With the development of smelting, small bells began to be cast. The art of casting later disappeared, resulting in riveted plate bells. In about the 8th century casting was resumed, and since that time all bells considered to have worthwhile musical tones have been cast of bronze, an alloy of copper and usually from 13 to 25 percent tin. Casting of a bell begins with building a core of bricks and covering it with soft clay that is molded to the outline of the inside of the bell. An outer mold, or cope, of clay is then made, shaped to the outline of the outside of the bell.
Molten metal is poured between the two parts of the mold and then left to harden and cool.
The bell is then removed and tuned by placing the rough casting on a lathe that shaves metal from the inside surface. The shape of the bell evolved in two different directions. The pot and bowl were favored in the Orient and the cup in the West. The pot, with straight sides and a height greater than its diameter, became the Chinese and Japanese barrel-formed bell with walls of equal thickness and of impressive tone but of no particular pitch. The bowl, of convex shape and with a diameter far greater than its height, gradually evolved into the gong.