A comparatively soft glass, a compound of flint or sand, red lead, and soda or potash. The materials are melted together, producing an almost clear glass, with a slightly bluish or greenish tinge (flux or frit). The degree of hardness of the flux depends on the proportions of the components in the mix. Enamels are called hard when the temperature required to fuse them is very high and it will not decompose as soft enamel would. Soft enamels require less heat to fire them and consequently are more convenient to use, but they do not wear as well. Enamel was first used in the Bronze and Iron Ages. It was often melted and united with gold, silver, copper, bronze, and other metals in a furnace. Enamel is colored white by oxide of tin, blue by oxide of cobalt, red by gold, green by copper. Different kinds of enamel are: 1) inlaid or incrusted, 2) transparent, showing designs on the metal under it, 3) painted as a complete picture. The various techniques practiced by craftsmen in the past differ mainly in the methods employed in preparing the metal to receive the powdered enamel. Some of those methods are cloisonné, champlevé, encrusted enameling, and painted enamels.