Everything from wood to glass and various metal alloys have been tried to create a percussive instrument that resonates according to the purpose at hand. Most modern bells are made of either cast iron or bronze. A few steel bells can be found. A bell that does not have magnetic attraction, confirmed by the presence of a natural green patina indicates a bronze composition. Most trained ears agree that bronze provides the most musical tone.
Bronze is an alloy made primarily of copper with the addition of tin to provide hardness and elasticity for a durable resonant material. An 80:20 ratio of copper to tin provides a starting point for the bell founder and other metals and chemicals are added to the molten metal to remove impurities and oxygen before casting. The bell bronze formula is often adjusted for different size bells.
The shape of a bell's cross section provides the primary distinction of bells from different foundries. Two primary patterns are used when creating a bell. One is the inner curves that define the cavity inside the bell's mouth, the second is the outer shape of the shell. The horizontal variations in diameter and thickness of the wall at various points in the vertical dimension is called the bell's profile.
Historic founders would adjust the curves of their patterns as they cast and tested their products. Many would claim to have perfectly tuned bells emerge from the molds.
While the profile of the bell can be adjusted to provide bells that are very good sounding, bells that ring together or are intended to be members of a chime or carillon instrument are machine tuned after casting.
In order to ring, the bell must be hung in such a way that it can freely resonate. Traditional bells are generally suspended from the top. This can be accomplished by simply providing holes for attaching bolts, or a crown top provided for attachment.