4. Firing

Porcelain is always fired twice:

The first firing is called biscuit firing. The unglazed porcelain is fired at about 950°C for twelve hours. The purpose of the first firing is partly to strengthen the material, since the porcelain clay that has simply dried at normal room temperature is extremely brittle and fragile, and partly to make the porcelain insoluble in water before it is decorated and glazed.

After the first firing the biscuit-fired items are inspected for cracks and other flaws. If it is suspected that an item has invisible cracks, it can be dipped in an aqueous solution of aniline dye, which is pink. The colour is absorbed by the porous body, and cracks will be revealed as hairlike white streaks against the coloured background. Flawed items are discarded, and the rest are sent on either for underglaze decoration or to be glazed. The pink colouring burns away during the second firing and does not damage the porcelain.

The second firing is called brightening, as this takes place after the porcelain has been glazed to give it its bright surface. Porcelain is fired in the brightening kiln for about twelve hours at 1400°C. The porcelain does not sinter until it is fired at this temperature. Sintering is a process in which the material begins to change to a glassy state, becoming dense and translucent with no open porosity. This firing takes place in a reducing atmosphere in which the proportion of oxygen is decreased, giving the porcelain its characteristic cool white color.