HgS. A bright red pigment composed of synthetically prepared red mercuric sulphide. Vermilion is chemically identical to the mineral cinnabar. There are two methods, dry-process and wet-process, for preparing vermilion. The dry-process was probably invented by the Chinese then imported to Europe about the 8th century. Dry-process vermilion is prepared by adding mercury to molten sulphur, which forms a black isomorph of mercuric sulphide. The black mass is ground, heated to sublimation, then condensed to form vermilion. Wet-process vermilion is made by placing the ground black mercuric sulphide isomorph into a solution of ammonium or potassium sulphide; this results in a red precipitate of vermilion. Vermilion is a very heavy pigment with good hiding power that has been used in oil, watercolour, tempera, and fresco paintings. When exposed to ultraviolet light and oxygen, vermilion can change from its normal red crystalline form to its black isomorph. This can result in dark, splotchy discolorations. By the 20th century, vermilion was superseded by cadmium red for most applications. Synonyms: cinnabar (mineral); minium (obsolete usage); Chinese vermilion; English vermilion; Pigment Red 106; CI 77766; chusha; vermillion (sp); vermiculus; red mercuric sulphide; Vermilion (Ger.); vermellon (Sp., Fr.); vermiglio (It.); shu (Jap.); tan-sha (Chin.). Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]