| Orton Firing Tip: Firing Handbuilt or Thick Cast
Ware |
Reprinted with the permission
of the Edward Orton Jr. Foundation
Most pinch pots, coiled or slab built ware generally have thicker
walls than their slip cast cousins, although molded pieces may be
cast heavily as well. With these types of pieces, the thicker walls
create some unique challenges for firing.
Basic problems that can occur when firing handbuilt or thick cast
ware include cracking (or exploding) and carbon burnout. Because of
the thicker walls it is important to fire slower and control heating
and cooling during firing. Preparation of the piece is important as
well.
During forming, stresses within the piece may result in hairline cracks
that appear during firing. It takes longer to fully dry a thick piece.
Uneven drying can result in warping or cracking.
For pieces properly prepared, handled and dried, the next critical
step is firing.
Firing Issues
- Is the ware fully dry?
Ware that is not adequately dried will crack or explode during
the early stages of firing. Water inside the pores of the ware
turns to steam, exerting pressure inside the ware. To fully dry
a thick walled piece, the ware needs to be warm for more than
12 hours.
- Am I firing too fast?
All bodies expand when heated and shrink when cooled. If the outside
wall expands more than the inner wall, stresses occur. If these
stresses are large enough, they pull the body apart and cause
cracking. A 1" thick wall can have more than a 10° F
difference in temperature between the hotter and cooler surfaces.
Firings need to be slowed down for thicker wall pieces. Likewise,
it is important not to cool too fast.
- Have I allowed enough time for carbon burnout?
It is important to burn out all carbon from the ware before higher
temperatures are reached (1200° F or 650° C). It takes
time for oxygen to move into the pourous body, react with the
carbon and then leave. If carbon remains, many problems can occur.
These include problems with colour, glaze fit, strength, blistering
and discolouration. Use of a downdraft vent system, combined with
slower heating, virtually eliminates carbon related problems.
Heating & Cooling Control
The best way to control cracking problems during firing is by controlling
the rate of heating and cooling for the kiln. During firing, materials
that make up the body undergo many changes. Special care must be taken
at temperatures below 1500° F (815° C) to heat the body uniformly.
Remember, the thicker the wall, the slower the heating should be done.
Above 1500° F temperatures can be increased more rapidly because
the changes are less likely to causes stress cracks within the ware.
- What Kind of changes occur?
All clays can many minerals contain water which does not leave
the body until above 700° F. Organic (carbon) materials need
to be oxidized (burned out). Other minerals, such as calcite,
break down and give off a carbon dioxide gas. Minerals such as
flint (silica) undergo a sudden expansion on heating to 1060°
F and contraction during cooling.
- How can I control my heating?
This depends on the controls for the kiln. With switches, leave
them on medium settings longer. It should take more than 3 hours
to reach red heat and even longer for thick pieces or a heavily
loaded kiln. Make sure the kiln is well vented below red heat
and closed up completely above red heat. Keep the kiln closed
during cooling for 8 hours or until well below red heat.
- When did cracking occur?
Often the crack itself can be examined to determine when it occurred.
If the edges are sharp, then it probably occurred during cooling.
If the edges are rounded or if the glaze has flowed into the crack,
then it occurred during heating.
- What else can cause cracking?
- Uneven heating is a primary culprit that causes cracking
during firing.
Hot and cold spots in the kiln can cause uneven heating of
pieces.
Use witness cones to diagnose hot and cold spots and then
adjust the switching or use a downdraft vent to help even
out the heating.
Careful loading of the ware in setters and on stilts can also
help heat circulate around the piece.
- Underfired bisque is not as strong and may crack more easily
during the glaze firing.
Use witness cones to assure a proper firing and prevent underfired
bisque.
- Gas expanding in air pockets which developed in the ware
during forming can cause large cracks during firing.
Want to learn more?
Read more about firing handbuilt and thickcast ware 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 &