iandol on fri 13 oct 00
Dear Chris Schafale,
Whatever the arguments put forward about the best bisque temperature, =
proponents of either view have to come up with a firing program which =
ensures two things.
One is that all the chemicals that are natural constituents of a clay =
which decompose under the influence of heat and release any gas, such as =
fluorides, sulphates, carbonates, hydrates and chlorides, have completed =
their reactions. Of these, I suspect Calcium fluoride and Calcium =
sulphate would be the last to decompose due to heating. I cannot find =
information about the temperatures which these reactions occur.
The second is that fluid phase sintering, the process which cements the =
grains of clay, sand, flux and grog together, has started but not =
advanced to a stage which could be regarded as incipient vitrification. =
This ensures that the bisque has enough porosity to suck glaze slurry to =
the surface by capillary action. A slow even firing ensures uniformity =
all around the surface of the pot and therefore an even glaze coating, =
assuming that the glaze slop is uniform in character.
The temperature at which the first condition is assured is about 900 Deg =
Celsius, a bright red heat, but it may be higher. Someone with access to =
the handbook of chemistry and physics might find these values.
The second condition, fluid phase sintering, depends upon many factors, =
not least of which is the presence of impurities in all contributing =
compounds, Sodium ions being the most important. Elemental Sodium has an =
average presence in crustal rocks of 2.3 * 10 exp 4 parts per million, =
or 0.023%. Some people might claim this, stated as oxide, is sufficient =
to initiate melting as low as 695 deg Celsius. In some felspars, sodium =
is recorded in significant proportions. So it is not improbable that a =
fragile bisque could form at a dull red heat. But it is the upper =
temperature which is the important judgement, the one at which =
vitrification has commenced and progressed sufficiently to give =
structural integrety but not to the extent that it occludes inherent =
porosity. This temperature is not always known.
Pundits who recommend high bisque temperatures are approaching this =
reaction point and leave little margin for error. Cautious potters go =
for the lower cone values. The idea of having a soaking period would =
seem to be a waste of fuel if there are no further chemical reactions =
taking place. Furthermore, soaking might continue to promote fluid phase =
sintering processes, so increasing the degree of vitrification and =
reducing porosity.
I have bisqued at cone 08 for about twenty five years regardless of the =
clay and, touch wood, so far not had any of the problems spoken of on =
this topic.
Ivor Lewis, Redhill, South Australia
| |
|
