|Monthly Tech-Tip |
These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Quartz is the most abundant mineral on earth, it is the main crystalline mineral form of silica (SiO