Permanency.—It should be stated what is meant by a permanent color. There is no color which is not to be influenced in some way. The most sound of pigments will change if the conditions favor the change. When we speak of a permanent color, we mean only one which under the usual conditions will stand for an indefinite time. By which is meant ordinary diffused daylight, not direct sunlight, and the ordinary air under normal conditions. If there be direct sunlight, you may expect your picture to change sooner or later. But one does not hang his pictures where the sun's rays will fall on them. If there is any exceptional condition of moisture in the air, the picture may suffer. Or if from any cause unusual gases are in the atmosphere, or if the picture be too long in a dark, close place, the picture may smother for lack of fresh air, just as any other thing, plant or animal, which depends on normal conditions of atmosphere would do.
Let us say, then, that what we mean by a permanent color is one which will stand unchanged for an indefinite length of time in a room which is of the usual condition of temperature and freedom from moisture, and where the light is diffused, and such that the direct rays of the sun are not on the picture often, or to any great extent. Cold will not hurt a picture if the canvas is not disturbed in that condition, but to bend or roll it while it is very cold will of course crack it, and sudden and extreme changes of temperature may have the same effect. In other words, some care must be used with all pictures as a matter of course.
COLOR LIST
Whites.—Zinc white is the only permanent white, but it lacks body and is little used. The lead whites, flake, silver, cremnitz, will darken in time, and will turn yellow with oil, and may change with or affect change in other pigments. The zinc white is liable to crack. We have no perfect white, so practically you may consider the lead whites as permanent enough, as other painters do.
Yellows.—Cadmium is permanent in all three of its forms. It is a color the permanence of which is of great importance; for its brilliancy is quite essential to modern painting, and if it were not permanent, the picture would soon lose the very quality for which the color was used. The chromes, which are of similar color-quality, are less permanent, and are almost sure to turn to a horny sort of yellow; and a green, which by their use was bright and sparkling, will, in a few months, lose its freshness—this cadmium will not do. Cadmium is also to be preferred to chrome, because it is of a much finer tonality. Greens and yellows made by the admixture of chrome are apt to be crude as compared with those in which cadmium was used.
Strontian yellow is a permanent and most useful light yellow, much to be preferred to all other citron yellows except the pale cadmium, and can be used in place of that if necessary. They are both expensive colors of about the same cost.
Naples yellow was a very prominent pigment with the older painters. It is still very much used, but in the simplification of your palette you may as well leave it out, as you can get the same qualities with cadmium and white. It is durable and safe, but adds another tube to your palette which you can well dispense with.
The ochres are among the oldest and safest of pigments. You can use them with any colors which are themselves permanent. There are several of them,—yellow ochre, Roman ochre, transparent gold ochre, and others. They are all native earths, and though they contain iron, they are sufficiently inert to be thoroughly sound colors.
The siennas, burnt and raw, are like the ochres, native earths, very old and permanent colors, and may be used anywhere.
The umbers are in the same class with the siennas and ochres. They should all rank among the yellows. The browns of umber and sienna will make greens with blues.