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Bellfounding is the casting and tuning of large bronze bells in a foundry for use such as in church bell, clock towers and public buildings, either to signify the time or an event, or as a musical carillon or chime. Large bells are made by casting bell metal in moulds designed for their intended musical pitches. Further fine musical tuning is then performed using a lathe to shave metal from the bell to produce a distinctive bell tone by sounding the correct musical harmonics.
Bellfounding in East Asia dates from about 2000 BCE and in Europe from the 4th or 5th century CE. In Britain, archaeological excavations have revealed traces of furnaces, showing that bells were often cast on site in pits in a church or its grounds. Centralised foundries became common when railways allowed easy transportation of bells, leading to the dominance of founders such as the Whitechapel Bell Foundry and John Taylor & Co of Loughborough.
Elsewhere in the world a number of foundries are still active, some using traditional methods, and some using the latest foundry techniques. Modern foundries produce harmonically tuned bells using principles established in the late 19th century; some of these are also highly decorative.
The earliest metal bells, with one found in the Taosi site, and four in the Erlitou site, are dated to about 2000 BCE.Falkenhausen (1994), 132, Appendix I pp. 329, 342. By the 13th century BCE, bells weighing over were being cast in China. After 1000 CE, iron became the most commonly used metal for bells instead of bronze. The earliest dated iron bell was manufactured in 1079, found in Hubei Province.
Bellfounding as a commercial trade followed later. Independent craftsmen set up permanent foundries in towns, such as London, Gloucester, Salisbury, Bury St Edmunds, Norwich, and Colchester. Although these attracted trade from the surrounding countryside, mediaeval founders did not confine themselves to bellmaking as their only source of livelihood. Instead, they often combined it with related trades, such as metal ware, utensil manufacturing and gunmaking. Some founders were itinerant, traveling from church to church to cast bells on site.
These early bells had poor tone, due to both their variable alloy composition and a lack of understanding of producing the correct shape for a harmonic tone; but over time the bell-shape was improved. The angles at the crown and soundbow were gradually flattened out and the waist became shorter, flaring more toward the mouth. Although tuning methods were still uncertain and empirical, sets of bells in diatonic scales were installed at important parish churches and monasteries.
Whilst most bell founders were men, some women were also part of the art, such as Johanna Hill who took over her husband's business, and then left it to her daughter.
Archaeological excavations of in Britain have revealed furnaces, which suggests that bells were often cast on site in pits dug in the building grounds. Great Tom of Lincoln Cathedral was cast in the Minster yard in 1610, and the great bell of Canterbury in the Cathedral yard in 1762. When the casting was complete, a tower was built over the casting pit, and the bell raised directly up into the tower. In some instances, such as in Kirkby Malzeard and Haddenham the bells were actually cast in the church.
The forces holding the tin and copper together cause vibrations rather than cracks when the bell is struck which creates a resonant tone. This metal combination also results in 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 the bell which coats it against further oxidation. The hardest and strongest bronze contains large amounts of tin and little lead though an alloy with more than 25 per cent tin will have a low melting point and become brittle and susceptible to cracking.
This low melting point proved to be the nemesis of Russia's third attempt at casting the Tsar Bell from 1733 to 1735. The bell was never rung, and a huge slab cracked off (11.5 tons) during a fire in the Kremlin in 1737 before it could ever be raised from its casting pit. Burning timber fell into the casting pit, and the decision was whether to let it burn and risk melting the bell or pour water on it and risk causing it cracking from cooling it too quickly. The latter risk was chosen and, as feared, because of uneven cooling, the bell was damaged. The present bell is sometimes referred to as Kolokol III (Bell III), because it is the third recasting; remnants from the old bell were melted down and the metal reused to cast the new bell. This practice was fairly commonplace, as the metal materials were very costly. Bell metal was considered so valuable that the first bronze coins for England were made in France out of melted-down old bells.
By popular tradition the bell metal contained gold and silver, as component parts of the alloy, as it is recorded that rich and devout people threw coins into the furnace when bells were cast in the churchyard. The practice was believed to improve the tone of the bell. This however is probably erroneous as there are no authentic analyses of bell metal, ancient or modern, which show that gold or silver has ever been used as a component part of the alloy. If used to any great extent, the addition would injure the tone not improve it. Small quantities of other metals found in old bell metal are likely to be impurities in the metals used to form the alloy.
Decorative bells can be made of such materials as horn, wood, and clay.
Instead of using a steel mantle and cement, the inner and outer moulds can also be made completely out of loam. In that case, the moulds are usually constructed inside out—first the inner mould on top of a coke, stone, or brick core, then the false bell including wax decorations as above, and finally the outer mould with added iron ring and fiber (e.g. hemp) reinforcements. At this stage the steel staple, from which the clapper will hang, is inserted. Separating agents are used to prevent the false bell from sticking too closely to both of the moulds. Finally, after lifting up the outer mould, the false bell can be destroyed and the outer mould lowered back down onto the inner mould, ready for casting.
The raw materials of copper and tin are melted in a furnace until they become liquid at a temperature of approximately . Often scrap bronze from old bells is added, especially if the bell being cast is a replacement for an existing bell, which is in effect being recycled.
The liquid metal is skimmed to remove impurities, then poured into the mould, using either a tilting ladle suspended from a crane, or else a system of brick channels constructed in the casting pit, which allows the metal to flow directly from the furnace into the individual moulds. As the metal enters the mould, holes in the top of the mantle ensure that gases are able to escape, otherwise there would be a risk the bell would be porous and susceptible to cracking. Porosity can also develop if the mould is damp, or is not at the proper temperature, or the metal when poured is not hot enough. The casting is allowed to cool for up to several days and large bells can take over a week to cool. Small bells, those under , can be removed from the moulds the following day.
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 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.
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