Hei, Greg !
You can start with a four color gum based on glass enamels. If you want to
laminate the image inside the glass, you will have to burn it after each
layer of emulsion, and still it is tricky, because many colors and the gum
itself will give you bubbles. Otherwise it is natural to try both ambrotype
(with some salts added) and calotype, but you will need to know a little
more about glass and its chemistry and physiology. Maybe it would be
easiest to order a full color enamel decal from a printing company ? Good
luck ! Regards, Galina Manikova. (galina@online.no)