the whole do not exceed in weight one third part of the Liver of Sulphur: raise the fire a little. As soon as the Liver of Sulphur is perfectly melted, it will begin to dissolve the Gold with ebullition; and will even emit some flashes of fire. In the space of a few minutes the Gold will be entirely dissolved, especially if it was cut and flatted into small thin leaves.
OBSERVATIONS.
The process here delivered is taken from M. Stahl. The design of that ingenious Chymist’s inquiry was to discover how Moses could burn the golden calf, which the Israelites had set up and worshipped while he was on the mount; how he could afterwards reduce that calf to powder, throw it into the water which the people used, and make all who had apostatized drink thereof, as related in the Book of Exodus.
M. Stahl, having first observed that Gold is absolutely unalterable and indestructible by the force of fire alone, be it ever so violent, concludes, that without a miracle Moses could not possibly perform the above-mentioned operations on the golden calf any way but by mixing with the Gold some matter qualified to alter and dissolve it. He then takes notice, that pure Sulphur does not act upon Gold at all, and that many other substances, which are thought capable of dividing and dissolving it, cannot however do it so completely as is necessary to render that metal susceptible of the effects related. He then gives the method of dissolving it by Liver of Sulphur, described in the process.
Liver of Sulphur dissolves likewise all the other metals: but M. Stahl observes, that it attenuates Gold more than any other metallic substance, and unites with it much more intimately than with the rest. This appears from what happens, on attempting to dissolve in water any of the mixts resulting from the union of another metal with the Liver of Sulphur: for then the metal separates, and appears in the form of a powder or fine calx; whereas, when Gold is united with Liver of Sulphur, the whole compound dissolves in water so perfectly, that the Gold even passes with the Liver of Sulphur through the pores of filtering paper.
If an Acid be poured into a solution of this combination of Gold with Liver of Sulphur, the Acid unites with the Alkali of the Hepar, and the Gold falls to the bottom of the liquor along with the Sulphur, which doth not quit it. The Sulphur thus precipitated with the Gold is easily carried off by a slight torrefaction, after which the Gold remains exceedingly comminuted. The Sulphur of this compound may also be destroyed by torrefaction, without the trouble of a previous solution and precipitation, and then also the Gold remains so attenuated as to be miscible with liquors, and floats on them, or swims in them, in such a manner that it may easily be swallowed with them in drinking. From all this M. Stahl concludes there is great reason to believe it was by means of the Liver of Sulphur that Moses divided, and in a manner calcined, the golden calf, so that he could mingle it with water, and make the Israelites drink it.
Process IV
To separate Gold from all other metallic Substances by means of Antimony.
Having put the Gold you intend to purify into a crucible, set it in a melting furnace, cover it, and make the Gold flow. When the metal is in fusion cast upon it, by a little at a time, twice its weight of pure crude Antimony in powder, and after each projection cover the crucible again immediately: this done keep the matter in fusion for a few minutes. When you perceive that the metallic mixture is perfectly melted, and that its surface begins to sparkle, pour it out into a hollow iron cone, previously heated, and smeared on the inside with tallow. Immediately strike with a hammer the floor on which the cone stands; and when all is cold, or at least sufficiently fixed, invert the cone and strike it: the whole metallic mass will fall out, and the under part thereof, which was at the point of the cone, will be a Regulus more or less yellow as the Gold was more or less pure. On striking the metallic mass the Regulus will freely part from the sulphureous crust at top.
Return this Regulus into the crucible, and melt it. Less fire will do now than was required before. Add the same quantity of Antimony, and proceed as at first. Repeat the same operation a third time, if your Gold be very impure.
Then put your Regulus into a good crucible, much larger than is necessary to hold it. Set your crucible in a melting furnace, and heat the matter but just enough to make it flow, with a smooth, brilliant surface. When you find it thus conditioned, point towards it the nose of a long-snouted pair of bellows, and therewith keep gently and constantly blowing. There will arise from the crucible a considerable smoke, which will abate greatly when you cease to blow, and increase as soon as you begin again. You must raise the fire gradually as you approach towards the end of the operation. If the surface of the metal lose its brilliant polish, and seem covered with a hard crust, it is a sign the fire is too weak; in which case it must be increased, till the surface recover its shining appearance. At last, when no more smoke rises, and the surface of the Gold looks neat and greenish, cast on it, by little and little, some pulverized Nitre, or a mixture of Nitre and Borax. The matter will swell up. Continue thus adding more Nitre gradually, till no commotion is thereby produced in the crucible; and then let the whole cool. If you find, when the Gold is cold, that it is not tough enough, melt it over again; when it begins to melt, cast in the same Salts as before; and repeat this till it be perfectly ductile.
OBSERVATIONS.
Antimony is a compound, consisting of a semi-metallic part united with about a fourth part of its weight of common Sulphur. It appears, in the ninth column of the Table of Affinities, that all the metals, Mercury and Gold excepted, have a greater affinity than the reguline part of Antimony with Sulphur. If therefore Gold, adulterated with a mixture of Copper, Silver, or any other metal, be melted with Antimony, those metals will unite with the Sulphur of the Antimony, and separate it from the reguline part, which being thus set free will combine and be blended with the Gold. These two metallic substances, forming a mass far heavier than the other metals mixed with the sulphur, fall together to the bottom of the crucible in the form of a Regulus, while the others float over them like a sort of scoria or flag: and thus the Gold is freed from all alloy but the reguline part of the Antimony.
As all the other metals have a great affinity with Sulphur, and Gold is the only one that is capable of resisting its action, one would think Sulphur alone might be sufficient to free it from the metals combined with it, and that it would therefore be better to employ pure Sulphur, in this operation, than to make use of Antimony; the reguline part of which remaining united with the Gold requires another long and laborious operation to get rid of it.
Indeed, strictly speaking, Sulphur alone would be sufficient to produce the desired separation: but it is proper to observe, that, as Sulphur alone is very combustible, most of it would be consumed in the operation before it could have an opportunity to unite with the metallic substances; whereas, when it is combined with the Regulus of Antimony, it is thereby enabled to bear the action of the fire much longer without burning, and consequently is much fitter for the purpose in question. Besides, if we were to make use of pure Sulphur, a great part of the Gold, which is kept in perfect fusion, and its precipitation facilitated, by the Regulus of Antimony, would remain confounded with the Sulphureous scoria.
Nevertheless, seeing the metals with which Gold is alloyed cannot be separated from it by Antimony, but that a quantity of Regulus proportioned to the quantity of the metals so separated will unite with the Gold, and that the more Regulus combines with the Gold, the more tedious, chargeable, and laborious will the operation prove, this consideration ought to have some influence in directing our process. Therefore, if the Gold be very impure, and worse than sixteen carats, we must not mix it with crude Antimony alone, but add two drams of pure Sulphur for every carat the Gold wants of sixteen, and lessen the quantity of Antimony in proportion to that of the real Gold.
It is necessary to keep the crucible close covered, after mixing the Antimony with the Gold, to prevent any coals from falling into it: for, if that should happen, the melted mass would puff up considerably, and might perhaps run over.
The inside of the cone, into which you pour the melted metallic mass, must be greased with tallow, to prevent its sticking thereto, and that it may come easily out. Striking the floor, on which the cone with the melted metal stands, helps the precipitation and descent of the Regulus of Gold and Antimony to the bottom of the cone.
Less fire is requisite to melt this compound Regulus, in order to add fresh Antimony, than was necessary before the Gold was mixed with the reguline part of the Antimony; because this metallic substance, being much more fusible than Gold, promotes its melting. The Antimony is mixed with the Gold by repeated projections, that the separation of the metals may be accomplished with the greater ease and accuracy. Yet the operation might be successfully performed, by putting in all the Antimony at once, and with one melting only.
The metalline mass found at the bottom of the cone after all these operations, is a mixture of Gold with the reguline part of the Antimony. All the rest of the process consists only in separating this reguline part from the Gold. As Gold is the most fixed of all metals, and as the Regulus of Antimony cannot bear the violence of fire without flying off in vapours, nothing more is necessary for this purpose but to expose the compound, as directed in the process, to a heat strong enough and long enough continued, to dissipate all the Regulus of Antimony. This semi-metal exhales in the form of a very thick white smoke. It is proper to blow gently into the crucible during the whole operation; because the immediate contact of the fresh air incessantly thrown in promotes and considerably increases the evaporation: and this is a general rule applicable to all evaporations.
The fire must be gradually raised as the Regulus of Antimony is dissipated, and the operation draws towards an end; because the mixed mass of Regulus of Antimony and Gold becomes so much the less fusible as the proportion of the Regulus is lessened. Though the Regulus of Antimony be separated from the Gold in this operation, because the latter is of such a fixed nature that it cannot be volatilized by the degree of fire which dissipates the Regulus; yet, as the Regulus is very volatile, it will undoubtedly carry up some of the Gold along with it, especially if you hurry on the evaporation too fast, by applying too great a degree of fire, by blowing too briskly into the crucible, and still more if you evaporate your mixture in a broad flat vessel instead of a crucible. All these things must therefore be avoided, if you would lose no more Gold than you needs must.
However, unless the evaporation be carried to the utmost, by the means above pointed out, a small portion of the Regulus of Antimony will always remain combined with the Gold, which defends it from the action of the fire. This small portion of Regulus hinders the Gold from being perfectly pure and ductile. In order therefore to consume and scorify it, we cast Nitre into the crucible when we perceive it to emit no more white vapours.
We know that Nitre has the property of reducing all metallic substances to a calx, Gold and Silver excepted; because it deflagrates with the phlogiston to which their metalline form is owing: but as this accension of the Nitre occasions a tumid effervescence, care must be taken to throw it in but by little and little at a time; for if too much be projected at once the melted matter will run over.
This operation might be considerably abridged by taking advantage of the property which Nitre possesses of thus consuming the phlogiston of metallic substances; as by means thereof we might destroy all the Regulus of Antimony incorporated with the Gold, without having recourse to a long and tedious evaporation. But then we should at the same time lose a much greater quantity of Gold, by reason of the tumult and ebullition which are inseparable from the detonation of Nitre. On the whole, therefore, if Nitre be made use of to purify Gold, great care must be taken to apply but very little of it at a time.
All the Silver that was mixed with the Gold, and indeed a little of the Gold itself, remains confounded with the sulphureous scoria, which floats upon the Golden Regulus after the addition of the Antimony: we shall shew in the Chapter on Silver how these two metals are to be separated from the Sulphur.