Copper—-+ \ + ➡ Cu_{2}S—+ / \ Sulphur—+ + ➡ × Cu_{2}S. yFeS ... _Matte_. \ / + ➡ FeS—-—-+ / Iron——-+ \ + ➡ FeO——+ / \ Oxygen—-+ + ➡ × FeO. ySiO_{2} ... _Slag_. / Silica ➡ SiO_{2}—+ The copper takes up sufficient sulphur to form Cu_{2}S; the remaining sulphur combines with any iron which is available, forming FeS. These two sulphides, dissolving in all proportions, constitute the _matte_ product of smelting. The iron in excess of that required by the sulphur becomes oxidised, and the resulting oxide combines with silica in the charge, forming the silicate _slag_ of the smelting operation.[5] It will thus be apparent that, in general, the larger the amount of sulphur present in a furnace charge, the more FeS will there be in the matte after melting, and the smaller will be the proportion of copper. In consequence, the grade of the matte will be lower. The proportion of sulphur in the charge thus controls the concentration of the copper by the smelting operation, and, in order to effect the desired concentration, oxygen is required in order to burn off sulphur and to oxidise iron. There are two general methods of supplying this necessary oxygen. (1) By a preliminary oxidation of the charge outside the smelting furnace—_Roasting_. (2) By oxidation inside the smelting furnace itself—_The pyritic principle_ (to be considered later). =Modern Practice as regards Roasting.=—In modern copper smelting, the tendency is to do away with roasting as much as possible. _Objections to Roasting._—(1) Expense involved by a separate preliminary process. This includes (_a_) Preparation of the ore for roasting. (_b_) Extra ground, and plant required for handling. (_c_) Labour, fuel, etc., required. (_d_) Extra handling of material before and after roasting. (2) Heavy mechanical and other losses during the process. (3) Loss of the fuel value of the iron and sulphur for smelting. (4) Necessity, in the majority of cases, of having the ore in a fine state of division in order to conduct efficient roasting, thus militating against its subsequent use in the blast furnace, unless the product receives preliminary agglomeration. Thus at Tennessee, the cost of roasting was about 40 cents, or 1s. 8d. per ton of ore (equivalent to ½d. on every pound of copper produced). The cost for the year 1903 amounted to £19,000, employing 170 men out of a total staff of 900 at mines and smelters. The conditions for roasting were here exceptionally favourable. The closing of the roast-yards set at liberty £34,000, which had been tied up in this manner.