Tommy Humphries on thu 29 nov 01
A while back, in another forum, plaster contamination in clay was being
bantered about. Many said that there would never be any plaster in their
studio's due to the bad things that happen around it.
Having worked with plaster wedging tables for the past 20 years or so, an=
d
never having seen any limepops in all that time, I decided to do a simple
experiment. I took a good sized chunk (1" dia.) of plaster and pounded i=
t
with a hammer till it was well crushed, with particle sizes ranging from
dust to 1/8" or so. This I wedged into 3 lb. of clay and with this threw =
2
pots of equal size and weight.
The first was fired in a decal firing, to 1000=BA C. (^06)- 3 hr start to
finish
The second was to 1223=BA C. (^6) singlefire 8 hr. start to finish
After the first came out of the kiln, I let it sit for a day...nothing, r=
ang
clear when tapped with a stick. Second day sank it in a bucket of water
till completely soaked. Sat it on a shelf to dry. 3 hr. later checked and=
it
looked like the surface of the moon... pop outs everywhere.
The second pot was fired 2 days later, in with a glaze firing up to ^6. W=
hen
the pot came out of the kiln, it was placed in bucket of water overnight,
then on the shelf for 3 days...nothing... still rang true, not a crack or
lime pop anywhere.
Took out the hammer and gave it a whack. There were many small glassy bi=
ts
in the walls of the pot, but no plaster present.
I surmised that in the quick bisque firing the plaster was simply
dehydrated, and when exposed to moisture swelled breaking the clay- class=
ic
lime pops.
In the glaze firing the plaster was reduced to calcium oxide, just as
whiting does, and became a flux and simply melted into the claybody.
This simple experiment shows that unless you are going to let the bisque =
sit
around the shop for a long time, there is really nothing to fear from
plaster. If you singlefire as I do, then there will be no problems
whatsoever.
Working with low-fire terra-cotta is another story though...be afraid of
plaster, be very afraid!
Tommy Humphries
the degree symbol =BA is produced by holding ALT and typing 167 on the
numerical pad on the keyboard---try it!
Ron Roy on fri 30 nov 01
Hi Tommy,
What you have said is true but there are some additional problems.
No matter how soon you apply glaze to bisque after the bisque firing you
add water. This means the popping happens during the glaze firing or
shortly after glazing.
It is also possible - if your clay is not well enough vitrified after the
glaze firing - sometimes it takes weeks and months depending on how long it
takes water to get into the clay and how "raw" the calcium oxide present
is. This can happen even where ware is glazed. It depends on with what the
body is fluxed.
When I was in Art college we had some contaminated fire clay - it happened
every once in a while - and I experienced the problem.
I don't use plaster and the clay I use is sieved through 30M before being
mixed so I have not seen a lime pop for at least 30 years.
Choose a clay supplier that sieves their raw clays to be sure - especially
the fire clays.
RR
>This simple experiment shows that unless you are going to let the bisque sit
>around the shop for a long time, there is really nothing to fear from
>plaster. If you singlefire as I do, then there will be no problems
>whatsoever.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Bob Hamm on fri 30 nov 01
Tommy wrote
This simple experiment shows that unless you are going to let the bisque =
sit
around the shop for a long time, there is really nothing to fear from
plaster.
I just want to support Tommy's conclusions. I've been throwing on =
plaster bats for about 24 years. I'm still using the first bats I made =
and they are worn down about 3/16" in the center. Until I switched clay =
bodies about five years ago I don't think I ever had any lime pops. I =
still don't get them very often, in fact rarely enough its not a =
concern. They never show up in a glazed pot, and only in bisque that =
sits on the shelf for weeks or months.=20
I've always bisgued slowly and glaze fired to ^6 or higher. I agree that =
earthenware is another story.
Plaster is a valuable tool in a pottery studio. It should be used =
properly and not feared.
Bob Hamm
Ph (250) 765-8876 Fax (250) 765-0497
email bobhamm@look.ca web site www.bobhamm-art.com=20
6750 Highway 33 East=20
Kelowna, B.C. Canada V1P 1H9
For information about the Kelowna Clay Festival 2001, go to =
www.bobhamm-art.com/clayfest=20
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----
Debbie Jestin on wed 6 mar 02
I found a plastic form that I thought would make a really neat slump mold,
so I draped clay on it,then poured plaster in a form to make a mold. I
pressed the plastic form with the clay slumped in it, into the plaster to
form a slump mold,the clay got plaster on it. What happens if you have
plaster in, or on clay and you fire it? Though I could make two things at
once.
Elca Branman on wed 6 mar 02
IF you have bits of plaster in your clay, and fire it, you won't need
firecrackers to explode it...bad karma.
As far as making two things at once, we all try to do that and generally
speaking, are always successful in making two things that won't work
ionstead of just one...lol
Don't be discouraged..you're at the beginning of a long pleasurable
learning trip..
Elca Branman
On Wed, 6 Mar 2002 09:57:03 -0500 Debbie Jestin
writes:
What happens if you
> have
> plaster in, or on clay and you fire it? Though I could make two
> things at
> once.
>
>
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elcab1@juno.com
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Dave Gayman on wed 6 mar 02
I'm not really clear on the sequence here. But it sounds like you got
plaster in the clay *original* for a mold. The clay used for *making* a
mold probably should be treated as expendable -- moldmakers use modeling
clay / plasticene (clay + wax + oil) or more up-to-date, polymer "clay" to
create originals from which to make molds. These are substances that are
far from anything that could actually be fired into a ceramic piece, and in
the case of plasticene, can be used over and over again.
For production, simply use a new lump of clay in your new plaster mold.
The problem with plaster in clay (as opposed to ON clay) is that the
plaster calcines during firing, and at some time after cooling, the
resulting plaster expands as it inevitably re-hydrates with moisture that
penetrates the wall, popping off clay or breaking the fired piece. It's
the same problem that many a purist potter faces with lime nodules in clay
gathered in the wild.
If the plaster is merely ON the clay, scrape it off without embedding any
within the wall of the piece. Plaster merely ON the clay will calcine and
should just fall off (or can easily be brushed off) as a powder.
But in the long run, just for insurance, any clay once contaminated with
plaster should be segregated from your working supply and never fired.
When you're using your new slump mold, watch to be sure you're not snapping
plaster off of any possible undercuts in the mold as you remove the clay
that's been formed in the mold.
Dave
At 09:57 AM 3/6/2002 -0500, you wrote:
>I found a plastic form that I thought would make a really neat slump mold,
>so I draped clay on it,then poured plaster in a form to make a mold. I
>pressed the plastic form with the clay slumped in it, into the plaster to
>form a slump mold,the clay got plaster on it. What happens if you have
>plaster in, or on clay and you fire it? Though I could make two things at
>once.
Tommy Humphries on wed 6 mar 02
What happens when you get plaster in the clay? Bad things. Maybe.
In over 20 years of using plaster around single fired stoneware, I have
never seen any problems.
But if you are bisque firing and have plaster in the clay, your pots will
have craters called "lime pops" all over them.
When the plaster is fired to low temperatures it calcines (looses all water)
and shrinks to a powder. When it is exposed to moisture (even just
humidity) the calcined plaster absorbs it and swells, busting out the sides
of the pot.
In single fired stoneware it seems that the plaster melts into a glass
inside the clay, and thus will not soak up moisture and swell.
This, at least is my experience...others may tell different
Tommy
Gwyn Ace on thu 7 mar 02
Plaster of Paris or specks of Limestone often have quite a delay before =
spitting out. I have had contaminated clay which gave me this trouble =
when fired to Cone 10 and this did not occur for a few weeks. A friend =
of mine was making some enormous terracotta jars and was supplied with =
grog which was contaminated with limestone....disaster !!
I have heard it recommended that if you are going to use plaster of =
paris for moulds etc. then use it in another room.. as far away from =
clay as possible and be extra careful !!! The specks which gave me =
trouble were only as large as a single bird seed... that was enough.
Similarly.. if you are adding whiting as a body flux.. then make it into =
a slip and add a lot of clay then put it through a fine sieve..so that =
no specks can exist.
Leave the explosions for the 4th of July.
GWYN in N.Z.
Louis Katz on sun 3 apr 05
Thanks for the reply. I am glad you found the site and the Calcia
article helpful. The page on Thailand is a general Thai information
site for people going to Thailand on my tour next year. Although the
tour is set up to view pottery, I think the real successes are getting
people to understand the culture of the place.
I am in the midst of watching Legend of Suriyothai a video about a 16th
century Thai Princess. Lots of nice pots in the movie. i am hoping to
scan it again after we finish it so that I can get another look at the
pots when I am not concentrating on the plot and language.
I feel like giving a real pitch for the tour, but if you are interested
you should ask Liz Willoughby. She went on our tour last year.
Louis
On Apr 3, 2005, at 2:17 PM, sincultura13 wrote:
> T
>
> http://www.tamucc.edu/wiki/Katz/ThaiLand
>
Louis Katz
WIKI site http://www.tamucc.edu/wiki/Katz/HomePage
Mitch on sun 3 apr 05
It is my understanding that pcs of plaster in clay cause cracking since =
the plaster "explodes" at high temperatures. Engineers call it =
"different coefficient of expansion". In other words the plaster =
expands more than the surrounding clay body. But I wouldn't worry about =
some plaster dust.
Mitch
John Jensen on sun 3 apr 05
My understanding of the problem of plaster is a little different, though I
might not have a full understanding.
When a bit of plaster imbedded in a pot is heated it goes through some
changes but nothing that would injure the pot. But after the firing is
complete and the pot is in service in the world, the plaster slowly absorbs
moisture and as it does so it expands slightly. This expansion can have
such force as to cause the fired clay around it to crack, creating what is
called a "lime pop." I think plaster bits have to be of a certain size to
cause problems...maybe over and eighth of an inch.
John Jensen, Mudbug Pottery
John Jensen@mudbugpottery.com
http://www.toadhouse.com www://www.mudbugpottery.com
>Mitch said:
It is my understanding that pcs of plaster in clay cause cracking since =
the plaster "explodes" at high temperatures. Engineers call it =
"different coefficient of expansion". In other words the plaster =
expands more than the surrounding clay body. But I wouldn't worry about =
some plaster dust.
Mitch
Malcolm Schosha on sun 3 apr 05
Pieces of plaster in low fired clay is bad news. It can cause pieces
of of the fired pot to break off, on either the inside or outside. If
you like to use plaster to wedge, and/or to dry clay, it is best to
cover the plaster with a fairly fine canvas (the type artists use for
painting). That keeps the clay and plaster separate. Personally I
always wedged clay on wood, or (in Italy) sometimes on marble. Even
in Italy, where wood is VERY expensive, most potters I knew prefered
wood.
Malcolm Schosha
..............
--- In clayart@yahoogroups.com, John Jensen wrote:
> My understanding of the problem of plaster is a little different,
though I
> might not have a full understanding.
>
> When a bit of plaster imbedded in a pot is heated it goes through
some
> changes but nothing that would injure the pot. But after the
firing is
> complete and the pot is in service in the world, the plaster slowly
absorbs
> moisture and as it does so it expands slightly. This expansion can
have
> such force as to cause the fired clay around it to crack, creating
what is
> called a "lime pop." I think plaster bits have to be of a certain
size to
> cause problems...maybe over and eighth of an inch.
>
> John Jensen, Mudbug Pottery
> John Jensen@m...
> http://www.toadhouse.com www://www.mudbugpottery.com
>
> >Mitch said:
> It is my understanding that pcs of plaster in clay cause cracking
since =
> the plaster "explodes" at high temperatures. Engineers call it =
> "different coefficient of expansion". In other words the plaster =
> expands more than the surrounding clay body. But I wouldn't worry
about =
> some plaster dust.
> Mitch
>
>
______________________________________________________________________
________
> Send postings to clayart@l...
>
> 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@p...
sincultura13 on sun 3 apr 05
Thanks for the replies, I feel more comfortable using the plaster
table now=85 Though I might try to get a slate slab in a near future=85.
Lois, after reading your article I remembered a teacher once told me
that plaster could be used in the formulation of glazes but that for
some reason people don't like to=85 I read another couple articles and
the Thailand link I found really interesting the first time I came
across your webpage so I took another look=85 Here is the link if
anybody is interested:
http://www.tamucc.edu/wiki/Katz/ThaiLand
Gary Navarre asked: "How did your brick collecting turn out? Did you
get a little kiln built?"
Gary, sadly I had the let the idea go for a while=85 I found no second
hand bricks and buying enough bricks was more that what I can afford
at the moment. I'm still keeping my eyes open for a trashed electric
kiln or any other secondhand material=85 One day, one day=85
thanks again
Sincultura
--- In clayart@yahoogroups.com, John Jensen wrote:
> My understanding of the problem of plaster is a little different,
though I
> might not have a full understanding.
>
> When a bit of plaster imbedded in a pot is heated it goes through
some
> changes but nothing that would injure the pot. But after the
firing is
> complete and the pot is in service in the world, the plaster
slowly absorbs
> moisture and as it does so it expands slightly. This expansion
can have
> such force as to cause the fired clay around it to crack, creating
what is
> called a "lime pop." I think plaster bits have to be of a certain
size to
> cause problems...maybe over and eighth of an inch.
>
> John Jensen, Mudbug Pottery
> John Jensen@m...
> http://www.toadhouse.com www://www.mudbugpottery.com
>
> >Mitch said:
> It is my understanding that pcs of plaster in clay cause cracking
since =3D
> the plaster "explodes" at high temperatures. Engineers call it =3D
> "different coefficient of expansion". In other words the plaster =3D
> expands more than the surrounding clay body. But I wouldn't worry
about =3D
> some plaster dust.
> Mitch
>
>
_____________________________________________________________________
_________
> Send postings to clayart@l...
>
> 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@p...
Jonathan Kaplan on mon 4 apr 05
Plaster is hydroscopic...means it absorbs water. As it expands it will
cause a pop out from the surface of the clay.
BTW.....and old slip casting technique is to add a very, read VERY
small amount of dry plaster to a blunger of slip to adjust rheology
issues. It works fine.
I have used plaster in our shop for well over 30 years with out any
cross contamination. Just good studio practice will avoid any problems.
BTW...... fear of plaster in the studio will eliminate one of the most
versatile materials a potter can use. IMHO
Jonathan
Jonathan Kaplan
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139
(please use this address for all USPS deliveries)
Plant Location:
1280 13th Street Suite K
Steamboat Springs CO 80487
(please use this address for all UPS, courier, and common carrier
deliveries only!!)
info@ceramicdesigngroup.net
www.ceramicdesigngroup.net
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