Analyzing the Causes of Crack Formation in Porcelain and the Ways to Eliminate Them

The studies deal with the porcelain products produced by casting finely-dispersed slips into gypsum molds. The causes of the occurrence of cracks on the edges of products after bisque and glazed firing were established and the ways of their elimination were proposed. As a result of the comprehensive research into the basic ceramic mass of the assigned composition and products from it, a series of the technological factors that cause cracking were determined. Determining the fineness of grinding of production slips of various batches showed that the residue on sieve No. 0063 ranged from 0.7 to 3.5 %. The direct correlation between the occurrence of cracks in products and an elevated indicator of the residue that contributed to the stratification of a ceramic slip in the process of settling in gypsum molds was established. This led to the emergence of internal stresses in the structure of a potsherd. It was established that another factor that caused cracking of products was the existence in the composition of the ceramic mass of large quartz grains, which in the heating-cooling process are capable of modification transformations. In addition, the low temperature of bisque firing did not contribute to the completion of the process of dehydration of clay and mica minerals, which strengthened the internal stresses in a potsherd. The research revealed that to ensure qualitative indicators of ceramic products, it is necessary not only to control the residue on a sieve but also to take into consideration the distribution of the fractional composition of a ceramic slip, while the content of the quartz component of 30–63 µm should not exceed 12 % by weight. This contributes to the formation of a dense homogeneous potsherd with a high content of a mullite phase. As a result of the research, it was also proposed to change the temperature of bisque firing from 660 to 800 °C. It is at this temperature that the processes of dehydration of layered silicates are completed and the shrinkage processes are stabilized. The obtained results can be applied at the typical production of household products from low-temperature porcelain.