Londraville,Richard L on mon 24 jan 05
All-
Can plaster go bad? I have some plaster that will not cure. I have
sifted the plaster into cold water through a screen, until it sits on
the surface for a 5-count before going under. I then mix and let it
slake until I can draw a line on the surface, then pour. It's been
sitting in the mold for days and is still liquid. I have used plaster
from this bag before with no problems, but not for about a year.
I've tried this 3 times now-no luck. =20
Ideas?
Thanks-
Rich
Dr. Richard Londraville
Associate Professor of Biology
University of Akron
Dept. Biology
Akron, OH 44325-3908
330-972-7151
londraville@uakron.edu
http://www2.uakron.edu/londraville/rll.htm
Kate Johnson on mon 24 jan 05
All-
Can plaster go bad? I have some plaster that will not cure. I have
sifted the plaster into cold water through a screen, until it sits on
the surface for a 5-count before going under. I then mix and let it
slake until I can draw a line on the surface, then pour. It's been
sitting in the mold for days and is still liquid.
Hi Richard! I had this same problem about a month ago--the archives are
full of good answers. Yes, the plaster can become hydrated and too old
to use. Fresh plaster GOOD. I had a huge wet mess in my mixing area,
got fresh and no problem at all.
How nice to be able to answer a question from recent personal experience....
Best--
Kate
John Rodgers on mon 24 jan 05
It sounds like your plaster could be dead, but then again, it also
sounds like your water to plaster ratio is way off.
The plaster mix must be done by weight to get it right.
The ratio is 1 lb of water to 1.5 lbs of plaster. That is close enough
and works pretty good for mixes under ten pounds. Over ten pounds you
really should use a plaster mix calculator (get from u.s. gypsum
corporation or maybe from your local gypsum supplier). The 1:1.5 ratio
will work for the bigger batch mix, but you will not have the strength,
hardness nor correct water absorption rate of the proper mix.
If the plaster is dead, it will react one of two ways. It will set
snap-set -- set in your bucket before you can get it thoroughly mixed.
or it will not set at all.
Try a small batch by weight so as not to waste the plaster. - 1 lb
water, 1.5 lbs plaster - and see what happens. If it does not set - you
might as well dump the plaster. and buy new.
Be sure your new plaster is not over 90 days old. Seal any unused
plaster in a waterproof plastic bag. Plaster will entrain moisture from
the air even on a dry day.
Regards,
John Rodgers
Chelsea, AL
Londraville,Richard L wrote:
>All-
>Can plaster go bad? I have some plaster that will not cure. I have
>sifted the plaster into cold water through a screen, until it sits on
>the surface for a 5-count before going under. I then mix and let it
>slake until I can draw a line on the surface, then pour. It's been
>sitting in the mold for days and is still liquid. I have used plaster
>from this bag before with no problems, but not for about a year.
>I've tried this 3 times now-no luck.
>Ideas?
>Thanks-
>Rich
>
>Dr. Richard Londraville
>Associate Professor of Biology
>University of Akron
>Dept. Biology
>Akron, OH 44325-3908
>330-972-7151
>londraville@uakron.edu
>http://www2.uakron.edu/londraville/rll.htm
>
>______________________________________________________________________________
>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.
>
>
>
>
Craig Clark on mon 24 jan 05
Richard, yes plaster can and does go bad. Had a bag of No.1 Pottery
Plaster do that a few years back. Called the folks that make the stuff
and one of their very helpful technical people informed that the
plaster, which had been sitting here in humid SouthEast Texas for
several months, had most likely absorbed moisture from the air and as a
result became "seeded." That just means it won't work right anymore.
He told me to get more plaster, which I purchase in 50lb bags from
our local ceramics store, and to start again but to dump the old "Island
in the Middle" of the water trick for what he called a far more accurate
method of weighing the water and the plaster. This is not coming from me
but from the folks who make the material. I followed his advice and it
works quite well.
Chances are that you are using a US Gypsum product. Give them a
call. The number is on the bag, on the back, near the top, under the
Heading labeled Mixing...for Tech/sales assistance call 1-800-487-4431.
The individual with whom I spoke was extremely helpful. He was able to
directly address all of the questions and concerns that I had for my
specific applications. He has been doing this for many years. I left a
message and was called back that day. Other times I was able to speak
with him immediately. My suggestion is to give the company a call and
talk with them. He will talk you throug things. Helps a lot!
I noticed that you said you were pouring your plaster into cold
water. How cold? I believe the ideal temp for creaming plaster is around
72 degrees F. Warmer and it will set faster. Cooler and it will set
slower. Don't know if there is "minimum" temp for curing or not. I do
know that temp of water, ratio of water to plaster, method of mixing,
and length of mixing all affect the quality of the cure.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
URL Krueger on mon 24 jan 05
When I was a young kid (about 1958) I was big-time into
chemistry. About that time our family visited White Sands,
New Mexico. I still remember the brilliance of the huge
white "snow" banks (dunes) and how the road looked like ice
but was not slick.
We brought home a jar full of the sand and having learned
that it was composed of calcium sulphate and that plaster
of paris was calcium sulphate that had been calcined I
proceeded to heat some of the sand to a less-than red heat.
It was fun to watch the particles of sand "dance" as the
water of hydration was driven off. After the sand had
cooled I found it was much softer and could be ground to a
powder rather easily. When mixed with some water it sat-up
(or set-up ?) just like plaster of paris.
So, if I had a batch of bad plaster (which I probably do)
and was not able to get any fresh quickly I might put the
powder in my kiln in a big bowl, run the kiln on low for a
few hours, or until no more water is seen escaping from the
kiln, and see if this re-freshed the plaster.
Has anybody actually tried this?
--
Earl K...
Bothell WA, USA
Volunteer U.S. Marine Corps 1967-1971
Making the premeditated decision to take the life of another
human being should only be considered under the most
extreme of circumstances and with sufficient cause.
Neither of these criteria have been sufficiently met to
warrant the killing that is taking place in Iraq.
Louis Katz on mon 24 jan 05
Yes,
I have had it work well twice and fail twice. I don't know why it =20
failed but the plaster did not get fluffy nor did it set well. I use =20
375 degrees or thereabouts in a programable kiln for many hours =20
(8-12). The water comes off in several stages starting pretty low =20
(125F?). I started with "bad plaster" but Pietric (sp) a former =20
instructor at PCA now University of the Arts Philadelphia says during =20=
the war the recycled all of their molds. If you get the bugs worked out =20=
of this one let me know your procedure.
Thanks
Louis
On Jan 24, 2005, at 12:02 PM, URL Krueger wrote:
> When I was a young kid (about 1958) I was big-time into
> chemistry. About that time our family visited White Sands,
> New Mexico. I still remember the brilliance of the huge
> white "snow" banks (dunes) and how the road looked like ice
> but was not slick.
>
> We brought home a jar full of the sand and having learned
> that it was composed of calcium sulphate and that plaster
> of paris was calcium sulphate that had been calcined I
> proceeded to heat some of the sand to a less-than red heat.
> It was fun to watch the particles of sand "dance" as the
> water of hydration was driven off. After the sand had
> cooled I found it was much softer and could be ground to a
> powder rather easily. When mixed with some water it sat-up
> (or set-up ?) just like plaster of paris.
>
> So, if I had a batch of bad plaster (which I probably do)
> and was not able to get any fresh quickly I might put the
> powder in my kiln in a big bowl, run the kiln on low for a
> few hours, or until no more water is seen escaping from the
> kiln, and see if this re-freshed the plaster.
>
> Has anybody actually tried this?
>
> --
> Earl K...
> Bothell WA, USA
> Volunteer U.S. Marine Corps 1967-1971
> Making the premeditated decision to take the life of another
> human being should only be considered under the most
> extreme of circumstances and with sufficient cause.
> Neither of these criteria have been sufficiently met to
> warrant the killing that is taking place in Iraq.
>
> =
_______________________________________________________________________=20=
> _______
> 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 =20
> melpots@pclink.com.
>
>
KE5CVK
Longitude: 97=B0 20' 47" W
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Grid: EL17
Monitors IRLP Node 3499
Snail Scott on tue 25 jan 05
At 08:53 AM 1/24/2005 -0500, you wrote:
>Can plaster go bad? I have used plaster
>from this bag before with no problems, but not for about a year.
Plaster will absorb moisture from the air and
set up in its powdery particulate form, looking
just like fresh plaster, but it won't set again.
If you've had plaster sitting around for a while,
test a bit in a paper cup to see if it's still
OK before committing it to a real project.
If it's a year old, it's probably gone bad,
unless you live in the desert or have it sealed
VERY well. Chuck it out and get more. A 100#
bag of plaster is about $15.00 - not worth the
time and effort you put into fighting with bad
stuff. If you have to buy plaster in smaller
quantities, it will be a little more expensive,
but your time is never worth less than the cost
of decent quality materials.
-Snail
Snail Scott on tue 25 jan 05
At 10:02 AM 1/24/2005 -0800, Earl K wrote:
>...if I had a batch of bad plaster (which I probably do)
>and was not able to get any fresh quickly I might put the
>powder in my kiln in a big bowl, run the kiln on low for a
>few hours, or until no more water is seen escaping from the
>kiln, and see if this re-freshed the plaster...
It will work, especially if you only need
a little, but isn't worth the effort if
you can get fresh instead. If you are truly
in need of plaster, you can get it 24/7 at
an all-night Wal-Mart, though you'll pay
plenty for it.
-Snail
Eleanora Eden on wed 2 feb 05
Plaster comes in 50 lb and 100 lb bags. Exactly because of all the
problems discussed on this thread I always buy 50 lb bags, no matter
how many bags that will be. The unopened bags are each wrapped in a
garbage bag and stacked off the floor. Used bag is rolled shut and
tied off with a mold strap and then wrapped tightly in a plastic bag
and sealed up.
Since I have started storing plaster this way I no longer have
problems. When an open bag is being used in the studio I close it up
tightly between pours even if I'll be using it again shortly.
If they tell you they only have 100 lb bags then go somewhere else to
buy it. Knowledgeable suppliers will buy 50 lb bags and only as many
as they can sell before next delivery.
Eleanora
>All-
>Can plaster go bad? I have some plaster that will not cure. I have
>sifted the plaster into cold water through a screen, until it sits on
>the surface for a 5-count before going under. I then mix and let it
>slake until I can draw a line on the surface, then pour. It's been
>sitting in the mold for days and is still liquid. I have used plaster
>from this bag before with no problems, but not for about a year.
>I've tried this 3 times now-no luck.
>Ideas?
>Thanks-
>Rich
>
>Dr. Richard Londraville
>Associate Professor of Biology
>University of Akron
>Dept. Biology
>Akron, OH 44325-3908
>330-972-7151
>londraville@uakron.edu
>http://www2.uakron.edu/londraville/rll.htm
>
>______________________________________________________________________________
>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.
Randall on wed 2 feb 05
>
>>All-
>>Can plaster go bad? I have some plaster that will not cure. I have
>>sifted the plaster into cold water through a screen, until it sits on
>>the surface for a 5-count before going under. I then mix and let it
>>slake until I can draw a line on the surface, then pour. It's been
>>sitting in the mold for days and is still liquid. I have used plaster
>>from this bag before with no problems, but not for about a year.
>>I've tried this 3 times now-no luck.
>>Ideas?
>>Thanks-
>>Rich
Rich,
I have never heard of plaster not setting and=20
I've worked with many types of it since 1977,=20
plaster doesn't "go bad" what happens is this; an=20
opened bag can absorb moisture from the air, when=20
this happens two things occur
1)It can become "lumpy", as long as you can break=20
up the hardened lumps in your fingers it's okay=20
but can be troublesome to mix and use if you=20
don't get EVERY lump all broken up and dissolved.
2)The set time will change with age and moisture=20
absorption, generally it seems to want to set=20
=46ASTER- quite a bit faster sometimes, so it can=20
happen unexpectedly and catch you off-guard if=20
you are used to a certain setting time.
If the bag has hard "rocks" in it beyond a few you can pick out- toss it.
I have had a half bag of hydrocal sitting on the=20
floor in my basement for 3 or 4 years, I went to=20
use it and it worked perfectly fine and had no=20
lumps or rocks, but did set a little faster than=20
I thought I remembered. It's not the age but how=20
much moisture you have in the room where it's=20
stored. My basement was about 40% RH or less at=20
70=BAF
So you need to keep the bags off the damp floor,=20
in a heated environment ( a tool shed or=20
unheated garage won't work well) and LOW=20
humidity, the humidity is the key.
Next, I NEVER use cold water- EVER, the maximum=20
solubility of gypsum cements and I assume plaster=20
of Paris is with the water at 100=BA F, you will=20
see a definite set time change between using cold=20
tap water and 100=BA water (or higher) get a=20
thermometer and check your water temperature and=20
see what you have been using.
Gypsum cements like hydrocal, hydrostone etc MUST=20
HAVE mechanical mixing with a drill and paddle=20
mixer usually for 4 minutes or so, you can't get=20
the properties or correct setting with hand=20
mixing the stuff.
Consider the possibility either your plaster has=20
been contaminated by something spilling in it-=20
laundry soap, detergent, talcum powder,=20
chemicals, or that your water has changed- excess=20
chlorine for example or maybe you added a new=20
water softener- sodium weakens plasters=20
considerably though it does shorten the setting=20
time.
On my plaster and rubber molds I use Plaster of=20
Paris for my mother molds, I use 100=BA water and=20
sprinkle in the plaster to ABOVE the water level=20
till no water at all shows. The plaster should be=20
at room temperature not 40 degrees from bringing=20
in from the garage or something!
I don't time slaking I just spend a minute or so=20
getting things ready and then I always use the=20
drill/paddle mixer and mix it smooth for maybe 30=20
seconds, plaster of Paris doesn't seem to depend=20
on the agitation of mixing energy to gain it's=20
properties like the gypsum cements require.
I only buy 100 pound bags, the 50 pounders are=20
nice for handling ease but you wind up paying=20
more per pound so your per piece costs increase.=20
The shelf life for the plasters should be several=20
MONTHS
With hydrocal, I do the same but it gets mixed=20
for 4 full minutes by the timer- it gets thinner=20
as you mix it.
One other issue is, you should find out the=20
recommended mix for your type plaster, USG has=20
charts- so many ounces/pounds of specific product=20
to so many ounces/pounds of water, mix a batch=20
exactly per the directive making sure you WEIGH=20
the plaster and the water- the goal is to make a=20
batch up exactly as recommended and get a "feel"=20
for how much plaster you NEED in a given amount=20
of water, eventually after a few batches you will=20
develop a "feel" for how the mix should look and=20
consistency and not need to weigh every batch. I=20
don't any more as I found my "feel" is so close=20
to dead-on specs it's not funny.
>At 5:34 PM -0500 2/2/05, Eleanora Eden wrote:
>If they tell you they only have 100 lb bags then go somewhere else to
>buy it. Knowledgeable suppliers will buy 50 lb bags and only as many
>as they can sell before next delivery.
Eleanora
I've never had a problem such as described, and I=20
use only 100 pound bags and some tend to sit=20
around opened for months- but in low humidity.
The supplier may or may not get fresh product,=20
the product could have been sitting in the=20
warehouse at USG or distributor's for months=20
before being shipped to your supplier. I have had=20
one time where I bought 100# sacks of plaster=20
from a lumber yard supplier in my former state, I=20
opened the first bag and it was full of "rocks"=20
so was the rest. I called and complained, they=20
came picked those up dropped off new bags, those=20
were the same, went to see them and asked if I=20
could open a bag before buying- no they said. but=20
I could return it- that was real efficient!
Turned out they were storing the stuff in an=20
unheated open sided lean-to and their entire=20
supply was rocks.
Randall
Webdirector of "Randall's Lost New York City"
A historical photo essay of lost buildings from NYC's architectural history
http://www.lostnewyorkcity.com/
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