Paul Herman on sun 26 dec 04
Hello Kate,
you wrote:
"Was pretty sure I knew the proportions, and I'm normally very good at
judging these things...
Opinions welcome, as always..."
My opinion:
That's where the difficulty came from.
There is a proper amount of water to mix with the plaster. The
manufacturer or supplier might tell you how much, and books can tell you
a ballpark figure. Perhaps it's on the container somewhere.
I would call or email the manufacturer, to get their recommendation.
By the way, I wish you luck in chasing that honey colored glaze, it
sounds like a lot of fun.....
Good castings,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: Kate Johnson
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Plaster problem...you KNEW that was coming...
>Date: Sun, Dec 26, 2004, 1:40 PM
>
>
> AT LAST, it started to warm and thicken, so when it got sort of like thick
> oatmeal, I started putting it in the molds.
>
> Where it's been for a half an hour.
>
> Oozing water. Exuding water. Water rising to the top. Much wiping of
> water, much sopping up of water.
>
> The stuff still on the sides of the bucket is grainy, not hard. This is NOT
> a good sign.
>
> So. What DOES happen when you get too much water in? Sure looked like it
> was going to get thick...
>
> Or maybe it's the old plaster. Nuts. These were going to be NICE molds...
>
> Trying to ignore them, on the theory that a Watched Mold Never Hardens...
>
> Opinions welcome, as always...
>
> Best--or maybe NOT so best--
> Kate
Kate Johnson on sun 26 dec 04
Admit it. You knew I'd find some way to, er, challenge myself...
I spent most of the afternoon getting 4 things ready to make molds from, all
nicely coated with diluted Murphy's, dried, coated again, buffed...found
things to put them on and contain the plater for nice neat edges, got out
the brand new plaster I got last week...
Was pretty sure I knew the proportions, and I'm normally very good at
judging these things...put water in the big bucket and started sifting
plaster into it. Used up the first container (I can't lift the 50 lb bags
so I got two 8 lb. tubs. Figured that would be plenty. No prob, I had
another. Opened it...sifted it in, too. Er...still didn't mound above the
water.
First indication of trouble.
Well, I didn't *want* to use the old plaster of paris, but whatchagonnado?
At that point I had a mess on my hands and nothing to lose...
The old plaster was a bit lumpy, but I kept squishing lumps with my
hands...at this point reasonably worried, because it was still VERY watery.
Kept mixing. Kept mixing.
AT LAST, it started to warm and thicken, so when it got sort of like thick
oatmeal, I started putting it in the molds.
Where it's been for a half an hour.
Oozing water. Exuding water. Water rising to the top. Much wiping of
water, much sopping up of water.
The stuff still on the sides of the bucket is grainy, not hard. This is NOT
a good sign.
So. What DOES happen when you get too much water in? Sure looked like it
was going to get thick...
Or maybe it's the old plaster. Nuts. These were going to be NICE molds...
Trying to ignore them, on the theory that a Watched Mold Never Hardens...
Opinions welcome, as always...
Best--or maybe NOT so best--
Kate
Bob Masta on mon 27 dec 04
Kate:
I wonder if maybe you didn't just take too long to sift all
that plaster in, such that the first part was already trying to set up
when you finally got to mixing it. Even though I have had good
luck with the mounding method in the past, I now always measure
things out. Takes all the uncertainty out of the process. What I
did was get two measuring scoops. Weigh out how much
plaster the first one holds when it is full to the brim. #1 pottery
plaster should be mixed at 70 parts water to 100 parts plaster,
so into the second scoop add water to get 70% of the weight of
plaster in the first one. (Easy with a balance with a tare adjust,
or with a 2-pan balance and 2 identical scoops.) Mark a line
on the second scoop exactly at the 70% level.
Now when I mix plaster, I fill the water scoop to the line
once for each scoop of plaster I will use, and put it all
into the mixing bucket. No plaster yet, so take as long
as you want for this stage. Then I use the plaster scoop
and add the corresponding number of scoops, working
quickly and sprinking evenly across the surface.
It may be you are just trying to mix more than you can
handle at one time, and you need to break the job into
smaller batches.
Hope this helps!
Bob Masta
potsATdaqartaDOTcom
Snail Scott on mon 27 dec 04
At 03:40 PM 12/26/2004 -0600, you wrote:
>...Used up the first container (I can't lift the 50 lb bags
>so I got two 8 lb. tubs. Figured that would be plenty. No prob, I had
>another. Opened it...sifted it in, too. Er...still didn't mound above the
>water.
Whether the plaster is good or bad, it will still fill
up the bucket the same way. The plaster DOES need to
be level with the waterline to work.If it's not kinda
creamy-thick after slaking, mixing won't make it
thicker. Don't mistake the stiffening of setting for
'thickening'. It needs to start off right. It's easy
to disbelieve just how much plaster is actually needed.
It can seem, especially if you are sifting it in a
handful at a time, that you will never get enough in,
and the plaster must be escaping through an
interdimensional warp in the bottom of the mixing bin!
Don't stop, though. If it's too thin, you will only end
up wasting whatever you already put in. Persevere!
>The old plaster was a bit lumpy, but I kept squishing lumps with my
>hands...at this point reasonably worried, because it was still VERY watery.
>Kept mixing. Kept mixing.
Sounds like your plaster was old and rehydrated
(i.e. gone bad). Even with too much water, it
should have set, though it would have been very
weak and had a 'pond' on top.
Small lumps should fall apart of their own accord
during slaking - if they haven't, it's a strong sign
that they're not just lumps - they're set. Plaster
that's lumpy in the bag is always suspect, so first
test a little in a paper cup to see.
Plaster isn't free, but it's pretty dang cheap. Even
if you're not selling your work for income, your time
is worth more.
And don't get those hobby-store packages of plaster!
They are quite often old and bad, and VERY overpriced.
Industrial plaster suppliers are the best, and their
usual rate for a 100# bag is around $15.00. I know
you said you can't handle large bags, but do you have
a friend who would buy half of it, and help you
repackage it into smaller plastic bins? Even if you
have to do it alone, try this: Put a tarp in the back
of your car, and when you buy the 100# bag, ask the
warehouse to load it for you. When you get home, cut
the bag open (still in the car) and use a scoop to
transfer it into lidded plastic bins that you can
carry. (Line the bins with trashbags.) Then just
throw away the now-empty 100# bag and clean off the
tarp.
If you don't want to deal with 100 pounds at a time,
you can try Home Depot or another hardware store. The
'Big Box' places don't pay much attention to their
stock, but their turnover of product is usually
sufficient to provide reasonably fresh plaster. They
are more expensive (about $6.50 for a 25# bag) but
they may beeasier to deal with (and locate) than an
industrial supplier - it depends on where you live.
And they are still MUCH cheaper than the hobby-sized
packages.
-Snail
Catherine Yassin on mon 27 dec 04
In a message dated 12/26/2004 5:58:12 PM Central Standard Time,
graphicart@EPSI.NET writes:
So. What DOES happen when you get too much water in? Sure looked like it
was going to get thick...
Well, if it does set, it will likely be pretty brittle and porous. I have
found that when my mixture was too watery that my end result of mold would be too
fragile. I had a large plaster slab that I made with a 2x4 frame that was too
watery and it seeped water for days before it finally got hard. If your frame
or original mold isn't porous then the water has no where to go but up and
may take some time to evaporate.
Good luck!
-Cat Yassin
San Antonio
Carol on mon 27 dec 04
Hmmm... I learned that the "setting" process of plaster isn't activated
without motion. That's why many of us let the whole thing slake 3-5
minutes without fear of it going off; the stirring after the slaking is
what starts the process. I agree, though, that weighing is the most
precise way of making plaster, leaving little room for error... I just
don't think you have to hurry through it!
Carol R
On Dec 27, 2004, at 9:14 AM, Bob Masta wrote:
Kate:
I wonder if maybe you didn't just take too long to sift all
that plaster in, such that the first part was already trying to set up
when you finally got to mixing it. Even though I have had good
luck with the mounding method in the past, I now always measure
things out.
Brant Palley NMCLAY.com on mon 27 dec 04
Plaster mixing problems are almost always caused by not weighing the
plaster and the water.
How to mix Plaster:
1. Always weigh out the amount of plaster you need, then look up the
water by weight percentage. For example USG #1 Pottery Plaster (for making
molds) uses 70% water to plaster. So 10 lb. of plaster requires 7 lb. of
water. First weigh out the plaster you need, take that amount and multiply
it by .7, (point seven, 70%) this gives you how much water to use.
2. Generally speaking the larger the percentage of water the softer
and more absorbent the plaster will be. The harder the plaster the less
water used.
3. Hot water will speed up the set time and cold water will slow it
down.
4. Place the water in a suitable sized container. Always add plaster
to water. Pour the plaster into the water and let slake for one minute.
Mix by hand or with a Jiffy Mixer, being careful not to suck air into the
plaster. When the mixture is lump free it is ready to pour; don=92t wait for=
it to start to thicken.
Brant Palley
www.nmclay.com
Kim Lindaberry on mon 27 dec 04
Never use generic plaster of paris if you value what you are doing. It
is generally junk and a waste of your time and money. There are
different grades or types of plaster and you should be using #1 Pottery
Plaster or Moulding Plaster.
Here are links to webpages that cover mixing plaster. This one has
quite a bit about mixing and uses of plaster;
http://www.plastermaster.com/usg/plasterfundamentals.htm
This link is to a USG pdf brochure about #1 pottery plaster;
http://literature.usg.com/pdf/IG1366.pdf
cheers,
Kim
Eleanora Eden on tue 28 dec 04
To Kate and all,
Here is an old post of mine about my method:
Eleanora's EZ Breasy Plaster Casting Method:
Having no patience at all with the weighing of water I do it in
the following manner:
1. Judge volume of pour (if you keep a log of your pours it will help).
2. Volume of plaster desired = volume of dry plaster. Only FRESH #1
pottery plaster should be used. Know your source. Ask how long the bags
have been around.
3. Volume of water = 2/3 volume of dry plaster.
so. measure your water and pour it in a container large enough. Dump
plaster in and mix for afew minutes. When it's thickened like cream it's
ready. When it's thickened like pudding it's more than ready.
Hope this helps.Eleanora
At 03:40 PM 12/26/04 -0600, you wrote:
>Admit it. You knew I'd find some way to, er, challenge myself...
>
>I spent most of the afternoon getting 4 things ready to make molds from, all
>nicely coated with diluted Murphy's, dried, coated again, buffed...found
>things to put them on and contain the plater for nice neat edges, got out
>the brand new plaster I got last week...
>
>Was pretty sure I knew the proportions, and I'm normally very good at
>judging these things...put water in the big bucket and started sifting
>plaster into it. Used up the first container (I can't lift the 50 lb bags
>so I got two 8 lb. tubs. Figured that would be plenty. No prob, I had
>another. Opened it...sifted it in, too. Er...still didn't mound above the
>water.
>
>First indication of trouble.
>
>Well, I didn't *want* to use the old plaster of paris, but whatchagonnado?
>At that point I had a mess on my hands and nothing to lose...
>
>The old plaster was a bit lumpy, but I kept squishing lumps with my
>hands...at this point reasonably worried, because it was still VERY watery.
>Kept mixing. Kept mixing.
>
>AT LAST, it started to warm and thicken, so when it got sort of like thick
>oatmeal, I started putting it in the molds.
>
>Where it's been for a half an hour.
>
>Oozing water. Exuding water. Water rising to the top. Much wiping of
>water, much sopping up of water.
>
>The stuff still on the sides of the bucket is grainy, not hard. This is NOT
>a good sign.
>
>So. What DOES happen when you get too much water in? Sure looked like it
>was going to get thick...
>
>Or maybe it's the old plaster. Nuts. These were going to be NICE molds...
>
>Trying to ignore them, on the theory that a Watched Mold Never Hardens...
>
>Opinions welcome, as always...
>
>Best--or maybe NOT so best--
>Kate
>
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