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[teacherartexchange] plaster hump mold release problems


From: allgaul (allgaul_at_TeacherArtExchange)
Date: Tue Nov 23 2010 - 04:30:14 PST

Hey everybody-
We are going to attempt the Empty Bowls project at my school so I decided
the easiest way to make all those bowls would be over plaster hump molds. I
had ordered pottery plaster to make the molds and got to it yesterday.
I greased up some heavy plastic bowls from the dollar store with Vaseline
and poured pottery plaster into them. I tried to get them to release and
they won't budge! They aren't too big, maybe 8" across and 5" tall. No
undercuts. Just simple heavy plastic soup bowls-some were rounded inside and
some had straighter sides and a flat bottom inside. The layer of Vaseline
was not too thick, but I definitely got the insides and top completely
covered. I tried to release them about 45 minutes after I poured the
plaster-it was still warm to the touch. The school day was over and I had to
leave for an appointment so didn't try again.
I haven't done this before with plaster and I am wondering if when plaster
cools down it contracts a bit? If I microwave the bowl, would it heat up the
plastic and allow me to get the mold out more easily?
I did 20 molds and really need them after the Thanksgiving break! I should
have put cheesecloth or plastic wrap to make it easier for myself, but of
course in hindsight...
anyway, I know I can cut into the plastic, it would be hard but it would get
them out. I was hoping to use the bowls again for more molds.


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