Bellfounding

Bellfounding is the casting of bells in a foundry for use in churches, clocks, and public buildings. The process in East Asia dates to about 2000 BCE^[1] and in Europe dates to the 4th or 5th century. In early times, when a town produced a bell it was a momentous occasion in which the whole community would participate. Archaeological excavations of churchyards in Britain have revealed furnaces, which suggests that bells were often cast on site in pits dug in the building grounds. In some instances bells were cast directly in the church. Before the nineteenth century, bellfounders tended to be itinerant, travelling from church to church to cast bells on site. More centralized foundries were established on foundation of railways. There are however examples of foundries producing bells prior to this, such as the Whitechapel Bell Foundry and John Taylor & Co of Loughborough.

Bells intended to be rung are usually made by casting bell metal (a high-copper bronze alloy) of a size appropriate for the pitch the bell is intended to produce. Fine tuning of metal bells is achieved on a lathe where a precise amount of material is removed from the inside of the bell in order to produce a true tone with correct harmonics. Bells are used often to play a chime sequence and so must be well tuned in order to produce a correct scale of musical notes.

History

Bianzhong of Marquis Yi of Zeng, dated 433 BCE

Bellfounding has been important throughout the history of ancient civilizations. Eastern bells, known for their tremendous size, were some of the earliest bells, cast many centuries before the European Iron Age. The earliest bells were made of pottery, developing later into the casting of metal bells.^[2] Archaeological evidence of bellfounding appears in Neolithic China. The earliest metal bells, with one found in the Taosi site, and four in the Erlitou site, are dated to about 2000 BCE.^[1] Portable bells came to Britain with the spread of Celtic Christianity, and most of those still remaining share an association with Scotland, Wales and Ireland.^[3] Bells are traditionally cast in foundries for use in churches, clocks, and public buildings. A practitioner of the craft is called a bellfounder or bellmaker.

Britain

Bellfounding in Britain, as with other scientific crafts, had its origins with monasticism and throughout the early mediaeval period and in centuries following, it was carried out predominantly by monks.^[4][3] Large bells in England are mentioned by Bede as early as 670 CE and by the seventh or eighth century the use of bells had become incorporated into church services. Nearly 200 years later, in the tenth century is the first record of a complete peal of bells.^[3] The chronologies of the abbot Ingulf suggest that Thurcytel, the first Abbot of Crowland, presented the Abbey with a bell named Guthlac, after which his successor, Egelric the Elder cast an additional six bells—two large, two of medium size and two small—to complete a peal of seven.^[3] The same period saw other ecclesiastics involved in the founding of bells. St. Dunstan, “The Chief of Monks”, was an expert worker in metals and known bell caster. Two bells were cast under his direction at Abingdon which also held two others cast by St. Ethelwold.^[5] Methods of moulding by lost-wax casting were described by the thirteenth-century Benedictine monk Walter de Odyngton of Evesham Abbey.^[3]

Bellfounding as a regular trade followed later. Independent craftsmen set up small, permanent foundries in towns. 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.^[6] Some founders were itinerant, traveling from church to church to cast bells on site; but the majority had settled works in large towns. Among other places London, Gloucester, Salisbury, Bury St Edmunds, Norwich, and Colchester were seats of eminent foundries.^[4]

These early bells had tonal discrepancies; a result of their weight and alloy composition as well as uniform thickness and profile—where the height was disproportionate to the diameter. The next century brought advances in all aspects of bellfounding where a better understanding of principles of bell design contributed to the introduction of a superior shape. The angles at the crown and soundbow were gradually flattened out and the waist became shorter, flaring toward the mouth. Although tuning methods were still uncertain and empirical, sets of bells in diatonic sequence were installed at important parish churches and monasteries.^[6]

Archaeological excavations of churchyards in Britain have revealed furnaces, which suggests that bells were often cast on site in pits dug in the building grounds.^[7] 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.^[7] When the casting was complete, a tower was built over the casting pit, and the bell raised directly up into the tower.^[8] In some instances, such as in Kirkby Malzeard and Haddenham the bells were actually cast in the church.^[7]

Bellfounding is a process that in Europe dates to the 4th or 5th century.^[9] In early times, when a town produced a bell it was a momentous occasion in which the whole community would participate.

Materials

The Tsar Bell showing crack caused by low melting point during casting

Bell metal

Functional bells, for the intention of producing sound, are usually made by casting bell metal, an alloy of bronze. Much experimentation with composition has existed throughout history; the bells of Henry II had nearly twice as much copper as tin, while much earlier Assyrian bronze bells had ten times the amount of copper to tin.^[10] The recognized best composition for bell metal though is a ratio of approximately 80 per cent copper and 20 per cent tin.^[11] Bell metal of these ratios has been used for more than 3,000 years, and is known for its resonance and "attractive sound".^[12] Both tin and copper are relatively soft metals that will deform on striking. By alloying the two elements a harder and more rigid metal is created but also one with more elasticity than the use of one of the metals alone.^[12] This allows for a better bell resonance and causes the bell to "vibrate like a spring when struck", a necessary quality as the clapper may strike the bell at speeds of up to 600 miles per hour. The forces holding the tin and copper together cause vibrations rather than cracks when the bell is struck which creates a resonant tone.^[12] 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.^[8]

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.^[8][13] This low melting point proved to be the nemesis of Russia's third attempt at casting the Tsar Bell from 1733 to 1735.^[8] 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 to pour water on it and risk causing it cracking from cooling it too quickly. The latter risk was chosen and, as feared, because of the low melting point of the bronze and uneven cooling, the bell was damaged.^[14] 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 re-used to cast the new bell. This practice was fairly commonplace, as the metal materials were very costly.^[13][15] Bell metal was considered so valuable that the first bronze coins for England were made in France out of melted-down old bells.^[16]

Other metals

Other materials occasionally used for bell casting are brass or iron. Steel was tried during the busy church-building period of mid-nineteenth England, for its economy over bronze, but was found not to be durable and manufacture ceased in the 1870s.^[17] They have also been made of glass, but although bells of this type produced a successful tone, this substance being very brittle was unable to withstand the continued use of the clapper.^[16]

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.^[16][18] Small quantities of other metals found in old bell metal are likely to be impurities in the metals used to form the alloy.^[16]

Decorative bells can be made of such materials as horn, wood, and clay.^[13]

Manufacturing process

Casting the mould

The craft of casting bells has remained essentially the same since the 12th century; bells are cast mouth down, in a two-part mould consisting of the core, and the shell, or cope, clamped to a base-plate.^[19][13] There are variations in the process, principally in the quality-control standards.^[12]

Measurement and templating

Firstly the bell design is calculated to precise specifications where the bellmaker determines the shape that the bell will need to take in order to resonate with the proper number of vibrations and create the right sound and pitch. The bell pattern is then cut out in two wooden templates called "strickle boards". One of the boards matches the dimensions of the outer bell (called the case or cope); the other matches that of the inner bell (called the core). The boards are used to create the inner and outer moulds of the final bell.^[13]

Bell moulds in the bell museum (Glockenmuseum) in Gescher, Germany

Constructing the mould

An exact stone model of the outer bell, sometimes called a false bell, is built on a base-plate using porous materials such as coke, stone or brick. It is then covered first with sand or loam sometimes mixed with straw and horse manure. This is given a profile corresponding to the outside shape of the finished bell, and dried with gentle heat. The false bell is then covered with molten wax and figures and inscriptions, also made of wax, applied on top by hand. The false bell is painted over with three coats of fireproof clay and then enclosed by a steel mantle overcasing. The empty space between the false bell and the mantle is filled in with cement and left to harden before the mantle is lifted off. The false bell is chipped away from the inner core to leave just the wax and cement. Any leftover scraps of the false bell are removed with a blow torch. The mould is then set over a coke fire to melt the remaining wax and to evaporate any water that has accumulated.^[13]

A model of the inner bell is then constructed of stone and coated with fireproof cement. It is then smoothed to remove any irregularities.^[13]

Instead of using a steel mantle and cement, both 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. 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.^[20]

Bell casting mould demonstrating the process at the Technical Museum Vienna, Austria, showing the mould being closed by lowering the lined case over the core. The two sections must correspond precisely or the bell cannot be uniform in shape and thickness.

Casting the bell

After the outer steel mantle has been cleaned, it is again lowered over the outer bell model. The mantle and the outer bell mould are then lowered over the inner mould and the outer and inner sections are clamped together, leaving a space between them. The clamped mould is supported, by being buried in a casting pit which bears the weight of metal and allows even cooling.^[21] Ingots of either ready-made bronze or its component metals are melted in a melting furnace and heated until liquid at a temperature of approximately 1,100 °C (2,010 °F). The liquid metal is then skimmed to remove impurities before the start of the casting process. When everything is ready, the molten bronze is transferred to the moulds using either ladles or a system of brick channels specially constructed in the casting pit, through which the hot metal can then flow from the melting furnace into the space between the two moulds.^[20] Holes in the top of the mantle ensure that gases are able to escape. If gas remained in the metal, the bell would be porous and susceptible to cracking.^[13] Porousness can also develop if the mould is damp, is not at the proper temperature, or if the metal, when poured, is not hot enough. The bell is allowed to cool for several days. Large bells can take over a week to cool completely. Small bells, those under 500 pounds (230 kg), can be removed from the moulding pit the following day.^[22]

Cooling

After the bell and equipment has cooled completely, the mould, containing the newly cast bell, is raised from the pit by the projecting trunnions of the bell case. The core plate is unclamped and the core broken out. The bell is then carefully extracted from the case. At this stage, any remaining loam still adhered to the bell is brushed away and flash (excess metal), which may have formed below the bell rim—owing to mould contraction in the presence of hot metal—is trimmed off. This completes the casting process.^[23]

Tuning

Bells are manufactured with exact formulas, so that using the diameter it is possible to calculate precisely every dimension of the bell, and in turn its musical note, or tone. Much experimentation and testing has been devoted to determining the exact shape that will resonate the best tone. 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.^[16] The thickness of a church bell at its thickest part (the "sound bow") is usually one thirteenth its diameter.^[8] If the bell is mounted as cast, without additional tuning after founding, it is called a "maiden bell". Russian bells are treated in this way and cast for a certain tone.^[8] "Tuned bells", which were common practice in Britain and Europe, are worked after casting to produce a precise note.^[24]

In the early days of bellfounding, bells were tuned using an imprecise method whereby the inside of the bell, or the edge of the lip, was chipped away.^[24] With the improvement of machinery, this was done using a lathe. The bell is cast with slightly thicker sides before being inverted, and gripped by vices, to keep it perfectly firm. The bell is then ground as it rotates on a circular lathe to acquire the precise tone. The bell tuner must be highly skilled as it takes years of experience to know how much metal to remove. By this means, bells can be very accurately tuned.

In casting, the tone of the bell is best left sharp, because it is much easier to flatten the tone than to sharpen it. A bell may readily be flattened one-eighth of a tone, or even more, but it cannot be sharpened so much; indeed, any sharpening is to be deprecated, and if at all possible should be avoided. The bell tone is tested frequently during the tuning process usually with tuning forks or an electronic stroboscopic tuning device commonly called a strobe tuner, which registers the vibrations as the bell is struck. If the tone is too low, the lathe operator grinds more metal off the lower edge of the bell. If the tone is too high, the bell is thinned with a file.^[13] 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.^[25]

Cutaway drawing of a bell, showing the clapper and interior

Fitting the clapper

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 crack.^[13] Holes are drilled into the top of the bell and the clapper is attached to the inside of the bell either by a metal link or by a leather strap. Finally the bell is installed in the tower.^[8]

See also

• Benjamin Hanks (1755–1824), goldsmith, instrument maker, and first maker of bronze cannons and church bells in America.^[26]

References

Literature cited

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