iandol on fri 23 may 03
Dear Snail Scott,=20
Thank you for treating us to some real time information.
Perhaps some of the elements and their compounds which we use and =
contaminate our kilns with are not quite as active as some former Clay =
Commentators would have us believe.
If Lead Oxide(Litharge)is volatilising at top firing temperatures it =
seems logical to suggest that it with flux kiln refractories and form a =
glass. What has being suggested is that heat will cause this glass to =
decompose. Does Heat make Glass decompose? Does heat make a Glaze =
decompose? I would like to know if these are good questions to ask in =
the light of the suppositions written to the list?
Best regards,
Ivor Lewis
Louis Katz on sat 24 may 03
My wife uses lead glazes on the exterior of here work. It is usually
commercial (Duncan) glazes. She overfires them by several cones. They
fume the surrounding clay. The non lead glazes don't seem to do this.
Louis
iandol on sun 25 may 03
Dear Louis Katz,
It is not possible to say without qualitative analysis of fired samples =
what is fuming from the glazes you wife is using.
To say that it is metallic lead or one of several lead oxides which is =
causing a fluxing effect on bare clay or other glazes is an assumption. =
It may be true or it could be untrue.
Remember that Sodium and Potassium compounds are both capable of =
volatilising at moderate to high temperatures. Both will cause an effect =
on nearby bare clay.
Though your comparison shows a difference only knowledge of the mineral =
composition of the glazes would enable us to understand what is =
happening. Perhaps the leadless glazes which do not give the effect are =
being underfired.
Perhaps it is a mistake to overfire a commercial product without =
reference to the maker. It might invalidate any guarantee.
Best regards,
Ivor Lewis
Louis Katz on mon 26 may 03
True enough. I am not looking for proof of anything. I just offered my
observations and my conclusions, which I recognize could prove to be
false.
It could be that the lead helps the sodium compounds to volatize. It
could be that the lead has nothing to do with it. The fact that it
happens with her Duncan 04 lead bearing glazes and not with the lead
free ones does seem to make a suggestion.
It might be a mistake under many circumstances to overfire a commercial
product, or even a non commercial one.. These are overfired
particularly for the fuming..The teapots thave no holes in the spouts.
If you prefer them to be called images of teapots thats ok too.
Perhaps I answered a question wrong, or read a post wrong if so I am
sorry.
On Sunday, May 25, 2003, at 01:28 AM, iandol wrote:
> Dear Louis Katz,
> It is not possible to say without qualitative analysis of fired
> samples what is fuming from the glazes you wife is using.
> To say that it is metallic lead or one of several lead oxides which is
> causing a fluxing effect on bare clay or other glazes is an
> assumption. It may be true or it could be untrue.
> Remember that Sodium and Potassium compounds are both capable of
> volatilising at moderate to high temperatures. Both will cause an
> effect on nearby bare clay.
> Though your comparison shows a difference only knowledge of the
> mineral composition of the glazes would enable us to understand what
> is happening. Perhaps the leadless glazes which do not give the effect
> are being underfired.
> Perhaps it is a mistake to overfire a commercial product without
> reference to the maker. It might invalidate any guarantee.
> Best regards,
> Ivor Lewis
>
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