James F on mon 6 oct 08
I just finished pouring my first 5-part slip casting mold. This is a mold =
of a very heavily textured squarish vessel=2C the texture on which would ha=
ve likely prevented draw if each wall were not molded separately. The mold=
also has a separate block which forms the foot of the vessel. This block =
is completely captured within the four wall sections=2C and was formed by p=
lacing a carefully fitted block of clay atop the foot of the inverted model=
before pouring the walls. After all four walls were poured=2C I removed t=
his clay block=2C then poured plaster into the resulting cavity.The strange=
ness occurred while pouring the foot block. After filling the void with pl=
aster=2C I want away to wash my mixing container. When I returned about 1 =
minute later=2C the plaster in the void had sunken in about an inch in the =
center=2C though the edges where it touched the plaster wall sections had f=
irmed and so had not sunk. I assumed that the floor of my model=2C which f=
ormed the floor of this void=2C had sprung a leak. I quickly mixed some mo=
re plaster and filled the void=2C but it sunk again=2C though this time onl=
y a half inch or so. When I opened the mold I fully expected to find a mes=
s=2C but in fact the block was clean and intact=2C with no leakage of plast=
er at all. I am certain there were no major air bubbles entrained in the p=
laster=2C and I am also certain that I did not create a bubble when pouring=
.Any ideas as to what happened? Since the foot block is now a different co=
nsistency from the walls due to the rapidly mixed and unmeasured patch=2C s=
hould I toss it and pour a new piece?Thanks for any help or insight you can=
provide....James
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Jeff Longtin on mon 6 oct 08
Hey James.
You're first 5 part...way to go!!
You've now stepped into the "Good luck teaching someone else to use your
mold" territory. You're very bold.
(Years ago a ceramic shop hired to me make a mold on behalf of a sculptor.
The resulting mold was a five parter and I had to show the ceramic shop how to
use it. "The mold doesn't work." was a very strange phone call.)
The only times I've had a plaster cave-in situation are when I didn't
properly seal the molded object and the model sucked the water out of the
plaster...or...I didn't mix the plaster sufficiently and the mix just shriveled up as
it dried
Are either of these factors possible in your situation?
If you don't know...the amount plaster shrinks is in reverse ratio to how
long you mix it. i.e. the longer you mix it the less it shrinks. (If I recall it
has to do with the particles being "wetted" and such.)
good luck
Jeff
In a message dated 10/6/2008 2:48:03 P.M. Central Daylight Time,
jsfreeman@HOTMAIL.COM writes:
I just finished pouring my first 5-part slip casting mold. This is a mold
of a very heavily textured squarish vessel, the texture on which would have
likely prevented draw if each wall were not molded separately. The mold also
has a separate block which forms the foot of the vessel. This block is
completely captured within the four wall sections, and was formed by placing a
carefully fitted block of clay atop the foot of the inverted model before
pouring the walls. After all four walls were poured, I removed this clay block,
then poured plaster into the resulting cavity.The strangeness occurred while
pouring the foot block. After filling the void with plaster, I want away to
wash my mixing container. When I returned about 1 minute later, the plaster in
the void had sunken in about an inch in the center, though the edges where
it touched the plaster wall sections had firmed and so had not sunk. I
assumed that the floor of my model, which formed the floor of this void, had sprung
a leak. I quickly mixed some more plaster and filled the void, but it sunk
again, though this time only a half inch or so. When I opened the mold I
fully expected to find a mess, but in fact the block was clean and intact, with
no leakage of plaster at all. I am certain there were no major air bubbles
entrained in the plaster, and I am also certain that I did not create a bubble
when pouring.Any ideas as to what happened? Since the foot block is now a
different consistency from the walls due to the rapidly mixed and unmeasured
patch, should I toss it and pour a new piece?Thanks for any help or insight
you can provide....James
_________________________________________________________________
Stay up to date on your PC, the Web, and your mobile phone with Windows
Live.
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James F on tue 7 oct 08
Thanks=2C Jeff.
The model was leatherhard clay and the rest of the mold was still wet=2C ha=
ving been poured only a few hours prior=2C so I don't think either one coul=
d have sucked that much juice out of the plaster. I guess this leaves insu=
fficient mixing as the likely culprit. And no=2C I did not know about mixi=
ng/shrinkage=2C so thanks. I mixed the plaster for the walls very well=2C =
but since the amount for the plug was very small it is entirely possible th=
at I mixed it insufficiently. I think I will stuff the mold with blocking =
clay then re-pour the foot=2C making sure to mix very well.
...James
> Date: Mon=2C 6 Oct 2008 20:03:54 -0400
> From: plasterjfl@AOL.COM
> Subject: Re: Strange Plaster Behavior
> To: CLAYART@LSV.CERAMICS.ORG
>=20
> Hey James.
>=20
> You're first 5 part...way to go!!
>=20
> You've now stepped into the "Good luck teaching someone else to use your
> mold" territory. You're very bold.
>=20
> (Years ago a ceramic shop hired to me make a mold on behalf of a sculptor=
.
> The resulting mold was a five parter and I had to show the ceramic shop h=
ow to
> use it. "The mold doesn't work." was a very strange phone call.)
>=20
> The only times I've had a plaster cave-in situation are when I didn't
> properly seal the molded object and the model sucked the water out of the
> plaster...or...I didn't mix the plaster sufficiently and the mix just shr=
iveled up as
> it dried
>=20
> Are either of these factors possible in your situation?
>=20
> If you don't know...the amount plaster shrinks is in reverse ratio to how
> long you mix it. i.e. the longer you mix it the less it shrinks. (If I re=
call it
> has to do with the particles being "wetted" and such.)
>=20
> good luck
>=20
> Jeff
>=20
>=20
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