Abstract
	Glaze cracks are defects, which are unwanted for the majority of ceramic wares, although in case of glazed fire-tiles it is only an aesthetic defect. 
	Cracks in the glaze are formed when the value of stress in the glaze exceeds the value of its strength. Strain in glaze is developed if i) thermal dilatation of melting-down glaze is significantly higher than that of fired ceramic body, ii) the cooling or the heating of the glazed surface is not uniform and is too fast iii) the fired body shows to moisture increase. Hence, the stress value depends mainly on the thermal expansion of biscuit and glaze, the cooling rate of glazed ware, the thickness of the glazed layer and on the interface layer which is formed between the body and glaze during firing.
	If the cracks appear in the glaze immediately after firing, it means that the glaze is not suitable for this body because the difference in average thermal expansion coefficient (TEC) in the temperature range room temperature-melting down glaze temperature between the body and  glaze is too high. 
	Dilatations of low-temperature and middle-temperature melting glazes were measured and the difference of the average TEC(t1=t2) of glaze were evaluated in the relation to two kind s of fired porous bodies (TEC(20-500) body from the plastic mass J = 50.10-7K-1, TEC(20-500) body from the casting mass N = 66.10-7K-1) produced by the 
company KACHLE, a.s. Košice. Because the TEC of bodies are different it is not possible to use the same glazes for their glazing. All tested glazes on the body from plastic mass J were exposed to the tensile stress and therefore they were cracking during or immediately after firing and cooling. On the other hand the glazes KK 112, KK 102 a KK111 stick very well on the body from the casting mass N and they are exposed to moderate compression stress.
	The stress in the glazes can change considerably in the course of use as in consequence of moisture increase of porous body. Volume increase of the body due to water sorption from the surroundings will increase tensile stress in the glaze layer. The moisture increase of the body J is twice as high as that of body from plastic mass N. Consequently the crack netting in the glaze on the body J develops easier. In the glazes KK 112, KK 102 and KK111 on the body made of mass N such moisture effects are compensated by the compression tension and these glazes do not tend to crack.