Glazing and scumbling are two techniques used for applying paint to the painting. A glaze is a thin transparent and generally darker layer of color applied over a lighter and mostly dry color or under painting. Glazing will affect the appearance of the underlying color. Some very striking effects can be achieved, because the light is going to be traveling through the glaze and bouncing off the underlying layer of paint and out to the viewers eye.
Many of the old masters used this method of painting, sometimes applying several layers of different colored glaze. They would also apply carious colored glazes over a monochromatic under painting.
The glaze is generally made by thinning the paint color with a mixture of one part copal or damar resin, one part stand oil and about five parts solvent. Another mixture is one part copal medium and one part solvent. Since this medium consists mostly of resin and solvent with very little oil from the paint color, it will have a tendency to dry rather fast.
You do not always have to wait for the glaze to dry firmly before painting on. However, if you do not wait for the glaze to dry firmly, do not use a medium that contains a solvent. The solvent may have a tendency to lift off the glaze. Note: It is best to allow a week or two for the glaze to harden more permanently before painting on.
The painting below is an example of glazing and scumbling, whereas the clouds were scumbled into the still wet transparent sky. Notice the brightness of the sky since it was done in glazes.
The imprimatura is a glaze applied to a white or toned ground canvas and allowed to dry before painting on. Some very interesting effects can be produced with an imprimatura applied over a toned ground, depending on the colors used by both.
Here’s another means of using the glaze. If you have a painting that you are not completely happy with, instead of destroying it, or painting over it completely with primer, you might try reworking it by adding a glaze. The glaze will tend to unify the whole painting by giving it an overall uniform color.
The scumble is the application of a lighter color over or into a darker color. The scumble can be done over a dry surface, or into a wet surface. It all depends on the effect you are trying to achieve. You can apply a scumble with a heavily laden brush over a wet or dry surface for a highly defined contrast. A dry brush over a dry surface to indicate the grain of wood or the sparkling sun off water. You can even scrape a palette knife lightly over the surface, allowing some of the paint to break, thus allowing some of the lower paint surface to show through.
Another scumble I have used is to apply a thin semi transparent layer of white mixed with a touch of ultramarine blue and burnt umber, (somewhat like a glaze), to depict fog or mist in a scene.
You might consider the application of glazing and scumbling as to how it could possibly enhance your next painting.