Vitreous paint
Vitreous paint is the paint used to draw and shade on stained glass. It is made from finely ground glass, a metal oxide for colour (usually iron or copper, giving brown or black) and a flux, and is mixed with water, gum arabic or oil to brush onto the glass. The panel is then fired in a kiln so that the paint fuses permanently to the surface.
Glass painters use it for outlines, for faces and hands, for folds in clothing and for the background patterns of grisaille. Techniques include tracing lines, matting a wash of paint over an area and then stippling or scratching highlights out of it with brushes, quills and needles.
Painted glass must be fired at a temperature high enough to fuse the paint but low enough that the glass does not slump, and fire is the reason that good painted glass survives for centuries.