Louis Katz on fri 1 apr 05
Plaster in stoneware under 30 mesh or so should not be a problem.
Softer bodies at bisque might present a problem when finer. I would
prefer it if all lime type contaminants passed through a 60 mesh seive.
Plaster Calcium Sulphate becomes CaO on heating. It picks up water
anywhere it can becoming Calcium Hydroxide. When this happens it
expands and can break off a section of the pot. If there are lots of
peices the pots can self destruct becoming a pile of shards. Happened
to me with my backyard clay in 1973. Very surprising.
More on my Calcia page mentioned twice in one day:
http://www.tamucc.edu/~lkatz/articles/calcia.html
Louis
On Apr 1, 2005, at 8:24 PM, Warren Heintz wrote:
> My understanding is that it has to do with clay's shrinkage. Any enert
> material inside the clay would cause it to crack. But what do I know,
> someone from this board probably has a better reason. W.
>
> Sincultura 13 wrote:I've read several times
> while searching the potters.org archive that if plaster gets into clay
> it can 'cause the unfired ware to break... Yet after searching for
> several hours I couldn' find out why... Does this happen even if just
> plaster dust gets into the clay? Or should I only worry about large
> size plaster particles?
>
> I asked my current ceramic teacher and he told me he has never had any
> problems with plaster tables and clay...
>
> thanks
>
> Sincultura
>
>
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Louis Katz
WIKI site http://www.tamucc.edu/wiki/Katz/HomePage
Sincultura 13 on fri 1 apr 05
I've read several times while searching the potters.org archive that if plaster gets into clay it can 'cause the unfired ware to break... Yet after searching for several hours I couldn' find out why... Does this happen even if just plaster dust gets into the clay? Or should I only worry about large size plaster particles?
I asked my current ceramic teacher and he told me he has never had any problems with plaster tables and clay...
thanks
Sincultura
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Warren Heintz on fri 1 apr 05
My understanding is that it has to do with clay's shrinkage. Any enert material inside the clay would cause it to crack. But what do I know, someone from this board probably has a better reason. W.
Sincultura 13 wrote:I've read several times while searching the potters.org archive that if plaster gets into clay it can 'cause the unfired ware to break... Yet after searching for several hours I couldn' find out why... Does this happen even if just plaster dust gets into the clay? Or should I only worry about large size plaster particles?
I asked my current ceramic teacher and he told me he has never had any problems with plaster tables and clay...
thanks
Sincultura
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______________________________________________________________________________
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You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
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Gary Navarre on sat 2 apr 05
Hay Sinculture and Crew,
On Fri, 1 Apr 2005 16:47:04 -0800, Sincultura 13
wrote:
>I've read several times while searching the potters.org archive that if
plaster gets into clay...
My bigest pet peeve besides needle tools when I worked is student
environments.
>it can 'cause the unfired ware to break...
Chunks will not shrink along with the clay. If they don't show up after
drying they will most often cause pop outs.
> Does this happen even if just plaster dust gets into the clay?
Not a good idea but you could mix some in a pound and see what what happens.
>I asked my current ceramic teacher and he told me he has never had any
problems with plaster tables and clay...
That's probably because they know not to scrape the clay off so much to
shave the plaster into the clay.
Personally I have no use for plaster. For molds I would someday like to try
making bisque molds but I'm still too much into throwing.
How did your brick collecting turn out? Did you get a little kiln built?
Good lick and stay in there!
G in da UP
Bob Masta on sat 2 apr 05
Sincultura asks:
> I've read several times while searching the potters.org
> archive that if plaster gets into clay it can 'cause the
> unfired ware to break... Yet after searching for several
> hours I couldn' find out why... Does this happen even if
> just plaster dust gets into the clay? Or should I only worry
> about large size plaster particles?
Only worry about partilces larger than about 80 mesh.
In fact, some folks say not to worry about anything smaller
than about 30 mesh. Dust is no problem at all.
The problem with plaster is "lime pops". Plaster is
calcium sulfate, which oxidizes in firing to calcium oxide.
If the particles are tiny, or the temperature is really high,
they melt into the clay matrix. You may have noticed that
many clazes use whiting (calcium carbonate) for the specific
purpose of adding calcium oxide to the glaze when the
carbonate oxidizes.
The problem, however, is when the calcium source is in
bits that are too large to melt at your firing temperature.
When the ware cools back down, these bits absorb
moisture and expand, popping out of the body.
You might see a little flake of the body pop off, and when
you look at the place it came from you will see a little white
spec. This is can happen in bisque or glaze fired wares.
Since this can be a real disaster, many folks cover their
plaster tables with canvas just to be safe. But bare
plaster has been used for years, with proper care and caution.
Best regards,
Bob Masta
potsATdaqartaDOTcom
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