The role of firing process on bubble formation in a glaze layer of sanitary ware

•TG–DTA was used for determination of the thermal behavior of raw glaze.•Characteristic points were determined by hot stage microscope.•Bubbles in fired glaze were observed by 3D depth of field microscope.•A solution was proposed to decrease bubbles via changing firing process.

Glaze is composed of glassy phase, pores and a small amount of crystal phase. The glazed product is endowed high technological properties such as low water absorption, good cleanability, high bending strength and abrasion resistance, excellent chemical and soil resistance. Nevertheless, surface degradation might lead to opening of closed bubbles in glaze thus diminishing the cleanability. As a result, a study of bubble formation in glaze layer would favor the improvement of mechanical property and cleanability of final products. In this paper, glaze slurry was prepared by milling for 30 min, then applied to green bodies and dried for later use. The same glaze was fired in hot stage microscope to measure melting behavior. Two characteristic points were obtained: start of melting temperature of 1184 °C; half sphere temperature of 1220 °C. The firing curves applied to glaze can be designed according to the characteristic points, and then the dried samples were fired. The bubbles in fired glaze were observed by 3D depth of field microscope. Microhardness was performed using Vickers hardness tester. A solution was proposed to decrease bubbles and increase microhardness via changing firing process. The results showed the formation of bubbles in glaze was determined by firing process.