Flashed glass
Flashed glass is clear or lightly coloured glass with a thin skin of strongly coloured glass on one face. The skin is so thin that it can be removed locally, by acid etching, sandblasting or abrasion, to reveal the lighter glass beneath. That makes it possible to put patterns, lettering, heraldry or highlights into a single piece of glass without lead lines.
Ruby red was the colour that most needed flashing. Solid red glass was dense and expensive, so a thin layer on clear glass produced a brighter red at lower cost. Blues, greens and yellows are flashed in the same way.
The method goes back to the medieval period and was widely used in the nineteenth century, when decorative acid-etched flashed glass was popular for windows and doors.