|Monthly Tech-Tip |
Materials are not always what their name suggests. These are Lincoln #60 Fireclay test bars fired at cone 10 reduction (top) and from cone 11 down to 6 oxidation (top to bottom). This clay already has stoneware density at cone 7 (3% porosity as indicated by our SHAB test). It vitrifies progressively from there upward (less than 3% porosity at cone 7 to near zero% by cone 9 oxidation. Maximum firing shrinkage happens at cone 8 and by cone 10 it is expanding (indicating decomposition has started) and it is bloating by cone 11 (melting is sealing the escape of gases of decomposition). Is Lincoln #60 a really fireclay? Absolutely not! But at cone 6 it is a credible plastic stoneware, all by itself!
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
In the ceramics industry, clays that are resistant to deforming and melting at high temperatures are called fireclays. Kiln bricks are often made from fireclay.
Lincoln 60 Fireclay