John Sankey on mon 20 aug 07
I've put up a description of how one can compare calculated glaze
thermal expansions with measured clay expansions at
http://sankey.ws/glazetest.html#COE
Critiques welcome, also any suggestions to make it clearer for
nontechnical Clayart readers. Experienced advice on differences
in expansions and the probabilities of shivering or crazing would
also be appreciated.
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Ivor and Olive Lewis on tue 21 aug 07
Dear John Sankey,=20
Accepting the evidence of W & S and others who have made a contribution =
to this topic there is one fact that puzzles me.
The Alkali Oxides have properties that prevent them been converted into =
bar specimens such as might be used in the equipment that Ron Roy uses.=20
I would like to know how Linearity values of these compounds were =
obtained. Are you able to explain this for me.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
John Sankey on thu 23 aug 07
Ivor Lewis asked:
"The Alkali Oxides have properties that prevent them been
converted into bar specimens such as might be used in the
equipment that Ron Roy uses. I would like to know how Linearity
values of these compounds were obtained. Are you able to explain
this for me."
They were done by starting with a base glaze, adding varying
proportions of the various oxides in turn, then measuring the
change in expansion coefficient. If you look at Appen, you will
see that he found this way that several oxides had variable rates
depending upon their concentration. However, Appen's value for
silica was done with a linear approximation from the maximum
concentration he could melt to the expansion for pure fused
silica. Ron Roy's glazes show that this transition occurs at a
higher concentration than Appen studied.
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Ivor and Olive Lewis on fri 24 aug 07
Dear John Sankey,
Thank you for that information.
I will need to work on that idea. .=20
Are you saying that, although the factor of a specific oxide is added =
fractionally to contribute to the total thermal linearity value of a =
glaze this relationship is not truly linear as temperature changes or as =
the fractional proportion changes?
If this is so, how can it be explained ?=20
Best regards,
Ivor
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