Mainly from two aspects, one is ceramic raw materials, one is burned. Ceramic raw materials are mainly body raw materials (mud), glaze raw materials (glaze), decorative raw materials (drawing). Generally speaking, the clay material of the body is porcelain clay (mainly kaolin) or clay, which are basically safe, because there is no chemical additive in general.
Natural clay or minerals, only grade quality difference. Glaze is formed after high temperature the glaze of approximate vitreous property (glass change), as long as it is formula raw ore, a small amount of chemical industry is no problem. Drawing material is more troublesome, basic now low-cost are chemical materials, color materials contain heavy metals (lead, mercury, etc.). Therefore, it is conceivable that underglaze color is relatively safe. Whether the mud glaze drawing material is safe depends on the firing temperature.
High temperature porcelain is better than medium and low temperature. High temperatures generally range from 1280 degrees Celsius to 1350 degrees Celsius. Under such temperature calcination, after oxidation and reduction atmosphere, the general harmful substances disappeared or transformed. To sum up, high temperature porcelain can almost be regarded as safe, if there is painting decoration, the underglaze is safer than the glaze.