j.a.velez on tue 19 mar 02
Ivor:
Thanks so much for your very informative posting on the coefficient of
thermal expansion for glazes. One thing I had noticed in reading John and
Ron's book was that a significant difference exists between the coefficient
of thermal expansion predicted by the computer programs and the measured
value. The calculated values being generally higher and in one example in
their book by as much as 25%. I am somewhat surprised that the existing
calculations obey only to a proratio of weight percent of the individual
oxides in the mix. Obviously these can only be used as an approximation on
glaze design to provide a starting point that must then be refined by actual
testing and observation. Although I realize there are an infinite number of
possible combinations I wondered if it would be possible to empirically
"frame the range" of results based on the limit formulas, for instance. If
sufficient data is accumulated a data base could be build that would
"correct" the calculated value based on an interpolation of empirical
results. I have seen a similar approach used succesfuly in other
applications. With time if enough data is accumulated the unpredicatble
becomes predicatble. I am not suggesting we undertake this effort, just
thinking aloud.
At any rate, thanks again. Your point in pursuing the source of information
is well taken.
Regards, Jose A. Velez
John Hesselberth on tue 19 mar 02
Hi Jose,
While it might make an interesting doctoral thesis for a ceramic engineer,
accurate numbers would have little practical importance to a practicing
potter. What is important is relative difference between different glaze
compositions. Defining an accurate model in a system with this level of
complexity would probably be a lifelong journey that would require the
assistance of several supercomputers. Also, don't forget that the
usefulness of calculated numbers ends with the domain of glossy glazes.
When crystals begin to precipitate out during cooling the calculated numbers
are considerably less useful and can even be misleading.
On the other hand, if you would rather be working on this than making pots,
have at it.
Regards,
John
on 3/19/02 1:12 AM, j.a.velez at j.a.velez@COX.NET wrote:
> The calculated values being generally higher and in one example in
> their book by as much as 25%. I am somewhat surprised that the existing
> calculations obey only to a proratio of weight percent of the individual
> oxides in the mix. Obviously these can only be used as an approximation on
> glaze design to provide a starting point that must then be refined by actual
> testing and observation. Although I realize there are an infinite number of
> possible combinations I wondered if it would be possible to empirically
> "frame the range" of results based on the limit formulas, for instance. If
> sufficient data is accumulated a data base could be build that would
> "correct" the calculated value based on an interpolation of empirical
> results. I have seen a similar approach used succesfuly in other
> applications. With time if enough data is accumulated the unpredicatble
> becomes predicatble. I am not suggesting we undertake this effort, just
> thinking aloud.
Web sites: http://www.masteringglazes.com and http://www.frogpondpottery.com
Email: john@frogpondpottery.com
"Art is not a handicraft, it is the transmission of feeling the artist has
experienced." Leo Tolstoy, 1898
 
