The methods of preparation of oxide reds vary slightly in detail according to the material from which they are made, but the general features of the processes are almost identical and eminently simple. The principal sources are impure native oxides of iron, such as the ochres, various waste liquors containing iron salts in solution, and copperas (protosulphate of iron). (1) Native oxides. The iron present in the ochres and similar native earths exists in the form of hydrated oxide, and has a brown red colour. For many purposes this hue is satisfactory, and the preparation of such a pigment consists simply in grinding the mineral in a wet mill, subjecting it to levigation till all grit is removed, and drying. In order to obtain a brighter red from the native oxides they must be calcined to effect dehydration. This can be accomplished in the most rudimentary forms of furnace, and many kinds are in use. The colour produced depends on the degree and duration of the heat to which the material is exposed, the shade becoming deeper as the roasting is prolonged or the temperature increased. As no two samples of ochre are just alike it is impossible to fix a precise time for the length of the operation, and therefore it is necessary to repeatedly draw samples in order to judge of the progress of the dehydration and development of the colour desired. When the requisite shade is attained, the charge is drawn and allowed to cool. (2) Waste Products. The pyrites cinders from sulphuric acid works afford an abundance of oxide of iron. When the pyrites has contained no copper, the cinders merely require grinding and levigating, the iron being present as oxide. But when the pyriteshas been treated for the recovery of the copper, by a second roasting with salt, the liquors contain the iron as chloride and sulphate, and lime has to be added to precipitate the oxide. This last is dried and calcined in the same manner as the native oxides, and grinding and levigation can be dispensed with. The liquors from galvanising works contain acid sulphate of iron (green copperas) in solution. To correct the acidity, more iron is added in the form of scrap. Then lime or other alkaline substance is introduced to throw down the iron as oxide, and this last is filtered out, dried, and calcined in the usual way.