Craig Martell on wed 19 nov 97
Hi:
The subject of blackcore and other problems stemming from the incomplete
burnoff of carbonates was recently hashed over and someone mentioned that an
electric kiln manufacturer stated that packing bisques too tightly could
cause this.
Anyway, I remembered reading in "Ceramic Science for the Potter" by Lawrence
and West about the subject of differential thermal analysis, which has a bit
of a bearing on this subject. In the book, this subject is discussed in the
chapter,"The Nature of Clays".
When subjected to heat, clays sometimes lag behind the kiln setting in
temperature while undergoing chemical change. This is called an endothermic
reaction. At other times, clays are actually evolving, or releasing heat
and are ahead of the setting in temperature. This is called an exothermic
reaction.
Certain types of clays exhibit endothermic and exothermic peaks that are
typical of that form of the clay mineral. In this way, differential thermal
analysis can be used to identify a type of clay. Kaolins do not exhibit an
exothermic effect between the temps of 572-932F. Ball clays do. There is a
gradual exothermic effect in ball clays that begins around 572F. and
continues until about 932F. This effect is due to the oxidation and burnoff
of carbonates. Lawrence states that this temperature range should be
traversed slowly to ensure that burnoff of the carbonates and the liberation
of CO2 is complete. Clay bodies containing ball clay, red clays, fireclays,
bentonites, and possibly secondary kaolins contain organic matter that will
oxidize and burnoff between these temperatures.
If you have a problem with incomplete burnoff, the temp climb can be
retarded or held up to 932F. or more until the job gets done. The more
pieces in the bisque, the more time it takes. However, kilns climb slower
anyway when there is more thermal mass to be fired.
This isn't the only big concern with bisque firing, and there are other
things to be concerned about but I just heard somebody nod off so I'd better
quit. Actually, I think it was me!
regards, Craig Martell-Oregon
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