Luminous paints require direct sunlight for some time, and the phosphorescence they display in the dark only lasts for a few hours. Luminous paints are usually made by heating oyster shells in the fire until they become white, and then placing them in a crucible with sulphur and melting. Another method is to mix thoroughly 100 parts chalk and 40 parts flowers of sulphur, and heat in a closed crucible until fumes cease to be evolved. Powder the residue of calcium sulphide, mix with the smallest possible quantity of gum water or glue size, and use it as a paint; it is said to be not so good if mixed with boiled oil or varnish. Luminous paints can also be made by using strontium carbonate in place of chalk.