It has already been stated, in dealing with the yellow ochres, that these colours can be toned by burning, part of the ferric hydroxide losing its water and changing into red ferric oxide. The more severe the burning, the larger the amount of ferric oxide formed and the nearer the colour of the product approximates to red. According, however, as the original ochre was yellow or brown, the tone of the burnt colour will lie between orange and brownish red. If the heating be pushed so far as to transform all the ferric hydroxide into oxide, the red will come more and more into prominence in proportion to the amount of hydroxide in the original material. If the product consists entirely of ferric oxide, as is the case with that obtained, as a by-product, in the manufacture of English sulphuric acid, a pure red ferric oxide (caput mortuum, colcothar, English red, etc.) will be obtained. If the heating be increased above a certain point, the pure ferric oxide will change colour, assuming a brown to violet tone according to the temperature employed. (_a_) _Burning in the Muffle_ Since, as a rule, the quantity of material treated in the preparation of these brown, violet to black ferric oxide pigments for the purposes of the painter on porcelain is not large, the same kind of muffle furnace (Fig. 29) as serves for making enamels can be used. The fire-clay muffle _M_ is inserted in a reverberatory furnace _O_, with a good draught, and is raised to a white heat. The finely powdered material to be burned is spread out evenly on plates of sheet-iron or fire-clay, and introduced into the white-hot muffle, where it is left for a period corresponding to the colour desired. To save time, the plates may be pre-heated in a second muffle arranged above the first. [Illustration: FIG. 29.] By this means a large range of tones can be obtained from one and the same material, by heating it to different temperatures; and the colours so produced are distinguished, not only by their warmth of tone, but also by very high stability. In fact, they may be regarded as permanent, because very strongly calcined ferric oxide only passes very slowly into solution even under prolonged boiling in the strongest acids. Owing to this excellent property, which is equalled by very few other pigments, and the low cost of preparation, these colours deserve the most careful consideration by all manufacturers who are in a position to obtain suitable material in sufficient quantities. (_b_) _Caput Mortuum, Colcothar_ Previous to the English method of making sulphuric acid by the oxidation of sulphur dioxide with nitric acid, this acid was manufactured by heating dehydrated ferrous sulphate (green vitriol); and even now, fuming sulphuric acid--oil of vitriol, or Nordhausen sulphuric acid--is largely obtained by the same process.