The Painter in oil

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 are 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 notable 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 in Transparent Colors.- Some colors, like the madders, have a jelly-like
consistency when mixed with oil, others, the earths among them, are dense than opaque.
We speak of them respectively as “transparent” and “solid” colors. These qualities, which
divided 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 in some
fugitive; and the same with 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 make 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
for 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.