Dialometric chart produced by a dilatometer. The curve represents the increase in thermal expansion that occurs as a glass is heated. Changes in the direction of the curve are interpreted as the transformation (or transition) temperature, set point and softening point (often quoted on frit data sheets). When the thermal expansion of a material is quoted as one number (on a data sheet), it is derived from this chart. Since the chart is almost never a straight line one can appreciate that the number is only an approximation of the thermal expansion profile of the material.
The 500-600C zone is the alpha-beta inversion of quartz. Notice the vitreous body experiences a bigger expansion change there. But in the 100-270C cristobalite inversion region the stoneware undergoes a much more rapid change (especially in the 100-200C zone). This information affects how ware would be refired in production to avoid cracking (slowing down in these two zones). In addition, that stoneware would not be a good choice for an ovenware body. Photo courtesy of AF
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
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.
|Tests||Co-efficient of Linear Expansion|
|Tests||Glass Transition Temperature|
|Tests||Frit Softening Point|