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 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.
=Frottée.=--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 and 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.
Copying may well be spoken of here, as it is in a sense a kind of manipulation. It is a means of study to the student, and a useful, sometimes necessary process to the painter. In the transferring of the results of his sketches and studies to the final canvas, the painter must be able to copy, and to know all the conveniences of it. Before the painting begins on a picture, the main figures in it must be placed and drawn on the canvas with reference to the plan of it, and their relation to that plan. This calls for some method of exact reproduction of the facts stored in the artist's studies for that purpose. The process of copying is that method.
From the side of study, the copy gives the student the most practical means of understanding the intent and the expression of the painter whose work he wishes to know. There is no way of understanding the why and the how of technical expression so sure and complete as to study with the brush and paint, following the same method and processes as the master you copy, and trying to comprehend the meaning and the expression at the same time.