Some colors are unaffected or little affected by light, and of course you will use them in preference to all others. The atmosphere affects the paint because of certain chemical elements contained in it, which tend to cause new combinations with the materials which are already in combination in the pigment. The action of the oxygen in the air is the chief agent in affecting the pigment, and it is here particularly that light, and especially sunlight, assists in decomposition. The air of towns and cities generally contains sulphuric and sulphurous acids and sulphuretted hydrogen. This latter gas is most effective in changing oil paintings, because of its action in turning white lead dark; and as white lead is the basis of many qualities in painting, this gas may have a very general action.
Moisture in the atmosphere is also a cause of change, but there is little to be dreaded from this, as the oil protects the colors.
Oil absorbs oxygen in drying, and so is apt to have an effect on colors liable to change from that element, and many vehicles contain materials to hasten the drying which further aid in the deterioration of the pigment. Bad oil will tend to crack the picture also. The greatest care should be used in this direction, as the most permanent colors may be ruined by bad vehicles.
Pigments will not have a deteriorating effect on each other as long as they are solid. But if one of them is soluble in the medium, then chemical action commences; but as most pigments are somewhat soluble, there is always some danger in mixing them. The best we can do is, as I said before, to try to have on the palette, as far as possible, only colors which are friendly to each other.
As a student you should not be much occupied, however, with all this. You must expect that all color will change somewhat. But you need not use those which change immediately or markedly, and you may use them in a way which will tend to make them change as little as may be. Colors have stood for years, and what is practical permanence, not perfect permanence, is all you need look for. If you think too much of the permanence of your colors, it will interfere with the directness of your study. Therefore, decide on a palette which is as complete and safe as you can make it, excluding the notably bad pigments, and think no more about it.
When you need to add a new color to your palette, choose it with reference to those already on it, and go ahead. This is what the whole subject resolves itself to, practically, for you as a student.
Opaque and Transparent Colors.—Some colors, like the madders, have a jelly-like consistency when mixed with oil, others, the earths among them, are dense and opaque. We speak of them respectively as "transparent" and "solid" colors. These qualities, which divide the paints into two classes, have no relation to their permanency. As far as that is concerned you use them in the same way, as some transparent colors are safe and some fugitive; and the same with the opaque colors.
The only difference is in the fact that, as a rule, the solid colors are better dryers. But you will notice that while you may mix these colors together as though this difference between them did not exist, in certain processes you use them differently. So you will see, farther on, that for a "glaze" you can use only the transparent or semi-opaque colors, for a scumble you naturally use the solid ones. You should know, however, for the sake of clearness, just what is meant when "solid" or "body" or "opaque" color is spoken of, and what is meant by "transparent" color.
Safe and Unsafe Colors.—Beyond what has been said of the causes of change in colors it is not necessary that you should know the chemical constituents of them. If you want to look into the matter further there are books, such as "Field's Chromatography," which treat fully of the subject, and which you may study.
But practically you should know which colors are to be depended on and which not. Let us consider the principal colors in detail then, merely as to their actual stability. I will speak of them in connection with the plates of colors at the end of this book. I would like you to compare what is said of each color with the corresponding color in the plates. Those colors in the plates which are not spoken of here, you may consider as useful in showing you the character of different colors which are made, but which may or may not be used, according as you may need them. I shall not attempt to mention all the pigments that are in the market. You need never use more than fifteen or twenty all told. Many painters use more, it is true; but if you know how to make the best use of that number, you may safely wait till you "grow to them" before you bother with more. And I shall speak only of those which you will find essential or most generally useful, and those which should be particularly avoided.