When and how to use glazes was something I found quite hard to get my head around. I’m still experimenting - in the end that’s the only way to understand what works and what doesn’t. Here are a few pointers that helped me:
What it is:
A glaze is a very thin film of colour made by mixing a little transparent tube oil with an oily medium.
Transparent paints include ultramarine blue, sap green, viridian, lemon yellow, alizarin crimson (semi-transparent), dioxin purple and burnt sienna.
For my medium, I mix equal quantities of Galkyd (alkyd resin) and Gamsol (odourless mineral spirit). It's best made in small quantities and stored in an airtight container (pour the Gamsol onto the Galkyd, stir, but do not shake or you will introduce air).
I use this mix for the first glazing layers. For later layers I reduce the amount of solvent (Gamsol) in the mix. For a more glossy "top coat" I mix Stand Oil with Gamsol; or use Neo Megilp for hard, gem-like colours. Both are slow-drying mediums, allowing time to add fine detail to the painting. Gamblin offer helpful advice on finding your happy medium here: https://gamblincolors.com/choosing-painting-medium
What it does:
You lay a glaze over dried paint to deepen, brighten or otherwise change the underlying colour without disturbing the brushwork. It has to be thin enough to allow the colours beneath to show through.
You can apply multiple glaze layers to intensify the effect and create the illusion of depth; each one must be completely dry before applying the next one.
A single glaze colour can be used to unify the whole painting, bringing together unrelated colours by glazing in a common hue.
The simplest glaze is a single transparent colour brushed over an opaque underpainting.
A glaze will emphasise a rough texture.
How to apply:
Apply with a soft flat brush over the whole painting, or the part you wish to affect, then wipe down gently before it dries.
Your underpainting must be completely dry before you apply a glaze.
Glaze for temperature and for value:
Use glazes to warm up a too-cool painting or tone down an overly hot one.
Use glazes to deepen darks and lighten lights.
For greatest luminosity start with a light underpainting. A glaze colour over white will dazzle.
A dark glaze gives more value to a painting immediately.
Glazing reduces contrast so keep darks really dark to compensate.
A blue glaze will intensify shadows; a yellow glaze will lighten a dark picture.
Do not add white to glazes.
Simple glazes always appear more luminous than multiple ones.
Multiple glazes tend to make the artwork darker and dull down colours, not make them more colourful.
But, multiple thin glazes of a single colour will increase its intensity.
Glazing with opaque colours:
A glaze made with opaque colours is called a wash. These are translucent rather than transparent.
Use to create a cloudy or hazy effect. Or to make something recede (glazed areas will look closer to the viewer than unglazed or opaque areas).
To increase the misty effect, add a cold wax medium and/or chalk - this is where it pays to experiment!
Adding a cold wax medium will also impart a matte finish to glazes and increase luminosity.
If you’re going for a soft cloudy effect, you need to start with a highly coloured underpainting.
Don’t rush it - build up colours and tones slowly and allow to properly dry.