2 CHINA STONE, Rostowrack china clay pit When William Cookworthy first discovered how to make hard paste porcelain from Cornish materials he used two ingedients from the granite - Kaolin, which we now call 'china clay', and 'Petuntze'. The latter is a type of granite with a very low content of dark coloured iron-bearing minerals, which is now called 'china stone'. The best china stone occurs in an area between St Stephen and St Dennis and was probably one of the later granites in the St Austell mass to move into place, about 275 million years ago. High quality porcelain nowadays uses china clay, china stone, ball clay (a kind of sedimentary china clay) and ground flint. In the case of bone china, animal bone was also added to the mixture. China stone is required in the 'body' because it supplies alkali-containing feldspar which acts as a flux to lower the firing temperature. The purple coloured mineral seen on one side of the boulder is fluorite - calcium fluoride, a mineral which can be used as a source of the fluoride in toothpaste. The feldspars in this granite are of two types - (1) a sodium containing feldspar, which is usually albite and, (2) a potassium feldspar which is usually in the form of a microscopically intergrown mixture of two types of feldspar, which is called a microperthite. The St Austell granite mass is a composite of many pulses of granite intrusion, spread over 15 million years; earlier granites in the St Austell mass, such as the biotite granites in the in the Luxulyan valley, have more potassium feldspar than sodium. The later granites had more time to 'stew' in the ground, giving time for the chemical composition to evolve, so they are lower in iron and have more sodium feldspar than potassium.. These are the granites which were later altered to form china clay or, if only very mildly altered, became china stone. The mica in these granites is a lithium-bearing variety (see explanation for boulder no. 5), iron bearing biotite mica is absent. Low iron also makes the granite unusually pale in colour and, in the past. it has been extensively used for building, when it was known as 'St Stephen's Stone'. The interior of Truro Cathedral is lined with this kind of granite, no doubt used because it helped to lighten the interior because of its pale appearance.