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kiln wadding / why silica?/ how much alumina?

updated tue 30 sep 97

 

Bill Amsterlaw on sat 20 sep 97

Hi Clayart:

The ideal wadding would be a plastic substance so refractory that it sticks to
nothing when fired.

I am looking at an "equilibrium diagram" that shows the melting point for
different mixtures of alumina and silica (in Cardew's _Pioneer Potter_ from
1969). Looking at this diagram, I would guess that epk alone should work well
as wadding (with a melting point over 3200 F). If you add alumina, you raise
the melting point; if you add silica, you lower the melting point.

From this, I don't understand why one would include silica in wadding.
Surely, it would not be a good idea to use a clay body (containing silica and
flux) as wadding. The silica and flux would lower the melting point, making
it more likely for the wadding to stick.

How much alumina would you need to add to raise the melting point of epk?
According to the diagram, kaolin (46% alumina, 54% silica) melts at 1800 C
(3272 F). For some reason, at this level the curve stalls: You need to raise
the % of alumina above 55% to raise the melting point.

How much alumina would you need to add to a 50-lb bag of epk to raise the % of
alumina to 55% (the point where the alumina will just begin to make a
difference)? If you do the math, you find that it would take 13.3 pounds of
alumina hydrate. In other words, you would need to add MORE than 13.3 pounds
of alumina hydrate to BEGIN to raise the melting point.

In order to raise the melting point to 1900 C (3452 F) you would need to get
the % of alumina to 63%. You would need to add over 30 lb of alumina hydrate
to your 50-lb bag of epk to get the alumina level to 63%.

I would assume from this that adding 1 lb of alumina hydrate to a 50-lb bag of
epk should have no effect on the melting point. The epk alone should do just
as well.

How about using the 1 lb of alumina to dust the surface of wads made of pure
epk? There may well be an advantage to dusting wads with a material that is
more refractory than the body of the wad - to give the wad a highly refractory
skin. Alumina hydrate powder should work well with a melting point of 2000 C
(3632 F). Zircopax should work even better with a melting point around 2700 C
(4900 F). Dusting with silica doesn't seem to make sense because it has a
melting point lower than epk alone.


- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY


Jonathan Kaplan wrote:
>>
I have been using the following wadding formula for the last I don't know how
many years not only to level deteriorating bottoms of kiln posts/props, but as
wads on top of each post to level the shelves: 50 pounds EPK, 1 pound Alumina
Hydrate. Store in a heavy duty plastic bag inside a 5 gallon bucket with lid.
<<

Barb Bihler wrote:
>>
I keep a bowl of dry kiln wash (50 kaolin/50 flint) near the kiln. It's my
"flour bowl". To wad kiln posts and level shelving while stacking a glaze
kiln, I use my regular stoneware claybody in the form of coils, "cookies',
whatever it takes, and dust them ("flour" them) with the powdered wash. It's
just enough to ensure release after firing.
<<

Kenneth D. Westfall on sun 21 sep 97

------------------
Bill,
To anwer you question why you would use silica and epk is cost and that=
it
works. Without doing a lot of book work, silica is more that refractory =
enough
for cone 11 firings. You mix four part of silica to one part epk , the epk
gives the mixture some plastisity without lowering its melting point below =
cone
11. The cost of flint is roughly =240.09 a pound, alumina hydrate is =
=240.55 a
pound, and epk is =240.15 pound, it doesn't take much to figure out that if =
it
works and is thrown away in the end you use what is the cheepest. This =
wadding
doesn't however work for salt firing because soda is attracted to the =
silica.

Kenneth D. Westfall
Pine Hill Pottery
pinehill=40ruralnet.org
Don't get stuck in the mud pies K=26T

David Hendley on sun 21 sep 97

Bill, I don't ever use wadding.
If the kiln post is deteriorted I toss it because it's so easy to make more.
After extruding my kiln posts I roll them in alumina hydrate
for a good thick coating on all 6 sides.
NOTHING sticks to them, and this is in a wood fired kiln.

Anyway, judging from this experience, I would say that
rolling in or dusting wads with alumina makes great sense.
The wads would do their job and then be easily removed.
David Hendley



At 09:41 AM 9/20/97 EDT, you wrote:
>----------------------------Original message----------------------------
>Hi Clayart:
>
>The ideal wadding would be a plastic substance so refractory that it
sticks to
>nothing when fired.
>
>I am looking at an "equilibrium diagram" that shows the melting point for
>different mixtures of alumina and silica (in Cardew's _Pioneer Potter_ from
>1969). Looking at this diagram, I would guess that epk alone should work
well
>as wadding (with a melting point over 3200 F). If you add alumina, you raise
>the melting point; if you add silica, you lower the melting point.
>
>>From this, I don't understand why one would include silica in wadding.
>Surely, it would not be a good idea to use a clay body (containing silica and
>flux) as wadding. The silica and flux would lower the melting point, making
>it more likely for the wadding to stick.
>
>How much alumina would you need to add to raise the melting point of epk?
>According to the diagram, kaolin (46% alumina, 54% silica) melts at 1800 C
>(3272 F). For some reason, at this level the curve stalls: You need to raise
>the % of alumina above 55% to raise the melting point.
>
>How much alumina would you need to add to a 50-lb bag of epk to raise the
% of
>alumina to 55% (the point where the alumina will just begin to make a
>difference)? If you do the math, you find that it would take 13.3 pounds of
>alumina hydrate. In other words, you would need to add MORE than 13.3 pounds
>of alumina hydrate to BEGIN to raise the melting point.
>
>In order to raise the melting point to 1900 C (3452 F) you would need to get
>the % of alumina to 63%. You would need to add over 30 lb of alumina hydrate
>to your 50-lb bag of epk to get the alumina level to 63%.
>
>I would assume from this that adding 1 lb of alumina hydrate to a 50-lb
bag of
>epk should have no effect on the melting point. The epk alone should do just
>as well.
>
>How about using the 1 lb of alumina to dust the surface of wads made of pure
>epk? There may well be an advantage to dusting wads with a material that is
>more refractory than the body of the wad - to give the wad a highly
refractory
>skin. Alumina hydrate powder should work well with a melting point of 2000 C
>(3632 F). Zircopax should work even better with a melting point around
2700 C
>(4900 F). Dusting with silica doesn't seem to make sense because it has a
>melting point lower than epk alone.
>
>
>- Bill Amsterlaw (wamster@msn.com)
> Plattsburgh, NY
>
>
>Jonathan Kaplan wrote:
>>>
>I have been using the following wadding formula for the last I don't know how
>many years not only to level deteriorating bottoms of kiln posts/props,
but as
>wads on top of each post to level the shelves: 50 pounds EPK, 1 pound
Alumina
>Hydrate. Store in a heavy duty plastic bag inside a 5 gallon bucket with lid.
><<
>
>Barb Bihler wrote:
>>>
>I keep a bowl of dry kiln wash (50 kaolin/50 flint) near the kiln. It's my
>"flour bowl". To wad kiln posts and level shelving while stacking a glaze
>kiln, I use my regular stoneware claybody in the form of coils, "cookies',
>whatever it takes, and dust them ("flour" them) with the powdered wash. It's
>just enough to ensure release after firing.
><<
>
>
David Hendley
Maydelle, Texas
See David Hendley's Pottery Page at
http://www.sosis.com/hendley/david/