Overglaze can reproduce far more colors than is possible with underglaze. The reason is that the paint is above the glaze on the overglaze figurine and below the glaze on the underglaze figurine.
The glaze on porcelain is a layer of glass that lies above the porcelain mass itself. The glaze is burned at over 1300 degrees Celsius, which is too high a temperature for many colors. Some colors simply dissolve and disappear. This is especially true for reds and golds that cannot handle the high temperature of underglaze burning.
This is avoided with overglaze decoration, where the glaze is burnt on the unpainted porcelain, after which the porcelain is painted and the paint is burnt at a lower temperature of approx. 850 degrees Celsius that does not dissolve the colors. Therefore overglaze can have many more colors, but this process is far more cumbersome and the risk of the porcelain being damaged in one of the many burnings is far greater.
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