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.
Bell founders in the 15th century made great progress in creating musical bells and carillon instruments. They guarded their techniques to the point that for four centuries creating good sounding bells was a lost art. During World War II, the Nazis went through Europe collecting bells to be melted down into cannon and other armaments (another common use for bell bronze). After the war, research was done in the camp where these bells were stored to determine which ones sounded best and why.
Research by Arthur Bigelow in the 1960s resulted in a scaling up of the profiles of smaller bells to improve their ability to compete with larger bells in the same instrument. It also allowed them to ring longer than bells with a light profile.
While the profile of the bell can be adjusted to provide bells that have a pleasant sound, bells that ring together, especially bells intended to be members of a chime or carillon instrument, are machine tuned after casting.
The tuning process generally involves a lathe. Horizontal shop lathes can be used for small bells, but large vertical engine lathes are used for large bells. This process always involves the removal of material and so it is an irreversible process. The risk of re-tuning valuable bells often does not justify the perceived benefits. Tuning individual partials of a bell is not as simple as tuning individual strings on a guitar for example. When one removes material from one area of the bell's profile, it may raise some partials while it lowers others. The tuner must be familiar with how each partial will react to the locations of cuts for the profile of a given bell.
The goal of what is called internal tuning is to bring the five main bell partials into conformity with one another. That works for stand alone bells, but for instruments of several bells, each bell must also be externally tuned to a musical note on a scale so it will combine with its siblings in the instrument in a musical way.
Modern carillon instruments are tuned to an equally tempered scale based on A=440 Hertz. Both the strike tone and other partials of bells in these instruments must conform to this scale in order to avoid beats and 'wolf tones' when the bells are played together.