Fabienne on sun 8 mar 98
Hello,
I have been doing some reading on Sio2 and its various phases and forms. I
think I am on track by affirming that the cooling cristobalite inversion is
crucial when considering glaze fit, i.e. no crazing or shivering. In other
words, the glaze should be under compression, which goes along with the
glaze having a lower thermal expansion than that of the body or the glaze
shrunk less than the body. Let me know if I am delirious.
Now, according to Hamer's Dictionary on p 310, this compression is
"desirable for earthenware and tableware ... not desirable for ovenware ...
not necessary for true stoneware."
I haven't read the whole book and may have missed something besides
misunderstood part of it. But, after reading a zillion cross-references
trying to understand why this statement, I ended up almost empty handed --
especially when dealing with stoneware, which is what I use, and tableware,
which is not defined and I am learning that anything goes. Does anyone
have an explanation for their statement?
TIA :)
--
Fabienne | Yes, I have learned from my
http://www.vivelafrance.org | mistakes...
| I can reproduce them exactly.
Ron Roy on mon 9 mar 98
Hi Fabienne,
This could be a long answer but I'll keep it short - if you need more let
me know.
1. Cristobalite goes through it's reversible expansion and contraction
during ovenware temperatures - my dilatometer says between 120C and 200C
approximately - so it should be kept to a minimum for this type of ware.
2. Never have seen any in porcelain.
3. Hardly ever seen any at cone 6
4. Usually some in stoneware at cone 10 reduction and oxidation - sometimes
a lot.
5. Never any at come 04.
There is a controversy among us as to whether some is desirable for glaze
fit. There are some glazes which will not craze only when there is some
present. Too much combined with a low expansion glaze will lead to dunting.
You can have glaze compression without it.
It is easier to fit glazes at higher temperatures because there can be more
alumina and silica in the glaze thereby lowering the expansion of glazes.
>I have been doing some reading on Sio2 and its various phases and forms. I
>think I am on track by affirming that the cooling cristobalite inversion is
>crucial when considering glaze fit, i.e. no crazing or shivering. In other
>words, the glaze should be under compression, which goes along with the
>glaze having a lower thermal expansion than that of the body or the glaze
>shrunk less than the body. Let me know if I am delirious.
>
>Now, according to Hamer's Dictionary on p 310, this compression is
>"desirable for earthenware and tableware ... not desirable for ovenware ...
>not necessary for true stoneware."
>
>I haven't read the whole book and may have missed something besides
>misunderstood part of it. But, after reading a zillion cross-references
>trying to understand why this statement, I ended up almost empty handed --
>especially when dealing with stoneware, which is what I use, and tableware,
>which is not defined and I am learning that anything goes. Does anyone
>have an explanation for their statement?
>
>TIA :)
>--
>Fabienne | Yes, I have learned from my
>http://www.vivelafrance.org | mistakes...
> | I can reproduce them exactly.
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
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