High Bisque, Low Glaze

The normal way for the studio potter to fire ceramics is usually to bisque the wares to somewhere between cone 09 and cone 06 (-- explanation of cone temperatures), glaze the work and then fire to the maturation temperature of the clay body and the glaze. This would go for earthenware as well as stoneware. However there is another method to fire ceramics, and that is a technique often used in industry, where the ceramics are bisqued to a higher temperature than the glaze firing.

This works best with earthenware or midfire clays bodies, as they are still somewhat porous when high fired. This means that they still have the capacity to soak up the glaze. A typical combination might be to bisque to cone 01 and then glaze fire the work to cone 06.

There are a couple of things that need to be taken into consideration. First of all, the clay should be strong enough for your needs after the bisque firing, as the subsequent lower temperature glaze firing will not further vitrify the clay much, if at all. Secondly, the lower fire glaze should be tested for a good glaze fit. This is not automatically guaranteed and the usual problems of crazing or shivering or pinholing can still occur, so previous testing is paramount. Underglaze decoration can still be done on greenware or bisque.

The technique is less suited to stoneware or porcelain although not impossible. The problem is that the higher fired ware has become vitreous, so it won't soak up any glaze. This excludes the normal methods of glazing such as dipping or under normal circumstances spraying. To overcome this problem, the vitrified work needs to be heated with a burner, then a light coat of glaze is sprayed on. The process is repeated until a sufficient layer of glaze has built up. The thickness of the glaze may need to be tested by scratching it with a pin or nail. In this case, it is also recommended to put some gum arabic or CMC into the glaze to stop it brushing of when handling to put in the kiln. In some cases, this technique will be desirable even for stoneware or porcelain, especially when applying glaze to bisque ware somehow influences the decoration, as would be the case with water-soluble metal salts.

More Articles