The White Flux is also very fit to promote fusion; but on this occasion the Black Flux is preferable, because the Phlogiston of the Black Flux prevents the Lead from being too soon converted to litharge, and so gives it time to dissolve the metallic matters. The White Flux, which is the result of equal parts of Tartar and Nitre alkalizated together, being no more than an Alkali destitute of Phlogiston, or containing but very little, doth not possess this advantage.
If Silver should be combined in the ore with Iron in its metalline state, which however does not commonly happen, then, in order to separate them, the Iron must be deprived of its Phlogiston, and converted to a crocus before the mixed mass be melted with Lead; which may be done by dissolving it in the Vitriolic Acid, and then evaporating the Acid.
We are necessitated to make use of this contrivance, because Iron in its metalline form cannot be dissolved either by Lead or by the Glass of Lead; but when it is reduced to a calx, litharge unites with it and scorifies it.
If you have not at hand the utensils necessary for performing the operation we have been describing in a test, and under the muffle; or if you have a mind to work on a greater quantity of ore at a time, you may make use of a crucible for the purpose, and perform the operation in a melting furnace.
In this case the ore must be prepared, as above directed, according to its nature, and mixed with a proper quantity of Lead and Glass of Lead; the whole put into a good crucible, leaving two thirds thereof empty, and covered with a mixture of Sea-salt and a little Borax, both very dry, to the thickness of a full half inch.
This being done, set the crucible in the midst of a melting furnace, raise the coals quite to the lip of the crucible; light the fire; cover the furnace with its dome; but do not urge the fire more than is necessary to bring the mixture to perfect fusion: leave it thus in fusion for a good quarter of an hour; stir the whole with a bit of strong iron wire; then let it cool; break the crucible, and separate the Regulus from the scoria.
The Salts added on this occasion are fluxes, and their use is to procure a perfect fusion of the scoria.
If the melted matters be left exposed to the fire, either in a test or in a crucible, longer than is above prescribed, the portion of Lead, that hath united and precipitated with the Silver, will at last vitrify, and at the same time scorify all the alloy with which that metal may be mixed. But as there are no vessels that can long endure the action of litharge, without being pierced like a sieve, some of the Silver may escape through the holes or fissures of the vessel, and so be lost. It is better, therefore, to complete the purification of your Silver by the operation of the Cupel, the description of which follows.