Care must be taken to chuse a cupel of a proper capacity. Indeed it should rather be too big than too little: because the operation is no way prejudiced by an excess in its size; whereas, if it be too small, it will be over-dosed with Lead, and at last the litharge, which destroys every thing, will corrode its cavity, and eat holes through the very body of the vessel. Add, that the ashes, of which the cupel is made, being once glutted with litharge, absorb it afterwards but slowly, and that the quantity of this vitrified litharge, becoming too great to be contained in the substance of the vessel, exsudes through it, and drops on the floor of the muffle, which it corrodes and renders unequal; and moreover solders to it the vessels set thereon. It may be laid down as a general rule for determining the size of a cupel, that it weigh, at least, half as much as the metallic mass to be refined in it. It is also of the utmost consequence that the cupel be well dried before the metal be put into it. In order to make sure of this point, it must be kept red-hot for a certain time, as is above directed: for though to the sight and to the touch it may appear very dry, it nevertheless obstinately retains a small matter of moisture, sufficient to occasion the loss of some of the metal; which, when it comes to melt, will be thereby spirited up, in the form of little globules, to the very roof of the muffle. The cupels that stand most in need of an intense heat to dry them, are those chiefly in whose composition wood-ashes are employed: for whatever care be taken to lixiviate those ashes before they are used, they will still retain a little alkaline salt; and that, we know, is very greedy of moisture, will not part entirely with it, but by the means of a violent calcination, and presently re-imbibes it when exposed to the air. A little Phlogiston also may still be left in the ashes of which the cupels are made; and that is another reason for calcining them before they are used. By this means the remaining Phlogiston is dissipated, which might otherwise combine with the litharge during the operation, reduce it, and occasion such a ferment in the matter as to make some of it run over; to these inconveniencies, which any remainder of moisture or Phlogiston may produce, we must add the cracks and flaws, which are very incident to cupels not perfectly freed from both those matters. It is of no less importance to the success of this operation, that a due degree of heat be kept up. In the process we have described the marks which shew the heat to be neither too strong nor too weak; when it exceeds in either of these respects it may be known by the following signs.