I.Lewis on sun 26 dec 99
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Louis H. Katz wrote
----------------------------Original message----------------------------
I always assummed that it was a reaction with the binder and silica and the
Soda, bubbling away just like any other glaze loaded full of Silicon =
carbide.
A little more information to hand on this one. Seems my speculation has some
substance after all, more to follow.
When dealing with technical problems I always ask about assumptions. Which
binder are we talking about here? Do you mean the stuff which glues the =
silicon
carbide grains together? Do you mean the siccative you put in the synthetic
reduction copper reds?
Facts from my chemistry book. Silicon carbide oxidises when it is heated to =
high
temperatures, such as those we have in a ceramic kiln. This leaves a residue=
of
silicon dioxide, silica, which tends to form a protective layer and prevent
further oxidation. However, in the presence of molten hydroxides or =
carbonates,
oxidation is much more rapid because of the formation of Na2SiO3. Free =
Chlorine
attacks silicon carbide vigorously, yielding Silicon Tetrachloride and =
Carbon
tetrachloride at temperatures above 1000C (1832F). My speculation is that =
sodium
chloride, which is an ionic compound, would also attack silicon carbide =
because
of the action of the chloride ion in the molten state on the exposed silica =
as
well as on the silicon carbide. Release of these gases would give the =
frothing.
Now, has any one ever seen the surface of clay frothing after they have been
salt glazing. Has any one thought to look for that. I believe that at the =
end of
the salt reaction that dissolved gas is released from the glaze as it cools =
and
that the bursting bubbles leave that typical orange peel effect because the
glaze has congealled sufficiently for the surface to stiffen so that is does=
not
flow out smooth.
So I hold to my original view. In the long term, for firing in salt or soda,
silicon carbide shelves would have a limited lifespan. If you can write them=
off
against tax breaks then the expense might be justified.
Ivor Lewis. Still inquisitive. Searching for answers.
Hank Murrow on mon 27 dec 99
>----------------------------Original message----------------------------
>Now, has any one ever seen the surface of clay frothing after they have been
>salt glazing. Has any one thought to look for that. I believe that at the
>end of the salt reaction that dissolved gas is released from the glaze as
>it cools and that the bursting bubbles leave that typical orange peel
>effect because the
>glaze has congealled sufficiently for the surface to stiffen so that is
>does not flow out smooth.
>
>So I hold to my original view. In the long term, for firing in salt or soda,
>silicon carbide shelves would have a limited lifespan. If you can write
>them off
>against tax breaks then the expense might be justified.
>
>Ivor Lewis. Still inquisitive. Searching for answers.
Dear Ivor; I have had lifespan problems in the salt fire with only one
batch of SiC shelves in 40 years. They were a batch supplied by
Harbison-Walker which failed after one to three firings at Anderson Ranch
in the early seventies. I am still using shelves purchased at that time
from Ferro refractories which did not fail then or now. I believe that the
'orange-peel' surface which you note is caused by the relatively high
Alumina content of the glaze formed by the body with Sodium. As evidence, I
offer the nearly orange-peel free surface of high Silica/low Alumina
porcelain bodies in the salt firing. Alumina has a very high coefficient of
surface tension, producing the characteristic 'pebbly' surface where it is
in high concentration in the body. Passing the millenium Quietly in Eugene,
Hank Murrow
Vince Pitelka on tue 4 jan 00
>So I hold to my original view. In the long term, for firing in salt or soda,
>silicon carbide shelves would have a limited lifespan. If you can write
them off
>against tax breaks then the expense might be justified.
Ivor -
I can only speak from my own experience. We scrape the thin, scaly foam off
our silicon carbide shelves with very little effort after every firing, and
there is no noticeable degradation of the shelf surface at all, after
hundreds of salt firings. The foam seems to be a surface deposit, and does
not interact with the internal structure of the shelves.
Best wishes -
- Vince
Vince Pitelka
Home - vpitelka@DeKalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
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