Ceramics-Silikáty 46 (4) 159-165 (2002)

MOISTURE EXPANSION OF POROUS BISCUIT BODIES - REASON OF GLAZE CRACKING

B. Plešingerová, M. Klapáč, M. Kovalčíková

This article presents the results of the thermal expansion coefficient (TEC) of the four porous ceramic bodies and their moisture expansion as well as the TEC of glazes and glaze cracking on these bodies. TEC (20-540°C) of the tested porous ceramic biscuits were relatively low, depending on the kind of the biscuits close to 52 x 10^-7K^-1 and 72 x 10^-7K^-1, respectively. If the TEC of the glaze applied on the biscuits is higher than the TEC of the biscuit by more than about 10 x 10^-7K^-1, the glaze is cracking in the kiln during the cooling of samples. The glaze on the biscuit is under high tensile stress. If the difference between the TEC of the biscuit and of the glaze is in the range of (-10 to 0) x 10^-7K^-1, the glazes can crack on the glazed body as late as several days after firing. The glazes are first under slight tensile stress, which slowly increases due to the moisture expansion of these porous biscuits. The moisture expansions of the tested biscuits are relatively high, from 0.05 up to 0.1% (100Dl/l_o) depending on the kind of the biscuits. Such moisture expansion leads to the development of hair-cracks on the glaze early after firing. If the TEC glaze is lower than that of the biscuit the glaze on the ceramic body is under compression stress. The cracks did not appear even after exposure of the glazed samples to vapour (98°C/12 hours).

Keywords: Glaze thermal expansion, Moisture expansion, Glaze stress, Crack