iandol on sat 5 may 01
Dear Kurt Wild,
Chlorine gas can be discharged form a salt glaze kiln (Not soda) towards =
the end of the process if the degree of reduction is so intense that =
there is no Oxygen in, or entering the kiln. Its presence in flue gas =
is obvious if it is there.
Daniel Rhodes in the 1st and 2nd editions of his book about clay and =
glazes that it is a hypothesis that salt and water react to give =
hydrochloric acid.
All the evidence which has been presented tends to point to Hydrogen =
chloride being formed during a secondary reaction. Barringer noted this =
as early as 1904. Silicon tetrachloride, Aluminium chloride, Ferric =
chloride all react with water to give acidic solutions of Hydrogen =
chloride.=20
By the way, when tested separately as pure materials Silica and alumina =
seem to resist the action of salt at cone 8. Kaolin is almost =
unreactive, ball clay a little more so. But potash felspar gives a =
superb surface finish. Which may explain why kaolin/aluminium hydrate =
mix makes a good resistive shelf wash in salt glaze kilns.=20
This is another technical aspect in ceramics arts where there is a lot =
of conjecture, assumption and opinion but very little in the way of =
definitive research to establish some facts.
Stephen Grimmer on sun 6 may 01
Ivor,
I did a little experiment this spring in our soda kiln. A slip of equal
parts (by weight) Kaolin and silica four was applied to tiles and fired in a
regular cycle. (Cone 11, moderate reduction, 4 lbs. soda ash in a 60 cubic
foot kiln.) Lo, and behold, this mix was as glassy and smooth as any
porcelain body in the kiln! Further, to eliminate any effect the underlying
body may have had, the slip was dried to plasticity and formed into tiles.
Same result: clear, smooth glass wherever soda contact occurred, but dry and
powdery on the bottoms of the tiles.
It would seem that feldspar is not needed for a good soda glaze.
Steve
--
Stephen Grimmer
Southern Illinois University at Carbondale
> From: iandol
> Reply-To: Ceramic Arts Discussion List
> Date: Sat, 5 May 2001 16:26:19 +0930
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: chlorine gas and salt firing
>
> By the way, when tested separately as pure materials Silica and alumina seem
> to resist the action of salt at cone 8. Kaolin is almost unreactive, ball clay
> a little more so. But potash felspar gives a superb surface finish. Which may
> explain why kaolin/aluminium hydrate mix makes a good resistive shelf wash in
> salt glaze kilns.
Iveragh Ceramics on sun 6 may 01
----- Original Message -----
From: "iandol"
To:
Sent: 05 May 2001 06:56
Subject: chlorine gas and salt firing
Ivor,
The magazine Ceramic Review some years ago had an article on this.
All I can remember is they had two kilns firing at the same time and were
measuring emissions, exactly which ones I cannot remember and as a member of
the "I don't like vapour glazes" club I was not very interested, but all
results were published in the article. You may be able to contact them about
this. I also remember the American Ceramics Monthly having an article on
this, I think from an industrial scientist, somewhere around the same time.
Sorry I cannot be more specific on dates but both articles did debunk some
of the myth regarding vapour glazing. No doubt more myth will be added in
time.
Regards,
Bob Hollis
This is another technical aspect in ceramics arts where there is a lot of
conjecture, assumption and opinion but very little in the way of definitive
research to establish some facts.
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