Wheal Martyn - Forming China Clay
Granite is one of the commonest igneous rocks but varies considerably in its composition from place to place. Granite mainly consists of three minerals; quartz, mica and feldspar. The quartz is never anything but quartz. The feldspar, however, can be a silicate of alumina with potash, soda or lime. The mica can take two forms. It can be either a potash-rich muscovite or an iron-rich biotite.
In some parts of the South West where the feldspar in the granite has a higher soda content than potash content, china clay deposits can be found today.
China Clay was created by a complex sequence of events. While the molten rock was still cooling it was attacked successively by steam, boron, fluorine and tin vapour, all of which acted on the feldspar’s alkali content and transformed it into china clay.
The South Western granite has been converted only in those areas where the feldspar had a high enough soda content. Though no china clay is to be found in the Scilly Isles, it occurs in some places in the Land’s End peninsula; at Tregonning Hill, near Helston; at Carnmenellis; on the St Austell moors around Hensbarrow Beacon; on Bodmin Moor and on parts of Dartmoor.
The greatest amount of china clay, however, is found on the St Austell moors. Deposits are roughly funnel-shaped, with the widest part at the top and the base as much as 300 metres deep.
In certain locations deposits occur in clusters and here are found the largest workings, including Blackpool, Goonbarrow, Littlejohn’s and Melbur Pits in the St Austell area, Stannon Pit on Bodmin Moor and Lee Moor Pit on Dartmoor. Some of the deposits cover many hectares at the surface, while the deepest of the clay pits currently being worked is some 130 metres. To work any pit beyond this would mean enlarging the pit in width as well as in depth.
China clay deposits take the form of china clay rock,which is a mixture of up to 15 per cent china clay and up to 10 per cent mica, the remainder being quartz. The clay quality itself varies considerably from pit to pit. In some deposits, such as those at Blackpool, Littlejohn’s and Goonbarrow, it is ideally suited for the filling and coating of paper, while in others, such as Treviscoe and Wheal Remfry, it is best suited for the making of ceramics.
The deposits of Cornwall and Devon are known as primary deposits, because the clay is found at the site where it was formed. This is an image of a stack of clay particles in the matrix from one of the Cornish clay pits.
They also exist in Czechoslovakia, France, Germany, Spain, parts of the USA and in the USSR. But in all these the feldspars in the granite have been broken down by weathering, so clay is strictly limited to the depth to which the weathering has penetrated. Because of this, reserves are much smaller than in Cornwall and Devon.
In Czechoslovakia in particular, where the china clay industry has existenced for two centuries, many of the largest deposits have become completely worked out and only the discovery of fresh ones has enabled the Czech industry to continue.
In some countries, including Greece, Hungary, Iceland, Italy, Mexico and Turkey, there are different deposits, which are also primary in nature. Here the rocks that have undergone alteration were of volcanic origin, and the alternative media have been sulphurous steam and fluids.
Consequently the clay contains minerals that make it unacceptable for either paper or ceramics.
Some china clay is found a long way from the site of its formation, having been created originally by weathering then washed away and transported by rivers to its eventual resting place.
Such occurrences are known as secondary deposits, with the largest in the world found in the US - in Georgia, South Carolina and Alabama. The clay occurs in beds up to 20 metres thick and is suitable for use after processing in paper, but not in ceramics, because of its high titanium content. Imerys is a major producer of china clay in Georgia.