Clays used in the Pottery Industry

China Clay 

China clay is kaolinised feldspar, which is one of the three main constituent minerals making up granite. Almost 300 million years ago, as a result of the great land masses of the era colliding, the sea bed fractured and folded, forcing masses of molten granite rock up through the crust of the earth. As the molten granite cooled over millions of years, water reacted with the feldspar in the granite transforming it into kaolinite.

China clays were first discovered in this country in the 1740s. In England the china clays are found near St Austell, on western flanks of Dartmoor and on the western and southern parts of Bodmin Moor. The quarrying technique is unusual. High pressure hoses are directed at the wall of the clay pits. The fine clay forms a slurry and is washed down. Most of the impurities are left behind.

China clays have poor plasticity so they are often used in conjunction with additives - usually ball clay and bentonite. As a general rule china clays are quicker to cast than sedimentary clays. They are highly valued for their whiteness, hence their use in bone china. Shrinkage of clays fired at 1300 C is about 12%.

Ball clays 

These clays are Tertiary (Mesozoic) deposits of sedimentary origin. They were initially dug out of the ground as blocks or balls - hence the name. Their geological age, chemical compositions and geographical locations vary greatly.

The main areas they can be found are:

• The Newton Abbot area of South Devon;
• Torrington, North Devon; and
• Dorset - Isle of Purbeck and Wimborne.

Both open cast and underground methods are used for extracting the clay. Underground workings rarely extend more than 100 feet below the surface. Ball clays are plastic so are suitable for throwing and machine use. For casting they need greater addition of defloculents (chemicals to make them behave as a liquid when water is added). The colours produced after firing vary depending on the clay.

Stoneware clays 

These clays are silica rich, plastic clays which vitrify easily.

Fireclays

The vast majority of British fireclays are Carboniferous, usually occurring within the Coal Measures. Consequently their distribution is a very wide one. They are either extracted by open cast methods or mined at the same time as coal.  Silica content ranges from 40% to 80%. Alumina content lies between 12% and 40%. It is the refractory nature of the high alumina fireclays that make them so valuable in the manufacture of firebricks and other heat resistant ceramics such as saggars.

• ECC International (English China Clays), produces a range of very useful booklets about different clays. Head Office - John Keay House, St Austell, Cornwall, England PL25 4DJ. Ceramic Body Plant: Shelton New Road, Cliffe Vale, Stoke-on-Trent Staffordshire. England ST4 7AR.