Ceramic materials exhibit thermal expansions just as fired bodies. Manufactured materials, like frits, have a consistent easily measured thermal expansion because they are completely melted during processing. They also calculate well, that is, the thermal expansion can be predicted. Raw materials, on the other hand are complex (expecially clays). Many raw materials are not homogeneous, thus when subjected to the heat of a kiln, different particles within the powder matrix will exhibit different thermal expansion characteristics (some of which can be multi-stage) and they interact in complex ways. For example, some may be refractory and resist melting, others may form a glass melt and take yet others into solution while still others may interact to create new crystalline species that survive for a period and then inturn interact for a time and finally go into solution or melt. If a material was fired alone the final thermal expansion would be a product of the extent to which all of this was allowed to proceed (according to the temperature the kiln reached). Thus, while it may be practical to publish the thermal expansion of a frit for calculation purposes, this is not practical with the vast majority of raw materials. It is better to calculate the expansion of the glaze into which they are incorporated based on the calculated quantities of each oxide. Of course, the reliability of the calculation is also based on whether the glaze completes melts or not.
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