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

11. Pottery Technology 2

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As mentioned earlier handles can be made by pressing clay into moulds, or made of slip in a plaster-of-Paris mould. They are removed when sufficiently stiff and luted on to the cup.

Casting handles - courtesy Worcester Porcelain Museum

Casting handles - courtesy Worcester
Porcelain Museum

Adding handles to cups - courtesy Worcester Porcelain Museum

Adding handles to cups - courtesy
Worcester Porcelain Museum

Nowadays clay is injected by impact into a steel die via an orifice, the die is then split to release the handle. This process is much faster than slip casting. Dies are often made from alumina-based ceramics that can last for 75,000 pressings compared with 1,000 for metal ones.

However, malleable clay is no longer essential in the manufacturing process. A spray drier is used to rapidly heat/dry the prepared slip, producing dry granules (3 to 4% moisture) that are suitable for very high-pressure “dust presses” that dry mould the constituents into the required form. This is known as “dry pressing”. This technique was invented by Richard Prosser, firstly for small items such as buttons, but then when Mintons bought the patent it was used for tiles, as a uniform pressure could be applied throughout the flat tile.

Tile manufacture. Encaustic (inlaid with clay of different colour), Minton dry pressed tile, one with inlaid lions, Minton 1812, and a hand operated tile press. Image courtesy of the Potteries Museum and Art Gallery, Stoke-on-Trent

Tile manufacture. Encaustic (inlaid with
clay of different colour), Minton dry
pressed tile, one with inlaid lions,
Minton 1812, and a hand operated tile
press. Image courtesy of the Potteries
Museum and Art Gallery, Stoke-on-Trent

Normal plastic clay allows particles to glide over each other to achieve a remarkably constant density. However, in more complicated shapes it is difficult to apply a constant pressure so parts have different densities and distort on firing. To overcome this problem, static powder presses have been developed using liquid (hydraulic) pressure to apply a uniform pressure.

So today many previously manual processes have been automated to increase productivity and consistency.

11.4 Slip Casting

Slip is not only used for sticking pieces of unfired pottery together and decorating, but is used extensively in manufacturing. Slip casting is a useful method of forming less-plastic clays or for complex shapes. A two or more piece mould is made of an absorbent or porous material such as plaster-of-Paris. Slip, the mixture of fine clay and water with the consistency of cream (25 to 30 % water), is poured into the mould orifice, and capillary action through the porous plaster-of-Paris causes solidification on the walls of the mould.

Pouring slip into dolls head moulds. Image courtesy of the Potteries Museum and Art Gallery, Stoke-on-Trent

Pouring slip into dolls head moulds. Image
courtesy of the Potteries Museum and Art
Gallery, Stoke-on-Trent

If left for about an hour the process will slow to a stop, with a wall thickness of about 0.5 inches, as the body forms a barrier against further moisture flow. After a period of time calculated to achieve the required clay thickness adhering to the mould wall, typically 10 to 15 minutes for the usual thickness of about one eighth of an inch (3 mm), the excess slip is poured off leaving a shell of drier clay attached to the mould. Further drying and with the help of shrinkage the mould can be removed leaving the hollow, moulded clay item. A typical plaster-of-Paris mould will last for about 100 operations before replacement. Care has to be taken to allow hollow bodies to vent during firing or the expanding air would cause them to burst. Slip casting was particularly used for low plasticity Parian bodies. Its earliest use was in ancient Palestine, and it was introduced in Staffordshire in 1745 AD.

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