The Think Tank has been created to give you an insight into the science behind the reactions and colours of our glazes.
Silica/ Quartz Inversion
Quartz or Silica inversion is the sudden change to the crystal structure of Quartz from α-quartz at room temperature to form β-quartz at 573ºC. This process is called inversion and has linear expansion of 0.45%. The inversion process can lead to cracking if ware cools too quickly through the inversion temperature. Cracks occur from varying thermal expansion within the clay body. As some of the quartz particles dissolve in the feldspar glass it creates different thermal expansion and contraction and heat may not be equally distributed throughout the whole piece. The cracking is more likely to occur when cooling rather than when the piece is heating up because the particles surrounding the quartz as they enter inversion are less capable of absorbing the volume change. To avoid this, we recommend cooling rates of 50 °C/hour.
The stress caused by the volume changes by sudden changes in temperature is called thermal shock. For example putting a frozen dish into a hot oven can trigger stress from the quick change in temperature. To avoid thermal shock it is important to make sure the thermal expansion of the glaze is compatible with the clay body. Ceramics can be easily tested for their resistance to thermal shock. An example would be the boiling water to ice water test.