iandol on tue 29 oct 02
Such an interesting discussion which seems to be devoid of factual =
information.
It also raises some alarming questions about the purpose or function of =
Barium Oxide in a glaze as well as the behaviour of Barium Carbonate =
which is the prime source of Barium used by we Potters.
Is there any one who has real values for the solubility of either of =
these materials in a silicate melt? Or is it almost insoluble in a =
silicate or borate melt at the firing temperatures we use? Does it =
remain as a free oxide which remains after the decomposition of the =
carbonate at some temperature below 900=B0 C without reacting in any way =
whatsoever with any other compound in the glaze recipe?
Since Barium oxide is freely soluble in water, we well might predict =
that any pottery surfaced with a high is Barium glaze is a disaster =
waiting to happen if put in any environment where the humidity is high, =
where it is subject to rainfall or if it is submerged in water.
Anyone faced with the prospect of putting Barium compounds into a glaze =
should have a clear answer which justifies their inclusion. If it is not =
your own recipe, ask why the inventor of that recipe incorporated it. =
There are only two acceptable answers as far as I can judge. The first =
is that it gives some special quality the glaze which is unobtainable =
from any other element. The second is that it gives colours from certain =
oxides which can be obtained in no other way.
Think carefully about what I have asked.
Best regards,
Ivor Lewis. Redhill, South Australia.
Edouard Bastarache on wed 30 oct 02
Hello Ivor,
have you read M.Banks'article on barium glazes?
http://perso.wanadoo.fr/smart2000/BaO.htm#MBanks_english
Later,
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
Philip Poburka on wed 30 oct 02
Part of me reads this as... "Bury 'um in Glazes"
...and...well...just a chuckle...as with so much Crematory
and 'Urn' considerations of late...the association
particularly seems to jump out...
Phil
el vee
----- Original Message -----
From: "Edouard Bastarache"
To:
Sent: Wednesday, October 30, 2002 10:03 AM
Subject: Re: Barium in Glazes.
Hello Ivor,
have you read M.Banks'article on barium glazes?
http://perso.wanadoo.fr/smart2000/BaO.htm#MBanks_english
Later,
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
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iandol on thu 31 oct 02
Dear Edouarde,
No fault to find with what Taylor says except to question the assumption =
that it could exist in a glaze as Barium Oxide inclusions.
It will depend on the temperature, and may depend on the partial =
pressure of Carbon Dioxide. Given that the partial pressure of Carbon =
Dioxide in a kiln fired with hydrocarbons will be fairly high in a kiln =
which was freely vented to atmosphere, then it would seem that the =
dissociation temperature of Barium Carbonate would be similar to that =
given by Greenwood and Earnshaw, which is 1360=B0C for a pressure of 1 =
atm CO2. As these authors say the value for Calcium Carbonate is 900=B0C =
it might be that the value for the decomposition of Barium Carbonate =
quoted in another popular source for potters of "At least 900=B0 could =
be in error.
An interesting point is the value given for the decomposition of =
Strontium Carbonate, of 1298=B0C at the same pressure, again, well above =
the popular range of cone values for glazing ranges.
So big questions hang; What is going on? Are we correct to consider =
that these two Alkali Earth Carbonates should be though of as fluxes? =
What about Strontium Carbonate? If it remains as a carbonate, will it =
also be attacked by dilute acids? Would substitution, as is popularly =
recommended, be a safe option?
Perhaps there is much in the compass of Clayart Knowledge which needs to =
be revaluated and tested.
Best regards.
Ivor Lewis.
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