Alteration of Pigments

One of the chief lessons to be learnt from this classification is this, that the members of each class, as a general rule, exert no action upon one another. This is explained easily. The oxides of Group II., having already taken up the full complement of oxygen which they can acquire under ordinary conditions, are not likely to be oxidized by admixture with other oxides of similar character. In the same manner the sulphides of Group III. neither give sulphur to, nor receive it from the other sulphides, for all but one of them have been produced in the presence of excess of sulphur.

It should be noted that members of each group, though presenting one or more characters in common, often exhibit certain chemical and physical differences of deportment. Here is a list of the chief changes which they are capable of suffering, with illustrative examples :

The effect of pulverization upon pigments may be mentioned in this connexion. Generally, the more finely an alterable pigment is ground, the more susceptible does it become to chemical injury: its colour becomes at the same time paler, and may even change in hue as well as in tone. Continued grinding, beyond the degree necessary to develop the proper colour, improves some pigments, but injures others.

In the chemical classification of pigments which has been sketched in the previous chapter there comes out in rather striking relief one point to which no allusion has yet been made. It is indeed a point which touches the chemist rather than the artist, and yet it seems to merit a passing reference in this place. I refer to the number of gaps in the table - to the number of elements and of classes of salts which have no place therein. The scientist will not, indeed, expect to find amongst these 9 groups any substances possessed, to a marked degree, of the property of solubility in water, for he will recognise the unfitness of such bodies for use as paints. But he will probably observe, with some surprise, that there are no compounds of fluorine, chlorine, bromine, or iodine in our lists, but two sulphates, only one substance containing tungsten, and not one containing nickel, uranium, molybdenum, or vanadium. Coloured insoluble compounds indeed exist of all these elements; several of such compounds have been proposed and even employed as pigments, but instability or injurious action upon other pigments has banished these candidates from the palette of the artist.

A few words may be added here as to the distinction between substantive and adjective pigments. Of the former class aureolin, vermilion and malachite may be cited as examples; to the latter all the lakes belong. Substantive pigments are homogeneous, definite substances, simple or compound, but not separable into two bodies, having differing optical qualities. On the other hand, adjective pigments contain two substances, one coloured, and another generally colourless, through which the former is disseminated or over which it is extended as on a background.