Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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11.3 Clay Preparation

Up to the middle of the 18th century the motive power in pottery factories was water, horse or man. Relatively crude stamping mills were used to break down coarse ingredients. To improve the speed of removal of water from the clay slip it was placed in canvas bags and squeezed.

Clay de-watering presses - courtesy Worcester Porcelain Museum and Image courtesy of the Potteries Museum and Art Gallery, Stoke-on-Trent

Clay de-watering presses - courtesy
Worcester Porcelain Museum and
Image courtesy of the Potteries
Museum and Art Gallery,
Stoke-on-Trent

A more recent example of clay preparation can be illustrated by the process used today for bone china. The dry constituents are ground in a ball mill, which is rather like a cement mixer having very hard balls of steel or ceramic constantly grinding the powders more finely. This process can take from 12 hours to 6 days, depending on the material.

Large ball mill

Large ball mill

The fine powder is then mixed with water to form a slip, which is finely sieved and passed under electro-magnets to remove unwanted fine iron particles. The slip is then pumped into a high pressure filter press, squeezing out the water and reducing its content from 50% to 20%, producing “filter cake” that can then be stored. Appropriate amounts of water are subsequently added to produce slip for “slip casting”, or the shredded filter cakes are put through a “pug” mill, replacing “wedging”, using a vacuum to remove any air and homogenising the mix, to produce workable lumps of prepared clay (body material).

Small pug mill

Small pug mill - courtesy
Brookhouse Pottery, Ruthin

This final process is especially important for high quality bone china. The prepared clay is then formed into the required shape. Shapes such as cups are made from body material; other items made from the body material are saucers, plates and dishes, while teapots, jugs, vases and figures are made from slip by casting.

The pottery industry was a leader in the industrialisation of its manufacturing processes in the 18th century. As an example, lathes were introduced in Staffordshire as early as the end of the 17th century in the Elers factory to finish his red stoneware. They were also mentioned in Aaron Shaw’s inventory in 1714, and Isaac Marsh had a turning house in his works in 1732. Thomas Wedgwood used them in 1737.

Other mechanisation methods were also developed in the 18th century. For example, adaptions to the wheel were made to produce plates and cups efficiently. The wheel head was formed as a mould shaped as the face of the plate. As it rotated the clay was shaped by being pressed against the mould by a flat piece of profiled metal that was brought into contact with the clay to form the back and foot ring of the plate. This process for plates was known as “jiggering”, whereas a similar process to form a cup was called “jollying”.

Turners making plates - courtesy Worcester Porcelain Museum and Image courtesy of the Potteries Museum and Art Gallery, Stoke-on-Trent

Turners making plates - courtesy Worcester
Porcelain Museum and Image courtesy of the
Potteries Museum and Art Gallery,
Stoke-on-Trent

An alternative process of forming shapes such as cups also used the potter’s wheel. A weighed lump of clay was thrown, hollowed and shaped to produce a cup-like shape called a “cup lining”. This was pressed into a plaster-of-Paris mould with a metal insert having the internal cup profile to ensure consistent shape, and excess clay was removed. When dried and shrunk slightly, it could be removed from the mould, dried further and the foot rim was formed by turning on a lathe. In the 20th century, machines were devised to carry out the entire production of cup bodies without manual intervention.

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