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Constructing the bell mould
The construction of the mould is different, depending on which of the three methods below are used.
The first two methods start with the building of a "core", which is an exact model of the inner face of the bell. It is built on a base-plate using a foundation of porous materials such as coke, stone, or brick to allow the escape of gases when the molten metal is poured. To produce an accurate and smooth surface it is covered first with loam (sometimes mixed with straw and horse manure) and then with a fine clay to form a smooth profile. This profile is produced by rotating a template called a "strickle" board around a spindle embedded in the centre of the core. It is then dried with gentle heat in a kiln.
False bell method
In this method a "false" or "model" bell is constructed on the core using clay, and is shaped by an (inner) strickle board, then dried, hardened and coated. Then clay is placed on this to produce a false bell using an outer strickle board to obtainthe right shape. This has any inscription and decoration applied, byplacing wax letters and decoration to the surface by hand. The false bell is painted over with clay and then enclosed by a steel "mantle" or "cope" casing. This causes the profile of the outside of the bell to be imprinted on the inside. The complete assembly is now baked in an oven to harden the core, the false bell in the inside of the cope. Any wax bell with core, false The false bell is chipped away from the inner core to leave 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 evaporate any water that has accumulated.[1] A model of the inner bell is then constructed of stone and coated with fireproof cement. It is then smoothed to remove any irregularities.[1]
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. 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.[2]
Individually shaped moulds
This practice evolved in the 20th Century in English bell foundries and does not require a pattern bell or a false bell. Each of the two moulds (outer and inner profiles of the bell) is constructed and shaped separately. The inner mould is built up on a core on a metal base plate, using bricks and loam, and the outer mould is built within an inverted cast iron cope. Each mould requires its own strickle board for the particular size of bell. The moulds are then clamped together and the bell metal occupies the space between them.
Pattern Bell method
This method depends on a re-usable false bell, called a pattern bell, which is constructed of a light but durable material auch as aluminium. The Verdin foundry in the USA casts their bells with the mouth upwards and uses modern silica sands and resins. This method does not required the use of strickle boards. A pattern bell is placed mouth upwards in a "drag" casing and packed tightly with moulding sand. The inside of the pattern bell is then built up with sand and bonded to the core contained in a cope which is lowered into the pattern bell. When all this is hardened, the cope is lifted, the pattern bell removed, and the core mould re-inserted. The bell metal is poured into the space left by the removal of the pattern.
