Effect of MgO on crawling and physical properties in ceramic glazes

Abstract

This study attempted to seek and verify the scientific aspects responsible for crawling in ceramic glazes. The selected additive was magnesium oxide (MgO). The glazes were applied on porcelain biscuits and fired between 1100℃ and 1300℃. The degree of crawling was enhanced with increasing amount of MgO, which was accompanied by formation of forsterite (Mg₂SiO₄) as a major phase. Complete island separation was achieved with 10 wt% MgO with good wettability and shiny surface. The morphology of the glaze demonstrated this phase as small crystals within the glaze layer. Addition of MgO resulted in a continuous decrease in the thermal expansion coefficient (as low as 11.76 ´ 10^‒6 1∙℃^‒1) likely due to crystallization of different phases. On the contrary, both glass-transition and softening temperatures (T_g and T_s) showed an initial decrease when adding 5 wt% MgO. However, the trend was reversed beyond the amount of 10 wt%. The in-depth study of the glass structure could not be carried out as the crystalline phase generating Raman peaks that interfered the deconvolution of the non-bridging oxygen (NBO) characteristics. The melting behavior displayed a similar trend to both T_g and T_s, signifying the complicated role of Mg cations in the glaze structure. Addition of MgO (5 wt%) could reduce the softening temperature from 1082℃ to 987℃ suggested that a small amount of MgO could likely behave as a network modifier. A higher amount modified the glaze structure to be closer to a glass-creamic. This change in phasic nature was likely to yield the glazes with high rigidity, high surface tension via melting difficulty which were closely reflected by the increase in the thermal expansion coefficient as well as T_g and T_s. Formation of crystalline phases due to MgO addition was believed to mainly cause crawling as evidenced by various, relevant characterization techniques.