The manner of case roasting has been noticed under the head of the articles alum and iron, to which the reader is re ferred.
Iron ore is thus roasted in the open air to free it from carbonic acid and water. Sulphuret of copper and iron (pyrites) are so treated to get rid of some of the sulphur : sulphates are, however, apt to fuse and run together into masses which are troublesome afterwards. Open air roasting is rarely as effective as that in the reverberating furnace. It is that best suited for powdery ores, and the flame has a powerfully oxidizing effect. On this, Dr. Ure remarks : But in every case where it is desired to have a very perfect roasting, as for blende, from which zinc is to be extracted, for :ulphuret of antimony, &c., or even for ores reduced to a very fine powder, and destined for amalgamation, it is properto perform the operation in a reverberatory furnace. When very fusible sulphurous ores are treated, the workman charged with the calcination must employ much care and experience, chiefly in the man agement of the fire. It will sometimes, indeed, happen, that the ore partially fuses; when it becomes necessary to withdraw the materials from tho furnace, to let them cool and grind them anew, in order to recommence the operation. The construction of these furnaces demands no other attention than to give to the sole or laboratory the suitable size, and so to proportion to this the grate and the chim ney that the heating may be effected with the greatest economy.
The reverberatory furnace is always employed to roast the ores of precious metals, and especially those for amalga mation ;. as the latter often contain arse nic, antimony, and other volatile sub stances, they must be disposed of in a peculiar manner.
The sole, usually very spacious, is di vided into two parts, of which the one farthest off from the furnace is a little higher than the other. Above the vault there is a space or chamber in which the ore is deposited, and which communicates with the laboratory by a vertical passage ; which serves to allow the ore to be push ed down, when it is dried and a little neated. The flame and the smoke which
escape from the sole or laboratory pass into condensing chaiiabers, before enter ing into the chimney of draught, so as to deposite in them the oxide of arsenic and other substances. When the ore on the part of the sole farthest from the pate has suffered so much heat as to begin to be roasted, has become less fusible, and when the roasting of that in the nearer part of the sole is completed, the former is raked towards the fire-bridge, and its nstulation is finished by stirring it over frequently with a paddle, skilfully work ed, through one of the doors left in the side for this purpose. The operation is considered to be finished when the vapors and the smell have almost wholly ceased ; its duration depending obviously on the nature of the ores.
The last department of metallurgy which need be noticed, is the assaying. Under the head of itenty a general out line of the mode of producing is given three modes of assaying may be followed. 1st. The mechanical assay. 2nd. Assay by the dry way. 3rd. Assay by the moist way.
1. Of Mechanical assays.—These kinds of assays consist iu the separation of the substances mechanically mixed in the ores, and are performed by a hand-wash in g, in a small trough of an oblong shape. After pulverizing with more or less pains the matters to be assayed by this process, a determinate weight of them is put into this wooden bowl with a little water ; and by means of certain movements and some precautions, to be learned only by prac tise, the lightest substances may be pretty exactly separated, namely,. the. earthy gangues from the denser matter or me tallic particles, without loosing any sensi ble portion of them. Thus .a echlich of greater or less purity will be obtained, which may afford the means of judging by its quality of the richness of the as sayed ores, and which may thereafter be subjected to assays of another kind, whereby the whole metal may be insula ted.