The Painter in oil

If you want to make a picture, or a part of it, brighter and lighter, bring it up in pitch
with body color first, with solid painting, and then glaze it.
Do not glaze on color which is not well dried. The drying of the under color and the
drying of the glaze are apt to be different in point of time, and the picture will crack.
If they vehicle is the same as was used in the under-painting and the drying qualities
of both paintings are the same, there is no danger. But when color dries, it shrinks and
flattens, and two kinds of colors shrinking differently are sure to pull apart, and that
causes cracking. If the under-painting is well dry, but not hard and glossy on the surface,
and is capable of still absorbing enough of the new color’s vehicle to bind the coats
together, your glaze will stand. But rather than have it too soft, have the under-painting
too hard, and then before you glaze over it with a little thin, quick drying varnish, and
glaze into that. The varnish will hold the two coats of paint together.
Glazing, as well as scumbling, implies the obligation to varnish your picture.
Whenever you use oil freely you will have to varnish your picture to keep it bright and
fresh in color.
It would be wise never to use a glaze as a final process. Glaze to get the tone or to
modify it, but paint into the glaze with body and color, and you keep the advantage of the
glaze without many of the disadvantages of it, and the picture has a more solid effect of
painting.
Frottées. - Closely akin to the glaze in manner, but very different in use, is the frottée
or “rubbing.” This is generally used on the fresh surface of the canvas, to “rub in” the
light and shade or the first coloring of the picture after the drawing is done. It is one of
the safest in wisest ways of beginning your picture. You can either rub in the picture with
a frottée of one color, as sienna or umber, or you can use all the colors in their proper
places, only using very little vehicle, and making something very thin in tint, somewhat
between a glaze and a scumble. You can make a complete drawing in monochrome in
this way, or you can lay in all the ground colors of the picture till it has much the effect of
a complete painting. Then, as you paint and carry the picture forward, every color you
put on will be surrounded with approximately the true relations, instead of being
contrasted by a glare of white canvas.
A frottée is a most sympathetic ground to paint over.