Abstract
	Thermal dilatation of ceramic material is a basic parameter for glaze selection, which increases the useful properties of ware as well as aesthetic appearance. It is important that the thermal expansion coefficient (TEC) of biscuit be the same or nearly the same as the TEC of glaze in the interval from the temperature of glaze melting on biscuit down the room temperature. If this condition is not kept the cracks on the biscuit glaze would appear either during cooling or shortly after firing. Sometimes, the cracks may appear after a long time after firing - weeks or even years later. Especially whiteware is susceptible to forming hair cracks. The volume increase of biscuit due to water absorption from the ambiance is the primary cause of formation of these hair-cracks. 
	This article presents the results of TEC measurements of the porous ceramic bodies for four kinds of biscuit. In addition, the biscuit volume growth is evaluated as the increase in size of biscuit. Dilatometry was used to measure the dimensions of biscuit up to the temperature of 575°C. The samples were measured after autoclave treatment during which they were exposed to water vapour of 143 kPa for 15 hours.
	TEC of fired bodies from the slip-cast and the plastic mass (N) are about 80.10-7 K-1 and these are about 20.10-7 K-1 higher than with coarse and fine grained plastic masses (H and J). These or similar values of TEC are obligatory also for glaze which are going to be deposited on the biscuit surface.
	The biscuit from the coarse- and fine-grained plastic masses (H and J) show two times higher moisture expansion than the slip-cast and the plastic ceramic mass (N). These values indicate the danger of early formation of hair cracks on the glaze. In case of bodies prepared from the slip-cast and the plastic ceramic mass (N) - the probability of cracks forming due to the humidity will be considerably smaller and also the time elapsed before the first crack appears will be longer.