Manufacture of the One-Piece Crotal Bell
The following paragraphs are based on an article published in Rescuing the Past (Countryman Books, 1970), in which the process used by the Whitechapel Foundry to mould and cast one-piece crotal bells is described in detail. Since first being employed, around the end of the 15th century, the process and the equipment used will have been subject to many improvements, but in principle the method has remained the same. As indicated elsewhere in this article, a development occurred sometime during the 17th century, which required that the pattern had a detachable ‘sprue-piece’. The use of an ‘oddside cup’, to ensure exact positioning of the pattern, is also likely to be a later refinement, as is the use of metal, rather than wooden moulding boxes. It should also be borne in mind that although a single bell is considered below, in practice a significant number of bells were moulded in each box, and cast at the same time.
The bells were cast in two-part moulds with the joint line at the girth rib of the bell. The patterns from which the moulds were made were probably of metal, as the decoration would need to be quite finely engraved.
The first stage of the process involves the use of an oddside cup, as mentioned above. This ensures that the mould joint line is positioned exactly at the centre-line of the bell. The cup is placed inside an open-ended moulding box, set on a moulding board as shown. Sand is packed around it, rammed tightly and levelled at the top.
Stage 1 - The oddside cup is set in a moulding box
The moulding box with the oddside cup is then turned over, and the pattern for the bell is placed in the cup. A second moulding box is placed on top of the first one and guide pins are fitted to ensure that there is no lateral movement between the two boxes when they are dissembled and reassembled. Moulding sand is then added to the upper box, rammed tightly around the pattern and levelled at the top of the box.
Pattern with detachable sprue-piece
Stage 2 - The pattern is placed in the oddside cup and the top half of the bell is moulded.
At the next stage the moulding box with the oddside cup is removed. It can be used repeatedly for other moulds, as it is not destroyed in the process. The second moulding box with the pattern in place is then turned over, and an empty box placed on top of it. This, in turn, is filled with moulding sand, rammed and levelled flush with the box.
The completed mould is turned over and the two boxes are separated. The pattern is removed, the sprue-piece being withdrawn from the top of the mould, and the body of the bell from the underside. A spherical sand core produced in a core-box and with an embedded iron pellet, is then placed in the lower half of the mould. It is supported on the ridge of sand that forms the sound bow of the finished bell. The upper half of the mould is then carefully lowered on to the lower half. The two small sand projections that will form the holes in the bell’s upper hemisphere ensure that the core is located centrally, so that the thickness of the bellchamber wall is uniform. A pouring cup is positioned on top of the completed mould, which is then placed on a bed of sand reading for casting.
Core of sand with embedded iron pellet
Stage 3 - The oddside box is removed and the bottom half of the bell is moulded.
Stage 4 - The pattern is removed, the core (with embedded pellet) is placed in the mould, and the bell is cast.
Molten metal is poured into the mould, which is then allowed to cool. It is then opened, the bell removed and excess metal trimmed from the sprue-piece. The core is removed as loose sand through the upper holes and sound bow, leaving the iron pellet trapped within the bellchamber. Finally, the bell is fettled and wire-brushed to complete the process.