Sheryl VanVleck on tue 30 sep 97
As with most things in life, I'm sure you can follow any number of mixing
ways and methods and still achieve a good product. I have been doing plaster
molds since 1993; taught by a great teacher, Katherine Ross at SAIC in
Chicago. Here is the way she taught us:
You use 1 QT WATER TO 2 LB. 12 OZ. #1 POTTERYPLASTER
Use Clean - room temp water
Pour plaster into water - she said that this was important.
let sit 2 minutes
then stir every couple of minutes for 10-15 minutes
Put your hand flat near or on the bottom of the bucket and use a side
to side motion to avoid any excess water
The plaster is ready to pour when X stays on surface (although I rarely wait
this long)
Pat the sides of the object to remove air and set in detail for 30 seconds
Wait for a 15 minute setup time
Hope this helps.
Sheryl VanVleck
Evansvile Wyoming
Where we again have clear blue skys and gorgeous sun rises
John H. Rodgers on thu 2 oct 97
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
I have been following this plaster mixing thread for a while, and here's my
$0.02.
I've done a lot of plaster work, and while there are many ways to mix the
plaster, I would encourage anyone planning to work with it to go to your
plaster supplier and ask for a plaster mixing scale. If you are using
plaster products made by .U.S. Gypsum Corp., they make a very good slide
rule device that will enable you to get your mix precise. And that is
important. Plaster density is an important factor in performance where
gypsum products are concerned so its important you get it right.
The slide scale matches the plaster by weight to the water by weight. So
many pounds of plaster requires so many pounds of water. No more, no less.
That way, you have the best combination of density v. strength. To dense (ie
.. to much plaster per pound of water, and absorption is affected), to little
plaster per pound of water and strength is affected. Talk about getting
plaster off your batt and into the clay ---- wow, crumbly plaster is the
pits, and comes from to much water per pound of plaster.
When I mix plaster, I use three tools. (1) A Rubbermaid 2-1/2 gal bucket( a
plastic sheetrock mud bucket works very well for larger batches). The
Rubbermaid is good for a 10 lb mix, and the mud bucket for 25 lb. mix.(2) An
18" one-gallon size jiffy mixer mounted in a variable/reversable electric
hand drill. (3) a large rubber mallet. (4) Two or three hardwood paint-mix
sticks to serve as scrapers.
I usually gestimate the amount of mix I need, then work out on the slide
scale the water/plaster required. I use enough plaster I generally don't
need to sieve my plaster into the water in the bucket. My plaster is fresh.
I use it up. But if you have any that has sat around several months it may
have lumps. Plaster should be used in three months and certainly inside of 6
.. When it gets old it reacts weirdly. On the one hand, it just might not set
, period. On the other, it will set in the bucket while you are still mixing
, almost as soon as the plaster enters the water. If it's like the fore -
just pour it out, if like the latter, don't try work around the fast set,
get rid of it. It's just going to cause you grief.
I shake my plaster into the water in the bucket, letting it settle. I allow
it to sit and soak for a couple of minutes, then I power mix it, keeping the
mixer head under the surface to reduce air entrainment. I remove the mixer
and give it a spin in a bucket of water that I didn't tell you about. That
removes the plaster so it won't set on the mixer head.Then I gently stir
with a paint stick, testing how the plaster runs off the stick. After some
experience you can tell by the increasing stiffness of consistency when it
is time to pour the mix.Medium to heavy cream is how I like it. But watch it
.. It can catch you off guard. Do not allow yourself to be distracted at this
point. I have had more than one bucket of plaster go hard on me just when I
thought "oh, it will be a few more minutes.!" During the gently stirring
period, I occasionally pick up the bucket and drop it about an inch or so to
the table or floor, like working a cake mix. Gets the bubbles to rise to the
surface. Then to get rid of the surface bubbles, I mist the surface with
alcohol. Instant bubble burst. After that its ready to pour. Pour steadily,
beginning in the deepest section of your mold first, slightly bumping the
mold as you go to break bubbles and smooth the plaster flow. When done, let
plaster set slightly, then rake off with a straight edge, ie, a paint stick,
or other.
When the plaster sets, it will heat up, when it reaches maximum heat it will
be the easiest to remove from the mold, because residual water will make a
flexible layer between the plaster and the mold. This aids in letting air in
underneath to help break any suction formed that tends to hold the plaster
in the mold.
When done, I let the plaster set in the bucket. Then, using the rubber
mallet, I pound the sides first, then the bottom. Plaster usually breaks
right out. Sometimes a little scraping is necessary, but not often. Finally,
dump the hardened plaster scrap into the empty gypsum plaster bag, you know,
the one I forgot to tell you to keep. USG always supplies a garbage bag with
each 100 lbs of plaster they sell. Comes built in.
If you can't get a slide scale, use a mix of 1.5lbs plaster to 1lb of water
.. That is good up to a ten pound mix. The ratio is not a true 1.5 to 1 so if
you go over ten pounds density errors are introduced.
This discussion has been based on the use of USG #1 Pottery Plaster, which
is about the standard for ceramic mold making industry. However there are
many other plaster types, each with their own characteristics, and they are
interesting to work with, should anyone want to pursue the work.
Goode luck.
John Rodgers
-------- REPLY, Original message follows --------
Date: Tuesday, 30-Sep-97 07:44 AM
From: Sheryl VanVleck \ America On-Line: (SAVanVleck)
To: CLAYART LIST \ Internet: (clayart@lsv.uky.edu)
Subject: Regarding Potters plaster mixing
----------------------------Original message----------------------------
As with most things in life, I'm sure you can follow any number of mixing
ways and methods and still achieve a good product. I have been doing
plaster
molds since 1993; taught by a great teacher, Katherine Ross at SAIC in
Chicago. Here is the way she taught us:
You use 1 QT WATER TO 2 LB. 12 OZ. #1 POTTERYPLASTER
Use Clean - room temp water
Pour plaster into water - she said that this was important.
let sit 2 minutes
then stir every couple of minutes for 10-15 minutes
Put your hand flat near or on the bottom of the bucket and use a
side
to side motion to avoid any excess water
The plaster is ready to pour when X stays on surface (although I rarely
wait
this long)
Pat the sides of the object to remove air and set in detail for 30 seconds
Wait for a 15 minute setup time
Hope this helps.
Sheryl VanVleck
Evansvile Wyoming
Where we again have clear blue skys and gorgeous sun rises
-------- REPLY, End of original message --------
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