Information from the Getty AAT
vermilion (pigment)
Synthetic pigment composed of red mercuric sulfide, chemically the same as the natural mineral cinnabar. There are two methods for preparing vermilion, a dry-process and a wet-process. The dry-process method, adding mercury to molten sulfur, was invented by the Chinese then imported to Europe ca. 8th century, seemingly via the Islamic world. The wet-process method, placing the ground black mercuric sulfide isomorph into a solution of ammonium sulfide or potassium sulfide, was developed in the late 17th century in Germany. Vermilion is a dense pigment with excellent hiding power, used in oil, watercolor, egg tempera, and fresco paintings. When exposed to ultraviolet light and oxygen, or chloride ions, vermilion can change from its normal red crystalline form to gray or black, resulting in dark discolorations.