| Orton Firing Tip: Firing Ceramic Bodies |
Reprinted with the permission
of the Edward Orton Jr. Foundation
Most bodies and glazes contain clay. These fine clay particles give
the body and glaze many desired properties and bonds other materials
together.
When the body is fired:
- clay and other minerals in the body start to change
- clay/minerals break down and react with other materials to produce
gases
- at 900° F (red heat), tightly held water molecules begin to break
free and leave
- gases such as sulfur oxides and some fluorine may be released
- as the temperature increases, clay and other minerals continue to
change and react with each other to form new compounds that will be
part of the final product
- some products form glass which will bond everything together
Gases
The gases which form need to be removed from the body. For example,
carbon is in the clay and organics are added to the body, glaze or
decoration to improve strength during handling or application. These
must be removed during firing to avoid defects.
Firing Conditions
Firing conditions can also determine many properties of the fired
product. Firing too fast at lower temperatures may not allow sufficient
time for materials to react and gases to leave the body or glaze.
Firing too fast can result in
- weaker bodies
- pinholing
- bubbling of the glaze
- color changes in the body
- color changes in the decoration
- mildewing of porcelain
- crazing or peeling of glazes if body is not properly mature
Earthenware
- typically fired from Cone 07 - 03
- made with talc, less expensive clays
- clays contain many impurities, need fired longer at lower temperatures
- low shrinkage
- porous after firing
- usually tan or red in color
- frequently glazed or stained
- sometimes used as-fired
Often, problems arise because bodies are underfired. The piece may
look okay, but is porous and weak. Also, underfired bodies may not
match the expansion of the glaze used in a later firing. This can
result in glaze fit problems or cracking of the body in use.
The high iron and carbon content of these clays requires plenty of
air during firing to maintain good color and to burn out all of the
carbon. If this is not done, many problems can occur when the product
is glazed and refired.
Stoneware
- typically fired between Cone 6 - 10
- large number of compositions
- contain clays and other minerals with many impurities, including
sand, feldspar and grog
- additives are used to provide plasticity, workability, strength,
color, and to reduce shrinkage
- colors depend on raw materials
Because of the additives and impurities, care needs to be given to
how stoneware is fired and to proper ventilation of the kiln early
in the firing to burn out organics.
Stoneware is vitreous and contains a high percentage of glass in the
fired product. For color variations, mature the ware under reducing
conditions.
Porcelain
- typically fired from Cone 3 - 10
- compositions vary, but contain high quality materials
- colorants may be added
- bodies are hard, white, translucent
- very high glass content
- narrow firing range - need to be fired close to slump or sag
point for best fired properties.
Because color is very important, these bodies need to be fired with
plenty of air below red heat to be sure all the carbon is removed.
Shrinkage is high and special care must be given to supporting porcelain
during firing or it will warp and distort.
Critical Firing Periods
For all clay containing bodies and for most glazes and decorations:
- be sure ware is dry before firing
- fire slowly below red heat (1100° F) where many changes
occur in the clay and other materials
- provide plenty of air below red heat for oxidation and burn
out organics and carbon
- do not to force cool the kiln while it shows red heat.
Want to learn more?
Read more about successfully firing ceramic bodies 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 |
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