Does anyone know the best way to simulate the effect of a colour glaze on a painting? Say, with photoshop over a photo of the painting, or some other method?

Thanks,
Richard

You could attempt to use layers combined with the “multiply” setting. Notice, however, that the optical effect is not exactly the same. Real-life glazing has a “staining glass” / 3d effect that heavily depends on how physical layers trap light and make it bounce back to your eyes.

You could paint a piece of clear plastic and hold it over your painting. Or maybe some tinted glasses. Or maybe paint your glasses

I do something similar in GIMP. As Patrick already noted, you must NOT preserve luminosity. Other than reflection from the glossy surface of a glaze, the glaze darkens what’s already there.

Scumbling, where a lighter and opaque color is placed atop the painting, is harder to simulate in software.

Both Photoshop and GIMP, and some other programs, have a “paint brush” tool, which allows you to “paint” a partially transparent layer, just where you need it. If done as layer (rather than directly on the existing image) you can retroactively change the transparency.

Maybe try laying one of these over the subject? They may be a bit too strong in color sometimes, but should at least give you an idea of where you can go with a given glaze. Nice thing here is that you could cut them down into smaller sizes for making a quick reference ring that you just flip through while holding it over the subject. Just a thought, I do wish they came in paler colors though.