Concept
Bell metal or bell bronze is an alloy used for making bells and related instruments, such as cymbals. It is a form of bronze with a higher tin content than most other bronzes, usually in approximately a 4:1 ratio of copper to tin (typically, 78% copper, 22% tin by mass). The higher tin content increases the rigidity of the metal, and increases the resonance. Historically, it was preferred for early cannons. Today, it also has industrial uses, being specified for valve bodies, piston rings, bearings, and bushings. Tin and copper are relatively soft metals that will deform on striking (though tin to a lesser extent than copper), but alloying the two creates a metal (bell metal) which is harder and less ductile and also one with more elasticity than either of the two original metals. This metal combination produces a tough, long-wearing material that is resistant to oxidation and subject only to an initial surface weathering. Verdigris forms a protective patina on the surface of bells which coats and protects them against further oxidation. Specifically, it is the combination of low internal damping and low internal sound velocity that makes bell metal specially suitable for resonant percussion instruments. Most commonly, as per its colloquial name, bell metal has been and is used for the casting of high-quality bells. The metal used is a high-tin alloy of copper and tin with approximately a 4:1 ratio of copper to tin (78% copper, 22% tin). This is a much higher tin component than that used, for example, in statuary bronze. The range of percentages of tin content can vary from 20% to 26%, depending on the founder who has arrived empirically at their own alloy ratio. It has been found that increasing the tin content increases the decay time of the bell strike, thus making the bell more sonorous. Taking into consideration overall properties, such as tensile strength, hardness, wear resistance, cast quality, sound, and cost, the optimum alloy can probably be obtained by having a type bronze composition as: ~20wt.%Sn, < 2wt.
