Bill Amsterlaw on sat 4 oct 97
Hi Ron:
How do you measure the amount of crystobalite in fired stoneware? How much
crystobalite is acceptable in stoneware?
- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY
Ron Roy wrote:
>>
I have yet to see a high iron clay body which does not have cristobalite. This
does not mean other types of bodies do not have cristobalite - that depends on
other factors which include amount of KNaO and fine silica.
I have never seen any cristobalite in porcelain.
<<
Ron Roy on mon 6 oct 97
>Hi Ron:
>
>How do you measure the amount of cristobalite in fired stoneware? How much
>cristobalite is acceptable in stoneware?
>
>
>- Bill Amsterlaw (wamster@msn.com)
> Plattsburgh, NY
>
>
>
>Ron Roy wrote:
>>>
>I have yet to see a high iron clay body which does not have cristobalite. This
>does not mean other types of bodies do not have cristobalite - that depends on
>other factors which include amount of KNaO and fine silica.
>
>I have never seen any cristobalite in porcelain.
><<
Hi Bill,
I have a dilatometer. The cristobalite shows up as a steeper incline
between 150C and 250C - the chart then proceeds in a more or less normal
incline till it gets to 573C where another "bump" happens as the alpha
quartz changes to beta quartz. The current Contact has an article by me on
the subject with copies of some charts showing how the information is used.
I can't answer the second question in a definitive way at the moment. As
usual the answer must be - it depends. If you have the same glaze inside
and out you can get away with a lot more. One of the problems with
cristobalite - because it goes through it's inversion at ovenware temps -
is a danger even if no glaze is present - one side goes through the
inversion before the other if it is heated unevenly. I guess you could say
for that purpose none or very little would be best.
If you have to work blind then testing your glazes on your clay will give
you the best indications of what works best. Testing methods should be
designed to aggravate the problems you want least.
I will be writing more on the subject as I progress with my experiments.
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
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