The Effect of Phlogopite Mica on the Sintering Point and Colour of a Claybody
Key Engineering Materials
In Finland the apatite mineral (calcium phosphate) is mined by Kemira Chemicals. Apatite occurs in small amounts in various rock types, but economical reserves in Europe are rare ones. In the Siilinjärvi apatite mine, situated in the Eastern part of Finland, the mined ore contains 10 % apatite, 65 % phlogopite, 20 % calcite and dolomite together and 5 % other minerals. The apatite concentrate is used the production of phosphoric acid. Phlogopite is a by-product and it is sold as a trade name
... mira Mica" for the different types of fillers (1). Apatite has been used in dental ceramics and as a raw material in manufacturing of glass. In the work being presented here, the application of phlogopite is investigated with clay bodies and glazes, fired at different temperatures. As a casting slip, phlogopite has been mixed with lowfired earthenware clay with high iron content (9%). The clay is common in Scandinavia and is fired in the temperature area, 1000 -1050 o C. The addition of phlogopite mica gives a higher temperature resistance and higher sintering area, 1100 -1150 o C. The casting properties of clay body including the phlogopite are excellent. Fired at a temperature of 1100 o C the water absorption is 8 %. The final softening and sintering point is 1150 o C. The fired colour turns from orange to red. At a high fired temperature, phlogopite was tested in white clay body with feldspar and china clay. It was noticed that phlogopite mica increases melting process in stoneware body and lowers the sintering temperature 100 o C. The absorption of water is 0,6% when fired at 1100 o C. The iron content of phlogopite causes a beige colour in the fired clay body. The use of phlogopite lowers the firing expenses and saves energy.