Bob Masta on sun 12 dec 04
Ron Roy wrote:
> Hi Bob,
>
> I have a dilatometer - easy to do some testing if you really
> want to know about that - you make the clay and fire it and
> I'll measure.
>
> From my experience - and I have measured a lot of clay to
> date - over 400
> samples of clay bodies - raw clay and glazes - I would think
> that firing higher would increase the chances of crazing.
>
> As more heat is applied - more of the quartz in the clay
> goes into solution. Free quartz in a clay is an anti craze
> because of the inversion at 573C. Melt more of that quartz
> and you lose some of the contraction during cooling.
>
Ron, thanks for the offer to do dilatometer tests.
However, what you say makes good sense to me,
and I'd think that if vitrification actually caused
an increase in contraction then you certainly
would have seen it by now.
> On the other hand - how much the glazes gets bonded to the
> clay has an influence on both crazing and shivering -
> perhaps that is the effect you are seeing.
>
> As you increased the temperature to cone 2 - did you also
> change the glaze?
>
Yes, I did change the glaze. I was using the "Simple
Colemanite" glazes from Behrens' "Glaze Projects"
booklet because he had a nice series using the same
ingedients at increasing cone ranges. But I never did
any calculations of my own, so maybe the effect I was
seeing is that the higher-cone glazes just worked out
to have lower expansion.
Thanks for your insights!
Bob Masta
potsATdaqartaDOTcom
Ron Roy on mon 13 dec 04
Hi Bob,
I still would like to know what increased temperature does to expansion of
clay bodies - I have not done any experiments on this so what I said is
theory - if you do want to so the firing my offer is still good - it would
make a good article if you would like to do that - help our collective
understanding.
If you care to take the time I would like to calculate the expansions of
your glaze series - it will shed light on what happened in your tests.
RR
>Ron, thanks for the offer to do dilatometer tests.
>However, what you say makes good sense to me,
>and I'd think that if vitrification actually caused
>an increase in contraction then you certainly
>would have seen it by now.
>
>> On the other hand - how much the glazes gets bonded to the
>> clay has an influence on both crazing and shivering -
>> perhaps that is the effect you are seeing.
>>
>> As you increased the temperature to cone 2 - did you also
>> change the glaze?
>>
>
>Yes, I did change the glaze. I was using the "Simple
>Colemanite" glazes from Behrens' "Glaze Projects"
>booklet because he had a nice series using the same
>ingedients at increasing cone ranges. But I never did
>any calculations of my own, so maybe the effect I was
>seeing is that the higher-cone glazes just worked out
>to have lower expansion.
>
>Thanks for your insights!
>
>Bob Masta
>
>potsATdaqartaDOTcom
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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