Antoinette Badenhorst on sat 29 jan 05
I was wondering what happens to a piece of pottery when refired. All the =
firing processes with the first firing get repeated, but the "clay" is =
already transformed to "stone". I recently refired a large platter =
successfully to ^10, but can not help but wonder if the piece is not =
weaker than it was before.
Antoinette Badenhorst
105 Westwood circle=20
Saltillo, MS, 38866
(662) 869-1651
www.clayandcanvas.com
Ivor and Olive Lewis on sun 30 jan 05
Dear Antoinette,
Given that a glaze is always fired so that all possible chemical
reactions are completed and that clay bodies are fired so that most
chemical reactions are incomplete at your maximum firing temperature
then there should be no further change in the nature of your clay
body.
What you must remember is that when a fully mature glaze is reheated
it begins to melt at a much lower temperate than its original maturity
temperature. From the information in Mastering ^6 glazes this takes
place in the region of 7-800 deg C.
So, if your platter was unglazed, there should be no further change
when reheating to the original firing temperature. But if the platter
was glazed, once the glaze melts it may begin to interact with the
clay on which it sits. This may cause problems.
One other thing. Be cautious. If the clay body contains free silica
then you may cause the pot to dunt during reheating if the rate of
temperature increase is rapid.
John or Ron may have more to say
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Antoinette Badenhorst on sun 30 jan 05
I've experienced these things that you mention here before Ivor,
particularly with larger plates and that is what make me think that one
might end up with a weaker product. I wonder about the silica specifically.
Is it still free after cooling took place. All the other ingredients in the
clay also changed.
It is interesting that you say the glaze start melting much sooner. I
wonder why? I've seen it running on the shelves before during a second
firing, but most of the times it improved the glaze appearance on the pot.
Antoinette.
----- Original Message -----
From: "Ivor and Olive Lewis"
To:
Sent: Saturday, January 29, 2005 9:10 PM
Subject: Re: refiring, the science process.
> Dear Antoinette,
> Given that a glaze is always fired so that all possible chemical
> reactions are completed and that clay bodies are fired so that most
> chemical reactions are incomplete at your maximum firing temperature
> then there should be no further change in the nature of your clay
> body.
> What you must remember is that when a fully mature glaze is reheated
> it begins to melt at a much lower temperate than its original maturity
> temperature. From the information in Mastering ^6 glazes this takes
> place in the region of 7-800 deg C.
> So, if your platter was unglazed, there should be no further change
> when reheating to the original firing temperature. But if the platter
> was glazed, once the glaze melts it may begin to interact with the
> clay on which it sits. This may cause problems.
> One other thing. Be cautious. If the clay body contains free silica
> then you may cause the pot to dunt during reheating if the rate of
> temperature increase is rapid.
> John or Ron may have more to say
> Best regards,
> Ivor Lewis.
> Redhill,
> S. Australia.
>
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Ivor and Olive Lewis on mon 31 jan 05
Dear Antoinette,
Clay, that is the Kaolin or the Ball Clay fraction of a clay body goes
through several solid state reactions as it is heated, loses it
hydroxyl radicles from its structure (which emerge from the kiln as
Water). Then there is a structural change to metakaolin. This
decomposes to give some free silica and Mullite. Much of the silica
freed by this reaction reacts with the flux mineral to form a glass
which cements the whole mass. Now if the clay body has been grogged
with fairly coarse sand this may not have enough time to dissolve in
the melt, so it is left as free Silica which can change phase and
cause dunting. Ron Roy will tell us the good oil on the variety of
silica called Cristobalite.
When we make a glaze all the minerals melt into one reasonably uniform
mass which cools with a glass structure. Now it is my belief that
either the Alkali ions Na+, K+ and Li+ terminate silicate tetrahedra
or interference from Boron prevents them from reuniting. This makes
amorphous silicate glass groups smaller. Moderate heating causes them
to separate. So the glass always melts at a lower temperature than the
original minerals, even if some of the ingredients in the glaze were
frits. (I need to extend my sabbatical leave to write this one up as
well.)
Your observation about refiring a glaze is interesting and important.
This has been noted by people who salt glaze. They have found that
when refired, Salt Glaze smooths out had the orange peel surface is
lost. If you are writing about returning to the original maturity
temperature then the glaze is given chance to complete its original
chemistry.
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