"Bob Masta" wrote in message
...
On Sun, 8 Jun 2008 01:59:08 -0700 (PDT), Gap
wrote:

Hello,
I want to make a plaster mold of a ball which is why I have 2 halfs of
polystyrene balls.
Gabi
(sorry - all good advice snipped for space sake)
Plaster is generally not supposed to shrink. ..... Bob Masta


Any plaster I have ever worked with expands! I mean that in a powerful way.
I have seen it push well nailed frames apart.

I would think you would want to cover the Styrofoam in something soft enough
that it is not going to damage the delicate structure, that hardens so some
degree and yet can be removed from the plaster when it is set. You can't
heat up plaster without damaging it but I don't know how high of heat is
involved for that to be an issue or I would suggest dunking the ball in
melted wax. In fact if you put it out in a hot room I bet that would be
warm enough to peel the wax away without damaging the plaster... Just a
thought.

Donna