Bells are usually cast from bell metal, which ranges in composition from three to four parts copper to one part tin. Copper-tin alloys tend to be hot-short and to crack during fusion welding. While being welded, tin oxidizes preferentially before copper and can reduce weld strength because of oxide entrapment. For these reasons, it is important that heavy bells be preheated to minimize stresses, and that the weld area be shielded to prevent oxidation of the tin.
Cracked bells have been successfully repaired by welding, without changing the tone of the bell. One case involved repair of a fifty-year-old, 750 kg (1650 lb) bronze bell. The bell was 1.5 m (5 ft) from lip to lip, with the sides about four inches thick. It was a copper-tin alloy, which chipped like hard brass and welded like cast iron.
Since it was not economically feasible to remove the bell from its mounting, a charcoal-fueled brick oven was built in the belfry to preheat the bell. The crack was located on the side adjacent to the supporting frame, so it was necessary to loosen the bell, turn and tip it until the crack was horizontal. The entire crack was ground out to form a V-groove. A piece of steel plate was clamped inside the bell under the crack to prevent the molten metal from dropping through.
Charcoal was placed in and around the bell in the oven. The charcoal was ignited with an oxyacetylene torch and blown with a molder’s hand bellows. The bell was heated to the point at which solder would melt when applied to the surface.
Three hours were required for welding. Two cylinders of acetylene, three cylinders of oxygen and 17 kg (37 Ib) of bronze filler rod were consumed in this repair operation. After the weld was complete, it was polished with a small portable grinder.
Other welding rods which are used for welding bells include bell metal, containing about 80% copper and 20% tin; phosphor-bronze, phos-copper; and other phosphor-bronzes which are high in tin content.