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 lustres.
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 a 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 to 1400 F.
The rate at which the carbon burns out is related to:
1) The amount of carbon present.
2) Amount of air available-air provides oxygen for burnout. Air needs to get to the carbon inside the body. This is impacted by several factors.
A load that is fired very quickly, or on fast, will not allow enough time for the oxygen to react with the carbon, form gases and leave the ware.
If ware is stacked during the bisque firing, oxygen may not be able to penetrate all surfaces or inside all the pieces.
If gases are not removed from the kiln and replaced with fresh air, then there may not sufficient oxygen to burn out the carbon. A kiln vent is recommended, not a over the kiln hood.
3) 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.
4) 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 than others. Oxidation should be completed below red heat which is around 1400 F.