Enamelled glass or painted glass is glass which has been decorated with vitreous enamel (powdered glass, usually mixed with a binder) and then fired to fuse the glasses. It can produce brilliant and long-lasting colours, and be translucent or opaque. Unlike most methods of decorating glass, it allows painting using several colours, and along with glass engraving, has historically been the main technique used to create the full range of image types on glass.
All proper uses of the term "enamel" refer to glass made into some flexible form, put into place on an object in another material, and then melted by heat to fuse them with the object. It is called vitreous enamel or just "enamel" when used on metal surfaces, and "enamelled" overglaze decoration when on pottery, especially on porcelain. Here the supporting surface is glass. All three versions of the technique have been used to make brush-painted images, which on glass and pottery are the normal use of the technique.
Enamelled glass is only one of the techniques used in luxury glass, and at least until the Early Modern period it appears in each of the leading centres of this extravagant branch of the decorative arts, although it has tended to fall from fashion after two centuries or so. After a brief appearance in ancient Egypt, it was first made in any quantity in various Greco-Roman centres under the Roman Empire, then medieval Egypt and Syria, followed by medieval Venice, from where it spread across Europe, but especially to the Holy Roman Empire. After a decline from the mid-18th century, in the late 19th century it was revived in newer styles, led by French glassmakers. Enamel on metal remained a constant in goldsmithing and jewellery, and though enamelled glass seems to virtually disappear at some points, this perhaps helped the technique to revive quickly when a suitable environment arrived.
It has also been a technique used in stained glass windows, in most periods supplementary to other techniques, and has sometimes been used for portrait miniatures and other paintings on flat glass.