for the scale, as this is dependent on casting tolerances. Because of this compromise large bells are therefore not always tuned to concert pitch. Much experimentation and testing was devoted over the centuries to determining the exact shape that would result in the best tone. In the early days of bellfounding, bells were profiled using empirical methods and the inside of the bell or edge of the lip was chipped away to adjust the tuning after being cast. With the invention of modern metalworking machinery, this was more accurately done using a vertical tuning lathe, which could remove metal at any position up the waist of the bell, thus allowing tuning of different harmonics, and the reliable introduction of harmonic tuning into the manufacturing process. Metal can only be removed during the tuning process; it cannot be added. Therefore, a bell is cast with slightly thicker profile than is needed for harmonic tuning. To tune the bell, it is placed on a vertical tuning lathe and metal removed by a cutting tool as it rotates. The bell tuner must be highly skilled and formerly used tuning forks to establish the tuning; this is now done electronically, but still requires great manual skill in the use of the cutting tool. Only by this means can bells be harmonically tuned. The bell's strongest harmonics are tuned to be at octave intervals below the nominal note, but other notes also need to be brought into their proper relationship.In general, the smaller the bell the higher the pitch, with the frequency of a bell's note varying with the square of its thickness and inversely with its diameter. The thickness of a church bell at its thickest part (the "sound bow") is usually one thirteenth its diameter. If the bell is mounted as cast, without any tuning, it is called a "maiden bell". Russian bells are treated in this way and cast for a certain tone. Fitting the clapper The preferred material for the bell clapper was wrought iron but because this is no longer obtainable wood or cast iron clappers are now used. The clapper or tongue is manufactured in a similar process as the bell. Special care is given to cast the clapper at the proper weight, as a clapper that is too light will not bring out the true tones of the bell and a heavy clapper might cause the bell to