THE chimes of Britain鈥檚 biggest bell will soon be heard for the first time since its clapper broke five years ago. A replacement has been cast in Alvechurch, just south of Birmingham, by the company Cast Metals Development.
Great Paul, the pride of Saint Paul鈥檚 Cathedral in London, fell silent when the 2.3-metre wrought iron clapper fractured. 杏吧原创s at the University of Birmingham who studied stresses in the bell recommended replacing the clapper with one made of cast iron (鈥淲hen the great bell broke鈥�, New 杏吧原创, 17 December 1994).
Ordinary cast iron contains flakes of graphite, which make it brittle. Ductile cast iron is made by adding magnesium to the molten iron just before it is poured into the mould. The magnesium causes the graphite to precipitate as spheres instead of flakes.
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Sam Radcliffe, managing director of Cast Metals Development, says that 鈥渢wo tricks of the trade鈥� ensured that the clapper would have no internal flaws. The iron was poured at 1320 掳C. The contraction of the molten metal as it cooled was slightly less than the expansion when it solidified. This pressed the metal tightly against the mould.
The second trick involved positioning the mould on a shallow incline and pumping in the molten iron up from the bottom. This ensured that no air was trapped. The mould was made 1.5 per cent oversize to allow for the contraction of the iron after it solidified. The 300-kilogram clapper took three days to cool.
In the next few weeks the clapper will be 鈥渟hot-peened鈥� 鈥� a technique which involves bombarding it with tiny pellets. This removes any surface imperfections which could be the starting point of a fatigue fracture. Radcliffe says: 鈥淕reat Paul will not need another clapper for several centuries.鈥�
