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Thermal Shock And Cracking

Avoid sudden temperature changes.
It’s exactly like when you’re taking a warm bath: you fill the tub and then you settle down slowly and comfortably.
That’s what your Italian ceramic plates like:
being tempered with warm tap water before serving very hot foods or liquids.
It’s important to avoid possible thermal shock and cracking.



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Cracks that show up in fired pottery or ceramics 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 pottery or ceramic during the heating and cooling process.
It comes from temperature differences in the piece and can cause small to large cracks in the piece or the piece may actually break.

Why Does Cracking Occur?

Some of the causes that makes a piece of pottery or ceramic more apt to be damaged by thermal shock are 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 and cooling.
When temperature changes rapidly, the outside of the piece and the kiln furniture become much hotter or cooler than the inside.
This causes stresses which may result in cracking or breaking.

What Causes Thermal Shock?

A fast heating rate or rapid cooling.
A sudden surge 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 a piece 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 hotter the ware is, the better able it is to resist cracks due to thermal shocking.
Weak pieces 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 hotter a piece is fired, the less porous it will be.

A piece of pottery or ceramic that expands and shrinks a great deal during heating and cooling is also more likely to be affected by thermal shock.
Most kiln shelves are made of cordierite because this material has a lower expansion rate than most of our pieces and so is less affected by thermal shock.

What Happens During Firing?

During heating and cooling, the body and glaze undergo many physical and chemical changes.

Moisture is driven out of the pottery and ceramic, and if this occurs too rapidly, cracking can occur.

Organic materials are 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 1063° F when heated 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.

Reducing 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.

Do whatever is necessary to avoid sudden temperature changes.

A programmable controller is the best solution to control the heating and cooling rates and to get a smooth temperature rise.

If you do not have a programmable controller, heat loss during cooling can be controlled to some extent by keeping the kiln closed until well below red heat, which is about 900° F.

To be sure that the pottery or ceramic is properly matured, be sure to use witness cones.
Under fired bisque will continue to shrink during the glaze firing and this can result in a poor glaze fit.

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