3.8 Preparation for Glazing

Firing in Jingdezhen

Glazing can be applied in dry stage (raw-clay) or bisque stage.

Preparation for Glazing Dust and oil on surface of a pot are bad for glazing. Therefore, ready to glaze pot should be cleaned with a wet sponge or brush inside and outside surface. Both raw-clay and bisque wares should be cleaned and moistened with damp sponge before glazing. Extra moisture helps to minimize pin holds formed and avoid the glaze dry too fast.

Glazing in the Raw-Clay Stage Glazing to raw clays is usually fired in a single operation at the mature temperature that eliminates bisque firing and cost less in labor and power. It is still very common practise in China. But only if handle properly, a single firing would have a better union between the clay body and the glaze. However, raw-clay pot is much more fragile and contains certain amount of moisture, which make it challenges for handling and glazing. Thin and delicate pieces are not recommended for dipping or pouring, instead spraying will be more suitable to prevent high moisture absorption, that will cause slumping or cracking. If dipping or pouring, glazing should be applied quickly to avoid uneven water absorption. If Spraying, glazing should be applied slowly to allow time building up coating and surface not getting too moist. Glaze with a high amount of clay content tend to fit with the body and work best in single firing.

Glazing in the Bisque Stage Today, most potters would have bisque firing in low temperature before glazing firing which make the pots stronger to be handled in the glazing operation. Bisque firing is recommended at cone 010 (900c), which is sufficiently porous to absorb glaze. If bisque firing is too high, that will make it difficult to absorb glaze. The techniques of glazing bisque is depended on the size and construction of a pot, small size of pots are best for dipping, bigger or more complicated structure are suitable for pouring or spraying.

After glazing, pots are ready to be fired in higher temperature. However, the bisque firing for high quality or fine porcelain ware is exceptionally fired higher than normal temperature. Due to that Chinaware with thin wall tendency to be warped easily, so their bisque firing is designed to reach in a high or maximum temperature. Meanwhile, the wares are also supported with fireclay rings inside their lips or stacked upside down during firing in order to minimize deformation. Later, the bisque Chinaware are glazed and then re-fired to lower temperature on their own foot. This process would reduce the warping rate since the form is matured at higher temperature.