| Orton Firing Tip: Avoiding Carbon Burnout Problems |
Reprinted with the permission
of the Edward Orton Jr. Foundation
Materials used in ceramics contain naturally occurring impurities
that can affect the color, appearance and maturing temperature of
the product.
Carbon, found in most clays, is normally considered one of these impurities.
Carbon can also be present in the additives and binders which make
up clay bodies, slips, decals and lusters.
How Carbon Burns Out
During heating (firing) the carbon reacts with oxygen to form carbon
dioxide and carbon monoxide gases. The carbon leaves the body as a
gas.
Binders are burned off at a relative low temperature: 300° F to 500° F.
Naturally occurring carbon in clay burns off (becomes gases) at higher
temperatures: up to 1200° F - 1400° F.
The rate at which this carbon burns out is related to:
- The amount of carbon present (that is, the amount of natural
contaminants in the body). Some bodies have more contaminants
than others, such as red clays. This needs to be considered when
planning the firing.
- Amount of air available (air provides oxygen for burnout)
- Air needs to be considered when planning the firing.
- This is impacted by several factors. A load that is fired
very quickly will not allow enough time for the oxygen to
react with the carbon, form gases and leave the ware.
- If ware is stacked during bisque firing, oxygen may not
be able to penetrate all surfaces of or inside all the pieces.
- Also, if gases are not removed from the kiln and replaces
with fresh air, then there may not be sufficient oxygen to
burn out the carbon.
- Thickness of the piece.
- Air has to penetrate through the entire thickness of the
piece and the gases have to escape the same way. It takes
longer for carbon to burn out of a thicker piece of ware.
- Time and temperature profile during the burn out period.
- Both time and temperature are important for proper burn
out of the carbon. Some carbons require much higher temperatures
that others. Oxidation should be completed below red heat
(1400° F).
- Carbon burns out from the surface first. As more oxygen
penetrates the body, them more carbon is reacted to form the
CO or CO2 gas and the burn
out process continues. If there is sufficient time, temperature
and oxygen, then complete burn out occurs. If these conditions
do not exist, the resulting incomplete burn out is referred
to as black coring (where the center of the piece has black
or gray cast).
Incomplete Burn Out
Incomplete burn out can result in several firing problems including:
- Bloating of the ware - if the temperature is hot enough, the
outside of the piece will seal up before all the gases can escape.
As the body becomes plastic due to glass forming, gases trapped
inside the body expand with heat and cause bloating and sometimes
cracking of the ware.
- Glaze defects, such as pinholes - the escaping gases will push
through the glaze surface and cause bubbles which pop. If these
do not heal, then pinholes will result.
- Appearance of fired bisque - where carbon burn out is incomplete,
the piece will have a grayish cast (white bodies) or may have
greenish cast (red bodies). The body will also be more porous
and weak.
Preventing Incomplete Burnout
- Slow Down Firing
- Be sure the kiln is vented adequately so there is sufficient
oxygen.
- Load the kiln with burn out requirements in mind.
Leave plenty of space between ware and shelves. Do not stack ware.
Use tile and plate stackers and invert pieces on top of one another
to help conserve space and insure proper burnout.
Want to learn more?
Read more about carbon related glaze and body defects 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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