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Carbon Burnout Problems And Solutions

Materials used in ceramics contain naturally occurring impurities that can affect the color, appearance and maturing temperature of the piece.



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Carbon which is found in most clays is 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 firing the carbon reacts with oxygen to form carbon dioxide and carbon monoxide gases.
The carbon leaves the pottery as a gas. Binders are burned off at a fairly low temperature such as 300º F to 500º F.
Naturally occurring carbon in clay burns and becomes gases at the higher temperatures up to 1200º F or 1400º F.

The rate at which this carbon burns out is related to the amount of carbon present in the clay.
Some clays have more contaminants than others such as red clays and this needs to be considered when planning the firing.

Air provides oxygen for burnout and needs to be considered when planning the firing. This is effected by several things, 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 piece of pottery.
If the pottery is stacked tight during bisque firing, oxygen may not be able to penetrate all surfaces of or inside all the pottery.
Also, if gases are not removed from the kiln and is replaced with fresh air then there may not be sufficient oxygen to burn out the carbon.

Thickness of the piece


Air has to penetrate the entire thickness of the piece and the gases have to escape the same way.
It always takes longer for carbon to burn out of a thicker piece of pottery.
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 which is about 1400º F.

Carbon burns out from the surface first and as more oxygen penetrates the body, then more carbon forms the CO or CO2 gas and the burn out process continues.
If there is enough 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, which is where the center of the piece has a black or gray cast.

Incomplete Burn Out

Incomplete burn out can result from several firing problems including, the bloating of the pottery, 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 pottery.

Glaze defects, such as pinholes, which are cause by the escaping gases, will push through the glaze surface and cause bubbles which pop.
If these do not heal, then pinholes will result.
When ever I glaze a piece of pottery inside and out I never glaze the bottom, which leaves an escape route for any gases or steam.
The pottery will have the appearance of fired bisque when carbon burn out is incomplete and a white clay piece will have a grayish cast and red clay pieces may have a greenish cast.
The body will also be more porous and weak.

Preventing Incomplete Burnout

The best solution for this is to just slow down the firing and be sure the kiln is vented adequately so there is enough oxygen.
When loading the kiln be sure and leave plenty of space between the pottery and the shelves.
Do not stack the pottery, use tile and plate stackers and turn the pieces upside down on top of one another to help conserve space and insure proper burnout.
Now, if you are glazing your pottery, you cannot stack them, because they will fuse together or to the shelf and you will have a mess.
You will have to stilt or dry foot them and make sure they don't touch each other.

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