a vessel used in metallurgy for absorbing the scori- a of metallic bodies when melted. See COPPER.

Some of the German writers recommend, both for tests and cupels, a sort of friable opake stone, called white spath, which appears to be a species of gypsum, or of the stones from which plaster of Paris is prepared. The spath is directed to be calcined with a gentle fire, in a covered vessel, till the slight crackling, which happens at first, has ceased, and the stone has fallen in part into powder: the whole is then re- duced into subtle powder, which is passed through a fine sieve, and moistened with so much of a weak solution of green vitriol as is sufficient for making it hold together. Gellert, however, finds, that if the stone is of the proper kind, which can be known only by trials, calcination is not necessary. Scheffer observes, that these kinds of tests are liable to soften or fall asunder in the fire, and that this in- convenience may be remedied by mixing with the uncalcin- ed stone somewhat less than equal its weight, as eight-ninths of such as has been already used and is penetrated by the scoria of the lead, taking only that part of the old test which appears of a green-grey colour, and rejecting the red crust on the top. Tests or cupels made of the spath are said not to require so much caution in sealing and heating them as the common ones; it appears, however, from Schef- fer's account, that they are less durable than those made of the ashes of bones, though greatly inferior to those of wood-ashes. Vegetable ashes, which stand pretty well the testing of silver, can scarcely bear any great quantity of gold, this metal requiring a considerably stronger fire than the other; but bone-ashes answer so effectually, and are among us so easily procurable, that it is not needful for the refiner to search for any other materials; though those who work off large quantities of lead, in order to gain a little sil- ver or gold contained in it, may possibly, in places remote from populous cities, avail themselves of substances similar to the spath above-mentioned.

The test, for its greater security, is fixed in the mould in which it was formed; which is sometimes a shallow vessel made of crucible earth or cast-iron, more commonly an iron hoop, with three bars arched downwards across the bottom, about two inches deep, and of different widths, from three or four inches to fifteen or more, according to the quantity of metal to be tested at once. The ashes or earthy powder, moistened as for making cupels, are pressed down in the mould so as to completely fill it or rise a little above the sides; with care to make the mass equally solid, and to put in at once, or at least after the bottom has been pressed close, as much of the matter as will be sufficient for the whole; for any additional quantity will not unite tho- roughly with the rest, but be apt to part from it in the fire. The edges are pared smooth, and a portion cut out from the middle with a bent knife, so as to leave a proper cavity, which is smoothed by strewing some dry powder on the surface, and rolling on it a wooden, or rather a glass ball.

The process of testing is often performed in the same manner manner as that of cupellation: but where great quantities of base metal are to be worked off from a little gold, recourse is had to a more expeditious method, that of testing before the bellows.

An oval teft is placed in a cavity, made in a hearth of convenient height, and some moistened sand or ashes pressed round it to keep it steady: the nofe of a bellows is directed along its surface, in such a manner, that if ashes are sprinkled in the cavity of the teft, the bellows may blow them completely out: some have an iron plate fixed before the bellows, to direct the blast downwards. To keep the surface of the teft from being injured in putting in the metal, some cloths or pieces of paper are interposed. The fuel consists of billets of barked oak laid on the sides of the teft, with others laid crosswise on these: the bellows impel the flame on the metal, clear the surface of ashes or sparks of coal, hasten the scorching of the lead, and blow off the scoria, as fast it forms, to one end of the teft, where it runs out thro' a notch made for that purpose. About two thirds of the scoured lead may thus be collected; the rest being partly absorbed by the teft, and partly dissipated by the action of the bellows. Care must be taken not to urge the blast too strongly, lest some portion of the gold should be carried away by the fumes impetuously forced off from the lead, and some minute particles of it entangled and blown off with the scoria.