This article presents the results of the thermalexpansion coefficient (TEC) of the four porous ceramic bodies and their moistureexpansion 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, dependingon the kind of the biscuits close to 52 x 10-7K-1 and 72 x10-7K-1, respectively. If the TEC of the glaze applied onthe 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 onthe biscuit is under high tensile stress. If the difference between the TEC ofthe 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 tothe moisture expansion of these porous biscuits. The moisture expansions of thetested biscuits are relatively high, from 0.05 up to 0.1% (100Dl/lo)depending on the kind of the biscuits. Such moisture expansion leads to thedevelopment of hair-cracks on the glaze early after firing. If the TEC glaze islower than that of the biscuit the glaze on the ceramic body is undercompression stress. The cracks did not appear even after exposure of the glazedsamples to vapour (98_C/12 hours).
