China Clay in Cornwall
What is China Clay
Simply speaking, China-Clay is decomposed granite. More precisely, China-Clay is granite where the feldspar crystals have been transformed to kaolin, a fine white clay, but leaving the quartz and the mica unchanged. This process happened a long time ago on geological scales and for some reason only affected some parts of Devon and Cornwall. In this regions, granite does not appear as a solid rock but as a soft growan (another word for decomposed granite), a gravel-like deposit of quartz sand mixed with mica and china-clay.
China-clay found little use until the mid-18th century, as it was difficult to separate from the matrix of quartz sand and it lacked the natural plasticity of a good potting clay. Its main property is an exceptional resistance to heat. Refractory clays, as heat resistant types are know, were ideal for the lining of tin-smelting furnaces and there is some evidence that china-clay was being mined on Hensbarrow, for this purpose, as early as the 16th century.
The mining of china-clay for the purposes of porcelain production, however, only began in 1745 in Cornwall when the apothecary William Cookworthy discovered clay deposits at Tregonning Hill and at Carloggas. Here we would like to emphasise that William Cookworthy was not the first person to discover the existence of china-clay in Britain. However, he was the first person in Britain to make the connection between china-clay and the production of porcelain. After his discovery of the kaolin deposits, Cookworthy experimented with the manufacturing of porcelain for about 20 years unitl, in 1768, he finally managed to patent a method to turn the clay into fine porcelain. Shortly afterwards he established the Plymouth Porcelain Factory, which later moved to Bristol. Other potteries in the UK followed suite and started to use china clay, also know as kaolin, from Cornwall.
By the early 19th century, the deposits around St Austell had emerged as the largest in the world and the Cornish kaolin industry had become big business, with a multitude of small companies selling their products on the open market, a market that was by then rapidly expanding. Apart from china, kaolin was also used in many other products such as paper, paint and rubber goods. By 1910 production was nearly one million tons a year, 75% of which was exported to North America or Europe. Before World War I, about 70 china clay producers were operating in Cornwall and the competition was cut throat. This often resulted in harsh working conditions for the thousands of men which were employed in the industry.
In 1919 the three largest china clay producers, namely the West of England China Clay Company (established 1849), Martyn Brothers (established 1849) and North Cornwall China Clays (established 1908), merged into English China Clays (EEC). At that time these three companies accounted for about 50% of the total china clay output. In 1932 ECC acquired its rivals John Lovering and HD Pochin. By the end of the 1960s ECC was producing around 2.5m tons of kaolin a year. In 1999 ECC was bought by the French company Imetal (which was founded in 1880 by the Rothschild family). Imetal was later renamed into Imerys, now a multinational company that specialises in the production and processing of industrial minerals.
The Wenford Dries were built in the early part of the 20th century (probably post-1907) by the Stannon China Clay Company, to serve the china clay pit at Stannon on Bodmin Moor some 7.2km to the north east. The liquid china clay slurry was carried in a pipeline from the pit to the settling tanks behind the dries. The choice of site was heavily influenced by the presence of an existing railway line leading from Wenfordbridge which was originally constructed to carry granite from the nearby De Lank quarries. in 1919 the Stannon China Clay Company was acquired by English China Clays – at this time the largest china clay producer in Cornwall.
After arriving from the Stannon Pit the china clay (slurry) had to be dried because of its very high water content. The drying of the china clay slurry was traditionally undertaken in plants called “pan-kilns” – Wenford Dries probably being the best preserved one in Cornwall.
Pan-kilns consist of 4 major areas, namely sand and mica-drags, settling pits, settling tanks and the “dry” building (or simply the “the dry”). The “dry” building is a long, narrow building with half granite, half brick chimney stacks at one end a coal-fired furnace at the other end. Internally, the dry consists of 2 terraces. The upper terrace is called the “pan” and was used for drying the china clay, while the lower terrace was called the “linhay” and was used to store the dried china clay before removal by rail or road. Thus, there is often a railway track just below the linhay.
Similarly to the Roman hypocaust, the pan was constructed in such a way that hot air was flowing under the floor. The pan’s heating systems consisted of horizontal flues that ran between a coal-fired furnace and the chimneys, which provided the up-draught to draw the hot air under the pan floor. The flues itself were covered by tiles, thus allowing for the wet clay to be spread out, which made it easier to dry.
Let us look at the drying process in more detail. After the china clay slurry arrived at the pan-kilns it passed through sand and mica drags. The sand drag are made up of concrete channels used to settle out the finer quartz sand from the slurry. Every two hours the drain plugs were removed and the sand drags scraped clean. The slurry now containing only very fine china clay particles, fine sand and flakes of mica enters a series of wooden channels called mica drags. Flowing slowly it drops the mica on to the floor of the channels while the fine clay flows on, the mica was cleared every eight hours and washed to the river.
From the mica drags the slurry flowed into settling pits, where the clay settle over a period of days and the the water was progressively run off until the clay slurry had about 12% solids. At this stage, the slurry was transferred to the settling tanks where this process was repeated until the solids reached about 30%.
Once the slurry had the right consistency, a plug was pulled and the slurry passed into the upper terrace of the dry-building or the pan. Here the slurry was spread out on the heated floor and allowed to dry for several days. This drying process could easily several days. Once the china clay was dry it was shovelled into the linhay, where it was waiting for removal either by land or rail.From the above it is evident that this was a slow and labour intensive process and more efficient methods for drying the clay were required in order to increase the production. Some of these processes will be described in The China Clay Dries in Par.
The Wenford Dries closed in 2002. Nowadays most of the building is just an empty shell which provides a nice playground for grafitti artists.