James on mon 7 mar 05
I have a question for any clay geniuses out there:
Our student workers accidently grabbed a sack of AP Green castable
refractory instead of AP Green fire clay. They added two scoops of the
castable to a batch of stoneware clay made of half reclaim and about 20
scoops of fresh materials (Goldart, Custer Spar, Flint, Redart, OM4, and
what was supposed to be fireclay).
Does anyone have a good guess as to whether or not this batch is now junk?
Assuming the castable contains some portland cement, will we get "lime pops"
if we try to use this stuff, or is "lime pop" actually a misnomer for gypsum
pops from plaster?
Thanks for any help you can provide.
...James
Snail Scott on tue 8 mar 05
At 05:07 PM 3/7/2005 -0500, you wrote:
>Assuming the castable contains some portland cement, will we get "lime pops"
>if we try to use this stuff, or is "lime pop" actually a misnomer for gypsum
>pops from plaster?
Cement can cause effects like 'lime pop',
but as with plaster, the effect only occurs
when actual chunks are present, not just
powder. Otherwise we could never use calcium
as a flux! If the cement in the mix was a
loose powder, it will merely become a flux,
too.
The higher the temperature of the firing, the
less critical this becomes, also, partly
because the strength of the fired clay is
greater, but also because small particles of
plaster or'lime' will be melted. That's why
lime pop is thought of mainly as an earthenware
problem. At earthenware temperatures, the plaster
is merely calcined, no melted, and is thus primed
to go through its process as moisture contacts
it and it sets (and expands) all over again.
That can't happen if it's been melted.
If you were making stoneware, you're fine. If
the cement was powder, you're fine. Just test
the result before making work out of it, since
it may behave differently now!
-Snail
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