Cracking and Thermal Shock

CRACKING AND THERMAL SHOCK

Cracks that appear in fired ware which were not caused by casting or drying problems may be the result of thermal shock. Thermal shock occurs when too much stress is created in a piece of ware during the heating and cooling process. It comes from temperature differences in the ware and can cause small to large cracks in the piece, or the piece may actually break.

The tendency of a piece to be susceptible to thermal shock is related to the strength of the piece and the thermal expansion of the material. Thermal shock can result when changes in temperature occur in the kiln during heating anc cooling. As temperature changes rapidly, the outside of the ware and kiln furniture becomes much hotter or cooler than the inside. This causes stresses which may result in cracking or breaking.
The following can effect thermal shock:

A fast heating rate or rapid cooling.
A sudden influx of cool air such as opening the kiln lid when the kiln has not finished cooling.
In a gas kiln, turning off the gas and allowing cool air from the burners to enter the kiln.

Thermal shock can also occur when ware is stressed in use, such as a casserole or dish that is taken from the freezer or refrigerator and put into a hot oven. The stronger ware is, the better able it is to resist cracks due to thermal shocking. Weak ware will be more likely to break when stressed. A piece that is porous will also be weaker, making it easier to crack. Water or condensation that enters pores in the ware can turn into steam and expand and this can cause cracking when heated. The harder (hotter) ware is fired, the less porous it will be.Ware that expands and shrinks a great deal during heating and cooling is also more likely to be affected by thermal shock. Most kilns shelves contain cordierite because this material has a lower expansion than most of our ware and so is less affected by thermal shock.

During heating and cooling, the body and glaze undergo many physical and chemical changes. Some of these include:

Moisture is driven out of the ware. If this occurs too rapidly, cracking can occur.
Organic material is oxidized and released from the material.
The glaze softens, melts and flows during heating and may trap gas.
The body expands as it is heated and contracts during cooling.
The glaze solidifies and contracts during cooling.

If the body or glaze contains silica, it will expand rapidly at 1063þF on heating and contract during cooling. If the heating or cooling is rapid near this temperature, this change can lead to cracking of the piece. Control of heating and cooling is especially critical when firing thick walled pieces or pieces with an irregular wall thickness.

There are several easy ways to minimize the potential for thermal shock:

Use a smooth, moderate heating rate.
Let the kiln cool naturally with the lid closed.
Use a controller to slow down the cooling time.
Avoid sudden temperature changes.

A programmable controller such as the Orton AutoFire is the best solution to control the heating and cooling rates and to get a smooth temperature rise. If instrumentation is not available, heat loss during cooling can be controlled to some extent by keeping the kiln closed until well below red heat (900ºF). To be sure that ware is properly matured, be sure to use witness cones. Underfired bisque will continue to shrink during the glaze firing and this can result in a poor glaze fit.

(Printed with permission of Orton firing Institute.)

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