From 1 to 10 per cent. of iron filings may be added for refining in case the copper contains no iron or other easily oxidizable metals.The process of fusing very poor copper ores in larger quantities is as follows: 10 to 15 grammes (154.32 to 231.48 grains) of unroasted ore are mixed with 15 to 20 per cent. of iron pyrites (free from copper), and 20 per cent. of sulphur, 100 per cent. of borax glass, 100 per cent. of glass, and 20 to 25 per cent. of resin are added, and a covering of common salt. The charge is then fused to matt (matt assay) in a clay crucible. The resulting matt is roasted, etc.-Easily decomposable sulphates (cupric sulphate) are decomposed, before they are subjected to the reducing fusion, by roasting with an addition of charcoal (p. 35); sulphates difficult to decompose (cuprous sulphate with calcium and barium sulphates) by preliminary fusion to matt.

3. Alloys of copper.-In case they do not contain too many and difficultly oxidizable components (nickel, tin, etc.), they are refined in the refining-dish or cupel; otherwise the wet method of treatment is preferable.

B. Cornish copper assay.'-This is an imitation of the English smelting process in the reverberatory furnace, and requires much skill. On account of its inaccurate results (involving a loss of from 20 to 40 per cent. of copper, according to the richness of the ore) the wet method has been substituted for it even in Cornish smelting works for valuing ore in quantity. The following operations are required for ores containing sulphur, antimony, or arsenic: A gentle roasting in a suitable crucible (Fig. 40, p. 66);

'Mitchell, Manual of Practical Assaying, p. 299.

fusing to crude matt in the same crucible. This is then roasted, and the roasted assay sample is fused to black copper. This is purified by fusing it with oxidizing and solvent agents, and the purified black copper is refined with oxidizing and solvent agents, and, finally, the slag is fused. All operations are carried out in the Cornish clay crucibles (Fig. 40, p. 66).

29. WET ASSAYS.

These, on account of their greater accuracy and simpler execution, have, as a general rule, been substituted for dry assays. The choice of one of the numerous wet assays depends chiefly on the foreign admixtures (antimony, arsenic, lead, bismuth, mercury, etc.), and somewhat on the richness of the sample. Colorimetric assays are especially adapted for poorer ores, and volumetric assays, if many are to be made in quick succession, are preferred to gravimetric assays.

A. Gravimetric assay.-The methods by which the copper is determined in the metallic state (Swedish and electrolytic assay) are simpler and more convenient to execute than those by which the copper is separated and determined in combination (determination of copper as cuprous sulphide, or as subsulphocyanide). The Swedish assay can be executed in less time than the electrolytic but it is done at the expence of accuracy, especially with poorer ores.

assay,

1. Modified Swedish assay.'-The cupriferous substance is brought into solution with sulphuric or hydrochloric acid (nitric acid must not be used, as the precipitated copper is again dissolved in it), and the copper precipi

B. u. h. Ztg. 1869, p. 12.

tated with iron or zinc, and determined either as metal or oxide. This plan is not admissible in the presence of metals, which are also precipitated by iron and zinc.

The dry mass

But such metals can be removed during the operation without injurious effect (lead as sulphate, silver as silver chloride, mercury by igniting the precipitated copper); or they must be removed by a preparatory operation (arsenic by roasting the assay sample with charcoal powder, or first by itself, and then with an addition of some iron pyrites at not too high a temperature; tin and antimony by heating with moderately diluted sulphuric acid, then adding nitric acid, and heating nearly to boiling, and an addition, if necessary, as in the case of metallic sulphides, of a few drops of fuming nitric acid. The solution is then evaporated to dryness until the fuming ceases. is dissolved in hot water and filtered, and the solution treated with some hydrochloric or nitric acid, etc.; or, the antimony is removed by fusing the assay sample with potassium hydrate or potassium carbonate in a silver crucible, lixiviating the potassium antimoniate, and dissolving the residue; or, the assay sample is dissolved, neutralized with soda, and digested with a solution of sodium sulphide to extract the antimony, arsenic, and tin in soluble form. It is now filtered, and the residue washed and dissolved as above. To separate bismuth by analytical methods is a very tedious operation.-Ores containing bitumen, for instance cupriferous schists (Kupferschiefer), must be ignited to remove the bitumen, before they are dissolved. Impure (black) precipitated copper can be further examined according to Parkes' and Fleitmann's volumetric method, to be shortly described.

a. Precipitation with iron.-1 to 5 grammes (15.43 to 77.16 grains), according to degree of richness of copper; generally, 2.5 to 5 grammes (38.58 to 77.16 grains) of the assay sample are decomposed in a suitable flask (Fig. 9, p. 38), which is placed in an oblique position, by heating with sulphuric acid, and adding from time to time some fuming nitric acid, or potassium chlorate, until the separated sulphur, inclosing particles of the ore, is oxidized as much as possible; or, the sample is at once dissolved in aqua regia (see also p. 39 for method of decomposing metallic sulphides). It is now evaporated to

dryness with some sulphuric acid, or until the sulphuric acid vapors appear in the flask. A few drops of sulphuric acid are added to the dry mass (to dissolve the basic salts), and then water is cautiously added; or, is at once added to the (cooled off) mass, while it still contains free acid. The fluid now entirely free from nitric acid is filtered into a glass flask, such as is shown in Fig. 15, p. 42. The residue is washed until the wash-water no longer produces a red stain upon a piece of bright sheet iron. Two pieces of iron wire, 3 to 4 centimeters (1.18 to 1.57 inches) long, are then added (or, in order to shorten the time required for the assay, the fluid may at once be filtered into a porcelain dish in which the iron wires lie, and copper will then be precipitated during the filtration). The filtrate is sufficiently diluted and gently heated until a pointed iron wire, when dipped into the fluid, shows no reddish stain of copper. The copper is twice decanted with cold water in a spacious beaker-glass (to prevent the separation of basic iron salts, which are more easily formed by hot water), and is then decanted three times with boiling water. The flask is now completely filled with cold water; a flat-bottomed porcelain dish, about 80 millimeters (3.15 inches) wide and 20 millimeters (0.78 inch) high, is placed bottom upwards on top of it. The flask and dish are then inverted. The mouth of the flask is held in an oblique position, and the water is allowed to. run into the dish until it is nearly full. The flask is left standing in the dish until all the copper and the iron wires have fallen into the water in the dish (small particles of carbon separated from the iron will remain floating on the water for some time). The flask is now quietly drawn over the side of the dish, which should be somewhat inclined for the purpose. The iron is freed from copper by rubbing with the fingers, which should be

rinsed off in the water. twice with boiling water. This is poured off as completely as possible from the copper, which is moistened with absolute alcohol, and dried on the water-bath, until it has assumed a pulverulent condition. It is allowed to cool in the desiccator, and is then brought upon the pan of the balance, or into a tared porcelain crucible with the aid of a fine brush, and quickly weighed. It is now dried for 10 or 15 minutes more, and again weighed until the results agree; or the copper is spread out upon a roasting. dish and ignited in the muffle-furnace, and the metal calculated from the amount of cupric oxide formed (100 cupric oxide 79.88 copper). If the water used in decantation shows a reddish sediment in the beaker-glass it should be filtered, the filter dried and ignited upon the scorifier, and the percentage of copper resulting from the cupric oxide should be added to the principal yield.

The copper is now decanted

Correction for iron that may be contained in the precipitated copper on account of a deposit of basic iron salts: The precipitated copper is ignited upon the scorifier until it becomes black. The cupric and ferric oxides formed are weighed and dissolved in hydrochloric or sulphuric acid. The ferric oxide is precipitated with ammonia. The solution is filtered upon a small filter of paper. The filter is dried and ignited, and the ferric oxide, which may be found, is deducted from the combined weight of the cupric and ferric oxides, and the copper calculated from the quantity of pure cupric oxide found. Instead of decanting the precipitated copper, it may all be filtered, dried, ignited, weighed, and dissolved, as above, for obtaining the percentage of iron.

Pure precipitated copper has a fine copper color. If the solution contains antimony and arsenic, it has first a copper color, which changes to black, by the antimony and arsenic which are precipitated later on. The largest portion of the antimony, after the precipitate has been evaporated to dryness with sulphuric acid and again moistened with water, remains as basic sulphate of anti