Materials in this class are added to bodies to reduce their thermal expansion by simply diluting a high expansion matrix by introducing particles having low expansion. It is thus implied that these materials are added as grog and are not fired high enough to change their mineralogy and therefore thermal expansion properties.
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.