Ron Roy on thu 30 jan 97
>----------------------------Original message----------------------------
>>Talc is used as a low fire body addition to encourage conversion of excess
>>free quartz to cristobalite to increase body expansion which reduces
>>crazing.
>>==========================================================
>>Tony Hansen
>
>Tony -
>OK, my turn again. I have always been under the impression that
>Cristobalite only forms above 2250 Fahrenheit. Am I right? And if so, how
>could talc encourage cristobalite formation in low-fire bodies?
>- Vince
First of all cristobalite starts to form when mullite starts to form. There
seems to be some confusion as to when this happens and my guess is - it
depends on the body composition. In most cases, I think it starts around
1100C (2000F.) When mullite is formed silica is ejected form the metakaolin
crystal (originally the clay crystal). This silica is a very small crystal
and forms the seed for further cristobalite to form on. This process
proceeds slowly at first and gathers speed. This and certain other factors
determine how much cristobalite is formed.
Second of all I have tested a low fire white body by itself and with
additions in our dilatometer. The body has 36.36% talc. There is no
cristobalite produced at cone 04. The only cristobalite I found was in the
samples to which I had added used firebrick grog. Those samples had some
cristobalite but it was there because it was in the grog.
I also measured those same tests fired to cone 6 and there was cristobalite
present in all of them. I have also found cristobalite in cone 6 bodies.
I have not finished my experiments but I must conclude that cristobalite
starts to form somewhere between cone 04 and cone 6.
Hamer says additions of Talc to low fire bodies inhibits crazing by being a
catalyst for early cristobalite production - and so do Taylor and Bull. As
you can see from the above experiments they could not prove it by me
although the Taylor and Bull text does say small amounts. I have seen
another reference which says this anti-craze action of Talc (or is it
Magnesia?) is due to the formation of another material but I can't relocate
it - does anyone else have any ideas.
Yeah! I know I said the Hamer book was great - and it is - I just goes to
show you - NO ONE is right all the time. I think it is better to want to
know than to just know anyway.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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