Glazing is the application of transparent layers of color over dry paint. There are many ways to do this and I'm sure that some artists would say that my way isn't right, but it does work. Simply stated a paint color is diluted with a medium and brushed over dry paint to add, enhance or change the color. The old masters painted a gray underpainting, called a grisaille, and added many layers of color glaze on top. The way light travels through all of these layers results in a luminosity or glow that cannot be duplicated with directly applied paint. Of course this greatly slowed the painting process because each layer must dry before another can be applied. We now have mediums with alkyd resins that dramatically decrease the drying time. Even so, glazing multiple layers on top of an underpainting is not as fast as painting alla prima.
I never do the above, but I have glazed some of my finished alla prima paintings to make improvements or corrections. The entire painting can be glazed or just a part. I either use Galkyd lite, a product made by Gamblin, diluted with a little clean Gamsol, or Liquin made by Winsor and Newton, again thinning with a little mineral spirits. These are added to the color I have mixed to make it transparent and the glaze is applied with a brush dabbing a bit with a towel if necessary. If I decide I don't like the result I can remove it right away with mineral spirits and try again or forget the whole thing.
Here is a painting from a recent workshop. The exercise was to use only 3 colors, in this case asphaltum*, yellow ochre and white. (I know, white isn't really a color, but you get it.) The object of the exercise was to get the values right. This is sort of like a grisaille, but the paint is thicker and is not just black or brown mixed with white. It is a tonal alla prima painting.
This is an architectural detail of a house in the Caribbean that my husband and I love and I wanted to keep it. The real house is pink so this little study didn't do the colorful building justice. I decided to glaze it later with several layers of pink and orange to show the warm glow of the end of day sunlight.
Though it may not be obvious in the photo, the painting is luminous and obviously more colorful. I used some thin darker glaze to vary the shadows cast by the palms. I also made a glaze using Naples yellow light to brighten and warm the lightest areas.
Here is an example where I used glazing to improve parts of a painting.
I glazed a warmer blue (horizon blue) on the lower sky to brighten it. This is difficult to see in the photos, but I glazed violet (ultramarine blue mixed with cad. red and a bit of white) on the shadow sides of the tree trunks and the shadow areas of the foreground. I warmed the weeds in the foreground with a light yellow glaze. I like it better with more lavender, which was already present in the upper portion of the sky.
And last is a painting that I glazed entirely with Indian yellow. I decided that the overall temperature was too cool and I wanted more color harmony.
I especially like what the glaze did to the asphalt and the car.
I hope this gives you some ideas. It is easy to experiment and then wipe if things aren't working out. Save your tonal studies so you can practice.
Thanks for reading!
*asphaltum is a transparent brownish-black made by Gamblin. We were supposed to use burnt umber in the exercise but I forgot to bring some so used this instead.