AGREEMENT BETWEEN THE GLAZE MEASURED THERMAL EXPANSION COEFFICIENT AND CALCULATED BASE ON THE XRF CHEMICAL ANALYSIS
Abstract
The match between the thermal expansion coefficient (TEC) of ceramic body and glaze is very important in order to avoid cracking of glaze on the ceramic body.
In the case of glazes the values of TEC, the tensile and compression strength and the coefficient of elasticity are considered to be the additive function of composition. Their approximate values can be calculated according to the equation suggested by Winkelmann and Schott .
Composition of glazes is variable and therefore the chemical analysis of the glazes is very difficult without the information about the kind of the glaze, i.e. without the knowledge of the qualitative composition of glaze. The XRF analysis method is unable to determine the lighter elements than sodium. Though the boron and lithium are often components of the glazes. In these cases, if the qualitative composition of glaze is not knowledge and lithium and/or boron are present in the glaze, the calculated values of TEC based on the XRF analysis could deviate from real data comparing to that obtained from the measurement. The content of 3 - 4 wt. % Li2O can increase the glaze linear TEC by about 15 x 10-7 K-1, while the content of 10 - 15 wt % B2O3 can decrease the TEC by about 10 x 10-7 K-1. In addition, the contents of boron in glaze can decrease the tensile strength. While the incomplete and inaccuracy chemical analysis of glaze can cause significant difference between the calculated and measured TEC, the tensile strength, however, the difference in the compression strength is negligible.
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