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water to plaster ratios

updated fri 15 sep 00

 

judy motzkin on tue 25 jul 00


this is a good table to have on hand, although i must admit that for most of my plaster needs, I use the visual method...I don't recall the source of this table.WATER TO PLASTER RATIOS AT A CONSISTENCY OF 73The following table of various volumes of watr and corresponding plaster weights, is based on plasters which work well as slip casting molds at a consistency of 73, especially USG No1 Pottery plaster. Use recommended soaking and mixing times, and 70 degree F water for best results. (mixing quantities smaller than 2 quarts is not recommended).WATER PLASTER (grams) pint 11 oz. (or 312 gr.)1 pt 1lb. 6oz (624gr)1 qt 2lb. 12oz (1248gr)1 qt 4 lb. 2 oz. (1972 gr)2 qt 5 lb 8 oz (2497 gr)2 qts 6 lb 14 oz (3021)3 qts 8 lb 4 oz (3744)3 qts 9 lb 10 oz (4388)1 gal 11 lb (4992)1 gal
` 16 lb 8 oz (7488)2 gal 22 lbs (9984)2 gal 27 lbs (12,480)3 gal 33 lbs (14,9763 gal 38 lb 8 ozI hope this h
elps.Judy

www.motzkin.com


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John Baymore on thu 27 jul 00


Water to plaster varies by the type of plaster used and the function of t=
he
casting. =


US Gypsum has a wonderful plethora of information on plasters.....
including an handy little sliderule type calculator. Contact them (don't=

have the address here at the computer but a little work should come up wi=
th
it ). They'll send the stuff to you. Your pottery suppleir might be
able to get it for you too.


BEst,

.................john

John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA

603-654-2752 (s)
800-900-1110 (s)

JBaymore@compuserve.com
John.Baymore@GSD-CO.com

"Earth, Water, and Fire Noborigama Woodfiring Workshop August 18-27,
2000"

David Hendley on sun 3 sep 00


I've been making some plaster batts this week and
I've found this table (below, sent to Clayart by Judy
Motzkin last month) of water/plaster ratios to have
too much water/not enough plaster for my tastes.

The table uses the ratio of 1lb. 6 oz. of plaster
(No. 1 Pottery Plaster) to 1 pint of water.
I prefer 1lb. 7 oz. of plaster to 1 pint of water.
This may seem like an insignificant difference, but it
adds an extra 1/2 pound of plaster to a gallon of water.

At the 1lb. 6 oz. ratio, it takes too long for the mix
to set up enough to pour, up to 15-20 minutes after
mixing. With the greater plaster ratio, it's ready to
pour 5 minutes after mixing.
(My criteria for 'ready to pour' is when a finger dragged
across the surface leaves a small wake on the surface
of the plaster).

Any comments from those of you more experienced in
plaster work?
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/




----- Original Message -----
From: judy motzkin
To:
Sent: Wednesday, July 26, 2000 12:14 PM
Subject: Fwd: water to plaster ratios


I hope this comes through readably...
>
> this is a good table to have on hand, although i must
> admit that for most of my plaster needs, I use the
> visual method...I don't recall the source of this
> table.

WATER TO PLASTER RATIOS AT A CONSISTENCY OF 73
The following table of various volumes of watr and
> corresponding plaster weights, is based on plasters
> which work well as slip casting molds at a consistency
> of 73, especially USG No1 Pottery plaster. Use
> recommended soaking and mixing times, and 70 degree F
> water for best results. (mixing quantities smaller than
> 2 quarts is not recommended).
WATER PLASTER (grams)
pint 11 oz. (or 312 gr.)
1 pt 1lb. 6oz (624gr)
1 qt 2lb. 12oz (1248gr)
1 qt 4 lb. 2 oz. (1972 gr)
2 qt 5 lb 8 oz (2497 gr)
2 qts 6 lb 14 oz (3021)
3 qts 8 lb 4 oz (3744)
3 qts 9 lb 10 oz (4388)
1 gal 11 lb (4992)
1 gal 16 lb 8 oz (7488)
2 gal 22 lbs (9984)
2 gal 27 lbs (12,480)
3 gal 33 lbs (14,976)

Tom Buck on mon 4 sep 00


David:
For a studio potter needing the occasional plaster form for
shaping clay, I like John B. Kenny's description of 1949. He gives enough
detail to allow a potter to make successful casts but doesn't get too
technical to confuse you. Kenny cites 2.75 lb of plaster powder to 1 US
quart (aka 1 litre) as the most serviceable mix. Mathematically massage
these numbers and you get 73 weight units of water to 100 weight units of
plaster powder. This gives a slightly soft form. If you need a harder form
for repeated casting, then 67-69 units of water to 100 units of plaster
powder would be better.
As for set time, this is a function of water temperature
(warm means faster), and the type of plaster of paris you use. US Gypsum
offers PP that is slow-set, normal-set, and fast-set. You choose, and
order same.
and happy slip-day to you too. Peace. Tom.

Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada

A Reid Harvey on thu 7 sep 00


David,
I can't recall for sure the exact plaster to water ratio recommended by
the manufacturer of No. 1 Pottery Plaster, but the 1 lb. 6 oz. plaster
to 1 pint water does ring a bell. Not to become encumbered with rules,
but the manufacturer suggests you follow their guidelines, in part, so
they can't be blamed if something goes wrong. In this case you should
follow their suggested ratio in order to get a stronger, more long
lasting mold.

I guess you could think of there being an optimal crystal lattice, where
the water and plaster have been combined in just the right amounts.
Anything more or less, one way or the other, and there are more flaws in
the lattice. You may not perceive this at all in your mold or in your
casting, but I would imagine it could make the difference in the
lifetime of the mold of, say 300 castings where you used the correct
ratio, and maybe 250 castings where you changed the ratio slightly.

There are other variables that might be causing your plaster mix to take
longer than you'd like, before you are ready for pouring. For example,
if it's as long as fifteen minutes maybe your water is a little on the
cool side. Manufacturers usually recommend room temperature water. Or
for that long a mix, maybe you're doing the mixing, let's say be hand, a
little too slowly. Interestingly enough, you can actually cut the mixing
time, the five minutes you consider optimal, down to about three
minutes, just by using a motorized mixer. What's happening on a micro
scale with a quicker mix is that there are more opportunities for the
plaster and the water to have collisions and form bonds. This hastens
mixing time.

So I think your batts would probably last longer if you used the right
ratio, used room temperature water, and gave a good steady mix, perhaps
a bit quicker than what you've been doing. There are other variable, but
I can't recall them at the moment, since my hand is not in plaster. On
the other hand, if following all the rules slows you down while making
the batts, taking a lot of energy, I suggest you do what you're
comfortable with.
Bye for now.
Reid Harvey
Ceramique d'Afrique (now relocated to Bangladesh)

David Hendley wrote:
I've been making some plaster batts this week and
I've found this table (below, sent to Clayart by Judy
Motzkin last month) of water/plaster ratios to have
too much water/not enough plaster for my tastes.

The table uses the ratio of 1lb. 6 oz. of plaster
(No. 1 Pottery Plaster) to 1 pint of water.
I prefer 1lb. 7 oz. of plaster to 1 pint of water.
This may seem like an insignificant difference, but it
adds an extra 1/2 pound of plaster to a gallon of water.

At the 1lb. 6 oz. ratio, it takes too long for the mix
to set up enough to pour, up to 15-20 minutes after
mixing. With the greater plaster ratio, it's ready to
pour 5 minutes after mixing.
(My criteria for 'ready to pour' is when a finger dragged
across the surface leaves a small wake on the surface
of the plaster).

Any comments from those of you more experienced in
plaster work?
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/

Louis H.. Katz on thu 7 sep 00


Oddly enough, water that is too cold will cause a greater expansion in your set
plaster. If you are making large molds the expansion can be enough to crack off
keys and destroy dit between two mold sections.
This property of plaster, to expand as it is setting, is used by pattern and mold
makers to make up for the contraction of metal as it is cooling. It allows a true
scale original to be made and a mold taken that will grow as it sets.
Louis

Jeff Lawrence on wed 13 sep 00


Hi,

I am in the process of replacing a bunch of molds that were mixed with a
soft mix 70 of 73 weight units water to 100 weight units plaster (USG No. 1
Pottery Plaster).

Other molds twice as old are stable and usable -- mixed with USG's
recommended mix ratio of 67 weight units water to 100 weight units plaster.
Meanwhile, the molds from the more watery mix shed Big Mac sized pieces of
themselves at every use.

Although the Big Mac has yet to be accepted as a standard unit in plaster
work, I use it because I need something concrete after all these recipes
that mix and match weights and volume units.

Jeff Lawrence ph. 505-753-5913
Sun Dagger Design fx. 505-753-8074
18496 US HWY 285/84 jml@sundagger.com
Espanola, NM 87532 www.sundagger.com