Ivor and Olive Lewis on sat 12 may 07
Dear Friends
Other than the observation of colouration on adjacent pottery is that =
any substantive evidence to indicate the nature of the material or =
substance responsible for this effect?
One possibility that may have been overlooked are the notes of Michael =
Cardew who provides information about Sintering (Pioneer Pottery pp =
64-5) giving a formula for calculating the temperature of the onset of =
the interaction between finely ground mineral particles without the =
necessity for a fluid to assist adhesion. This is known as the Tamman =
Temperature.
A more complete explanation for the interactions that promote the =
sintering process is given by Kingery et al (Introduction to Ceramics pp =
469-75). In this text the authors describe several physical processes to =
account for the transfer and interaction of the substances involved. One =
of these is Evaporation-Condensation due to the transport of vapour from =
one surface to another.=20
Knowing that vapour can emanate from a material and that the temperature =
at which this will happen can be ascertained, consider the temperature =
at which Chromium Sesquioxide, Cr2O3 mp 2435 deg C, will commence =
sintering. Using the simple calculation given by Cardew leads to a range =
of 1262 to 1352 deg C. Consider black Copper oxide, CuO mp 1235 deg C. =
The range within which sintering, and hence evaporation from the solid =
will commence would be at some temperature between 592 to 632 deg C.
This information provides evidence to support the opinion of Ron Roy =
about the nature of Copper oxide but to me it suggests that there must =
be some other mechanism, some form of Chemistry, to explain motion of =
Chromium sesquioxide, which is green, across space to create a red =
blush.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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