One of the more interesting things about this style of painting (which
was popular in the 19th C) is that the color is only vibrant and rich
when viewed by the light reflected off the foil. In the examples I've
seen, the reflecting surface is more directly in contact with the
painted glass.
If you remove the glass from the frame and view the color by transmitted
light, the colors look pale and washed out. Apparently, the color is
enriched by the fact that light must pass through it, bounce off the
reflector, and pass though it again before your eye sees it. This is
analogous to the difference between viewing an image directly on film
(transparency) versus one on a print.
I should think that gum emulsions would be well suited to this process,
assuming that you could get it to adhere to the glass.
Carson Graves
carson@ileaf.com