Bisque Firing

December 18, 2008
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Fast bisque firing caused the bottom of this cup to explode.

Bisque firing of ceramic forms is an intermediate step in the process of an eventual glaze firing. It makes the clay body stable for a water-based glaze application without deforming the pottery. Even though this is a necessary step in pottery production, many potters do not fully understand the relationships involved with mechanical and chemical water found in the clay body. Fast-firing the bisque kiln can cause pottery to crack or blow up.

Even when pottery has dried out in the studio for months or years, it still contains the atmospheric moisture of the surrounding environment. In this sense, the pottery is never really dry at this stage.

Moist clay goes through several important steps in the bisque firing process:

212 to 392°F
When pottery is heated in this temperature range, mechanical water (which is found on the particle surfaces of the clay) is driven off. When fast-firing occurs during this period, water can go off as high-pressure steam, causing a crack in the clay.

842 to 1112°F
At increasing temperatures, chemical water is removed from the clay. Keep in mind that the theoretical formula for clay is Al203 2 Si02 2 H20. The clay shrinks slightly and increases in porosity. At this stage, the clay body cannot be slaked down with water to achieve its original plastic properties.

1063°F
Quartz in the clay body changes in form, and the quartz crystals actually pivot on their axis to cause a large expansion. The clay body can crack if fired or cooled too fast in this temperature range, because not enough time is being allowed for the physical movement of the quartz transformation.(1)

The clay body goes through other changes, but the period from 212°F to approximately 1000°F (dull red heat in the kiln) statistically produces the greatest number of defects from fast-firing the bisque kiln.

At any point during these temperature ranges, the pottery can crack if fired too fast. The question then becomes: How fast is too fast? As a general guideline for most functional pottery forms not over 14 in. in height and/or thicker than ¼ in., a total firing time of 14 hours to cone 06 (1828°F) is recommended. Larger ceramic forms or sculpture that exceeds these dimensions should be bisque fired longer. Keep in mind that a longer bisque firing to prevent possible cracking poses no danger.

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