III. THE COPPER ASSAY. The composition of natural and artificial substances containing copper, is always to be considered before the commencement of the operation, as upon this the difficulty of the examination. depends. For the same reason, cupriferous substances are to be divided into the classes of, Ores, Minerals, and Furnace Products : a. Containing volatile ingredients; b. Containing the Copper in an oxidized state, with or without acids and water, vitrified, or washed in another manner with earthy constituents. ¶ B. Metallic Compounds, the Copper of which is either a principal or an accidental constituent : a. b. c. Plombiferous Copper, and Cupriferous Lead; Stibiferous Copper; d. Stanniferous Copper. For the ores, minerals, and furnace products belonging to the first class, roasting is necessary, previously to the fusion of the copper, in order to eliminate sulphur and arsenic; but it is not so with those of the second class. The impossibility of producing pure copper immediately from ores, minerals, and furnace products, containing several volatile metals, many of which cannot be expelled by roasting, is well known from the fusion of copper ores on a large scale, as well as from the common test in the dry way. As it may be necessary to give particulars concerning this point, the manipulation for obtaining the copper, as correctly as possible, by the aid of the Blowpipe, will at one be proceeded with. ¶▲. Ores, Minerals, Products of Furnaces and Manufactures. a. EXAMINATION OF SUCH AS CONTAIN VOLATILE INGREDIENTS, SULPHUR AND ARSENIC. To this class belong all copper ores prepared on a large scale; of minerals,—Kupfer Glanz, Purple Copper, Tennantite, Grey Copper, common sulphides of copper and tin, Bournonite, Melanglanz, et cetera; and of furnace products, kupferstein, kupferleg, cupriferous rohstein, bleistein, tutty, et cetera. Of these substances, the necessary ground ore is prepared, and one decigramme weighed for examination. As they must be roasted previously to the fusion, the sample is mixed in the agate mortar, either with three times its volume of dry pulverized charcoal, or with 20 to 25 milligrammes of graphite, which in most cases, particularly for substances containing much arsenic, is more advantageous than charcoal. It is then placed upon a clay basin, and besmeared with red ochre-sesquioxide of iron. A charcoal prism of sufficient size is introduced into the charcoal holder, and tightened; a cavity is then made in it, and from its contiguous sides so much cut out with a knife as is prescribed by the orifice b, Fig. 27, page 27, in order to allow the flame access to the cavity. The platinum foil and wire is placed in it, the clay basin, taken with the pincers by the rim, is sunk in a horizontal position, until it reposes upon the platinum wire. For obtaining a horizontal position of the basin, it is necessary to hold the pincers so that their arms can pass through the orifice. After these preparations, the Blowpipe is provided with a point of medium size, then an oxidating flame, not too strong, is directed through the orifice upon the open space below the basin. In order to produce a red heat on the surrounding space, as well as on the basin, the charcoal holder should be at the dis tance of an inch and a half from the wick of the lamp, for if it is too near the flame, sufficient heat is not produced; whereas, at a greater distance, by the introduction of a large supply of heated air into the cavity, the combustion of the charcoal is too much accelerated, generally impairing the operation. The basin is kept for some minutes in a state of redness, though care must be taken to blow moderately, in order to prevent a partial or perfect fusion of the sample. In this operation, the presence or absence of volatile ingredients may be detected from the smell of the eliminated gases. Further; the charcoal, added to the ore, is wholly destroyed, which can be ascertained by examination with the iron spatula, heated over the free flame of the lamp, in order to prevent the adhesion of small particles of the substance. The basin is removed from the holder, its contents placed in the agate mortar, and reduced to powder. Generally the ore changes its color in the first roasting, forming a porous mass, which may be easily detached from the basin, and seldom requires the use of the spatula. In this roasting, which is terminated in about ten minutes, the greater part of the volatile ingredients is removed—sulphur, arsenic, and antimony-the addition of charcoal preventing the formation of sulphates and arseniates. For the complete elimination of the volatile substances preventing the reduction of the copper, a second roasting is necessary. The residuum of the first roasting is again mixed with three times its volume of pulverized charcoal, the basin, prepared as before, placed on the holder; if the charcoal, mixed with the substance, is in full incandescence, a somewhat stronger heat is applied, and as soon as half the charcoal is consumed, the assay is examined for volatile ingredients. If no smell indicating sulphur or arsenic is perceived, the remainder of the charcoal is allowed to burn under continued insufflation, after which the roasting is considered to be completely finished. On the other hand, if the volatile constituents are not wholly eliminated, a third roasting is necessary. It very seldom occurs, except in substances containing much arsenical nickel, that a fourth roasting is required; two are generally sufficient. In order to learn when the roasting is finished, it is better to weigh the basin several times successively, as after the assaying its weight remains constant; this, however, is difficult, and occupies too much time; therefore the smell exhaling from the roasted substance usually serves for ascertaining this period. Regarding the richness of substances roasted with pulverized charcoal, conclusions may be derived with sufficient certainty, from the color of the sample. The blacker the color, the richer it is in copper; if reddish or whitish, it is poor. If, instead of charcoal, graphite is employed, the assay is kept at a red heat, until the gases disengaged become inodorous. The graphite being destroyed slower than charcoal, and consequently remaining longer in immediate contact with the substance, reduces continuously the volatile ingredients; preventing, at the same time, the formation of sulphates or arseniates. As soon as the smell of the escaping gas ceases, the basin is taken from the charcoal, and the metallic substance, mixed with the remainder of the graphite, is triturated again in the agate mortar. This operation is indispensable, as, generally, during the roasting, the upper is sooner consumed than the lower portion of the graphite, in which unaltered metallic particles might remain, and these are again brought into contact with new portions of graphite. The sample is respread on the clay basin, and submitted to a new roasting in a higher temperature. At the commencement of this second operation, the smell of volatile matter is often perceived for some moments, indicating the volatilization of the remaining traces of arsenic and sulphur. The blowing is continued until nearly all the graphite is destroyed; then the basin is removed from the charcoal. Absence of smell from an assay roasted with graphite, is a sufficiently certain sign of the termination of the process. The constancy of the weight cannot be taken as a characteristic as the consumption of the whole graphite is not necessary. The possibility of adding a surplus of graphite to an assay, is the chief advantage of its employment in roastings. It is necessary to employ it in a pure state. If a cupriferous mineral, prepared on the large scale, contains heavy spar or gypsum, the sulphuric acid cannot be driven off by roasting; though the heavy spar is reduced, in a red heat, to sulphide of barium, and the gypsum to sulphide of calcium, both substances re-oxidize after the combustion of the charcoal. For the reduction of the copper contained in one decigramme of the substance, roasted in the manner here described, the following dressing was adopted, videlicet :100 milligrammes ; Soda, The soda reduces the oxide of copper, and other easily reducible metallic oxides; and the borax dissolves the oxides of iron manganese, cobalt, and the earthy constituents. The roasted substance, mixed with the soda and borax in the agate mortar, are enveloped in a soda paper cornet, and pressed into a cavity, prepared on the diametrical section of a good piece of charcoal, near one of its corners. These preparations being effected, a pure and strong reducing flame is deflected directly upon the sample, until the paper, the substance roasted, and the graphite are destroyed, and metallic copper is seen floating, either on the scoria,-with its peculiar greenish-blue color,-or separately near them, in the form of a metallic globule. In rich substances the reduced copper speedily appears, but more slowly, and sometimes not at all, if the ore is poor. Sometimes, after a long continued strong insufflation, the copper unites to one globule. It, however occurs oftener, that minute metallic particles remain in the scoria. It is therefore preferable to interrupt the process of reduction, so soon as the melted copper appears in the perfectly glossy scoria; to withdraw the scoriæ cautiously from the charcoal, if not colored red, which indicates vitrified suboxide of copper; reduce it to fragments between folded paper, upon the anvil; detach the visible globules; levigate and lixiviate the mass in a porcelain basin, with a view to remove dross and charcoal; and desiccate the basin with the copper, over the lamp flame. If the scoriæ are not completely separated, they remain united with the copper, and adhere to the basin, from which they must be detached by the spatula. In this case, it is necessary to re-levigate with water, and to desiccate a second time, after its affusion, when the copper may be easily detached. If the copper |