The days of using ovens – which took an age to fire – in production have long gone. They have been replaced by much smaller and more economic quick-fire kilns very often using off-peak power and very precise automated systems. But the actual process of making fine bone china thimbles differs little from when the great Josiah Spode introduced porcelain at the beginning of the 19th century.
It is a fact that although some mechanisation has been forthcoming, the pottery industry as a whole seems not to have noticeably changed. The process used to make fine bone china thimbles is known as slip casting, which does not involve mechanisation at all as every stage is carried out by hand.
In a typical production process the clay arrives at the factory in slab form, known as cake clay. It is then made into liquid clay or slip by way of a blunger – a large mixer where the clay is broken down and, with the addition of water and silica, made up into specific recipes. This is an important stage in the production process, as the attained density or pint weight is very much a matter of individual mixing.
At the casting benches the slip is poured into moulds made from plaster of Paris which form the basic thimble shape. Being absorbent, the plaster soaks up the liquid clay and allows a shell to form, which the caster leaves in the mould until dry, the surplus liquid clay having been poured back into a slip bucket. The thickness of the thimbles is controlled by the length of time the slip is left in the mould – the shorter the period – the finer the finished thimble will be.
When the thimbles have been removed from the moulds, they are in what is known as a “green” state. Although a recognisable shape, they are fragile, larger than the finished product and rough around the edges. Spongers, as the name suggests, wipe them smooth for their first firing.
Any rejects at this stage can be reconstituted into slip and reused for casting. The thimbles are placed by the spongers onto batts incorporating specially made formers to retain the product’s shape during firing and they are carefully placed in kilns for the biscuit firing.
Fired at temperatures in excess of 1200°C for about eight hours, a biscuit thimble is strong white, translucent, but still rough to the touch. It has to be dipped in glaze, have any surplus solution removed from its foot to prevent it sticking in the kiln, and then be placed in a glost kiln for a second firing.
The finished thimble is a white glazed blank which can be gilded and transfer-printed, the kiln having been placed and fired on four separate occasions in order to produce this tiny artifact, the humble thimble.