When anhydrous ferrous sulphate, FeSO{4}, is exposed to a very high temperature--strong white heat--it is decomposed into sulphur trioxide, SO{3}, sulphur dioxide, SO{2}, and a residue, mainly composed of ferric oxide and a little basic ferric sulphate, which remains behind in the heating-pan. In fact, even at the highest possible temperatures obtainable in the furnaces used for the distillation of the green vitriol, it is impossible to recover the whole of the sulphuric acid, a small portion being tenaciously retained by the iron. This red residue is sold under various names--colcothar, caput mortuum, English red, Indian red, etc.--and is used as a low-grade pigment, and also as a polishing agent. The name caput mortuum is a survival from the time of the alchemists, and was probably applied to indicate a dead-burned product, from which all the active ingredients had been removed. Although, in former ages, this substance was held in low estimation as a pigment, attempts have been made in recent times to convert it, by suitable treatment, into a more valuable product; and these attempts have been crowned with success, affording another instance of how a high commercial value can be imparted to a waste product by proper manipulation. (_c_) _Calcining Ferric Oxide_ In order to obtain a series of tones of colcothar, it is subjected to repeated calcination, but not by itself, since it would require an extremely large quantity of fuel to effect any change of tone in view of the very high temperature the material has already been exposed to in the sulphuric acid plant. If, however, salt be added, then a variety of tones can be obtained without recourse to any particularly high temperature. It is frequently stated that the only effect of the presence of salt is to keep the calcining temperature uniform, inasmuch as the salt volatilises at a strong red heat, and when that temperature is reached, the whole mass cannot get any hotter until the whole of the salt has passed off, all the heat applied being consumed in transforming the salt into the state of vapour. As a rule, however, the amount of salt added does not exceed 6% of the weight of the charge to be calcined; and this quantity does not seem to be sufficient to keep the temperature at a uniform level through the several hours required for the calcining process. The author is therefore of opinion that the salt also has a chemical action on the material during the calcination. As already mentioned, colcothar is by no means pure ferric oxide, but always contains basic ferric sulphate. Now, it is feasible that some reaction may take place between the basic sulphate and the sodium chloride at calcination temperature, with the formation of caustic soda, which, being a far more powerful base than ferric oxide, deprives the latter of sulphuric acid, sodium sulphate being formed. The chlorine of the salt combines with the iron to form ferric chloride, which volatilises at a glowing heat.