Ceramic Design Group on sat 1 jun 02
To set the record straight on some of the plaster questions posed lately:
1.The basic rule of plaster is that it not only sticks to itself, it sticks
to anything. To insure that it doesn't. the model, the original, whatever
you want to take a mold of needs to be sealed and then parted.
You can seal a porous model with a variety of preparations, Its important
that the surface be completely non-absorbant and smooth afterwards, and
thats why vaseline/petroleum jelly is a poor choice. It leaves marks.
Understand that your original model is just that and if you seal it and part
it afterwards, it can't be used for any other claywork. Thats why you make a
mold of it. We currently use several spray compounds to seal porous models
in plaster, bisque etc.
2. After sealing, you need to "part" the model. There are many commercial
products that work...Fels Naptha Soap, Murphy's Oil Soap, etc. etc. The best
we have found and have used for well over 25 years is Polyurethane Parting
Compound" from Permaflex Mold Company in Columbus Ohio.
Use the right stuff and follow the right procedures, and the results will be
successful. Plaster work then doesn't take on any mythic or difficult
proportions. Its just stuff to work with.
Jonathan
--
Jonathan Kaplan
CERAMIC DESIGN GROUP
PO Box 775112
Steamboat Springs, CO 80577
(970) 879-9139
info@ceramicdesigngroup.net
www.ceramicdesigngroup.net
John Rodgers on sat 17 nov 07
Number 1 Pottery Plaster is the only one on your list that will absorb
water the way that you will need it to. All the others have specific
applications, and greater densities than the #1 and therefore the
differences in the absorption rates.
For stamps, most all on the list are OK.
For molds, where good absorbancy is required to release the clay, stick
with #1.
Like I said, all have different densities, therefore different
applications, but they all do very well when used for the purpose for
which they were intended.
For release agents - I use tincture of green soap cut 50/50 with water.
However, this is for plaster to plaster release. For plaster to a
non-porous surface like a pyrex dish or bowl, you need something other
than soap. PAM, WD 40, Polyease 2000 (best) all work, but Polyease works
best for me. It is a silicone based release agent and will interfere
least with later clay work with the mold. The oil type releases will
become entrained in the plaster and interfere with release of clay based
on water absorption. It will release by simply drying of the clay, but
that can take quite a bit longer. USG has quite a bit of literature
listedon their website and I suggest y ou give it a read. Also, pony up
and get yourself a copy of Moldmaking for Ceramics by Donald Frith. This
is the definitive book on all things plaster. Best book on the market
about the subject.
God Luck.
Regards,
John Rodgers
Chelsea, AL
Shula wrote:
> In the next few days, I am thinking about making a trip to my not so local clay supply store (Aardvark in Santa Ana, CA). One of the items I plan to pick up is plaster. I want to make plaster stamps, molds from things like shells and my grandmother's button collection, etc., and perhaps a slump or hump mold. I will use the molds I create pottery from stoneware or porcelain.
>
> Since I live in the desert now, where the humidity is low, low, low, I assume I will be able to successfully store the plaster for a long time (as Marcia wrote she was able to do when she lived in Montana where the humidity is low). If you disagree, please tell me why.
>
> I've checked the archives, but may have missed some of the answers I am seeking, so here goes.
>
> - Release agent. I may make a mold from a pyrex bowl. Do you think spray Pam will work or should I get mold release or (you fill in the blank)?
>
> - I am really confused about what type of plaster (or not in the case of some of the following) that I should buy. My choices are:
> USG No 1 Pottery Plaster
> Ceramical Gypsan Cement
> USG Puritan Pottery Plaster
> Hydrocal B-11
> Ultracal 30
>
> Any light you can shed on my questions will be greatly appreciated.
>
> Thanks.
>
> Shula
> in Desert Hot Springs, California USA
> where, according to weather.com, it is currently 70 degrees F and 30 % humidity at 9am.
>
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Shula on sat 17 nov 07
In the next few days, I am thinking about making a trip to my not so local clay supply store (Aardvark in Santa Ana, CA). One of the items I plan to pick up is plaster. I want to make plaster stamps, molds from things like shells and my grandmother's button collection, etc., and perhaps a slump or hump mold. I will use the molds I create pottery from stoneware or porcelain.
Since I live in the desert now, where the humidity is low, low, low, I assume I will be able to successfully store the plaster for a long time (as Marcia wrote she was able to do when she lived in Montana where the humidity is low). If you disagree, please tell me why.
I've checked the archives, but may have missed some of the answers I am seeking, so here goes.
- Release agent. I may make a mold from a pyrex bowl. Do you think spray Pam will work or should I get mold release or (you fill in the blank)?
- I am really confused about what type of plaster (or not in the case of some of the following) that I should buy. My choices are:
USG No 1 Pottery Plaster
Ceramical Gypsan Cement
USG Puritan Pottery Plaster
Hydrocal B-11
Ultracal 30
Any light you can shed on my questions will be greatly appreciated.
Thanks.
Shula
in Desert Hot Springs, California USA
where, according to weather.com, it is currently 70 degrees F and 30 % humidity at 9am.
Snail Scott on sun 18 nov 07
On Nov 17, 2007, at 11:00 PM, Automatic digest processor wrote:
> Date: Sat, 17 Nov 2007 09:12:56 -0800
> From: Shula
> Subject: Plaster questions
>
> Since I live in the desert now, where the humidity is low, low, low, I
> assume I will be able to successfully store the plaster for a long
> time...
Yes, though not indefinitely.
>
> - Release agent. I may make a mold from a pyrex bowl. Do you think
> spray Pam will work or should I get mold release or (you fill in the
> blank)?
Avoid using greasy release agents if you intend to use the mold with
clay, since it will interfere with the absorbency of the plaster. This
is
less critical with press molds, but essential for slip-casting. Instead,
use mold soap. Mold soap can be purchased from many suppliers,
but I find that Murphy's Oil Soap works very well, and is obtainable
quite cheaply from Wal-Mart and similar stores in the floor-cleaning
products aisle.
Do NOT use so-called 'liquid soap' products, since they are not
actually soaps at all, but detergents. For washing, it makes little
difference, but for molds, detergents will not work. It needs to be
actual, real, old-fashioned soap.
When using mold soap, apply a thin, even coating and allow it
to dry before applying plaster. For smoothest results, buff lightly
first.
If making a multi-part mold, you can use soap as a parting agent
between the sections. Apply one coating of soap to the surface of
the plaster that the next part of the mold will touch. Allow it to soak
in, then apply more and let dry as above, then proceed with plaster.
>
> - I am really confused about what type of plaster (or not in the case
> of some of the following) that I should buy. My choices are:
> USG No 1 Pottery Plaster
> Ceramical Gypsan Cement
> USG Puritan Pottery Plaster
> Hydrocal B-11
> Ultracal 30
For all ceramics mold-making, pottery plaster is best, since it is
more absorbent. Molding plaster or plaster of paris will work
well enough for press molds, but stick with pottery plaster for slip
casting. I usually use #1 pottery plaster, but the Puritan will work
fine, too.
Gypsum cement, Hydrocal, Ultracal, Hydrostone and such are
designed to be very hard and strong, but are not very absorbent
at all. Avoid them for molding clay. They are very useful, but not
for this. They are better for pouring INTO molds to make solid
objects, and a lot of mass-produced lawn sculpture is made that
way. They are actually so hard that it would be difficult to smooth
the edges of the mold after it sets, or do any additional carving
or revisions to your mold.
-Snail
Jamie Yocono on sun 18 nov 07
Hi Shula,
I also live in a dry desert climate, and I usually have a big bag of USG No
1 Pottery Plaster on hand. I make a lot of slump molds, for making simple
dinnerware sets. Here's a link to one of my blog posts about molds, it might
be helpful to you: http://wooditis.blogspot.com/2007_05_01_archive.html
There is much information out there in the ceramic magazines regarding types
of plasters and which to use. After reading everything I could get my hands
one, I decided the USG No 1 Pottery Plaster was what I needed for simple
molds, stamps, and even a small wedging table top. The Hydrocal and Ultracal
were much more expensive, and are mostly used when ultra fine detail is
needed. If your stamps are not finely detailed, then the No 1 will probably
do what you need, for a reasonable price.
I always weigh my plaster and measure the water, instead of just dumping and
mixing it until it feels right. Weighing gives you the proper ratios for the
plaster to develop it's strength. Pottery Making Illustrated recently had a
reprint of an article that is VERY helpful on plaster mixing. I would guess
it's available online. It give various water amounts, and how much plaster
to use. There is a ratio formula, too, but I just look at their chart. And I
make notes, so that if I mix up a quart, I know how many soy bowl molds (or
whatever) it will make. Make notes!
I have a very large trash can that I keep devoted to storing my plaster. I
line it with a plastic trash liner, and put the plaster (still in it's bag)
inside. As I use the plaster up, I always keep a running total of what's
left in the bag. For example, if I use 12 pounds, I will write on the bag
100-12=88 pounds left. It helps me know how much I've got on hand, so if I
need to mix some up, I won't get caught short. Maybe I'm just anal retentive
about it, but I always know how much I have, so I don't start mixing up a
batch and then run out.
I've had a bag for nearly 8 months, it's still perfect, no dampness. Even
though I'm in the desert, I use a plastic bag, and then place it in a
plastic storage trash can.
Release agents? I've used Pam, WD-40, and even Formula 409, that soapy spray
stuff you can buy at the grocery store. I never use a thin cloth between the
mold and the clay, but I know some people do.
Last thing- I make a ton of molds out of plastic and foam disposable food
containers, as you'll see on my blog. If the container is flexible, I don't
bother spraying it with a release, I just pop it out of the mold when it's
hardened. You will need to spray a Pyrex bowl, though, as it won't come out
on it's own. I use a Surform rasp to clean up and soften any edges, so that
no plaster chips off when I'm working with the molds. And after pouring
molds, I dry them in the sun, so that they lose all their moisture.
Hope that helps!
Jamie in Vegas
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