Priscilla Wilson and/or Janice Lymburner on thu 18 jan 01
Hi from Sautee, Georgia - I'm wondering if anyone has an opinion about =
whether leaving pots in molds overnight will cause the plaster to =
deteriorate beyond the normal effect of the deflocculants. It kind of =
seems to me that the damage probably happens when the clay is touching =
the molds and might not keep happening as the clay shrinks away. =
Sometimes it's convenient for me to cast a second time before I leave my =
workshop and not have to remove until morning, when it is much easier to =
get certain pieces out anyway. They have lots of texture and are =
soemtimes a problem even without undercuts.
Thanks,
Priscilla
Aerni on thu 18 jan 01
I would think there would be no deleterious effects from the slip remaining
in the mold overnight, as long as the molds get a rest every now and then.
Generally, when they get too wet, the plaster will start growing mold, or
fungus, or something like that. Also, when they get wet, they will stop
effectively absorbing the water from the slip and you'll find your
production slowing. But, I've seen plaster forms that have been used
several times per week for many years that are still working fine.
Richard Aerni
Bloomfield, NY
----- Original Message -----
From: "Priscilla Wilson and/or Janice Lymburner"
To:
Sent: Thursday, January 18, 2001 1:10 PM
Subject: Deterioration of plaster molds
Hi from Sautee, Georgia - I'm wondering if anyone has an opinion about
whether leaving pots in molds overnight will cause the plaster to
deteriorate beyond the normal effect of the deflocculants. It kind of seems
to me that the damage probably happens when the clay is touching the molds
and might not keep happening as the clay shrinks away. Sometimes it's
convenient for me to cast a second time before I leave my workshop and not
have to remove until morning, when it is much easier to get certain pieces
out anyway. They have lots of texture and are soemtimes a problem even
without undercuts.
Thanks,
Priscilla
____________________________________________________________________________
__
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.
mudlark on fri 19 jan 01
I have found the most deterioration occurs if the molds are not allowed to dry between castings. Maybe it would be best to pop the molds before you leave in the evening.
Priscilla Wilson and/or Janice Lymburner wrote:
> Hi from Sautee, Georgia - I'm wondering if anyone has an opinion about whether leaving pots in molds overnight will cause the plaster to deteriorate beyond the normal effect of the deflocculants. It kind of seems to me that the damage probably happens when the clay is touching the molds and might not keep happening as the clay shrinks away. Sometimes it's convenient for me to cast a second time before I leave my workshop and not have to remove until morning, when it is much easier to get certain pieces out anyway. They have lots of texture and are soemtimes a problem even without undercuts.
> Thanks,
> Priscilla
>
> ______________________________________________________________________________
> 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.
Jeff Lawrence on sat 20 jan 01
Priscilla was asking about deterioration of plaster molds when left
unmolded.
Hello Priscilla,
I think the answer to your question may lie in your particular
circumstances, ordered by what I'd bet are the most important criteria:
1 what kind of slip you use
2 where you are on the defloc curve
3 how quick to dry is your slip (less clay dries faster, more clay dries
slower)
4 how much green strength your slip has (i.e. does it want to crack or hold
together as it dries?)
5 what shape your molds are in
In our case, with a 50:50 (ballpark) ball clay:talc recipe, 1.76-1.79
density, slightly underdeflocculated for thicker castings, most of our
castings sit overnight covered in the molds. In fact, it is hard to unmold
them on the same day because they are often too flabby to handle (maybe a
function of my defloc preferences). We often cast five days a week and leave
the Friday castings covered over the weekend. If you cast your molds too
many times, the slip will mechanically erode the interiors and produce
unusable castings if you need that texture. We have texture-free pieces, so
this isn't that much of an issue for us.
Some other maybe salient points:
- Our conditions here are very dry -- if we leave molds uncovered, the part
near the opening dries completely while the interior parts are still wet.
This invites deadly flaws.
- There is no texture in these molds, and indentations work as long as they
are rounded off so the shrinking clay doesn't catch on them and crack.
- Other slips I've tried (with less clay) dry so quickly the ware cracks in
the mold.
- Newer molds dry the ware a bit faster -- that fuzz on the outside of older
molds indicates deflocculant permeating the pores and coming out the outside
with the respiring water. I've concluded that the water passes a little more
slowly through those molds than the old ones. With good slip, however, this
difference is minimal.
hope this helps,
Jeff
Jeff Lawrence ph. 505-753-5913
18496 US HWY 84/285 fx. 505-753-8074
Espanola, NM 87532 jml@cybermesa.com
| |
|
