Bell-metal is a compound of tin and copper, which becomes not only more sonorous, but heavier than either of the separate ingredients which compose it. While the proportions of tin and copper in the compound differ, ordinarily 23 lbs. of tin are mixed with 100 lbs. of copper, the latter being somewhat increased if the bells are large. Brass, spelter, and even lead are sometimes added. Silver, which much improves the tone of the metal, is infrequently used.