A glaze is a thin, transparent film of oil paint applied over a layer that has already dried. Light travels through it, bounces off what lies underneath and returns to the eye through the colour. This is why a hue built up with glazes glows differently from the same hue mixed beforehand on the palette.
Yellow mixed with blue on the palette produces one finished green. Yellow glazed over dried blue produces a green in which both pigments remain separate and keep working on their own. The difference is clearest in the shadows: mixed, they turn matt and heavy; glazed, they stay deep and see-through.
Each new layer has to hold more binder than the one below it, otherwise it dries faster than its support and cracks. In practice this gives a simple order of work:
The layer has to be dry all the way through, not merely dry to the touch. Depending on the pigment and the thickness that means anything from a few days to a fortnight. It is the most inconvenient feature of the technique and the reason a glazed painting takes weeks. Running several canvases side by side is what works for me.
Glazing deepens a shadow, pulls a scattered palette together, darkens an area without losing the drawing beneath it, or shifts the whole picture towards a warmer or cooler key at the very end. It is no use for lightening, though: white stops being transparent and the layer turns milky.
Oil paintings with meadow and plant motifs are in the paintings gallery.