Michael McDowell on wed 25 jun 03
While I'm still not feeling up to making pots after my brain tumor
was removed last January, I've been trying to do various chores
around the pottery that usually get pushed onto the back burner. One
of those projects is making some more plaster bats. I purchsed
plaster for this project several years ago and then didn't get
around to it. Its been sitting on the foor of my studio in the
unopened bag all this time. Now its my understanding that plaster
rehydrates in storage and would probably not set up properly. I've
purchased fresh plaster now, but facing the prospect of putting 100
lbs. of unused plaster directly in the trash I'm wondering if it is
possible to revive it by recalcining it in my electric kiln. Can
anybody advise me on this?
Michael McDowell
Whatcom County, WA
Michael@McDowellPottery.com
http://www.McDowellPottery.com
Longtin, Jeff on thu 26 jun 03
Mike,
Wow! a brain tumor? Welcome back dude! I'm recovering from a auto accident
and think that's bad enough. My hats off to ya!
Truthfully I don't think you really need to be too concerned about your
plasters' age. Yes it is old and yes it has hydrated to a degree but since
your not making slip-casting molds I don't think it will be too bad. I used
some Hydrostone that had been in a bucket about five years without any real
discernable difference. Yes, your plaster will set quickly but other than
that it shouldn't be that big a deal. I use new plaster when making
slip-casting molds but for prototypes and models I use up any old plaster
that may be sitting around without much problem.
Years ago I thought I needed to calcine some zinc to solve some glaze
defects. It went in powder it came out lumpy gravel. (Not the intended
result!) Luckily I was able to find some calcined zinc at A.R.T in Chicago
in powder form. Recalcining your plaster, I think, would be way more work
than its worth.
Take care
Jeff Longtin
-----Original Message-----
From: Michael McDowell [mailto:michael@mcdowellpottery.com]
Sent: Wednesday, June 25, 2003 11:31 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Reviving old plaster
While I'm still not feeling up to making pots after my brain tumor
was removed last January, I've been trying to do various chores
around the pottery that usually get pushed onto the back burner. One
of those projects is making some more plaster bats. I purchsed
plaster for this project several years ago and then didn't get
around to it. Its been sitting on the foor of my studio in the
unopened bag all this time. Now its my understanding that plaster
rehydrates in storage and would probably not set up properly. I've
purchased fresh plaster now, but facing the prospect of putting 100
lbs. of unused plaster directly in the trash I'm wondering if it is
possible to revive it by recalcining it in my electric kiln. Can
anybody advise me on this?
Michael McDowell
Whatcom County, WA
Michael@McDowellPottery.com
http://www.McDowellPottery.com
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Snail Scott on thu 26 jun 03
At 09:31 PM 6/25/03 -0700, you wrote:
>its my understanding that plaster
>rehydrates in storage and would probably not set up properly...=20
Test it first - in your climate is dry (I don't know which=20
side of Washington you're on) it may be fine. Plaster=20
actually works best with some humidity in it. Just mix some=20
up in a dixie cup and see how fast it sets.
If it takes forever (or never), you can recalcine it - about=20
400=BAF is sufficient.=20
-Snail
John Rodgers on thu 26 jun 03
Michael, good to hear you are past the tumor thing. And on the way
toward full recovery, even though you haven't reached peak yet. Hang in!
As for calcining the plaster ... do a test cast to see how it performs.
If it sets in a reasonable time, don't worry about it. Besides, here it
only costs $19/100lbs, so were I doing the task, at a rate of $20/hour,
it's would be a waste of money to me to do all that work. I would toss
it if it doesn't work right. Wouldn't consider calcineing. You might
consider that.
Regards,
John Rodgers
Birmingham, AL
Michael McDowell wrote:
>While I'm still not feeling up to making pots after my brain tumor
>was removed last January, I've been trying to do various chores
>around the pottery that usually get pushed onto the back burner. One
>of those projects is making some more plaster bats. I purchsed
>plaster for this project several years ago and then didn't get
>around to it. Its been sitting on the foor of my studio in the
>unopened bag all this time. Now its my understanding that plaster
>rehydrates in storage and would probably not set up properly. I've
>purchased fresh plaster now, but facing the prospect of putting 100
>lbs. of unused plaster directly in the trash I'm wondering if it is
>possible to revive it by recalcining it in my electric kiln. Can
>anybody advise me on this?
>
>Michael McDowell
>Whatcom County, WA
>Michael@McDowellPottery.com
>http://www.McDowellPottery.com
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
>
>
Roger Graham on fri 27 jun 03
For Michael, who was asking about reviving stale Plaster of Paris by
re-heating it. Probably more trouble than it's worth, but it would be fun to
experiment a bit. Here's the accepted wisdom (from my ancient copy of
Sherwood Taylor's "Inorganic and Physical Chemistry").
Plaster of Paris begins life as the mineral Gypsum. Chemically, it's calcium
sulphate di-hydrate, CaSO4.2H2O You'd read this aloud as "See-ay Ess Oh
Four Dot Two Aitch Two Oh" The "dot" bit in the middle is the chemist's
way of saying, there's two molecules of water joined up with each molecule
of CaSO4 in making up the crystal structure. (And don't you serious chemists
jump out and tell me the crystal is really made of ions not molecules... I
know, I know. Let's agree to think of them as molecules.)
If gypsum is heated gently, somewhere between 100 and 200 degrees C, PART of
this water is driven off, to leave what they call "Calcium sulphate
hemi-hydrate". The formula is usually written (CaSO4)2.H2O Looks less
confusing if you can use subscript letters for the little numbers below the
symbols. But all it means is, only one molecule of water for every two
molecules of CaSO4 This hemi-hydrate stuff is powdery and dry, but it has
the useful property of absorbing water and re-combining with it to form long
needle-shaped crystals of the original di-hydrate version. And of course,
it's this hemi-hydrate stuff that is called "Plaster of Paris", which sets
hard as the long crystals form an interlocking mass.
But... if you heat any form of calcium sulphate really hot, like red hot,
you drive off ALL the water and end up with plain dry useless CaSO4. That's
"anhydrous" calcium sulphate. It won't recombine with water in the way you
want, to set hard again.
The book says there's an intermediate stage between these two extremes. To
quote the exact words "If the gypsum is heated to a higher temperature than
that needed to make plaster of Paris, but not to a very high temperature, a
plaster is obtained which sets very much more slowly, form a hard and
resistant surface suitable for floors etc. Still stronger heating destroys
the setting properties altogether."
So... if you really want to try reviving stale plaster, it would probably
work to cook it at say 150 degrees C for an hour or two. Nothing to lose.
Let us all know if you do, and what you get.
Roger Graham, near Gerringong, Australia
http://members.optusnet.com.au/~rogergraham
masta@UMICH.EDU on fri 27 jun 03
I had the same question recently, when I wanted to make some small
molds.
I had some plaster that was 14 years old. Part of it was in a container
with a fairly-tight plastic lid, and that was just fine. But most of it (maybe
40 pounds) was in the original bag, inside a metal garbage can with a
poorly-fitting lid. It got enough Michigan humidity to turn it semi-solid; it
could be scraped out, but really lumpy. Needless to say, it was dead.
I tried calcining some at 350F in the kitchen oven, and that seemed to work
OK. Then I found on the Clayart archives (and a few other sites for
confirmation) that it should be done between 260-300F, since if it gets
over 325F it goes to the anhydrite form, which is not useful as plaster any
more. I checked my oven, and it looks like the average temperature was
actually closer to 325, but the thing that may have saved me was that I had
ball-milled it first, so it probably insulated itself fairly well. Next time, I'd use
300F for a couple of hours or so.
Considering the energy cost and the work of milling it, this is fool's work,
especially since plaster is so cheap. But it's good to know these vital
skills in case I'm ever stranded on a desert island with a gypsum mine!
Robert Masta
tech@daqarta.com
D A Q A R T A
Data AcQuisition And Real-Time Analysis
Shareware from Interstellar Research
www.daqarta.com
Chris Morgan on sat 28 jun 03
Every bag of plaster I've ever seen had a plastic liner that served as a
vapor barier. If the plastic is in good shape, the plaster may be as
well...
Chris Morgan
PS
A shout out to all the texans. I'll soom be joining you.
Michael McDowell on mon 30 jun 03
Many thanks for the several responses, helpful hints and good wishes
that I received in response to my question on the possibility &
advisability of reviving old plaster. The first advice that I took
was to try some and see if there was a problem. Well, when I went to
open the bag the contents had so stiffened over the years that I had
to use a hammer to break it up before I could get it out of the bag.
BTW: though this was #1 pottery plaster in it's original, direct
from the manufacturer package, there was no plastic liner. There
were clods that wouldn't break down when mixed. And I didn't have
long to mix because the setting time was greatly reduced. Some
suggested that it might take longer to set up, but my experience was
the opposite. The first bit I tried was mixed along with about 80%
new plaster and as I was trying to get the clods to break down as I
mixed it nearly hardened in the mixing bucket. I got a bat poured
with that batch though I question its eventual longevity. Next I
tried mixing some straight after putting it through a 40 mesh screen
to break clods. It still felt like 40 mesh cottage cheese while
mixing and set so fast I didn't have time to pour it. The best
advice I ended up getting was to consider the value of my time
against the cost of fresh plaster. The rest of the stuff is still in
the bag, bag in the trash now. I'm not even giving it the time to
break it up small enough to spread around the yard! But thanks to
you all, this was an informed decision!
Michael McDowell
Whatcom County, WA
Michael@McDowellPottery.com
http://www.McDowellPottery.com
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