| Orton Firing Tip: Cracking and Thermal Shock |
Reprinted with the permission
of the Edward Orton Jr. Foundation
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.
Why Does Cracking Occur?
The tendency of a piece of be susceptible to thermal shock is related
to:
- the strength of the piece
- the thermal expansion of the material
Thermal shock can result when changes in temperature occur in the
kiln during heating and 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 kiln shelves
contain cordierite because this material has a lower expansion than
most of our ware and so less affected by thermal shock.
What Happens to Ware During Firing?
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 1063° F
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.
Reducing Thermal Shock
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 (900° F).
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.
Want to learn more?
Read more about cracking and thermal shock in the Orton Firing Line
and Technical Tips publications. Each issue is packed full of articles
to help you learn more about firing. Members of the Orton Firing Institute
receive these publications at no charge. Single copies are available
to non-members at a per issue rate. Orton's 80 minute video, Key
Principles of Successful Firing, is also an excellent resource
on firing.
For information on Orton products, see your Orton dealer or distributor.
For information on the Firing Institute or publications, contact:
Orton Firing Institute
PO Box 460
Westerville, OH 43086
(614) 895-2663 |
|
Bulletin Board &