A glaze is a thin, semi-transparent layer of paint. Glazing is a popular technique in oil painting, where glazes are applied on top of an opaque layer of paint which has been allowed to dry.
The general practice is to create a monochrome under-painting using opaque colors and then gradually building up glazes on top, allowing each layer to dry in-between. This practice is obviously very time consuming due to the slow drying times of oil paint. Depending on how many glazes are used, the time is takes to create an oil painting using this method could be anywhere from a few weeks to years.
When glazing, it is best to use paints which have more of a translucent quality. White for example is not great for glazing as it is very opaque.
When there are multiple layers of glazes, the colors optically blend as if they were all combined, without the pigments actually mixing. This creates interesting effects which are very effective for portrait painting.
As there are many layers of paint, care needs to be taken in relation to the fat over lean rule. Each subsequent layer of paint should have more oil then the prior layer to avoid cracking in the paint. This is because whilst oil paint may be dry to the touch, it may not be completely dry.
Many painters use both thin glazes and thick, impasto brushwork to create variance in the painting. The impasto areas would appear pushed forward in the painting compared to the softer glazes.