What is flashed glass in stained-glass making?

Flashed glass is glass with a thin colored layer fused over a differently colored body.

Glassmakers create it by gathering a layer of one molten glass color over a gather of another color and then blowing or shaping the combined material. The resulting sheet may have a thin red, blue, or other colored surface over clear or differently colored glass.

Because the outer layer is thin, an artist can remove part of it by engraving, abrading, or etching. This technique reveals the contrasting layer underneath and permits detailed two-tone decoration. Medieval and later stained-glass artists used flashed glass for controlled highlights, lettering, heraldic details, and other visual effects.

Flashed glass is not the same as surface paint or enamel. Its color is part of the glass structure, although only a thin layer carries the contrasting color. It is also distinct from ordinary pot-metal glass, which is colored throughout its thickness.