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Cornwall geology and pottery

During our summer holiday in Cornwall we had a lovely walk along the SW coast path from Perranporth to Portreath, passing several abandoned mines and spoil heaps, great places to look for rocks and minerals.

Cornwall, Godrevy lighthouse

We started our 12 mile walk at Perranporth, south of Newquay on the north Cornwall coast. At Cligga Head, we passed an abandoned tungsten mine, where they mined wolframite, a black mineral containing tungsten, iron and manganese. There was also a lot of greisen, decomposed granite, on the first step towards becoming china clay. While the granite mass was solidifying 300 million years ago, hot gases caused it to decompose. It is now easily weathered by the rain and sea and we could see white granules of quartz everywhere under the heather. The hot gases forced the granite up through cracks in the older Devonian slate rocks and caused minerals to dissolve and reform in seams. The ends of mine shafts can be seen in the cliff face, which is stained red with iron oxide.

Rocks and pebbles found on our walk. Bottom right, altered granite; the large white crystals are feldspar. Bottom left, turquoise rock stained with copper. Middle left, red rock stained with iron oxide. The black rocks and pebbles are Devonian slate (400 million years old), with white veins of quartz running through them.

Rocks from the SW coast path

At St Agnes head, there is a granite outcrop which has been weathered into a clay and sand deposit where clay is dug for the Leach Pottery. Further along the coast path is the remains of an old tin mine, Wheal Coates, where the chimney of the Victorian pump engine house still stands at the top of the cliffs. Tin ore was found in seams below sea level, and so water had to be pumped from the mine shafts. We tried throwing a rock down one of the old mine shafts and it took a very long time to hit the bottom.

We also had a lovely visit to the Leach Pottery in St Ives, where we were shown round the working studio by trainee potter Lexie Macleod. The potters use a mix of St. Agnes clay, ball clay from Devon and iron oxide to give the dark, toasted colour they want. They use several glazes, a green ash glaze, black tenmoku, a dolomite white and a white/orange shino. They often throw a hundred mugs in a day, which then take several more days to finish and add handles. Their outlets include Seasalt, who fund an apprentice, and David Mellor in London and Sheffield.

The Leach Pottery studio
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Cornish clay pits

I have been visiting clay pits in Cornwall, researching for my next book Science for the Potter. I visited three clay pits. The first was in St Erth near St Ives, a disused clay pit where sand and red clay were once dug. The pit has been turned into a nature trail but you can still see a rusted metal trolley embedded in the ground. It is very overgrown but there is still visible red clay and sand, which Bernard Leach used in his earthenware clay body.

I also visited a china clay pit, Wheal Martyn near St Austell. Unlike other types of clay, china clay is not found in an immediately usable form. It is part of a crumbly white rock, which is decomposed granite. The clay is washed out of the rock by directing a powerful jet of water onto the newly exposed rock. The water is pumped out of the mine and left to settle in tanks. The sand and mica settle out, leaving china clay, which is drained off and dried. At Wheal Martyn you can see a working pit now owned by Imerys, as well as the old settling tanks and drying sheds once used to process the china clay. There are displays of old machinery and earthenware pots made by Lake’s pottery in Truro. On the way home we stopped in Bristol, went to the Royal west of England academy of art and by chance saw the painting of The Clay Pit by Harold Harvey of the Newlyn School, painted in 1923.

Wheal Martyn China clay pit
Lake’s Pottery display

On on the last day of the holiday I drove to Doble’s clay pit in St Agnes. I got lost down a long rutted track but finally found the clay and sand pit with some directions from Mr Doble. I bought a bag of his stoneware clay and had a look at the clay pit, grey fireclay which lies directly under the gorse and heather on the headland near St Agnes Beacon. Doble’s has supplied the Leach Pottery with stoneware clay since the 1960s. I’m looking forward to trying the clay out with my glazes.

Doble’s clay pit St Agnes
Store of fireclay at Doble’s