Vince Pitelka on tue 3 mar 98
> The reason for not using mild steel for pugging porcelain is a simple
>one. It is rusts, most people who use porcelain want a clean white clay
>body free of iron speckles. The usual practice is to leave clay in the
>mill and pug as needed. Mild steel sitting for and length of time will
>rust with wet clay in it.
Kenneth -
There is a lot more to this than just the rusting of steel. I hope that
someone more knowledgeable about it than me will post an explanation. I
keep stoneware clays in my pugmill permanently with no problems. Way back,
just after I built the mill, I tried using it on porcelain, and very soon
the shredder screens got completely clogged with hard blue-grey chunks of
solidified porcelain. After cleaning the shredder screens a number of times
in a very short period (which is very unusual for this mill unless you are
processing dirty recycle) I finally disassembled the mill and cleaned it.
That involved TEDIOUSLY scraping off thick hardened blue gray deposits from
all the steelsurfaces. This was not any normal rusting of steel. The
porcelain clay in direct contact with the steel was being affected to a
depth of up to 1/4".
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Gavin Stairs on wed 4 mar 98
At 01:01 PM 3/3/98 EST, you wrote:
>----------------------------Original message----------------------------
>> The reason for not using mild steel for pugging porcelain is a simple
>>one. It is rust ... Mild steel sitting for and length of time will
>>rust with wet clay in it.
....
> From Vince:
>There is a lot more to this than just the rusting of steel.
> ... thick hardened blue gray deposits from
>all the steel surfaces. This was not any normal rusting of steel. The
>porcelain clay in direct contact with the steel was being affected to a
>depth of up to 1/4".
I can hazard a guess about this. First, the presence of blue grey deposits
means that the iron is in a reduced condition, like Fe2O3. The likely
culprit would be Al(OH)3 coming from the unsatisfied edges of kaolin
grains. This could ion exchange with the Fe, producing both Fe2O3 and Al.
The Al would plate to the iron surface, while the Fe would become Fe3+
(Fe2O3) in the kaolin. I can't remember what this is called, but it's
something like olivine or chlorite. The Al would galvanically oxidize back
to Al(OH)3. This would use up some water, making the whole mass more
sticky and dense, and leading to the accretion Vince describes. Somehow,
this plus other contaminants is making something like a cement, a hydration
curing solid. I've probably got some of that wrong.
Stoneware already has some iron and other contaminants in it, and it is
presumably less active. The iron tends to collect at the edges of the
kaolin grains, which makes the aluminum a little less likely to hydrate and
pull little diffusion driven exchange tricks like this.
Gavin Stairs
Toronto, Canada
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