The truth is the basics of painting glazes is easy to understand, though it is a painting technique that does require some patience because each layer of paint must be completely dry before a new glaze is applied and some knowledge of the colors you’re using in order to ‘predict’ the colors glazing will produce. As a result, beginners (and not-such-beginners) often don’t discover the fabulous results glazing can bring for far too long.
Glazing is the term used for a thin, transparent layer of paint, particularly in oil painting and acrylics. Glazes are used on top of one another to build up depth and modify colors in a painting. It must be completely dry before another is applied on top, so the colors don't physically mix.
In watercolor painting, a glaze is often called a wash. A glaze done with an opaque pigment is called a velatura.
Each glaze tints or modifies the color of the paint beneath it. When you look at a painting, the color is mixed optically giving a deep, rich color. For example, painting a glaze of red over blue gives a richer purple than you’d get if you mixed the red and blue paint together on your palette before you applied it. To rather over-simplify the science, the purple you’re seeing is created by light bouncing back from the canvas, through the blue and then the red layer, into your eye, producing a deeper color than if it’d just bounced back from the surface of one layer of mixed paint.
No, there’s no painting rule that says you must paint using glazes. But it’s a painting technique that shouldn’t be rejected without spending some time learning the basics and giving it a go, as the results can be spectacular. (The terms ‘glowing’ and ‘luminous’ are commonly used to describe the effect.)
A single glaze is a single layer of color. How many layers you glaze, depends on the results you’re after and comes with practice. A glaze works best when each color you use is made from only one pigment, not a mixture of two or more. The more pigments or colors you use, the sooner you’ll end up with a brown and gray (or tertiary colors).
Using paint colors that contain a single pigment rather than a combination of pigments also makes it easier to learn/predict the result of glazing with that particular color, helps retain color saturation, and reduces the risk of inadvertently creating dull or muddy colors. The paint tube label should tell you what pigments are in a particular color.
It depends on what the final color is you’re trying to produce. If for instance, you’re glazing a red over a blue to produce a purple, additional glazes of the red will make the purple deeper, richer, and redder. You glaze as many times as is necessary to get the color you want.
Again, there’s no hard-and-fast rule. It’s the result that counts.
Paint pigments or colors are classified as transparent, semi-transparent, or opaque. Some colors are so transparent that used thinly they barely showing on top of another color. Others are extremely opaque, totally obscuring what’s beneath when used straight from the tube. Glazes work best with transparent pigments. If you’re not sure whether a color is opaque or transparent and the paint tube label doesn’t tell you, you can do a simple paint opacity test.
You can use opaque colors for glazing – the results just aren’t the same as with transparent colors, producing a misty effect that’s ideal for painting fog for instance. Try glazing with all the colors in your palette and get to know their characteristics and the results they produce. Paint up a sample glaze chart, recording what colors you used, so you have a record you can refer to.
Glazing is about putting down thin layers of paint, so the paint should be fluid (thin) or you need to ensure that you spread it thinly when you paint. You can buy glazing mediums for both oil paint and acrylic. (If you add too much water to acrylic paint you run the risk of the paint losing its adhesive qualities; see this Acrylic Painting FAQ.) A common ‘recipe’ among oil painters is to mix 50:50 turpentine and oil. Some bought oil painting mediums (such as Liquin) will help speed up the drying time of oil paint.
You can glaze with any brush, but if you’re new to glazing, start with a soft brush which makes it easier to paint smooth glazes, without visible brush marks.
Just like some artists don’t like mixed media, some don’t like mixing techniques such as impasto and glazing. It’s up to you whether you like the result the combination gives you. You don’t need to glaze across the whole painting either; you can just do it in part of a painting.
Smoother surfaces reflect more light, so hardboard painted white is ideal. But that’s not to say you can’t paint glazes on other grounds, such as canvas.
If you’ve tried glazing and don’t get good results, check that you’re not glazing over a layer of paint that hasn’t completely dried. Also check whether you are using transparent, single-pigment colors. Then try again. I recommend starting with a blue and a yellow, glazing to make various shades of green.