This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1855 Excerpt: ...slowly into the heated muffle of the furnace; the most refractory specimens foremost, and those after wards which melt easily. The ores which contain much siliceous matter or iron are refractory: those which contain pyrites chiefly, are very fusible. In a short time, say five or eight minutes after exposure to the heat ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1855 Excerpt: ...slowly into the heated muffle of the furnace; the most refractory specimens foremost, and those after wards which melt easily. The ores which contain much siliceous matter or iron are refractory: those which contain pyrites chiefly, are very fusible. In a short time, say five or eight minutes after exposure to the heat of the muffle, the ore begins to roast, that is, exhale sulphur, arsenic, and other volatile substances, and the lead begins to melt. When ore and metal are thus heating, the mouth of the muffle is shut by a piece of hard charcoal, and the dishes, gradually exposed to a stronger heat by stirring the fire: this must not be done too fast, because some ores foam in melting and flow over the lead, thus spoiling the assay. The principal object of this operation is to form a slag of the ore, and bring at the same time melted lead in contact with it in its molten state, which may absorb the precious metal. In case the ore does not contain sufficient flux for melting, some of the lead is oxidized by the heat, and forms a flux for it. It is therefore evident that too much heat is disadvantageous to the ore, because some of it may boil over the-edge of the vessel and is lost. The heat must not be too slow to act, because in that case, the oxide of lead, which is generated abundantly, will eat through the dish. The heat in the muffle is easily regulated by putting more or less coal at the mouth. It is therefore advisable to have the fire rather too strong than too weak. After the lapse of fifteen or twenty minutes we observe a bright metallic spot in the middle of the dish. It is the melted lead surrounded by the fluid ore, which is now in the form of a slag. The lead is all the time burning and evaporating; and if these vapours of it draw over the...
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