Elements of the Theory and Practice of Chymistry, 5th ed.

by Pierre Joseph Macquer

Smear the inside of a good crucible well with soap. Put your Luna Cornea into it; cover it with half its weight of Salt of Tartar, thoroughly dried and pulverized; press the whole hard down; pour thereon as much oil, or melted tallow, as the powder is capable of imbibing; set the crucible thus charged, and close covered, in a melting furnace, and, for the first quarter of an hour, make no more fire than is necessary to make the crucible moderately red: after that raise it so as to melt the Silver and the Salt, throwing into the crucible from time to time little bits of tallow. When it ceases to smoke, let the whole cool; or pour it into a hollow iron cone, warmed and tallowed.

OBSERVATIONS.

The process here delivered furnishes us with the means of procuring Silver in a degree of purity which is not to be obtained by any other method of treating it whatever. That which is refined on the cupel always retains a small portion of Copper, from which it cannot possibly be separated in that way: but if it be dissolved in aqua fortis, and precipitated thence in a Luna Cornea by the Marine Acid, the precipitate will be an absolutely pure Silver, unalloyed with that small portion of Copper which is retained on the cupel. The reason of this effect is, that the Copper remains as perfectly dissolved in Spirit of Salt and in aqua regia as in aqua fortis: so that when the Silver, and the Copper with which it is alloyed, are dissolved together in the Nitrous Acid, if the Acid of Sea-salt be mixed with the solution, part of this latter Acid unites with the Silver, and therewith forms a new compound, which not being soluble in the liquor, falls to the bottom. The other part of the Acid mixing with the Nitrous, forms an aqua regis, in which the Copper remains dissolved, without separating from it.

Fresh Acid is poured on the precipitated calx of Silver, in order to complete the solution of the small portion of Copper that may have escaped the action of the first solvent. It is indifferent whether the Spirit of Salt or the Spirit of Nitre be employed for this purpose, because they both dissolve Copper alike, and because Silver precipitated by Spirit of Salt is not soluble in either.

After this it is necessary to wash the precipitate well with pure water, in order to free it entirely from the particles of aqua fortis adhering to the Silver; because they may possibly contain something of Copper, which would mix with the Silver in melting, and taint its purity.

If this precipitate of Silver be exposed to the fire, unmixed with any other substance, it melts as soon as it begins to be red; and, if the fire be increased, part thereof will be dissipated in vapours, and the rest will make its way through the crucible. But being poured out as soon as melted, it coagulates into a cake of a purplish red colour, semi-transparent, ponderous, and in some degree pliable, especially if it be very thin. It bears some resemblance to horn, which hath occasioned it to be called Luna Cornea.

As Luna Cornea is not soluble in water, recourse must be had to fusion, in order to reduce it, by separating from the Silver those acids which give it the above-mentioned properties. Fixed Alkalis and fatty matters are very fit to produce that separation.

We directed that the inside of the crucible, in which the reduction is to be made, should be carefully smeared with soap, and that the Luna Cornea should be quite covered with a Fixed Alkali and fat, to the end that when the heat is strong enough to dissipate it in vapours, or to attenuate it so as to render it capable of penetrating the crucible, it may be forced to pass through matters qualified to absorb its Acid, and reduce it.

Luna Cornea may also be reduced by being melted with such metalline substances as have a greater affinity than Silver with the Acids wherewith it is impregnated. Of this kind are Tin, Lead, Regulus of Antimony: but the Luna Cornea rushes so impetuously into conjunction with those metalline substances, that a vast many vapours arise, and carry off with them part of the Silver: if therefore you chuse to effect the reduction by the interposition of such metalline substances, you must employ a retort instead of a crucible.

But this method is attended with another inconvenience; which is, that some part of those metalline substances may unite with the Silver, and adulterate it: for which reason it is best to keep to the method first proposed.