As the resident artist at Avalon Stained Glass School and Creativity Center in Munfordville, KY, I teach glass painting and I use 3 methods in my workshops.
1. When we are doing a piece of folk glass such as painting on a recycled window pane in a window frame I use acrylic paints. If we are just painting on a piece of glass that will be put in a frame we still use acrylic paint. However, you sort of need to paint in reverse as you will want to have your painting on the back side of the glass.
2. When we are doing a piece of glass that is going to be fired in the kiln then we use special paints for that. This paint is the kind that will be used in a high kiln firing.
3. When we paint on porcelain, ceramics or tiles then we again use a special paint that is even different from my answer in #2. However, both of these paints come in a powder or pigment form and they have to be mixed special in order to use them.
Come to one of my vacation retreat workshops and learn some more.