Craig Clark on wed 11 sep 02
William, if you are planning on using the plaster on the counter to dry out
your slip try using regular Pottery plaster. You don't need to go for the
hard stuff like hydrocal unless your going to particularly rough with the
surface. Cover it with some canvas to keep it from being accidently abraided
or chipped.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
----- Original Message -----
From: "william wertenbaker"
To:
Sent: Wednesday, September 11, 2002 5:31 PM
Subject: what kind of plaster should i use?
> i am setting up a studio and plan on having a slab of plaster as part of
the counter and was wondering if plaster of paris is fine or i should use a
more specialized type.
>
> Liam
>
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william wertenbaker on wed 11 sep 02
i am setting up a studio and plan on having a slab of plaster as part of the counter and was wondering if plaster of paris is fine or i should use a more specialized type.
Liam
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John Kimpton Dellow on thu 12 sep 02
I have been using casting plaster as a wedging surface for
years.
william wertenbaker wrote:
>
> i am setting up a studio and plan on having a slab of plaster as part of the counter and was wondering if plaster of paris is fine or i should use a more specialized type.
>
John Dellow "the flower pot man"
Home Page http://www.welcome.to/jkdellow
http://digitalfire.com/education/people/dellow/
Jeff Longtin on thu 12 sep 02
Liam,
Quickly I'll throw out two bits of knowledge on your plaster choices. If you
want to use your plaster slab strictly as a wedging table its best to use
either Hydrostone (USG) or Densite (GP). These plasters, cements really, have
great durability and some absorbtion. If you want a surface upon which you
can easily dry out reclaimed clay then you should use either Pottery Plaster
(USG) or K-60 (GP). Both of these plasters are very absorbant.
On durability: It is very impotant that you follow the suggested water to
plaster ratio for best results. To make sure your proportions are correct it
is best to weigh out your materials. It is also important to mix the plaster
for the specified period. Add the plaster to the water-allow to sit 2
minutes. Then stir this mixture for 2 minutes. Then allow this mixture to sit
1 minute before pouring. (That last minute by the way allows air bubbles to
come to the surface.) If you're mixing fresh plaster you probably can stir
the mixture longer than the 2 minutes. I often stir the mixture until it
reaches a "pancake batter" like consistency before I pour.
Little known secret: Durability of cement and plaster is greatly increased
the longer it is mixed. Yes water-to-plaster ratio is important but you can
produce weak molds if you pour fresh plaster just after the 5 minute mix
cycle I've descibed above. If you've ever looked at setting plaster and
noticed a thin layer of water coming to the surface you've seen plaster that
could have/should have been mixed longer.
Good luck
Jeff Longtin
Complex Molds Made Easy
Les Haworth on thu 12 sep 02
I recommend two plasters that are well suited for constructing a wedging
table. Hydrocal B-11 is my favorite. Less absorbency than No 1 pottery, but
increased strength. It is also resistant to chipping. Another plaster I
recommend is Cerami-cal. This plaster is made specifically for Ram press
molds, however it also makes an excellent wedging table. If your looking for
the stronger of the two, I'd say go with the Cerami-cal. One last thing!
Remember this, when you mix plaster always add dry to wet. And (this is the
important one!) to mix your plaster there is a prescribed ratio of water to
plaster. If you increase the amount of water in the plaster, you increase
the porosity and lose strength. If you decrease the water in the plaster,
you increase strength but lose porosity. By the way, the ratio of plaster to
water for the Hydrocal B-11 is 100:44. (Expressed 100# of plaster to 44# of
water.) And for the Cerami-cal the ratio is 100:40. When I watch people
mixing plaster by sight I get a little sick. With all the literature out
there on mixing plaster and people are still mixing it wrong. Generally on
the high porosity and less strength side. Not so good for a wedging table.
Les H.
Lester R. Haworth III
Sales and Technical Support
Laguna Clay Co.
14400 Lomitas ave
City of Industry, CA 91746
1(800) 4-LAGUNA ext. 229
(626)330-0631 ext. 229
les@lagunaclay.com
www.lagunaclay.com
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of william wertenbaker
Sent: Wednesday, September 11, 2002 3:32 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: what kind of plaster should i use?
Importance: High
i am setting up a studio and plan on having a slab of plaster as part of the
counter and was wondering if plaster of paris is fine or i should use a more
specialized type.
Liam
---------------------------------------------
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Craig Clark on fri 13 sep 02
Jeff, a question concerning a possible plaster molding problem. I am
familiar with standard pattern making, mold making and casting techniques.
I've primarily cast bronze, aluminum, and iron. Very little experience with
slip casting though.
A student called me and stated that she has done a series of 6 small
plastincine models of funky looking cars with different types of hats for
tops. She wants to do multiple slip castings of each one. I haven't seen the
pieces yet but I suspect that undercuts will be a problem.
My first thought was to use a flexible rubber molding system. The
question I have is one; does slip casting work with flexible rubber molds?
If this the best route to take?
Any suggestions would be greatly appreciated
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Jeff Longtin"
To:
Sent: Thursday, September 12, 2002 10:39 PM
Subject: Re: what kind of plaster should i use?
> Liam,
> Quickly I'll throw out two bits of knowledge on your plaster choices. If
you
> want to use your plaster slab strictly as a wedging table its best to use
> either Hydrostone (USG) or Densite (GP). These plasters, cements really,
have
> great durability and some absorbtion. If you want a surface upon which you
> can easily dry out reclaimed clay then you should use either Pottery
Plaster
> (USG) or K-60 (GP). Both of these plasters are very absorbant.
>
> On durability: It is very impotant that you follow the suggested water to
> plaster ratio for best results. To make sure your proportions are correct
it
> is best to weigh out your materials. It is also important to mix the
plaster
> for the specified period. Add the plaster to the water-allow to sit 2
> minutes. Then stir this mixture for 2 minutes. Then allow this mixture to
sit
> 1 minute before pouring. (That last minute by the way allows air bubbles
to
> come to the surface.) If you're mixing fresh plaster you probably can stir
> the mixture longer than the 2 minutes. I often stir the mixture until it
> reaches a "pancake batter" like consistency before I pour.
> Little known secret: Durability of cement and plaster is greatly increased
> the longer it is mixed. Yes water-to-plaster ratio is important but you
can
> produce weak molds if you pour fresh plaster just after the 5 minute mix
> cycle I've descibed above. If you've ever looked at setting plaster and
> noticed a thin layer of water coming to the surface you've seen plaster
that
> could have/should have been mixed longer.
> Good luck
> Jeff Longtin
> Complex Molds Made Easy
>
>
____________________________________________________________________________
__
> 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.
Ron Roy on fri 13 sep 02
I don't recommend canvas on working surfaces - just another way to get clay
dust into the air you breath.
RR
>William, if you are planning on using the plaster on the counter to dry out
>your slip try using regular Pottery plaster. You don't need to go for the
>hard stuff like hydrocal unless your going to particularly rough with the
>surface. Cover it with some canvas to keep it from being accidently abraided
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Dupre Mr Marcy M on fri 13 sep 02
Jeff,
I make plaster and hydrostone bats for my Thomas Stuart wheels, and I make
molds for tiles and forms.
In using plaster and other variations on a theme (hydrostone, et al), I
follow the instructions, I mix by hand, I "float" the dry powder into the
water, I mix for the indicated amount of time and more--slowly, ever so
slowly to avoid bubbles, and I STILL get bubbles and the water film on top.
Should I possibly sift the powders through a sieve or colander to float them
more thoroughly into the water? Should I use a powered mixer to blend the
materials, then de-air the liquid with my hand to get the bubbles out?
I have a slide rule type calculator that came with a book, "Making Molds for
Ceramics" that seems to do a good job of explaining the proportions of
substance to water, but (as the printed word is prone to do) falls short on
being able to explain the complete process.
Thanks for any light you will shed,
Tig
in Springfield, VA, where it cannot make up its mind to be late summer or
early fall
-----Original Message-----
From: Jeff Longtin [mailto:plasterjfl@AOL.COM]
Sent: Thursday, September 12, 2002 11:40 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: what kind of plaster should i use?
Liam,
Quickly I'll throw out two bits of knowledge on your plaster choices. If you
want to use your plaster slab strictly as a wedging table its best to use
either Hydrostone (USG) or Densite (GP). These plasters, cements really,
have
great durability and some absorbtion. If you want a surface upon which you
can easily dry out reclaimed clay then you should use either Pottery Plaster
(USG) or K-60 (GP). Both of these plasters are very absorbant.
On durability: It is very impotant that you follow the suggested water to
plaster ratio for best results. To make sure your proportions are correct it
is best to weigh out your materials. It is also important to mix the
plaster
for the specified period. Add the plaster to the water-allow to sit 2
minutes. Then stir this mixture for 2 minutes. Then allow this mixture to
sit
1 minute before pouring. (That last minute by the way allows air bubbles to
come to the surface.) If you're mixing fresh plaster you probably can stir
the mixture longer than the 2 minutes. I often stir the mixture until it
reaches a "pancake batter" like consistency before I pour.
Little known secret: Durability of cement and plaster is greatly increased
the longer it is mixed. Yes water-to-plaster ratio is important but you can
produce weak molds if you pour fresh plaster just after the 5 minute mix
cycle I've descibed above. If you've ever looked at setting plaster and
noticed a thin layer of water coming to the surface you've seen plaster that
could have/should have been mixed longer.
Good luck
Jeff Longtin
Complex Molds Made Easy
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
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Moderator of the list is Mel Jacobson who may be reached at
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Alan D. Scott on fri 13 sep 02
When going for longer mixing times, keep a sharp eye on the mix. The first
time I tried mixing Hydrostone for the time recommended in the USG
literature, it hardened up right there in the mixing bucket... Use your
head, not a blind eye on the timer. :-)
Jeff Longtin on sat 14 sep 02
Craig,
Slip-casting in a rubber mold is a no-go, however I did have a client who
used her rubber molds as press molds. These were doll head molds and she was
using porcelain clay.
I've never pressed clay into rubber molds, I would think it would stick, but
she swore by it. I mentioned this awhile back on the list and it seems others
do it with success as well.
Good Luck
Jeff
Snail Scott on sat 14 sep 02
At 10:48 PM 9/13/02 -0500, you wrote:
>...My first thought was to use a flexible rubber molding system. The
>question I have is one; does slip casting work with flexible rubber molds?
In short, no. Slip casting relies on the absorbency
of the mold material to stiffen the clay to a usable
consistency. The slip closest to the (plaster) mold
gets the moisture drawn out of it, leaving a wall
of solid clay up to about 1/4" thick while the inner
volume of slip (further from the mold) remains
liquid and can be poured out, leaving the hollow
shell of hardened slip in the shape of the mold. A
non-porous mold will not permit this to happen, and
the entire volume of slip will remain equally liquid.
However, flexible rubber molds can be used for press-
molding! Since the clay used is already at 'normal'
hardness, and is already the right thickness, it can
air-dry easily enough to a stiffer, manageable level
for assembly. Nan Smith does much of her work using
this method.
-Snail
Rick Monteverde on sat 14 sep 02
>At 8:47 AM -0400 9/14/02, Jeff Longtin wrote:
>Slip-casting in a rubber mold is a no-go,
Ever tried a thin latex skin mold with a paster backing? I've had some success with that. Latex is porous to moisture so the clay can become drier against the mold.
- Rick Monteverde,
Honolulu Hawaii
knik at kodiak on sat 14 sep 02
Can you make a slip of regular clay to use in slipcasting with molds? Do
you need to add sodium silicate?
Marilynn
Snail Scott wrote:
>At 10:48 PM 9/13/02 -0500, you wrote:
>
>>...My first thought was to use a flexible rubber molding system. The
>>question I have is one; does slip casting work with flexible rubber molds?
>>
>
>
>In short, no. Slip casting relies on the absorbency
>of the mold material to stiffen the clay to a usable
>consistency. The slip closest to the (plaster) mold
>gets the moisture drawn out of it, leaving a wall
>of solid clay up to about 1/4" thick while the inner
>volume of slip (further from the mold) remains
>liquid and can be poured out, leaving the hollow
>shell of hardened slip in the shape of the mold. A
>non-porous mold will not permit this to happen, and
>the entire volume of slip will remain equally liquid.
>
>However, flexible rubber molds can be used for press-
>molding! Since the clay used is already at 'normal'
>hardness, and is already the right thickness, it can
>air-dry easily enough to a stiffer, manageable level
>for assembly. Nan Smith does much of her work using
>this method.
>
> -Snail
>
>______________________________________________________________________________
>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.
>
>.
>
Snail Scott on sun 15 sep 02
At 12:04 PM 9/14/02 -1000, you wrote:
>Ever tried a thin latex skin mold with a paster backing? I've had some
success with that. Latex is porous to moisture so the clay can become drier
against the mold.
This will work to some extent, depending on the depth of the
undercuts. The reason that flexible molds work with deeply
textured or undercut forms is that the rubber can be pulled
away from the shape without distorting the casting. But, the
plaster backer is still rigid, and if the rubber isn't thick
enough to fill up the undercuts, the plaster will lock in
just like with a rigid mold. And it will still be less
absorbent than bare plaster, so allow for a MUCH longer set
time for the slip.
-Snail
Snail Scott on sun 15 sep 02
At 05:44 PM 9/14/02 -0700, you wrote:
>Can you make a slip of regular clay to use in slipcasting with molds? Do
>you need to add sodium silicate?
You can make slipcasting bodies out of a wide range of
clay recipes. Any of a wide range of deflocculants will
do the job, but I find sodium silicate to be difficult
to work with, because of the tiny amounts involved.
I'll let folks who do more of it recommend particular
recipes - what temperature range/color are you looking
for? (A number of books have recipes, too.)
-Snail
Jeff Longtin on sun 15 sep 02
Marilynn,
To quote Snail "In short no".
Deflocculants create a reverse charge to your clay particles and enable you
to turn a clay into a casting slip with less water.
If you were to take water and simply add it to a dry clay body you would need
to add about 50% to turn it into a slip. This is the case with decorative
slips. In a casting mold however this amount of water will cause severe
cracking. By adding deflocculants the to water before you add the dry clay
you can reduce the water amount to about 30-35%. At this level the clay will
not crack in the mold.
Throwing clay by comparison is about 30-35% water. Considering the fact that
throwing clay and casting slip have roughly the same amount of water you can
see that deflocculants have a powerfull effect indeed.
Jeff Longtin
Complex Molds
Jeff Longtin on tue 17 sep 02
Tig,
Oddly, the theory of your practice should work, its the reality of your
practice that doesn't.
An aside:
The book to which you refer, the Donald Frith book "Making Molds for
Ceramics", came out in the mid 80's. Many people bought it, at that time,
thinking it would explain all there is to know about mold making, which it
does. However, it neglects to connect theory with reality and so many people
starting calling me with the " The book says I should do this, so I did that,
but when I did that I just came up with a big mess", type of questions so I
spent alot of time connecting fiction with reality.
Despite these little irratants these experiences did provide me with some
interesting bits of knowledge. A production potter friend of mine bought the
book thinking he could easily and simply learn how to make molds. Once he
realised that was not the case he contacted me. After answering his many
questions, and solving his many problems, he made me aware of some
interesting resources mentioned in the book.
One resource mentioned was R.T Vanderbilt, supplier of Darvan no.7 and Darvan
811. With the sales rep I learned about...
(right about now I'm having a mental block!)
Another resource mentioned was Lightnin Motors. Lo and behold I call up
Lightnin and they send me a catalog. After looking over the catalog I call up
the sales rep and we talk. Lightnin, if you don't know, makes motors that can
mix continuously for 24 hours a day, for 30-50 years, without needing oil.
I called the rep just because this fact blew me away. A motor running 50
years without an oil fill, truely amazing!
Anyway, the catalog has all these types of mixing blades. I ask the rep "Why
on earth do you offer so many expensive mixing blades?" and the rep explains
that different materials need different types of mixing abilities. Some
materials, like clay, need high velocity mixing blades designed to
intersperse the materials quickly and easily, in the initial mixing stages,
but then, once mixed, they need gentler mixing blades to keep the particles
in suspension.
Which brings me around to answering your question:
Unfortunately your attempts to prevent air bubbles from entering your plaster
mix is contributing to your poorly setting mix. Plaster needs to be
vigorously agitated to mix thoroughly.
Paster particles do not become "wetted" with gentle agitation!
Mix your plaste/water mixture like hell for the proscribed time, 2 minutes in
most cases, and then allow the mixture to sit for a minute before pouring and
you shouldn't have any problems. Agitate longer if you're using new plaster
or if the mix still appears to be very fluid.
I usually pour once it hits the "pancake batter" consistency.
Yes you will create air bubbles but that's alright. Like you mention, just
gently vibrate the mixture and the air bubbles should come to the surface. If
you get lots of air bubbles just spray a 50/50 mixture of rubbing alcohol and
water to break up the bubbles. Works like a charm every time!
Good luck
Jeff
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