iandol on wed 5 mar 03
Dear Fredrick Paget,
You say <shelves.>>
Would you please explain the chemistry on this one. The inference is =
that Talc will melt. I have always understood Talc to be a heat =
resistant material which could only be fused with great difficulty. =
Hamer says, in 1 st Ed, that it decomposes to become Magnesia and =
Silica. Someone might confirm this from an independent source tell us =
what he says in the latest edition.
Best regards,
Ivor Lewis
Fredrick Paget on wed 5 mar 03
Greetings Ivor,
I am not sure about the reason but we have had sad experiences at the
college with students bringing in a low fire talc containing body and
putting their creation in the hi fire loads. Bill Abright, our teacher,
always exhibits a nasty melted low fire pot melted into a puddle and stuck
firmly to two rather nice stoneware sculptures of other students.
These low fire bodies are clay and talc which is what the mistake kiln wash
consisted of. So I was extremely wary of firing it onto the new shelves.
You are right about the magnesia of course. It is one of the most
refractory oxides but talc and clay must play by some different rule than
the lone ingredients. Some sort of eutectic perhaps?
As always I appreciate your depth of knowledge,
Fred Paget
>Dear Fredrick Paget,
>You say <>Would you please explain the chemistry on this one. The inference is
>that Talc will melt. I have always understood Talc to be a heat
>resistant material which could only be fused with great difficulty.
>Hamer says, in 1 st Ed, that it decomposes to become Magnesia and
>Silica. Someone might confirm this from an independent source tell us
>what he says in the latest edition.
>Best regards,
>Ivor Lewis
From Fred Paget, Marin County, California, USA
Fredrick Paget on fri 7 mar 03
Dear Ivor,
You are right. The talc - kaolin mix does not melt and run.
Last night I was firing a cone 10 test kiln load and I put in a sample of
50-50 talc-EPK mixed in water and brushed very heavily on a tiny bisque
porcelain test bowl. with another tiny piece on top.
The mixture sintered and got very hard but flakes off when pried with a
pocket knife blade. The piece on top was not really stuck to the one
underneath.
That said, I don't think this mixture makes a very good kiln wash and I am
glad they got it off the shelves with only expenditure of some labour.
Always the experimentalist,
Fred
PS
An interesting thing happened this morning as I finished this email to Ivor
with the test bowl sitting on the table beside the computer. It suddenly
spontainously shattered itself into 4 pieces. The talc-EPK fired on it must
have a very different expansion like some of the glazes RR is often warning
us about. I took the test piece out of the cooling kiln at about 200 C.
using tongs and it was just finished cooling as I wrote.
>Dear Friends,
>This is the essence of a message I sent to Fred Paget
><>temperature of the body but I doubt if freedom of melting has anything
>to do with a true Eutectic. To get those you with only Silicon Dioxide
>and Aluminium Oxide as the other precursor molecules you would need to
>have Cordierite and Tridymite or Cordierite and Forsterite as the
>other compounds and even then, you are melting above 1300=B0C. It would =
>not work just as a raw mix of Kaolin, Silica and Talc. However, you will =
>get sintering
>and you might get a secondary reaction from glaze residues on a shelf
>fired to cone 6 or higher using Talc in a kiln wash.
>I am sure a Low fire Talc Body would have had some other fluxing
>agent in it. A quick squiz through Chappell leaves me with a suspicion
>that most Low temp Talc bodies incorporate a frit. In addition, some
>employ Sodium Carbonate. Once these melting fluxes activate they form
>a solvent in which Talc will freely dissolve. So there would be a
>massive increase in fluid volume, though viscosity and surface tension
>might be fairly high if there were significant volumes of Talc, there =
>would be sufficient fluid to allow the work to deform under the effects =
>of gravity.>>
>Best regards,
>Ivor Lewis
>
>Nice to have a chat.
>Best regards,
>Ivor.
>
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From Fred Paget, Marin County, California, USA
iandol on fri 7 mar 03
Dear Friends,
This is the essence of a message I sent to Fred Paget
<temperature of the body but I doubt if freedom of melting has anything
to do with a true Eutectic. To get those you with only Silicon Dioxide
and Aluminium Oxide as the other precursor molecules you would need to
have Cordierite and Tridymite or Cordierite and Forsterite as the
other compounds and even then, you are melting above 1300=B0C. It would =
not work just as a raw mix of Kaolin, Silica and Talc. However, you will =
get sintering
and you might get a secondary reaction from glaze residues on a shelf
fired to cone 6 or higher using Talc in a kiln wash.
I am sure a Low fire Talc Body would have had some other fluxing
agent in it. A quick squiz through Chappell leaves me with a suspicion
that most Low temp Talc bodies incorporate a frit. In addition, some
employ Sodium Carbonate. Once these melting fluxes activate they form
a solvent in which Talc will freely dissolve. So there would be a
massive increase in fluid volume, though viscosity and surface tension
might be fairly high if there were significant volumes of Talc, there =
would be sufficient fluid to allow the work to deform under the effects =
of gravity.>>
Best regards,
Ivor Lewis
Nice to have a chat.
Best regards,
Ivor.
Ron Roy on sat 8 mar 03
Hi Fred - as I said - high MgO equals low expansion.
One use of this technology is for kiln shelves and furniture - It's called
corderite. The combination is also used for flame ware - anywhere a low
expansion body or glaze is called for.
Lithium can have the same kind of results - best for all of us to
understand this particular subject - what we don't want is our ware
cracking and/or sliveres of razor sharp glaze popping off into food.
It is why some potters welcome the occasional bit of crazing - they know
they are not dealing with a low expansion glaze. We can do better than that
now - expansion can be calculated for many of our glazes so we can tell in
advance if we can expect problems.
RR
>Fred
>PS
>An interesting thing happened this morning as I finished this email to Ivor
>with the test bowl sitting on the table beside the computer. It suddenly
>spontainously shattered itself into 4 pieces. The talc-EPK fired on it must
>have a very different expansion like some of the glazes RR is often warning
>us about. I took the test piece out of the cooling kiln at about 200 C.
>using tongs and it was just finished cooling as I wrote.
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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