Ditmar on wed 29 dec 04
In my opinion plaster should always be mixed by weight. Period !!!!
The "island" method is an abomination that should die and go away.=20
Weighing is faster and the results are consistent. One side of the mold =
won't absorb faster than the other. One part won't be harder or have =
properties different than another.
( granted..if you use the same plaster time after time all the time, and =
are used to it...well, island away. )
There are MANY different plasters and gypsum cements. Each with it's own =
specific water to plaster ratio for optimum results. ( USG products)=20
e.g. Hydrostone 100 parts to 32 parts water =20
Hydrocal 100 parts to 45 parts water
# 1 Casting 100 parts to 65 parts water
#1 Pottery 100 parts to 70 parts water
Hydroperm 100 parts to 80 - 100 parts water
If you want consistent results and the best plaster has to offer, weigh. =
Hell, it's not rocket science.
Ditmar
karen gringhuis on sat 7 apr 07
If you do much plaster mixing, you may want to get
your hands on a cc of Don Frith's "Mold Making for
Ceramics." It's the bible & includes industrial
quality info. (He was honored at the recent NCECA conf.)
Karen Gringhuis
KG Pottery
Box 607 Alfred NY 14802
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Lynne and Bruce Girrell on sat 7 apr 07
Lately I've been mixing up some plaster for making some slump/drape molds. I
am attempting mix according to USG guidelines, but am unable to meet the
times that they suggest.
Details:
I am mixing 10 lbs water and 14.5 lbs plaster for a 69% water:plaster ratio.
I sift all plaster through a 30 mesh sieve prior to beginning work
It takes about 3 minutes to get all of the plaster distributed into the
water
Water temperature is about 60 F
Slake for 1 minute
Mix with Jiffy Mixer in a 1750 rpm drill
USG recommends up to 9 minutes of mixing. The minimum time in their chart is
3 minutes, providing the lowest strength. By the time I have mixed for 3
minutes the plaster is threatening to set up. When I pour the plaster, it
comes out as glops, not liquid.
I am running minimum times on everything. I want the highest strength forms
that I can make, but my times push everything toward minimum strength. Any
ideas on what I might be doing wrong?
Bruce Girrell
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John Britt on sat 7 apr 07
Bruce,
Sounds like your plaster is old. If it is older than about three months it
absorbs moisture and sets up really quickly.
Try buying some fresh stuff.
Hope it helps,
John Britt
www.johnbrittpottery.com
John Rodgers on sat 7 apr 07
It sounds like maybe your plaster is a bit old. The older the plaster
the faster the setup rate. Get new plaster and you will be better off.
Also, if you want to continue trying to work with the plaster you have,
you might try a bit of Sodate Retarder. Used a lot in concrete work.This
dry powder can be found at most concrete supply places. It slows setting
times. It is often mixed with water then sprayed on fresh poured
concrete to kill the set in the surface of the concrete. Then when the
concrete is hosed off, it leaves the gravel in the concrete exposed
nicely. Makes for an interesting surface. . You will have to experiment
with it to get the performance you want.
Regards,
John Rodgers
Chelsea, AL .
Lynne and Bruce Girrell wrote:
> Lately I've been mixing up some plaster for making some slump/drape
> molds. I
> am attempting mix according to USG guidelines, but am unable to meet the
> times that they suggest.
>
> Details:
> I am mixing 10 lbs water and 14.5 lbs plaster for a 69% water:plaster
> ratio.
> I sift all plaster through a 30 mesh sieve prior to beginning work
> It takes about 3 minutes to get all of the plaster distributed into the
> water
> Water temperature is about 60 F
> Slake for 1 minute
> Mix with Jiffy Mixer in a 1750 rpm drill
>
> USG recommends up to 9 minutes of mixing. The minimum time in their
> chart is
> 3 minutes, providing the lowest strength. By the time I have mixed for 3
> minutes the plaster is threatening to set up. When I pour the plaster, it
> comes out as glops, not liquid.
>
> I am running minimum times on everything. I want the highest strength
> forms
> that I can make, but my times push everything toward minimum strength.
> Any
> ideas on what I might be doing wrong?
>
> Bruce Girrell
>
> _________________________________________________________________
> Can’t afford to quit your job? – Earn your AS, BS, or MS degree online
> in 1
> year.
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>
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Ivor and Olive Lewis on sun 8 apr 07
Dear Bruce Girrell,=20
I will not repeat what has been said already.
I think you have overworked the plaster during the mixing stage. IF you =
gave the plaster 9 mins with an electric drill mixer you are making a =
trowelling mix, the material used by Plaster Moulders who tool it to =
produce cornices, frames and other ornamentations. This has to be done =
while the plaster is setting otherwise it would not retain the shape =
that is imposed on it. It has to have a certain degree of plasticity so =
that it will not slump or fall away from vertical or overhead surfaces.
Years ago I had a year 11 class and we needed to cast two litre blocks =
for learning elementary stone carving techniques. They were told to =
slowly stir their bucket of wet plaster for twenty rotations after it =
has slaked, then tap it to eliminate any bubbles they might have =
introduced.
All except one young lass accomplished this. Then she called to me to =
complain her mixture was beginning to stiffen. I looked across and she =
was still stirring. We managed to get it into her mold and then I asked =
why she was still stirring. "I knew I had to count to twenty but every =
time is listened to Xxxx I forgot where I had got to, so I started to =
count to twenty again". I suppose that plaster had only been stirred for =
five mins !
A good reference for you and anyone else seeking more knowledge about =
this wonderful resource should look for Jack C. Rich. "The Materials =
and Methods of Sculpture" Oxford University Press 1947. It was reprinted =
many times.
By the way, the chemical process that causes Gypsum Plaster to set takes =
time. I used to allow about 48 hours for it to develop its full =
strength. Then it has to dry. The heat of the reaction must dissipate =
before the cast is disturbed and it must remain wet throughout. It must =
be poured when it is creamy and not be disturbed after that, other than =
a brief tap to assist bubbles to rise.
Hope things work out as time goes by.
Best regards,=20
Ivor Lewis.
Redhill,
South Australia.
Tom Witman on sun 8 apr 07
One clue might be the use of a stirring device in a high speed drill. I
think you are putting a lot of heat into the mix with the drill/mixer.
You said you are doing this for several minutes, which would really heat
things up. Just stir it by hand and look for the telltale 'groove' when
you stroke a finger across the surface.
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