The treatment may be stopped when the solution acquires a slight brownish colour. When the soot has been purified so far it has lost its brown shade and now appears as a velvety black powder, very soft and very light, and distinguished by extraordinary covering power. It is not so easy as it appears at first sight to judge of the quality of a black pigment by its appearance. To the unaccustomed eye a pigment may appear to be of an unexceptionable black, which to the expert appears decidedly brown; only long practice can give the eye the requisite keenness. In addition to the test already given—of smearing on white paper—there is another especially to be recommended to the inexperienced for discriminating between pigments. A small quantity of the black under examination is intimately mixed with a white pigment; white lead or zinc white is very suitable. If the mixture has a pure grey shade, the black may be regarded as of good quality, but if it contains brown substances the mixture has an indefinite, dirty shade instead of a pure grey. This is a sure sign that the black requires further purification. Although caustic soda is now very cheap, its use for the purification of soot black is tolerably costly, because it entails much labour. This process is therefore only used for the finest qualities which are to be employed for copper-plate inks and black coach colours. For inferior qualities the method of calcination is used, which produces, when properly carried out, a black of such purity that it can be used for the preparation of even the finest black paints. _Calcination of the Soot._—The substances which give the soot its brown colour are products of dry distillation, and hence are all volatile at a certain temperature; they can be separated from the soot by heating it in the absence of air. The temperature necessary completely to volatilise these compounds is fairly high. The soot must be heated to a good red heat to obtain a safe result. When the soot is heated too quickly or to too high a temperature it undergoes an alteration which affects the quality of the product. By too long or too vigorous ignition the soot changes its flocky consistency to a sandy nature; it will then require much longer grinding with oil to produce a uniform mixture than is the case with the light flocculent soot, which very readily mixes with oil.