Sulphides are small ornamental objects made of a glass-clay paste and encased in glass. The term "sulphide" (or, in French, sulfure) is improper, since no sulfur is involved in the making of these objects. The name probably resulted from confusing these glass items with plaques made of sulfur and wax, which were popular during the mid-18th century. The latter, however, cannot be embedded in glass. The technique of making encrustations of medallions in glass was introduced in France about 1798 by H.-G. Boileau at a small glasshouse near Paris called Gros-Caillou. Before 1810, the technique was much improved at a larger factory in Montcenis, and additional refinements were made about 1819 by Honoré Boudon de Saint-Amans. He initially worked at the Montcenis factory, but he later sold his patent to the English glassmaker Apsley Pellatt. Beginning about the 1830s, encrustations were also made in Bohemia and Germany. Shortly thereafter, advances in mechanization led to mass production and a subsequent decline in the popularity of sulphides. Pellatt's Falcon Glassworks in London produced quality lead glass tableware and lighting devices. Its glass was displayed at the 1851 and 1862 world's fairs. These door plates are from a set of four, each of which bears a representation of one of the cardinal virtues as they evolved through ancient Greek philosophy and Christianity: temperance, fortitude, justice, and prudence.