~Ores containing much Sulphides.~--Ores of this class may be easily recognized, either by the appearance of the minerals they contain or by the odour of sulphurous oxide (SO_{2}) which they evolve when roasted on a spatula. The sulphides most commonly present, in addition to the sulphurized minerals of silver, are pyrites, galena, blende, and mispickel. When they are present in only a moderate amount, their effect is simply to increase the weight of the button of lead; and this is easily counteracted by reducing the amount of flour, or by omitting it. When in larger amounts, they not only yield large buttons, but also render the metal sulphury, sometimes even giving a button of regulus instead of lead. This last evil may be remedied (1) by putting in a rod of iron as soon as the charge has fused, or (2) it may be counteracted by a proper addition of nitre, or (3) when the sulphides present are only those of iron or copper the sulphur may be removed by calcining, and the ore converted into one of the class containing metallic oxides. The calcination is effected as follows:--Weigh up 20 grams of the powdered ore and place it in a wide-mouthed crucible sufficiently large to perform the subsequent melting down in. The roasting must be done at a gentle heat at first, so as to avoid clotting: the mouth of the crucible should project considerably above the coke, and should slope forward towards the worker. The charge must be occasionally stirred with the stirrer (fig. 10) so as to expose fresh surfaces to the action of the air, and to prevent adhesion to the sides of the crucible. The stirrer should not be removed till the calcination is finished. The temperature should be raised at the end to a good red heat; and (to ensure the decomposition of any sulphate that may be formed) the roasted ore should be rubbed up in a mortar with a pinch of anthracite, and again calcined. It is then mixed with fluxes as described, and fused in the same crucible.