Brian O'Neill on fri 26 sep 03
Dear Clayarters,
It's been years since I mixed any plaster and even then it was small
amounts for bats and molds.
I just tried to mix and pour a 30 x 30 x 3.5 inch plaster slab for
drying clay. I'm using potter's plaster of paris and am pretty sure I
got my proportions right. Long story short--I couldn't sift/sprinkle the
p.o.p. into the water fast enough (106 pounds!) before it started to
warm and stiffen up in the mixing container. I dumped it and now have a
miniature Mount Rainier in my back yard.
Have I surpassed the limit of what one potter can mix by one's self?
Would it be better to have 3 or 4 helpers to mix smaller batches while I
do a continuous pour like they do with concrete?
Any experience with this would be greatly appreciated.
Thanks in advance,
Brian
In glorious western Washington state, where the warmth and sun have not
yet departed, which they usually do around this time.
Earl Brunner on sat 27 sep 03
First, use as cold of water as you can. That will help slow the process
down a bit. I recently made a couple of large wedging table tops. I
used a 2X4 form 24X48 for each one. I mixed the plaster in smaller
batches, in a 5 gallon bucket as fast as I could and added them on top
of the other batch, even though the earlier batches were beginning to
set up, they seemed to go together well.
I always hate the top "finished" surface when I mix plaster so when they
had set for a day or two I flipped them over in the form. The bootom
made a much better finished surface.
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Brian
O'Neill
Sent: Friday, September 26, 2003 10:51 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Plaster Fiasco
Dear Clayarters,
It's been years since I mixed any plaster and even then it was small
amounts for bats and molds.
I just tried to mix and pour a 30 x 30 x 3.5 inch plaster slab for
drying clay. I'm using potter's plaster of paris and am pretty sure I
got my proportions right. Long story short--I couldn't sift/sprinkle the
p.o.p. into the water fast enough (106 pounds!) before it started to
warm and stiffen up in the mixing container. I dumped it and now have a
miniature Mount Rainier in my back yard.
Have I surpassed the limit of what one potter can mix by one's self?
Would it be better to have 3 or 4 helpers to mix smaller batches while I
do a continuous pour like they do with concrete?
Any experience with this would be greatly appreciated.
Thanks in advance,
Brian
In glorious western Washington state, where the warmth and sun have not
yet departed, which they usually do around this time.
________________________________________________________________________
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Randy O'Brien on sat 27 sep 03
A couple of years ago I decided to pour a 4 foot x 4 foot plaster slab for
a wedging/recycling table. I wanted it to be poured all at once so as to
avoid the unsightly layers of plaster on the side of the slab. (I have to
look at it every day you know.) Except for one small surprise, it went
fairly well.
I made a form from a 4' x 4' piece of melamine and had 4" walls screwed in
on the sides. I cast the slab upside down because I wanted the
beautifully flat pristine surface that plaster can have when cast against
glass or other smooth surface.
Now to cast it in one piece I bought an inexpensive 33 gallon plastic trash
can. I figured out the maximum amount of plaster and water that the
container would hold using a ratio of 67. (100 parts plaster to 67 parts
water by weight.) I don't remember the exact amount now, but it was in the
neighborhood of 200 pounds of plaster and 134 pounds of water. This made a
slab that was 2.5" thick. Thinner than what I wanted, but I haven't had
any problems with it not being strong enough. I positioned the container
on wood blocks so that one edge was overhanging the mold, weighed out the
plaster and water, put the water in the container, dumped all the plaster
into the water, waited 2.5 minutes, and mixed for 2.5 minutes with a
turbomixer attached to a drill.
Up to now my plan went flawlessly. Now to pour the plaster into the mold I
took a knife and cut a 3" hole near the bottom of the trash can expecting
the plaster to pour effortlessly into the mold. What I hadn't taken into
consideration was the force that the plaster would have as it came out with
334 pounds of liquid above it. It was like filling the mold with a fire
hose. Plaster was splashing everywhere. Luckily I was alone so I didn't
have any witnesses to the slapstick show. But when the excitement
subsided, what wasn't on me or the wall of my studio did make it into the
mold. I used myself as a human shield to deflect the gush of plaster from
splashing over the mold wall. Surprisingly the plaster slab turned out
beautifully and without any apparent air bubbles. The plaster was still
fluid enough that the air bubbles could rise to the surface.
I welded a table frame ( with locking wheels) upside down on the plaster
slab and with the help of a friend was able to turn the entire plaster
table assembly upright. It has turned out to be a great general purpose
work surface. If I had to do it again, I would make the opposite walls of
the mold somewhat higher than 4 inches.
Good luck!
Randy O'Brien
http://www.dakotacom.net/~rdobrien
Maurice Weitman on sat 27 sep 03
At 10:51 PM -0700 on 9/26/03, Brian O'Neill wrote:
>I just tried to mix and pour a 30 x 30 x 3.5 inch plaster slab for
>drying clay. I'm using potter's plaster of paris and am pretty sure I
>got my proportions right. Long story short--I couldn't sift/sprinkle the
>p.o.p. into the water fast enough (106 pounds!) before it started to
>warm and stiffen up in the mixing container. I dumped it and now have a
>miniature Mount Rainier in my back yard.
Whoa! That's a lot of plaster, Brian. With 106 pounds of plaster,
you'll end up with a 180 pound mixture, no?
My ideas: I wouldn't bother sifting at all. Just scoop both hands
full of plaster out of the bag and squish it with your fingers to
break up the largest clumps and make it as powdery as possible going
into the water while working quickly.
Make sure the water is cold. The colder the water, the longer it
will take to set.
For mixing, get one of those larger paint mixer whirlythings in an
electric drill to thoroughly mix the plaster into the water. Be sure
to not introduce air into it by keeping the mixer inside the mixture
and not coming up for air.
You probably know this from your smaller pieces, but keep testing the
consistency of the mix so as to not end up with another unintended
sculpture. It might take a while to pour it all. And don't forget
to dam the form well... that's a lot of weight and pressure on it.
Good luck!
Regards,
Maurice
Maurice Weitman on sat 27 sep 03
One thing I forgot...
If you have one of those more vigorous hand massagers, or an orbital
sander, or anything that vibrates strongly, after you've poured the
plaster, hold the vibrating appliance tight against the form for a
couple of minutes to help any air bubbles that might have been
introduced in mixing or pouring to rise to the surface. It really
works well!
Regards,
Maurice
John Rodgers on sat 27 sep 03
Brian,
You may well have had some old plaster. Old plaster either will not set,
or it will set a a much faster rate, sometimes almost as fast as the
water is added.
I have made several large plaster tables for wedging and other stuff.. I
always make them upside down on a special mold table I have built that
is strong enough to have no bending under the load of big plaster
batches. Also the table is covered with plastic laminate to give me a
perfectly smooth plaster working surface when finished.
I always use USG #1 Pottery Plaster for this project. No Plaster of
Paris .... it is way to soft and does not hold up over time.
For the large batches that I mix, I use the USG plaster slide scale to
determine the plaster/water ratio and the weight of each for the batch.
For reasons unknown to me, the ratio of water to plaster is not a
straight line but a slight curve, as the total weight increases. But for
plaster tables/wedging tables built of #1 pottery plaster the ratio of
1:1.5 - i.e., 1 lb water to 1.5 lbs plaster works fine. If you were
making molds I would not depend on that, but it's good for the table. I
always make sure I have plaster that is less than three months old. This
is a hassle sometimes because supply houses are more interested in
moving the oldest stock and you really have to stick to your guns and
hold their feet to the fire to get the fresh stuff sometimes. Old
plaster will often times have lumps anywhere from the size of pea gravel
on up, and will have really screwed up setting times.
You can slow setting times by adding a little "Sodate Retarder" to the
plaster, obtainable at your concrete supply houses. It will reduce the
hardness of the plaster just a bit. And you may have to play with it to
get just the right amount. I have also heard that salt will do this as
well, but I have never tried it.
I mix in 5 gallon plastic buckets, and I weigh out my water for each
batch into each of several buckets, then I weigh out the plaster the
same way. A bit of a pain, and takes a number of buckets, but I try to
get a single solid cast if I can, and getting multiple batches going at
once requires multiple buckets. I mix my plaster with a big drill motor
and a heavy duty paint mixer with a double blade setup. That thing
really does the job in a minute or so.
I build the frame of the table from 2X6 material, and nail in a 1X2
around the bottom edge for the plaster to sit on after the casting and
after the table has been turned right side up.
I begin by turning the fame upside down on the mold table, then I seal
the frame to the table with oil clay around the outside of the table. I
don't want a really heavy load of plaster to come squirting out under
the frame. Once it starts, you can't stop it so long as the plaster has
not set.
I lube the frame with a light coat of vaseline, to allow movement as the
plaster dries, so as not to induce chipping or cracking. I also drill
holes for two pieces of pipe to be inserted into the frame crossways at
90 degrees to the long side of the frame. The pipes are inserted, then
well lubricated with vaseline, so they can be pulled out right after the
plaster sets. Later I insert a piece of All-Thread into the holes and
snug the frame a bit as the plaster dries. This keeps space between the
frame and the plaster from occurring and getting filled with junk later
after drying.
After the table is cast, and everything is set, I install 2 inch thick
corner boards, and on them threaded stanchion plates that are big enough
to accept at least 2 inch or bigger steel pipe. These are the table
legs. On the floor ends of the legs I install rubber end caps that I get
at the hardware store. This protects the floor. By having threaded steel
legs, the legs can be turned to level the table. When all is in place, I
turn the table right-side up.
I do all of this by myself with the exception of turning the newly cast
table over right-side up. I do solicit some help for that....... and to
put it in it's proper location.
The plaster table will be very heavy with all the water that is in it,
but the free water will evaporate eventually and the table will be
considerably lighter in weight.
Aftrer the table has dried (not set.....but dried!!) I fasten a piece of
heavy painters (art) canvas over the surface to protect it from wear
and scratches that will inevitable occur.
Hope this helps a bit.
Regards.
John Rodgers
Birmingham, AL
Brian O'Neill wrote:
>Dear Clayarters,
>
>It's been years since I mixed any plaster and even then it was small
>amounts for bats and molds.
>
>I just tried to mix and pour a 30 x 30 x 3.5 inch plaster slab for
>drying clay. I'm using potter's plaster of paris and am pretty sure I
>got my proportions right. Long story short--I couldn't sift/sprinkle the
>p.o.p. into the water fast enough (106 pounds!) before it started to
>warm and stiffen up in the mixing container. I dumped it and now have a
>miniature Mount Rainier in my back yard.
>
>Have I surpassed the limit of what one potter can mix by one's self?
>Would it be better to have 3 or 4 helpers to mix smaller batches while I
>do a continuous pour like they do with concrete?
>
>Any experience with this would be greatly appreciated.
>
>Thanks in advance,
>Brian
>
>In glorious western Washington state, where the warmth and sun have not
>yet departed, which they usually do around this time.
>
>______________________________________________________________________________
>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.
>
>
>
Tony Olsen on sat 27 sep 03
Brian,
How about just forgetting the plaster and go to Hardy Backer board? I =
use them for bats and also for a wedging table and for reclaiming clay =
from the slop bucket. absorbent and works great. Also no chance of =
getting plaster blow-outs.
Just a thought, YMMV
Tony Olsen, Galveston TX, USA
neslot@houston.rr.com
http://tonyolsen.com/up/
Stephani Stephenson on sun 28 sep 03
First: Congratulations Snail on your success at the
Sacramento show!!!! Way to go!!! Also congrats to Valice!
Now, Plaster.....
1. If there are any bits of plaster which have already 'gone
off' in the bag or in the bucket, this will cause the mix
to 'go off' very quickly...i.e pay attention to whether
there are any hardened lumps in the bag, even minute sized
ones.
this is a sign of old plaster .
Use your dry plaster container for dry plaster only,
Use the water container for water only
Use a clean and separate bucket for mixing .
you can put some "Pam" on the inside of the mixing bucket to
help it clean up better,
If you use a scratched plastic bucket, old plaster will
collect in the scratches .
This can be enough to cause a new batch to go off too
quickly.
2. larger batches tend to set up faster. make smaller
batches...
3. use cold water.
4. Are you letting the plaster slake for about 4 minutes
before you start to mix?
Good idea to do so.
5. again use FRESH plaster.
Stephani Stephenson
steph@alchemiestudio.com
May Luk on sun 28 sep 03
For getting the bubbles out, I do a little samba number next to the table
while pouring the plaster. With plaster bucket over the mould, my hip shakes
the table and bubbles rise to the surface. Very comical, but works well. I
heard that Salsa would do too.
Best Regards
May
London, UK
Maurice Weitman on mon 29 sep 03
At 12:50 AM +0100 on 9/29/03, Janet Kaiser wrote:
>Well the mental picture of potters all over the Western world
>using a dildo on full power to mix their next batch of plaster,
>because "Maurice Weitman said it would work"... just.... Well,
>sorry... But... Oh, dear me....
Here we go again with those quaint Brit words, eh Janet? And save
your guffaws for the "My dog ate the dildo" thread that's bound to be
next.
I guess I should have bought a Brit English dictionary; my Amerikan
dictionary defines dildo thusly:
\Dil"do\, n. (Bot.) A columnar cactaceous plant of the West Indies.
So, no, I wouldn't use a cactus to mix my plaster, and I would hope
"potters all over the Western world" would kindly refrain from doing
so as well. That would be an egregious misuse of a lovely plant.
Besides, I'm not here on clayart to make a name for myself as so many
have before me. Not me, Mel, honest.
Regards,
Maurice
Janet Kaiser on mon 29 sep 03
Oh, good grief, Maurice! Your post had me in stitches... First
the only "vibrating appliance" in this house is a foot bath we
bought for my mother some years ago... I smiled at what Himself
would say if I filled it with water and used it to mix plaster.
Then I thought about what else you could mean as something that
one would have lying around the house & home... Yes, you
guessed...
Well the mental picture of potters all over the Western world
using a dildo on full power to mix their next batch of plaster,
because "Maurice Weitman said it would work"... just.... Well,
sorry... But... Oh, dear me....
Sniggering
Janet Kaiser
*** IN REPLY TO THE FOLLOWING MAIL:
>One thing I forgot...
>If you have one of those more vigorous hand massagers, or an
orbital
>sander, or anything that vibrates strongly, after you've poured
the
>plaster, hold the vibrating appliance tight against the form for
a
>couple of minutes to help any air bubbles that might have been
>introduced in mixing or pouring to rise to the surface. It
really
>works well!
*** THE MAIL FROM Maurice Weitman ENDS HERE ***
***********************************************************
The top posted mail was sent by Janet Kaiser
The Chapel of Art : Capel Celfyddyd
8 Marine Crescent : Criccieth : Wales : UK
Centre of Excellence for The Arts
Home of The International Potters' Path
Tel: ++44 (01766) 523570 http://www.the-coa.org.uk
Open: 13.00 to 17.00hrs : Tuesday to Saturday
************** AVG Virus Protected ********************
Janet Kaiser on wed 1 oct 03
Just goes to show, Maurice... According to my Concise Oxford
Dictionary, the word has apparently been in use since the 17th
century (origin unknown), so some very strange people
with particularly sadistic or sado-masochistic tendencies must
have gone around the West Indies either giving names to the local
flora and fauna or bringing native names back for "re-cycling"
here... If it were not too difficult for me to get out, I would
consult Band I of my facsimile edition of the 1900-and-something
Compact Oxford Dictionary to see what the definitive Victorian
take on the word was...
Until which time, your reputation surely remains intact?
Janet :-) Just found I have a very prudish spell check...
*** IN REPLY TO THE FOLLOWING MAIL:
>>Well the mental picture of potters all over the Western world
>>using a dildo on full power to mix their next batch of plaster,
>>because "Maurice Weitman said it would work"... just.... Well,
>>sorry... But... Oh, dear me....
>my Amerikan dictionary defines dildo thusly:
> \Dil"do\, n. (Bot.) A columnar cactaceous plant of the West
Indies.
>So, no, I wouldn't use a cactus to mix my plaster, and I would
hope
>"potters all over the Western world" would kindly refrain from
doing
>so as well. That would be an egregious misuse of a lovely
plant.
>Besides, I'm not here on clayart to make a name for myself as so
many
>have before me. Not me, Mel, honest.
*** THE MAIL FROM Maurice Weitman ENDS HERE ***
***********************************************************
The top posted mail was sent by Janet Kaiser
The Chapel of Art : Capel Celfyddyd
8 Marine Crescent : Criccieth : Wales : UK
Centre of Excellence for The Arts
Home of The International Potters' Path
Tel: ++44 (01766) 523570 http://www.the-coa.org.uk
Open: 13.00 to 17.00hrs : Tuesday to Saturday
************** AVG Virus Protected ********************
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