"Gap" wrote in message
...
Bob and Donna,
thanks for all your input!
It is not a problem to make the parting line exactly at half of the
ball because they do come in 2 halves.

I also thought it is because of the porosity of the polystyrene which
is tini balls melted together and I
understand what you mean with undercut.
That is probably not a good surface finish to start with..

I have asked my pottery teacher in Germany who has made a lot of these
balls and he said clay slip would be also a good medium for
separation, so I might try that..
Someone else said I should put gladwrap over the balls before I put
the plaster on. That would make the surface smooth but would give me
lines where the gladwrap is "folded".
I have also thought about painting the ball with some paint or even
fluid latex which I have here.

I guess I just have to try different things and see which works best.

Thanks again hand happy pottering and/or mould making!
Cheers from Gabi

I love the idea of a thick coat of latex paint! But I think the clay slip
if you can manage not to have brush marks sounds the most practical (again
you can dip coat it...). It would be easy to remove from the plaster after
it was set.

I assume you are building a box that the half sphere sits in, with the flat
of the half sphere laying on the bottom of the box. I would think you would
want to at least glue around the edge of the flat half so that plaster does
not seep under or shift the sphere out of shape. If you had a router the
size of a dowel you could easily build in your key. Router out the shape
and in one pour leave the dowel out and the next pour put the dowel in.

Ok, I have obviously gotten carried away here but it was fun the think
about. Good luck and please let us know how it worked out.

Donna