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. The following characteristics of each group may prove useful in the study of their chief members:

Group I.: Elements

All the black pigments in ordinary use consist of or contain the element carbon, and are not subject to change: graphite is a form of carbon and is unalterable and inert. Gold, if pure or nearly free from alloy, is not liable to chemical change, but silver readily tarnishes by combining with sulphur; drawings in silver-point are frequently found to have altered in hue from this cause. The best metal-point for fine drawing is pure platinum.

Group II.: Oxides

These have generally been prepared at a high temperature, and are not easily amenable to chemical or physical change: they are, moreover, not liable to affect other pigments, being practically inert.

Group III.: Sulphides

Some of these may give up sulphur to the metallic bases of other pigments. Thus cadmium yellow blackens emerald green, producing copper sulphide. One of these pigments, vermilion, is prone to a molecular change, whereby the red crystalline form passes, without chemical alteration, into the black amorphous variety. The members of this group sometimes contain free sulphur, or injurious sulphur compounds.

Group IV.: Hydrates

The water present in these compounds exists in two states, essential and hygroscopic. Sometimes a part of the former may be lost, and a change of hue occur in consequence, but the alteration is rare, save in the pigments which owe their colour to the presence of copper hydrate. In the case of raw umber, the water present acts rather in aiding the oxygen of the air, under the influence of sunlight, to oxidize some of the peaty or bituminous matter sometimes present in this pigment.

Group V.: Carbonates

Three out of the four carbonates included in this group are liable to suffer change on account of the metal they contain (lead or copper) combining with sulphur, and so forming a brown or black sulphide.

Group VI.: Silicates

These are generally inert bodies little prone to suffer or cause change. Some of the ochreous earths contain silicates of iron, manganese, and alumina, as well as the hydrates of the two former metals, and so might be placed in this group.

Group VII.: Chromates

This group consists of compounds rich in oxygen. When in contact with some of the more alterable organic pigments belonging to Group VIII. the chromates occasionally lose oxygen. On this account they show a tendency to acquire a greenish or greyish hue, the result of the reduction of the red oxide of chromium to the green oxide. This change is accompanied by a corresponding injury to the colour of the organic pigment which has been involved in the reaction.

Group VIII.: Various Inorganic Salts

A number of miscellaneous compounds having no chemical relationship have been thrown into this group. One, a sulphate, is insoluble and inalterable; another, potassium cobalti-nitrite, is generally inert, but, owing to its nitrous constituent, acts injuriously upon some organic pigments, notably on indigo. The members of the group which contain lead or copper may darken in the presence of sulphuretted hydrogen and of some other sulphur compounds.

Group IX.: Organic Compounds

This group includes many more pigments than any other: not one of its members possesses the permanency belonging to the majority of the mineral pigments, while some are so fugitive that they may even be used for producing a photographic picture by being exposed to sunlight under a negative. This fading is generally due to the combined action of water and oxygen : in oily and resinous media it is lessened, retarded, or even prevented by the hydrofuge character of these vehicles.

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 present 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.