Art 150 Beginning Ceramics - Basic Ceramic Technology

CLAY

Clay is a very common mineral formed in geologic time by the weathering and decomposition of granite-type rocks. Clay has two properties that make it unique:  

1.         It is plastic when wet. Plasticity is the ability to be shaped and hold to its shape.
            Plasticity is due to the size and shape of the clay particles.  

2.         It is hard and permanent once it is fired.

Plastic clay is approximately 25% water. Joining slip is approximately 50% water.  

As the clay dries, the water between the particles evaporates. This stage is called leatherhard. Leatherhard is the last stage at which clay can be joined together. The clay particles themselves are still wet. Between the plastic stage and leatherhard, the clay shrinks about 6%. Grog is added to the clay body to create pores which allow the evaporating water to escape.  

When the clay particles have dried completely, the clay is said to be bone dry. Bone dry is only as dry as the atmosphere.

Plastic, leatherhard and bone dry are the three stages of greenware. Greenware refers to unfired clay.  

FIRING

BISQUE FIRING

When the clay is bone dry, it is ready for the first firing, called the bisque firing. The bisque firing must proceed very slowly to allow any remaining moisture in the clay to escape without forming steam, which may cause the clay form to blow up. Bisque firing hardens the ware without glaze. The clay has been permanently changed; it cannot be made plastic again.  

TYPICAL BISQUE FIRING

First the kiln is loaded. In a bisque firing, work may be in contact with other work, as nothing fuses. This is not the case in a glaze firing. There are some simple guidelines for loading a bisque kiln. After the kiln is loaded, it is preheated overnight to a temperature of 212 degrees F., the temperature at which water boils. On the morning of the next day, the temperature is raised slowly, approximately 100 to 150 degrees per hour, until it reaches 1600-1800 degrees (pyrometric cone 08-06). The kiln is turned off and allowed to cool overnight. It will usually be cool enough by the next morning to be unloaded.  

Pyrometric cones are compounded to measure work heat, a combination of time and temperature.

GLAZE AND GLAZE FIRING

Glaze is a mixture of minerals and water, applied to the bisque fired clay form, that melt to form a glassy coating, with a variety of color and surface qualities.  

GLAZE FIRING

This is the second firing, after the application of glaze to the bisque fired clay ware. In the glaze firing, the glaze melts and fuses to the clay body. The wares cannot be in contact with each other when they are loaded, or they will be fused together after the firing. In the glaze firing, the ware shrinks another 6%, making the total shrinkage approximately 12%.

TYPICAL GLAZE FIRING

The glaze firing may or may not be preheated overnight. This firing may proceed more quickly in the beginning, as there is no real danger of blowing up the work. After the kiln reaches approximately 1600 degrees, clay body reduction is begun for one hour. Reduction is a firing which takes place with insufficient oxygen. After one hour, the kiln is adjusted to a slight reduction or neutral atmosphere until cone 8 begins to bend. Then the glaze reduction is begun and continued until the end of the firing, when cone 9 to 10 is achieved. It is best if the end of the firing proceeds slowly, about 1/2 hour between cones, to allow the glazes to mature properly. The gas is then shut off, the damper is closed, and the kiln is allowed to cool overnight. After 12 to 18 hours, the kiln should be ready to be unloaded.