In my book, I give definitions of scumbling and glazing, but it is not published yet.
Briefly, in simple terms:
A glaze is a transparent passage of darker value than what it is applied over. A color shift occurs when light from below shows through a transparent layer (glaze), producing a "warmer" sensation. This effect is most pronounced when the glazing colors are composed of paints whose pigments are transparent by nature, rather than opaque pigments thinned with transparent medium.
A scumble is a lighter passage composed of opaque paints applied thinly enough and/or thinned with medium as to be somewhat translucent, over a relatively darker underlayer. A color shift occurs here as well, though with a scumble the shift is toward blue.
Neither of these optical effects can be adequately described by the Munsell system. There are subtleties such as undertones that the Munsell system (used in the Frank Reilly method) does not cover.
I agree that there is too much confusion over terminology, and it would be well if it could be cleared up. Many painters use the term, "scumble" to mean an irregular scrub-on of paint in a sort of drybrush manner, without regard to whether it is darker or lighter than what it is applied over. A scumble can indeed be done this way, but it does not need to be.
I'm holding off submitting my book proposal to the next publisher until April, when my friend Mark Gottsegen will introduce me to his editor at Watson-Guptill. W-G is his publisher, and he has just signed a contract with them for a revised edition of his book, The Painter's Handbook. I'm hoping an introduction and endorsement from him will give me a leg up in getting my proposal accepted, so for the time being, I think it's best to wait. One of these days, my book will see print, and I hope it will help clear up a lot of the confusion, rather than add to it.
Virgil
