Chap. VI Of Lead
Process I.
To extract Lead from its Ore.
Having roasted your Lead ore reduce it to a fine powder; mix it with twice its weight of the black flux, and one fourth of its weight of clean iron filings and borax; put the whole into a crucible capable of containing at least thrice as much; over all put Sea-salt four fingers thick; cover the crucible; lute the juncture; dry the whole with a gentle heat, and set it in a melting furnace.
Make the crucible moderately red: you will hear the Sea-salt decrepitate, and after the decrepitation a small hissing in the crucible. Keep up the same degree of fire till that be over.
Then throw in as many coals as are necessary to complete the operation entirely, and raise the fire suddenly, so as to bring the whole mixture into perfect fusion. Keep up this degree of fire for a quarter of an hour, which is time sufficient for the precipitation of the Regulus.
When the operation is finished, which may be known by the quietness of the matter in the crucible, and by a bright vivid flame that will rise from it, take the crucible out of the furnace, and separate the Regulus from the scoria.
OBSERVATIONS.
All Lead ore contains a good deal of Sulphur, which must be first separated from it by roasting: and as this kind of ore is apt to fly when first exposed to the fire, it is proper to keep it covered till it be thoroughly heated. Another precaution to be used, in roasting this ore, is not to give it too great a heat, but to keep the vessel which contains it just moderately red; because it easily turns clammy, which occasions it to stick to the vessel.
The Iron that is added, and mixed with the flux, absorbs the Sulphur which may happen to remain, even after roasting: it helps also to separate from the Lead some portions of semi-metal, especially of Antimony, which are frequently mixed with this ore.
There is no fear least the Iron mix with the Lead in fusion, and adulterate it: for these two metals are incapable of contracting any union together, when each has its metalline form.
Nor is there any reason to apprehend lest the Iron should, by its refractory quality, obstruct the fusion of the mixture; for though this metal be not fusible when alone, yet, by the union it contracts with the matters it is designed to absorb, it becomes so to such a degree as in some measure to perform, on this occasion, the office of a flux.
The government of the fire is a point of great consequence in this operation. It is necessary to apply but a moderate degree of heat at first: for, when the metallic earth of the Lead, combining with the phlogiston, acquires the metalline form, it swells up in such an extraordinary manner, that there is great danger least the matter should overflow, and run all out of the containing vessel. With a view therefore to avoid this inconvenience, we direct a very large crucible to be used. This heaving of the Lead, at the instant of its reduction, is attended with a noise like the whistling of wind.
Notwithstanding all the precautions that can be used to prevent the reduction from taking place too hastily, and so occasioning the effusion of the matter, it often happens that, on raising the fire in order to bring the mixture into fusion, the hissing suddenly begins again, and is very loud. In that case all the apertures of the furnace must immediately be shut close, in order to choak and suffocate the fire: for, if this be neglected, the matter in the crucible will swell up, make its way through the luting of the juncture, nay, push up the cover, and run over. This accident is to be apprehended during the first five or six minutes after you raise the fire in order to melt the mixture. This effusion of the matter is accompanied with a dull flame, a thick, grey and yellow smoke, and a noise like that of some boiling liquor. When you observe these several phenomena you may be sure the matter is run out of the crucible, either in the manner above described, or by making its way through some cracks in the vessel, and consequently that the operation is spoiled.
Moreover, this event infallibly follows whenever a bit of coal happens to fall into the crucible; and this is one reason why it is necessary to cover it.
You may be certain that the operation hath succeeded if the scoria be smooth when cold, and have not in part escaped through the lute; if the Lead be not dispersed in globules through the whole mass of the matter contained in the crucible, but is, on the contrary, collected at the bottom, in the form of a solid Regulus, not very shining, but of a blueish cast, and ductile. Moreover, the scoria ought, in the present case, to be hard and black, and should not appear full of holes like a sieve, except only in that part which was contiguous to the Salt.
Here it is proper to observe, that the Sea-salt doth not mix with the scoria, but floats upon it. After the operation it is black; which colour it gets, no doubt, from the charred parts of the flux. The absence of these signs shews the operation to have miscarried.
When the ore to be smelted is pyritose and refractory, it may be roasted at first with a much stronger degree of fire than is used for ores that are fusible; because the martial earth, and the unmetallic earth, which are always mixed in pyritose matters, hinder it from growing readily soft in the fire. Besides, such an ore requires a greater quantity of the black flux and of borax to be mixed with it, and a higher degree of fire to fuse it.
It is generally needless to mix iron filings with this sort of ore; because the martial earth, with which pyritose matters are always accompanied, is reduced during the operation by the help of the black flux, which for that purpose is mixed with it in a large proportion, and furnishes a quantity of iron sufficient to absorb the heterogeneous minerals mixed with the Lead.
Yet, if it should be observed that the pyrites which accompany the Lead ore are arsenical, then, as such pyrites contain but a small quantity of ferruginous earth, iron filings must be added; which are, on this occasion, so much the more necessary for absorbing the Arsenic, as this mineral remains in part confounded with the ore, is reduced to a Regulus during the operation, unites with the Lead, and destroys a great deal of it by procuring its vitrification.
The Lead obtained from such pyritose ores is commonly not very pure; it is blackish and scarce ductile; qualities communicated to it by a small mixture of Copper in the pyrites, which always contain more or less thereof. We shall presently shew the method of separating Lead from Copper.
Lead ore may also be reduced by melting it amidst coals. For that purpose first kindle a fire in the furnace in which you intend to melt your ore; then put a layer of your ore immediately upon the lighted coals, and cover it with another layer of coals.
Though the melting furnace used for this operation be capable of giving a considerable heat, yet it is necessary further to increase the force of the fire by the means of a good pair of perpetual bellows, which will produce an effect like that of a forge. The ore melts, the earth of the Lead unites with the phlogiston of the coals, and so is reduced to metal, which runs through the coals, and falls into an earthen vessel placed at the bottom of the furnace to receive it. Care must be taken to keep this vessel well filled with charcoal-dust, to the end that the Lead may be in no danger of calcination while it continues there; the charcoal-dust constantly furnishing it with phlogiston to preserve its metalline form.
The earthy and stony matters that accompany the ore are scorified by this fusion, just as they are by the other which is performed in a close vessel. With regard to the Sulphur and Arsenic, they are supposed to have been first accurately separated from the ore by roasting. This is the method commonly employed for smelting Lead ore at the works.
Process II
To separate Lead from Copper.
With luting earth and charcoal-dust make a flat vessel, widening upwards, and large enough to contain your metalline mass. Set it shelving downwards from the back towards the fore-part; and in the fore-part, at the bottom, make a little gutter communicating with another vessel of the same nature, placed near the former and a little lower. Let the mouth of the gutter within side the upper vessel be narrowed, by means of a small iron plate fixed across it, while the loam is yet soft; so as to leave a very small aperture, in the lower part of this canal, sufficient to discharge the Lead as it melts. Dry the whole by placing lighted coals around it.
When this apparatus is dry, put your mixed mass of Copper and Lead into the upper vessel: both in that, and in the other vessel, light a very gentle fire of wood or charcoal, so as not to exceed the degree of heat necessary to melt Lead. In such a degree of heat the Lead contained in the mixed mass will melt, and you will see it run out of the upper vessel into the lower; at the bottom of which it will unite into a Regulus. When in this degree of heat no more Lead flows, increase the fire a little, so as to make the vessel moderately red.
When no more will run, collect the Lead contained in the lower vessel. Melt it over again in an iron ladle, with a degree of fire sufficient to make the ladle red; throw into it a little tallow or pitch, and while it burns keep stirring the metal, in order to reduce any part of it that may be calcined. Remove the pellicle or thin crust which will form on the surface; squeeze out all the Lead it contains, and then put it to the mass of Copper left in the upper vessel. Check the fire, and in the same manner take off a second skin that will form on the surface of the Lead. Lastly, when the metal is ready to fix, take off the skin that will then appear on it. The Lead remaining after this will be very pure, and free from all alloy of Copper.
With regard to the Copper itself, you will find it in the upper vessel covered with a thin coat of Lead: and if the Lead mixed with it was in the proportion of a fourth or a fifth part only, and the fire applied was gentle and slow, it will retain nearly the same form after the operation that the mixed mass had before.
OBSERVATIONS.
Lead frequently remains mixed with Copper after the reduction of its ore, especially if the ore was pyritose. Though Copper be a much more beautiful and more ductile metal than Lead, yet the latter by being alloyed with the former is rendered eager and brittle. This bad quality is easily discovered by the eye on breaking it: for the surface of the broken part appears all granulated; whereas when it is pure it is more evenly, and resembles a congeries of solid angles. If the Lead be alloyed with a considerable quantity of Copper, its colour hath a yellowish cast.
Considering the bad qualities which Copper communicates to Lead, it is necessary to separate these two metals from each other. The method above laid down is the simplest and the best. It is founded on two properties belonging to Lead: the first is that of being much more fusible than Copper; so that it will melt and run in a degree of heat that is not capable of making the Copper even red-hot, which yet is very far from being able to melt it: the second is, that Lead, though it hath an affinity with Copper, and unites very perfectly therewith, yet is not able to dissolve it without a greater heat than the degree barely necessary to fuse Lead. Hence it comes that Lead may be melted in a Copper vessel, provided no greater degree of heat be applied than that purpose requires. But when the Lead becomes so hot as to be red, fume, and boil, it instantly begins to dissolve the Copper. For this reason, it is essential to the success of our operation that a moderate degree of heat only be applied, and no greater than is requisite to keep the Lead in fusion.
Charcoal-dust is made an ingredient in the composition of the vessels used on this occasion, in order to prevent the calcination of the Lead.
The iron plate, with which the entrance of the gutter within the upper vessel is narrowed, serves to prevent the larger pieces of Copper, which the Lead may carry along with it, from passing through: it stops them, and allows the Lead to run off alone.
But as these parcels of Copper may entirely choak the passage, care must be taken, when any happen to be stopt, to remove them from the entrance of the gutter, and push them back into the middle of the vessel. It is also necessary to observe whether or no the Lead fixes any where in the passage; and, if it does, the heat of that part must be increased, in order to melt it and make it run off.
Notwithstanding all the precautions that can be taken, to hinder the melted Lead from carrying off any Copper with it, it is impossible to prevent this inconvenience entirely; and therefore the Lead is melted over again, in order to separate the small portion of Copper with which it is still adulterated.
As Copper is much lighter than Lead, if these two metals happen to be so blended together that the Copper, without being in fusion and dissolved by the Lead, is only interposed between the parts of the melted Lead, so as to swim therein, it is then precisely in the case of a solid body plunged into a fluid heavier than itself, and must rise to the surface, like wood thrown into water. It is proper to burn some inflammable matter on this melted Lead, in order to reduce such parts thereof as are constantly calcining on its surface while it is in fusion; for without this precaution they would be taken off together with the Copper.
The Copper remaining after this separation is, as we took notice before, still mixed with a little Lead. If you desire to separate it entirely therefrom, you must put it into a cupel, and expose it under the muffle to such a degree of fire as may convert all the Lead into litharge. This cannot be so done but that some of the Copper also will be scorified by the heat of the fire, and by the action of the Lead: but as there is a very great difference between the facility and readiness with which these two metals calcine, the portion of Copper that is calcined, while the whole Lead is turning into litharge, is scarce worth considering.
The Lead, though carefully separated from the Copper by the process here delivered, is not yet absolutely pure: sometimes it is alloyed with Gold, and almost always contains some Silver. If you would free the Lead as much as possible from any mixture of these two metals, you must convert it into glass, separate the remaining bead, and afterwards reduce this glass of Lead. But, as these two perfect metals are of no prejudice to the Lead, it is not usual to separate them from it, unless they be in a sufficient proportion to defray the charge, and produce some profit besides.
When we examine by the cupel the just proportion of Gold and Silver that an ore or a mixed metalline mass will yield, we make a previous assay of the Lead to be employed in the operation, and afterwards, in our estimate, deduct a proper allowance for the quantity of fine metal due to the Lead made use of.
Process III
The Calcination of Lead.
Take what quantity of Lead you please; melt it in one or more unglazed earthen pans: a dark grey powder will be found on its surface. Keep stirring the metal incessantly till it be wholly converted into such a powder, which is the Calx of Lead.
OBSERVATIONS.
As Lead is a very fusible metal, and in that respect greatly resembles Tin, most of the observations we made on the calcination of Tin may be applied here.
In the calcination of all metals, and particularly in this of Lead, there appears a singular phenomenon which is not easily accounted for. It is this: though these matters lose a great deal of their substance, either by the dissipation of their phlogiston, or because some of the metal, perhaps, exhales in vapours, yet when the calcination is over their calces are found to be increased in weight, and this increase is very considerable. An hundred pounds of Lead, for example, converted into Minium, which is nothing but a calx of Lead brought to a red colour by continuing the calcination, are found to gain ten pounds weight; so that for an hundred pounds of Lead we have one hundred and ten pounds of Minium: a prodigious and almost incredible augmentation, if it be considered that, far from adding any thing to the Lead, we have on the contrary dissipated part of it.
To account for this phenomenon Natural Philosophers and Chymists have invented several ingenious hypotheses, but none of them entirely satisfactory. As we have no established theory to proceed upon, we shall not undertake to explain this extraordinary fact.
Process IV
To prepare Glass of Lead.
Take two parts of Litharge, and one part of pure crystalline Sand; mingle them together as exactly as possible, adding a little Nitre and Sea-salt: put this mixture into a crucible of the most solid and most compact earth. Shut the crucible with a cover that may perfectly close it.
Set the crucible thus prepared in a melting furnace; fill the furnace with coals; light the fire gradually, so that the whole may be slowly heated: then raise the fire so as to make the crucible very red, and bring the matter it contains into fusion; keep it thus melted for a quarter of an hour.
Then take the crucible out of the furnace, and break it: in the bottom thereof you will most commonly find a small button of Lead, and over it a transparent Glass, of a yellow colour nearly resembling that of amber. Separate this Glass from the little button of metal, and from the saline matters which you will find above it.
OBSERVATIONS.
Pure Lead, being exposed to a strong fire without any additament, turns to Litharge; which is a scaly sort of substance, more or less yellowish, shining, and soft to the touch. This is the first advance to the Vitrification of Lead. The large refineries of Gold and Silver by the means of Lead furnish a great quantity of this material. It is sometimes whitish, and is then called Litharge of Silver; sometimes yellow, and then bears the name of Litharge of Gold. The difference of its colour depends on the degree of fire it hath undergone, and on the metalline substances vitrified with it.
Litharge alone is very fusible, and being exposed to the fire is easily converted into glass: but this Glass of Lead, made without additament, is so active, so penetrating, and so apt to swell, that it can scarcely be made use of when pure. We are obliged in some sort to clog it, by uniting it with some vitrifiable matter that is not so subtile, such as sand; and it is for this reason, not to render the mixture more fusible, that we have directed the addition of one third part of Sand to two thirds of Litharge.
The Nitre and Sea-salt, prescribed as ingredients in the mixture, are designed to procure an equal fusion of the whole. For, as the sand is lighter and less fusible than the Litharge, it will partly rise towards the upper part of the crucible when that matter first begins to flow; in consequence whereof the contents of the upper part will be much more difficult to melt, and form a Glass much more compact than that below: but the Nitre and Sea-salt possessing the upper part of the crucible, because they are still lighter than the Sand, and being in their own nature very efficacious fluxes, on account of their great fusibility, they quickly bring about the fusion of those particles of sand, which, having escaped the action of the Litharge, may have risen unvitrified to its surface.
The most difficult thing to procure, and yet the most necessary to the success of this operation, is a crucible of earth so firm and compact as not to be penetrated by the Glass of Lead, which corrodes and makes its way through every thing.
The precaution of chusing a crucible, that shall contain a good deal more than the matter to be vitrified, is a necessary one, because Litharge and Glass of Lead are very apt to swell.
The rule to keep the crucible close shut is also indispensably necessary, to prevent any bit of charcoal, or other inflammable matter, from falling into it: for when this happens it occasions a reduction of the Lead, which is always attended with a sort of effervescence, and such a considerable heaving, that commonly most of the mixture runs over the crucible. For the same reason it is very proper, before you expose the mixture to the fire, to examine whether or no it contains any matter capable of furnishing a phlogiston during the operation; and if it does, to remove that matter with great care.
The little button of Lead, found at the bottom of the crucible after the operation, comes from a small portion of Lead that is commonly left in Litharge, unless you prepare it carefully yourself, and do not take it from the fire till you are sure of having destroyed all the Lead. Besides, this small portion of Lead can be of no prejudice to the operation, because it cannot communicate its phlogiston to the rest of the matter.
The revivifying of Litharge, of the Calx, and of the Glass of Lead, may be obtained by the same processes as the reduction of its ore.
Process V.
Lead dissolved by the Nitrous Acid.
Put into a matrass some aqua fortis precipitated like that used to dissolve Silver; weaken it by mixing therewith an equal quantity of common water; set the matrass in a hot sand-bath; throw into it, little by little, small bits of Lead, till you see that no more will dissolve. Aqua fortis thus lowered will dissolve about a fourth of its weight of Lead.
There is gradually formed upon the Lead, as it dissolves, first a grey powder, and afterwards a white crust, which at last hinder the solvent from acting on the remaining part of the metal; and therefore the liquor should be made to boil, and the vessel should be shaken to remove those impediments, by which means all the Lead will be dissolved.
OBSERVATIONS.
Lead very much resembles Silver, with respect to the phenomena which attend its dissolution in Acids. For example, the Nitrous Acid must be very pure and uncontaminated with the Vitriolic or Marine Acid, to qualify it for keeping the Lead in solution: for, if it be mixed with either the one or the other of these Acids, the Lead will precipitate in the form of a white powder as fast as it dissolves; which is just the case with Silver.
If the Vitriolic Acid be mixed with the Nitrous, the precipitate will be a combination of the Vitriolic Acid with Lead; that is, a Neutral Metallic Salt, or Vitriol of Lead. If the Acid of Sea-salt be mixed therewith, the precipitate will be a Plumbum corneum; that is, a Metallic Salt resembling the Luna cornea.
When all the Lead is dissolved as above described, the liquor appears milky. If it be kept warm over the fire till little crystals begin to appear on its surface, and afterwards left to stand quiet, in a certain time there will be found at the bottom a greyish powder, which being tried on Gold is Mercurial enough to whiten it. Little globules of Quick-silver are even discernible in it.
We owe this observation, together with this manner of proving the existence of Mercury in Lead, and of procuring it from thence, to M. Grosse, who hath given an account of his process in the Memoirs of the Academy of Sciences, from whence we have copied the description of the operation in hand.
The solution being quickly poured off by inclination from the grey mercurial precipitate is still milky, and deposites another white sediment. When this second precipitate falls the liquor becomes clear and limpid, and is then of a fine yellow colour, like a solution of Gold. On this gold-coloured solution, and on the two precipitates above-mentioned, M. Grosse made several observations, the chief of which we shall here insert.
The yellow liquor affects the tongue at first with a taste of sweetness; but afterwards vellicates it very smartly, and leaves on it a strong sensation of acrimony, which continues for a long time.
Alkalis precipitate the Lead suspended in this liquor, just as they do all other metals dissolved by Acids; and this precipitate of Lead is white.
Sea-salt, or Spirit of Salt, separates the Lead from its solvent, and precipitates it, as we observed before, into a Plumbum corneum: but this precipitate differs from the Luna cornea, as being very soluble in water; whereas the Luna cornea will not dissolve in it at all; or at least dissolves therein with great difficulty, and in a very small quantity. This Plumbum corneum dissolved in water is again precipitated by the Vitriolic Acid. M. Grosse observes, that this forms an exception to the eighth column of Mr. Geoffroy’s Table of Affinities; in which the Acid of Sea-salt is marked as having a greater affinity than any other Acid with Metallic substances.
Our solution of Lead is also precipitated in a white powder by several Neutral Salts; such as Vitriolated Tartar, Alum, and common Vitriol. It is by the means of double affinities that these Neutral Salts effect this precipitation.
Even pure water alone is capable of precipitating the Lead of our solution, by weakening the Acid, and thereby disabling it from keeping the metal suspended.
Lastly, as all the solutions of metals in Acids are nothing but Neutral Metallic Salts in a fluid form, so if the solution of Lead be evaporated over the fire, it will shoot into very beautiful crystals, about the bigness of hemp-seed, shaped like regular pyramids having square bases. These crystals are yellowish, and have a sweet saccharine taste: but what is most singular in them is, that, as they consist of the Nitrous Acid combined with Lead, which manifestly contains a great deal of phlogiston, they constitute a Nitrous Metallic Salt, which has the property of deflagrating in a crucible, without the addition of any other inflammable matter. It is extremely hard to dissolve this Salt in water.
The grey mercurial precipitate which whitens Gold, and in which little globules of running Mercury are perceivable, is far from being pure Mercury. This metallic substance makes but a small part thereof: for it is an assemblage, 1. of little crystals of the same nature with those afforded by the evaporated solution; 2. of a portion of the white matter, or powder, which renders the solution milky; 3. of a grey powder, which M. Grosse considers as the only mercurial part; 4. and lastly, of little particles of Lead that have escaped the action of the solvent; especially if a little more Lead than the Acid is capable of dissolving were added with a view to saturate it entirely, as in the present process.
By means of motion and heat the small parcels of Mercury may be amalgamated with the Lead.
That Mercury should be found entire and in globules in the Spirit of Nitre, which very easily dissolves that metallic substance, will not be surprizing to those who reflect that, in the present case, the Acid is saturated with Lead, with which it has a greater affinity than with Mercury; as appears by M. Geoffroy’s Table of Affinities, where, in the column that hath the Nitrous Acid at top, Lead is placed above Mercury. Agreeably to this, if Lead be presented to a solution of Mercury in Spirit of Nitre, the Lead will be dissolved, and as the dissolution thereof advances the Mercury will precipitate.
Hence it appears that, in order to find any Mercury in the spontaneous precipitate of Lead dissolved by the Nitrous Acid, it is necessary that the Acid be entirely saturated with Lead; or else that portion of the Acid which remains unsaturated will dissolve the Mercury.
With regard to the white powder that renders the solution milky, and afterwards precipitates, it is nothing but a portion of the Lead, which, not being intimately united with the Acid, falls in part of its own accord. It is a sort of calx of Lead, which being exposed to the fire becomes partly glass, and partly Lead, because it still retains some of its phlogiston.