Traditionally, glass painting referred to painting on the surface of a sheet of glass to be included in a stained glass work. This kind of painting, which is actually closer to drawing than painting, was done to add details such as faces and folds of clothing that couldn't be added with traditional lead lines. It was also used to cover up portions of stained glass works so that light was kept from shining through.

In most cases, the glass paints used for stained glass painting are predominately browns and gray-blacks. The colors tend to be water or gum arabic based, and can be applied with a brush in a method similar to the way watercolors are applied. In most cases, these paints are fired onto the glass using a kiln. The heat of the kiln causes them to bond permanently with the glass.

There are several major types of traditional stained glass paints, including vinegar trace paint, matt paint, silver stain, and oil based paints.

• Vinegar trace paint

This paint, which is dark and completely blocks out the light in the areas where it is applied, is most often used for figure or design lines. It is fairly thick and must be mixed with water, vinegar, and gum arabic to use. Gum arabic, which helps the paint stick to the glass, is usually purchased in powder form and must be mixed with water or alcohol before using.

Vinegar trace paint must be applied "wet on wet"; that is, both the brush and the glass surface must be wet. You can't apply more paint to a particular place once it dries; if you do, the paint is likely to flake when fired in the kiln.

Painting with vinegar trace paint requires practice. The hardest part is learning to apply just the right amount of paint. Too much on the brush and it will blot, too little and it will dry before the stroke is complete.

When dry, vinegar trace paint is often scraped or scratched with a small stick or quill. This gives the paint a texture and depth that can't be gotten from the paint alone. Once prepared, the paint is fired to around 1100 degrees F. It becomes shiny after firing.

• Matt paint

Matt paint, which uses a base of either water and gum arabic or water and vinegar, is easier to apply than vinegar trace paint. It can be applied thickly or thinly and can even be "blended" and stippled or worked with a second brush to give it an interesting texture. Some artists even rub it with their fingers to achieve more unusual effects.

Because it is more transparent than vinegar trace paint, matte paint is generally applied over tracing paint. Often, two firings are required, one for the tracing paint and a second for the matt paint.

Matt paint is most frequently used for filling in backgrounds and adding shadows. As with vinegar trace paints, the color selection is somewhat limited, consisting primarily of blacks, brown, blues, and greens.