Robert Klander on fri 11 jul 03
Here's a novice question from a novice potter, and I'll be checking the
archives when I have time and energy in a couple of days...That said, if
anyone wants to throw me a bone...
I have been exploring some glazes as I add them to my glazechem software.
I've lately taken begun to at least looking at the limit formula graph
whenever I have investigated a recipe. I've been reading clayart for
almost a year, and try to read all I can about glaze formulation as it comes
up. (Whether I understand it or not, I read it.) I was noticing that a
couple of cone 6 glazes fit within Cushing's cone 9-10 limits, yet a couple
of elements are outside cone 6 limits... Is it possible that these glazes
could perform well at a higher temperature?
I plan to mix a few small test batches and give them a try, but thought it
would be interesting to post the question to the group...
Thank you, one and all. Have a great weekend.
Robert Klander
Paul Lewing on sat 12 jul 03
on 7/11/03 7:34 PM, Robert Klander at papabo@CITLINK.NET wrote:
> I was noticing that a
> couple of cone 6 glazes fit within Cushing's cone 9-10 limits, yet a couple
> of elements are outside cone 6 limits... Is it possible that these glazes
> could perform well at a higher temperature?
Yes, it's possible. Keep in mind that any set of limit formulas is a very
general, rule-of-thumb kind of guideline. Almost every set of limit
formulas out there is based on what will melt, not on any kind of scientific
testing. The one exception I know of to this is the set in Ron and John's
"Mastering Cone 6 Glazes" book. They actually did some testing. Keep in
mind also that almost all the really cool glazes have at least one oxide
that's outside of normal limits.
One of the most popular cone 6 glazes out there, Pete's Weathered Bronze, is
actually a cone 10 glaze. Some glazes have a really broad firing range,
some as much as 6 or 7 cones, in which they look pretty much the same.
Others are radically different looking with only one cone of variation in
temperature. If there is a way to tell which are which from looking at any
set of numbers, be it recipes, molecular formulas, or limit formulas, I
don't know it.
And keep in mind that there's not much agreement on what "performs well"
might mean.
So you're right, you'll have to test them all. But you knew that, right?
Paul Lewing, Seattle
iandol on sun 13 jul 03
Dear Robert Klander,
Limit formulae or ranges are for guidance.They are not impressed in =
tablets of clay and fired for all posterity.
Perhaps it is important to be critical when you are evaluating glazes. =
If things seem out of kilter be sure you are aware of the purpose for =
each substance in a glaze as well as the purpose of each element, =
expressed as an abstract oxide. There are several other questions to =
think about. Why are some compound given low limit maxima while others =
are given much higher values?...What is the order in which these =
materials will melt?...How does each one affect the thermal behaviour of =
the other ingredients?... What are the qualities each brings to the =
vitreous melt?...What qualities does each bring to the cooled =
glaze?...Are some things infinitely soluble in a Silicate Melt or do =
they have a limited solubility?...How will increasing or decreasing the =
maturity temperature change the qualities of the glaze and its =
association with the clay it sits on?...What are the effects of =
variations in firing programs?...How do you determine if a dwell time at =
temperature should be introduced?...or to hold at a particular =
temperature to get Isothermal Transformations?
Only one way to find out. Consistent advice is "Test..Test.. Test.. Be a =
critical observer and keep scrupulous records"
Best regards,
Ivor Lewis. Redhill, South Australia.
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