A glaze is a thin, transparent layer of color brushed over a layer that has already dried. Light passes through it, reflects off the paint underneath, and comes back out through the glaze on the way to your eye. The color you see is made in the light rather than on the palette, and it does not look like the same two colors stirred together.
That is the entire idea. Everything else is technique. Here is how to do it in acrylic, how it differs in oil, and the handful of mistakes that turn a glaze cloudy.
Why glazing looks different from mixing
When you mix two colors on a palette, the pigments sit side by side and the result is duller than either one started out. Some light is absorbed by every particle it meets, and the more particles you add, the more you lose. That is why heavily mixed color goes gray.
A glaze keeps the colors in separate layers. The light travels down through the transparent one, bounces off the opaque one beneath, and travels back up. Nothing is muddied on the way, so the color arrives with a depth and a slight inner glow that mixed paint cannot reproduce. Painters call this optical mixing.
It is an old technique. Before modern pigments, certain colors were reachable only by laying one transparent color over a different one beneath, and the habit stuck because the result is genuinely better than the mixed version.
The layer underneath matters most
A glaze only works if there is something opaque under it to reflect the light back. Glaze over bare canvas and there is nothing to bounce off, so you get a thin stain rather than a glow.
Light underlayers give the brightest results. A glaze over white or off-white will sing. The same glaze over a dark area will barely register, because there is not much light coming back through it. If you want a passage to have depth, get it light and opaque first, then glaze down into it.
Which colors actually glaze
This is where most attempts fail. Transparency is a property of the pigment, not something you create by adding medium. Thinning an opaque paint does not make it transparent; it makes it a thin, milky, opaque paint.
Colors that glaze well are the naturally transparent ones — the quinacridones, the phthalos, dioxazine violet, the transparent earth colors. Most tubes state transparency on the label, and many have a small printed square that is solid for opaque, hollow for transparent.
Colors to keep out of a glaze: titanium white above all, the cadmiums, cerulean blue, and most opaque oxides. White in a glaze is the single most common mistake in this technique. It does not lighten the passage, it fogs it.
Glazing in acrylic
Use an acrylic glazing medium. It is sold as glazing liquid or glazing medium, in gloss and satin, and it is made for exactly this: it carries a small amount of pigment, stays transparent, extends the working time a little, and keeps enough binder in the mix for the layer to hold together.
Two things people are told to use instead, both wrong:
- Linseed oil is not an acrylic medium. It is an oil-painting medium. It will not combine with waterborne acrylic and has no place in an acrylic glaze.
- Flow improver is not a glazing medium. It is a surfactant that reduces surface tension, it contains no binder at all, and manufacturers say to use it sparingly and properly diluted. Loading a glaze with it leaves pigment with too little binder to form a sound film.
Water on its own is acceptable in small quantities in the early layers, where the ground is still absorbent enough to take it. For the late glazes that sit on top of a sealed, glossy surface, use medium rather than water.
The method
- Make sure the layer underneath is completely dry. Acrylic is touch-dry in minutes and fully cured much later; for glazing, an hour is a sensible minimum and overnight is safer on thick paint.
- Put a quantity of glazing medium in a container and add a very small amount of transparent color — far less than looks right. You can always add another layer; you cannot take one off.
- Stir thoroughly, then test it on scrap painted the same color as the area you are about to glaze. This step takes two minutes and saves paintings.
- Brush it on with a wide, soft brush in long even strokes, going in one direction. Do not scrub. Scrubbing drags the layer, leaves brush marks in the film and can pull at paint that is not fully cured.
- Let it dry completely before the next one. Two to four glazes is typical. Past that, the layers start cancelling each other out and the passage goes flat.
Acrylic dries too fast to rework a glaze once it is down. Decide before the brush touches the canvas, and correct by glazing again rather than by fiddling.
Glazing in oil
The principle is identical, the handling is not.
Oil glazes are thinned with an oil painting medium, and an alkyd medium will speed the drying between layers considerably. Work over a layer that is dry to the touch, and observe fat over lean: each successive layer should contain more oil than the one below it. A lean layer over a fat one dries at the wrong rate and will eventually crack.
The advantage oil gives you is time. A glaze stays open for hours, so you can move it, feather the edges, and lift paint back out with a brush or a soft rag to pull a highlight through. That kind of subtraction is a real part of oil glazing and it is simply not available in acrylic.
Where it goes wrong
- Too much pigment in the mix. The commonest fault. It stops being a glaze and becomes a thin coat of paint, and the layer beneath disappears.
- An opaque color, especially white. Cloudy every time.
- Glazing over a layer that is not dry. The two blend and you lose the separation the whole technique depends on.
- Too many layers. Every glaze shifts the color a little. Enough of them and everything drifts towards the same brown.
- Glazing the whole canvas. You do not have to. Glaze the passages that need depth and leave the rest alone.
Glazing a finished paint by numbers canvas
This is one of the few genuinely useful things you can do to a completed numbered canvas, and hardly anyone knows about it.
Paint by numbers gives you flat areas of solid color with visible boundaries between them. A thin glaze brushed across a group of neighboring sections ties them together, softens the edges between them, and adds the depth that flat color lacks. A warm glaze over a sky, or a deep transparent tone across the shadowed side of a scene, can pull a finished canvas together in twenty minutes.
Wait until the canvas is fully dry, mix it weak, and test on the edge first. If you want to go further with layered color, our guide to layering acrylic paint covers building up tone in stages, and the color wheel is useful for working out which glaze color will warm or cool a passage rather than gray it out.
If you are looking for a canvas to try it on, the kit collection is the place to start.
