I.Lewis on thu 7 oct 99
------------------
If I am right to believe that, regardless of particle size, plaster =
otherwise
known as Gypsum, Anhydrite or Calcium Sulphate and it=92s relation, Calcium
Carbonate in the form of marble, limestone, whiting will loose their acid
components as volatiles when the clay is fired leaving behind a residue of
Calcium oxide, then, as air permeates unglazed clay lime powder will first
absorb water vapour then carbon dioxide changing back to calcium carbonate. =
This
change is accompanied by a significant increase in volume. The pressure
generated is enough, as the Roman legionaries knew, to split massive rocks.
Quicklime was used for centuries prior to the advent of blasting powders to
fracture stone and mine ore bodies.
I presume that at stoneware temperatures finely divided whiting reacts with =
free
silica to form Wollastonite so it causes no problems. Perhaps someone can
confirm this as a fact?
Had it happen to me=21
Ivor Lewis.
Louis H.. Katz on fri 8 oct 99
I believe that,
The difference bisque and stoneware in resistance to lime blows has more to due
with body strength. It takes far less pressure to pop off a piece of soft bisque
than hard stoneware.
I don't deny that some of the calcium oxide goes into solution with the clay fro
the CaO-Alumina-Silica eutectic but experience tells me that only the surface of
chunk of whiting will melt into the clay. Any sizable lump will still be a probl
Louis
BTW
In a book, it could have been "Ceramics for the Archaeologist", I read that ther
was a great controversy over organic temper that was used in a class of pots. So
pots of the same period had lime temper(shells or limestone I don't remember)
(bonfired below 1500 degrees F) the others had just empty spaces. Seems the
restorers had treated the Pieces with HCl to remove soluble salts and had also
removed the lime temper.
"I.Lewis" wrote:
> ----------------------------Original message----------------------------
> ------------------
> If I am right to believe that, regardless of particle size, plaster otherwise
> known as Gypsum, Anhydrite or Calcium Sulphate and it s relation, Calcium
> Carbonate in the form of marble, limestone, whiting will loose their acid
> components as volatiles when the clay is fired leaving behind a residue of
> Calcium oxide, then, as air permeates unglazed clay lime powder will first
> absorb water vapour then carbon dioxide changing back to calcium carbonate. Th
> change is accompanied by a significant increase in volume. The pressure
> generated is enough, as the Roman legionaries knew, to split massive rocks.
> Quicklime was used for centuries prior to the advent of blasting powders to
> fracture stone and mine ore bodies.
>
> I presume that at stoneware temperatures finely divided whiting reacts with fr
> silica to form Wollastonite so it causes no problems. Perhaps someone can
> confirm this as a fact?
>
> Had it happen to me!
>
> Ivor Lewis.
Michael Banks on fri 8 oct 99
------------------
Ivor, Thanks for the interesting historical background on quicklime
rock-breaking.
All these lime salts revert to calcium oxide at bisc temperatures. Whether
this CaO is involved in solid-state reactions, enters fusion or remains
unreacted, depends on it's grainsize, the peak temperatures reached and
duration of the peak temperature (heatwork). Minus 200 mesh (-75 micron)
whiting never seems to cause problems in ware properly bisced at 1000o C,
suggesting that the whole grains react with clays, silica and other fluxes
easily at this size. These essentially solid-state reactions produce
silicates and aluminosilicates (not just wollastonite).
But at sufficient concentrations of other fluxes, particularly alkalies,
lead etc, this CaO forms low eutectics and will enter fusion readily,
producing a glass. Significant molar proportions of CaO (up to 0.4 of flux
unity) are useful in very low temperature glazes below 900oC, unlike
magnesia (a close relative on the Periodic Table) which does not flux easily
at these temperatures.
Predicting the critical mesh size above which calcium salts can cause
lime-popping faults, is tricky, not knowing the bulk body composition. But a
safe minimum for stonewares appears to be finer than 70 mesh.
Michael Banks,
Nelson,
New Zealand
----------------------------Original message----------------------------
------------------
=3EIf I am right to believe that, regardless of particle size, plaster
otherwise
=3Eknown as Gypsum, Anhydrite or Calcium Sulphate and it=92s relation, =
Calcium
=3ECarbonate in the form of marble, limestone, whiting will loose their acid
=3Ecomponents as volatiles when the clay is fired leaving behind a residue =
of
=3ECalcium oxide, then, as air permeates unglazed clay lime powder will =
first
=3Eabsorb water vapour then carbon dioxide changing back to calcium =
carbonate.
This
=3Echange is accompanied by a significant increase in volume. The pressure
=3Egenerated is enough, as the Roman legionaries knew, to split massive =
rocks.
=3EQuicklime was used for centuries prior to the advent of blasting powders =
to
=3Efracture stone and mine ore bodies.
=3EI presume that at stoneware temperatures finely divided whiting reacts =
with
free
=3Esilica to form Wollastonite so it causes no problems. Perhaps someone can
=3Econfirm this as a fact?
=3EHad it happen to me=21
=3EIvor Lewis.
Lily Krakowski on fri 13 sep 02
There is a pot in my kitchen --all my own pots in my own kitchen are
seconds--that was made from the contaminated clay I spoke of in a previous
post. Being a big pot it is relatively thick. It went on lime popping for a
good six months if not longer. The determining factor was how often it got
washed. Maybe in a dry place, never washed/soaked the pops would have
remained hidden "forever"-- just gnawing at my conscience at night....
i
Tommy Humphries writes:
> yeah, I ran a few tests to test out that theory a while back...I had good
> results with it. I made a few pots with various sized plaster pieces wedged
> into them, and fired them to bisque and ^6. within a week all the bisque
> pots had pop outs, and as of now the pots that were single fired to ^6 only
> had one pop out...this was with a really large (1") chunk of plaster. I
> concluded through these experiments that the occasional bit of plaster from
> a plaster sink, or wedging table would not be enough to make me worry,
> especially since I do mostly single fire work.
>
> Tommy
>
>
> ----- Original Message -----
> From:
> To:
> Sent: Thursday, September 12, 2002 7:04 AM
> Subject: pig in a poke-primal-mommy-vince
>
>
>> I remember reading on this list that in low fire, plaster casues popouts,
>> becasue it will absorb moisture after it is fired, and in time pop out
>> pieces of clay. I also remember reading that if it was high fired, it
>> would form a glassy bead in the clay, and would be no problem.
>
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Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
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