Lusters are metallic salts that are applied over already-fired glazes and then the ware is re-fired to cone 022 to cone 018. The result is an extremely thin layer of metal on the surface of the glaze. The fired luster is very fragile and easily scratched.
Lusters were probably invented in pre-Muslim Egypt. They do need to be distinguished from gold leaf, which is a type of paint. Gold leaf was often used on pottery during the 18th and 19th centuries in Europe. Lusters are also distinct from luster glazes.
Both lusters and luster glazes can change colors as the metals used oxidize over time. For example, a copper metallic surface can oxidize to a green color in three to four years.
Overglazes (China Paints)
Overglazes are sometimes also known as overglaze enamels or china paints. They are applied very thinly over an already-fired glaze. Their use allows for very fine detail in the painting and they have an almost unlimited color range. Overglazes are usually fired at or about cone 018.
Luster and Overglaze Hazards
Both lusters and overglazes release toxic fumes during both the application process and the firing. Whenever they are being used, you need to use the correct type of respirator and work in a very well ventilated room.