_Advantages of Roasting._—Illustrative of the conditions under which roasting is advantageously conducted in modern practice, the case of the Butte second-class ores may be quoted. These ores contain about 5 per cent. of copper in the form of sulphides, finely disseminated through large quantities of siliceous gangue. Direct smelting in a blast furnace would not yield a matte of the desired “converter” grade, except at very heavy expense and difficulty. The ore is, therefore, wet-dressed up to 9 to 10 per cent. copper, and the coarse concentrates now help to yield a good matte, when smelted in the blast furnace. By the wet-dressing treatment, however, a considerable quantity of fine material is unavoidably produced, for which the most convenient treatment in such large quantities, under prevailing conditions, is in the reverberatory furnace. The atmosphere of this type of furnace being to a great extent neutral, the charge would tend simply to melt down without very much reduction of sulphur, resulting in the production of very low-grade matte. Roasting of these fine concentrates is, therefore, desirable for reducing the sulphur to such an extent as will yield a high-grade converter matte.[6] Roasting being thus often advisable as a preliminary, its inclusion in a smelting scheme under suitable conditions entails the following advantages over the direct reverberatory treatment of unroasted ores:— (1) It ensures satisfactory concentration on smelting. (2) It leaves reverberatory furnace smelting practically a remelting operation, and so affords exact control of the concentration effected. (3) The roaster gases may be utilised for making acid. In modern practice the work of the reverberatory plant is controlled at the roasters. The reverberatory foreman smelts whatever mixture is sent from the roasting plant, and if the grade of the resulting matte is not satisfactory, it is in the roasting operations that the required change is made for the correct adjustment of the sulphur and for controlling the consequent tenor of the matte. =The Reactions of Roasting.=—The operation of roasting is the exposing of a substance to the effects of heat and air, in order to oxidise it, and to render it more suitable for subsequent smelting operations.[7] In the case of the ordinary sulphide copper ores, roasting not only (_a_) reduces sulphur, and so ensures good concentration on smelting, but (_b_) by oxidising the iron, provides a ready flux for siliceous gangues. The more important reactions occurring to the usual constituents of the copper ores which are roasted, may be summarised as follows:— _Iron Pyrites._—First loses free sulphur at a low temperature: it is generally assumed that FeS is left, but the residual sulphide rarely attains this composition— FeS_{2} ➡ FeS + S.