In practice the roasting is never carried to such a degree that all the sulphur is eliminated, since it is essential to retain some sulphur in order to collect the copper in the form of matte, and also because the time, and the cost of the fuel required to roast all of it off, would be prohibitive. Consequently, the products from the roasting of chalcopyrite consist principally of oxides of iron and copper, together with a certain amount of copper sulphate, very little iron sulphate, and some undecomposed sulphides. The actual form in which the sulphur is present at the end of the roasting operation is not usually of very special importance in practice, especially where the previous experience with the roasted material determines the extent to which the roasting is conducted, since the greater part of the sulphur eventually produces the sulphide and constitutes the matte, on smelting the roasted charge; although some is also eliminated as SO_{2} by interaction with oxides. In modern roasting practice, therefore, all that is usually required is to roast the ore down to, say, 5 per cent., 6 per cent., 8 per cent., or whatever proportion of sulphur is necessary to yield the required grade of converter-matte in the reverberatories, as judged by previous experience of the furnace plant and working. Much SO_{2} is evolved during the roasting, though it is usually largely diluted with nitrogen from the air used up. _Other Foreign Constituents of Copper Ores—Zinc Sulphide._—ZnS is sometimes present. Some remains unchanged on roasting, as the heat in ordinary practice is not great enough to thoroughly decompose it. Some oxide and some sulphate are also produced. 2ZnS + 7,O ➡ ZnO + ZnSO_{4} + SO_{2} is suggested by Peters as a probable reaction occurring to this material under roasting conditions. _Lead Sulphide_ is also occasionally present with copper ores. It melts readily, and is not entirely decomposed at the temperatures employed for the roasting of copper ores. The reactions on oxidation are largely analogous to those for other sulphides. ┌─ │ PbS + O_{2} ➡ Pb + SO_{2} ─┤ │ Pb + O ➡ PbO └─ or, PbS + 3.O ➡ PbO + SO_{2}. Also, 2PbO + SO_{3} ➡ PbSO_{4}.PbO (basic sulphate). _Arsenides_ are partly left as the corresponding oxides, whilst some As_{4}O_{6} is evolved, and some basic arsenate generally remains. Roasting Practice. _Favourable Conditions for Successful Roasting._ (_a_) The sulphide should be in a finely divided form, so as to ensure good contact with the air. (_b_) The air should be supplied in a gentle current, so as to continually provide fresh oxygen, and sweep away the inert gases which are produced. (_c_) The ore should be heated to a dull red heat, which is a condition favourable for commencing the ignition and reactions. The temperature should, of course, be well below a melting heat (_Peters_).