Craig Martell on wed 7 jan 98
Hello Tom:
Thanks for responding to my post. I know about body-glaze relationships as
they relate to glaze fit and problems that arise from too great a difference
in the two. I'm not an expert on that, or a rock solid chemist, so I do
thank you for the time you took to type that post and help me out.
The thing I was having trouble with was the calculated expansion and
shivering problems that arise from too much lithium in some glazes. As I
understand it, 8.8x10-6 is a very high expansion glaze which would craze on
most bodies, yet the Wild Rose Tenmoku is shivering. Your idea of problems
arising in the area of the interface or boundary layer sounds very plausible
to me. I have done some reading in Parmelee and he says that Lithium has a
"high coefficient of dispersion" and on raw glazed ware will penetrate
several inches(must be on some really thick pots) into the porcelain
body. This action would surely affect the expansion of the glaze through
interaction with alumina and silica in the porcelain, as you suggest.
Another point Parmelee makes is that when replacing potassium in a glaze,
lithium reduces expansion when supplied in amounts up to .14 moles. Above
that amount, the reverse is true and the expansion increases. As I recall,
the Wild Rose glaze had somewhere around .35 or more moles of lithium and
this may account for the 8.8 expansion figure. But, the shivering is going
to originate in the interface of glaze and body, where the lithium is
interacting with the clay?? I also read that lithium has very small
molecules and and is a very compact oxide and this would allow penetration
into silicate chains to happen much easier than for molecules that are larger??
Anyway, thanks for the good thinking. I wouldn't have thought of that
myself. Well, at least not for a long time.
regards, Craig
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