Any ceramics manufacturer who has experienced crazing of their glazes will be familiar with the issue of thermal expansion.
Crazing is caused by the different contractions of the body and glaze during cooling in manufacturing, or thermal cycling in service, e.g. oven to tableware, or exterior tiles.
Measuring thermal expansion allows manufacturers to predict stress within joined materials or single materials under conditions of changing or non-uniform temperature. It can also serve as an indicator of phase composition or changes in structure.
How can thermal expansion be measured?
In the case of glazes, if a glaze has a higher expansion than the body, on cooling it will contract more than the body and the resultant tensile stress can be enough to fracture or craze the glaze.
Similar expansion rates between the glaze and body would of course be ideal, but as most ceramic bodies are porous, it’s inevitable that moisture from the atmosphere will penetrate into the body eventually. As the body expands due to the water ingress, ultimately the glaze will craze.
Conversely, if a glaze has a lower expansion than the body, the body will contract more than the glaze and put it into compression. If the compression stress on the glaze is great enough, the glaze may peel from the edge of the ware (sometimes called ‘shivering’).
A careful balance needs to be struck – and it’s for this reason that much gloss ceramic ware is purposely produced with the glaze in slight compression; firstly because it’s less likely to fail, and secondly because the compression will be alleviated as moisture causes the body to expand to meet it.