When the Sulphur and Arsenic are dissipated by roasting, the ore must be treated like that which is rendered refractory by stony and earthy matters; for as the pyrites contain much iron, there remains, after the Sulphur is evaporated, a considerable quantity of martial earth, which is difficult to scorify. The pyrites, as well as the cobalts, contain moreover an unmetallic earth, which is hard to fuse. The general rule therefore is, when the ore is rendered refractory by any cause whatever, to mix it with Glass of Lead, and to add a larger quantity of granulated Lead. Yet some ores are so refractory that Lead alone will not do the business, and recourse must be had to some other flux. That which is fittest for the present purpose is the _Black Flux_, composed of one part of Nitre and two parts of Tartar deflagrated together. The Phlogiston contained in this quantity of Tartar is more than sufficient to alkalizate the Nitre. This Flux, therefore, is nothing more than Nitre alkalizated by Tartar, mixed with some of the same Tartar that hath not lost its Phlogiston, and is only reduced to a sort of coal. 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.