METALLURGY, in its most extended signification, means the working of metals in every different way. It therefore includes gilding, parting, smelting, assaying, &c. in its more limited sense, however, it is used to denote the process of smelting, or the separation of metals from their ores, for the purposes of art, and the assaying of these, with the view ol ascertaining their value. In this last. sense it is here used.
Almost all the ores employed in metallurgy, are em bedded in quartzy matter, from which they must be freed as much as possible, before they are subjected to the process of smelting The methods followed in the reduction differ in the different ores.
The assay of ores is performed in two different ways, with the view, either of ascertaining the quantities of all the different substances which the ore contains, or only the proportion of the metal for which it is wrought. The latter of these is used in the arts, and is more properly termed assay, the former being called ana lysis.
\Vhen assaying an ore, for the purpose of ascertaining with tolerable precision the quantity of metal contained in a large collection of it, we must take pieces from different parts of it, and put these together, by which we procure a mixture, containing the mean quantity of metal in the different pieces. In all the different assays, the ore must be reduced to powder, or to small fragments before it is weighed, and care must be taken that none of it is thrown out of the vessel during the operation, and the product should not be removed from the crucible till it is quite cold. If the assay be well performed, the metallic button obtained, will have a convex sur face, and will be of uniform density ; the scoria will be compact, and without the admixture of metallic grains. We must never trust to one assay, but repeat the pro cess several times, and, as in general there will be a slight. difference in the results, we must take the mean of the whole as the quantity of metal in the ore.
In the following article, under each metal will be in cluded the different methods practised in the smelting of its ores, and the assays which are performed on them, with the view of ascertaining the quantity of metal in them for which they are used. The metals employed in the arts, are Iron.
For the reduction of iron ores, see Inox.
?ssay. The assay of iron ores differs a little, accord ing to the ore employed.
The common magnetic iron-stone is assayed by mix ing it, when reduced to powder, with about twice its weight of a flux, composed of 1 of charcoal, 6 of chalk, and 8 of bottle glass. The mixture must be exposed to a very strong heat in a blast furnace, for about an hour, after which the iron will be found at the bottom of the crucible. If the ore contain sulphur, it must be roast ed previous to its mixture with the flux.
The other ores of iron arc assayed nearly in the same way. When very little earthy matter is present, they may be reduced by using charcoal without the flux ; and it the ore contain much calcareous matter, the quantity of chalk in the flux may be diminished.
Geld.
Gold is always found in its native state, generally al loyed with silver and copper, and occasionally with tel lurium. Some of the iron pyrites, and galena also, con tain a sufficient quantity of this metal, to make them va luable as ores of gold.
When gold is found alloyed with silver mixed only with stoney matter, the method of extracting the metal is very simple. In working a mine containing gold in this state, the whole of the matter procured from the vein, is collected and broken into pieces about the size of a nut, which are arranged into heaps, according to their richness. The small fragments, and the re fuse of the different processes, arc also collected and arranged according to their value. The quantities thus procured, are afterwards broken into smaller pieces, and freed as much as possible from impurities. They are then reduced to powder, and made into a thick paste with salt and water. Mercury is now squeezed through a leather bag on the mixture, and as the metal flows in, in very minute globules, it is intimately blend ed with it by means of wooden spatules. \Vhen the requisite quantity of mercury is added, the whole is beat together, and kept at about the temperature of boiling water, for two or three days, by which the union of the gold and mercury is promoted. The earthy matter is then carried off by levigation, and the superfluous mercury removed, by squeezing the amal gam through a leather bag. What remains is subjected to distillation, by which the mercury is driven off, and the gold is obtained, perhaps containing a little silver. The method of separating these will be explained when treating of silver.