I.Lewis on mon 27 dec 99
------------------
After reviewing the information available to me I think I would consult the
manufacturer about the suitability of Silicon Carbide as a desirable =
material to
use as shelving in either Salt or Soda Glazing.
Although potters using Silicon carbide shelves and bricks in a variety of
situations may experience and ignore foaming, believing that it is of no
consequence, I continue to believe that there may be potential for grain
boundary degradation which could eventually lead to failure. The problem =
would
be associated with the binding material which is used in fabrication as well=
as
the gross attack on the Silicon Carbide.
>From what I can gather there are three possible binding agents used in the
manufacture of Silicon Carbide kiln furniture. These are Bonding Clay, Ethyl
Silicate and Silicon Nitride.
Bonding clay forms a ceramic cement during firing and I understand they are =
high
alumina, low silica materials. My experience of testing Kaolin is that they =
are
almost inert in a salt firing but I have not repeatedly refired any test =
pieces.
Ethyl Silicate decomposes when heated leaving micronised highly active =
silica
behind which sinters the silicon carbide into a solid mass. Presumably, =
Silicon
Nitride also acts as a sintering agent.
It is known that Silicon Carbide reacts strongly with oxygen forming carbon
monoxide and free silica at temperatures above 1000C (1832F). In an Air
atmosphere the silica which forms acts as a self protective coating which
prevents further attack. But free silica reacts vigorously with molten
hydroxides, and carbonates of alkali metals to form silicates. It is also =
known
that silicon carbide reacts with all of the Halogens. With Chlorine, Silicon
Carbide forms Carbon tetrachloride and Silicon tetrachloride. It seems to be=
a
common experience that Sodium chloride also attacks it. My assumption is =
that
gases which evolve, including water vapour and carbon monoxide are =
responsible
from creating the foam. And I can see no reason why silica based binding =
agents
should be immune to this degradation.
Regards to all,
Ivor Lewis. Inquisitive as usual, knowing that the wrong answer may come if =
we
do not ask the right question.
I.Lewis on tue 28 dec 99
------------------
Silicon carbide shelves and salt glazing
After reviewing the information available to me I think it would pay to =
consult
the manufacturer about the suitability of Silicoan Carbide shelving for =
either
Salt or Soda Glazing.
Although there may be potters using Silicon carbide shelves and bricks in a
variety of situations who experience and ignore foaming, believing that it =
is of
no consequence, I continue to maintain that there may be potential for grain
boundary degradation which could eventually lead to failure. The problem =
would
be associated with the binding material which is used in fabrication as well=
as
the gross attack on the silicon carbide.
>From what I can gather there are three possible ways of making Silicon =
Carbide
kiln furniture, all of which involve pressing a mixture of the synthesised
refractory with a lubricant and a binding agent. I know of three possible
binding agents. These are Bonding Clay, Ethyl Silicate and Silicon Nitride.
Bonding clay forms a ceramic cement during firing and I understand they are =
high
alumina, low silica materials. My experience of testing Kaolin is that they =
are
almost inert in a salt firing but I have not repeatedly refired any test =
pieces.
Ethyl Silicate decomposes when heated leaving micronised highly active =
silica
behind which sinters the silicon carbide into a solid mass. Presumably, =
Silicon
Nitride also acts as a sintering agent.
It is known that Silicon Carbide reacts strongly with oxygen forming carbon
monoxide and free silica at temperatures above 1000C (1832F). In an Air
atmosphere silica which forms acts as a self protective coating which =
prevents
further attack. But free silica reacts vigorously with molten hydroxides, =
and
carbonates,of alkali metals to form silicates. It is also known that silicon
carbide reacts with all of the Halogens. With Chlorine, Silicon Carbide =
forms
Carbon tetrachloride and Silicon tetrachloride. It seems to be a common
experience that Sodium chloride also attacks it. My assumption is that gases
which evolve, including water vapour and carbon monoxide,are responsible =
from
creating the foam. And I can see no reason why silica based binding agents
should be immune to this degradation.
Regards to all,
Ivor Lewis. Inquisitive as usual, knowing that the wrong answer may come if =
we
do not ask the right question.
Craig Martell on wed 29 dec 99
Good Day:
Ivor Lewis sez:
>Although there may be potters using Silicon carbide shelves and bricks in a
>variety of situations who experience and ignore foaming, believing that it
>is of
>no consequence, I continue to maintain that there may be potential for grain
>boundary degradation which could eventually lead to failure.
>Ivor Lewis. Inquisitive as usual, knowing that the wrong answer may come if we
>do not ask the right question.
Good grief!! I don't think that any of us could expect to fire a vapor
kiln without some degrading of the refractories and furniture used in that
kiln. You can't eliminate, only minimize the attack on bricks, posts, and
shelves. If one takes precautions a vapor kiln can and will last for a
long while. So will silicon carbide shelves.
I use a 75% alumina 25% kaolin wash on the working side of silicon carb
shelves. No foaming whatsoever. Almost no wasting away of the
shelf. Many potters use ITC coating on refractories and shelves with good
results. As I said in a previous post, don't introduce copious amounts of
soda or salt into the kiln at any given time. Let the vapor react with the
ware and then clear the chamber a bit before continuing to introduce soda
or salt. When I've over salted the kiln per individual application, I then
notice more foamy slag.
In the US, I would recommend talking with Donna at Firebrick Supply in St.
Paul, Minn. about grades of silicon carb that are better for vapor
kilns. She has given me some good info on this and will do this for any of
you that are interested. I have been using alumina carbide shelves in the
salt kiln a bit and they don't take any sort of glaze or foam at all. They
are at zero level for maintenance. You have to be careful with these
shelves because they are prone to thermal shock cracking. Norton makes a
"sandwich shelf" that has a silicon carbide core with an high alumina
covering that may be good for vapor glazing. The alumina carbide shelves
that I have were made by Ferro Vesuvius and the engineer was and is Greg
Patterson. If anybody wants to pursue this you could probably talk with
him about the shelves.
Personally, I don't think this issue is worth worrying about or taking time
out of pot making to deal with. When and if I need more silicon carbs or
other types of shelves for the salt kiln, I'll talk to Donna, or someone
else that is experienced with furniture and refractories and get their
uptake on things. At the rate my silicon carbs are deteriorating now, I
should be about 150 years old when I need to replace them. I hope to be
retired by then. Know what I mean?
later, Craig Martell in Oregon
Earl Brunner on wed 29 dec 99
Ivor, potters use a variety of materials in a variety of ways that are different
than the original intent of the manufacturers. The fact is that even with as ma
potters as there are that buy and use ceramic supplies, it still remains that ma
many of the things we use we have adapted to our use. We are a very small segme
of the market for some manufacturers. If it works, I don't see the problem. I
had silicon carbide and shelves made out of other materials fail on me in normal
usage in normal gas, reduction firing. Why would I expect it to be any differen
a salt firing? The kiln and the kiln furniture should go through a thorough
inspection every time the kiln is fired. Several potters have indicated that th
have used silicon carbide shelves in salt, soda and wood firing and that the she
hold up reasonably well. The weed burner that has been modified for use in a ra
kiln, gets the job done.
I.Lewis wrote:
> ----------------------------Original message----------------------------
> ------------------
> Silicon carbide shelves and salt glazing
>
> After reviewing the information available to me I think it would pay to consul
> the manufacturer about the suitability of Silicoan Carbide shelving for either
> Salt or Soda Glazing.
>
> Although there may be potters using Silicon carbide shelves and bricks in a
> variety of situations who experience and ignore foaming, believing that it is
> no consequence, I continue to maintain that there may be potential for grain
> boundary degradation which could eventually lead to failure. The problem would
> be associated with the binding material which is used in fabrication as well a
> the gross attack on the silicon carbide.
>
> >From what I can gather there are three possible ways of making Silicon Carbid
> kiln furniture, all of which involve pressing a mixture of the synthesised
> refractory with a lubricant and a binding agent. I know of three possible
> binding agents. These are Bonding Clay, Ethyl Silicate and Silicon Nitride.
>
> Bonding clay forms a ceramic cement during firing and I understand they are hi
> alumina, low silica materials. My experience of testing Kaolin is that they ar
> almost inert in a salt firing but I have not repeatedly refired any test piece
> Ethyl Silicate decomposes when heated leaving micronised highly active silica
> behind which sinters the silicon carbide into a solid mass. Presumably, Silico
> Nitride also acts as a sintering agent.
>
> It is known that Silicon Carbide reacts strongly with oxygen forming carbon
> monoxide and free silica at temperatures above 1000C (1832F). In an Air
> atmosphere silica which forms acts as a self protective coating which prevents
> further attack. But free silica reacts vigorously with molten hydroxides, and
> carbonates,of alkali metals to form silicates. It is also known that silicon
> carbide reacts with all of the Halogens. With Chlorine, Silicon Carbide forms
> Carbon tetrachloride and Silicon tetrachloride. It seems to be a common
> experience that Sodium chloride also attacks it. My assumption is that gases
> which evolve, including water vapour and carbon monoxide,are responsible from
> creating the foam. And I can see no reason why silica based binding agents
> should be immune to this degradation.
>
> Regards to all,
>
> Ivor Lewis. Inquisitive as usual, knowing that the wrong answer may come if we
> do not ask the right question.
--
Earl Brunner
http://coyote.accessnv.com/bruec
mailto:bruec@anv.net
I.Lewis on fri 31 dec 99
------------------
Well said Craig Martell.
Thanks for confirming my suggestion that potential users should consult
manufacturers. Excellent wisdom.
The product made from Aluminium Carbide is a new invention to me and I know
nothing about it. However, I would speculate that it forms a self protecting
surface in an oxidising atmosphere. You may be able to find out more from =
the
maker and pass the information around. In addition, again based on tests and
several years use, I know that the kiln and bat wash you are using does not
readily react with salt vapour. In fact, is seems to be totally inert. Under
oxidising conditions, without the intervention of sodium chloride vapour,
Silicon carbide forms an impervious layer of silicon dioxide which also =
prevents
further oxidation of the carbide. It should last for generations used with =
care.
Reduction would have no effect on this as far as I know.
As you say, confirming available knowledge, excess salt will cause damage. =
Since
you admit to it=92s occurrence in your own situation, you take adequate
precautions and advise others to do likewise. Again, I support you in this
admonition. Chemists provide the reasons for the effect. And I would still =
like
to know which gases are causing the foaming.
And in relation to Bat Wash, my suggestion is that a kaolin be selected =
which
has very low alkali and iron contents.
Still seeking answers
Ivor Lewis. Who also knows about the way termites work, silently and in the
dark.
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