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.
I got them sent to Australia from my german pottery friend and I made
one mold already approx 20 cm diameter which worked so far so good.
The only problem was to get the halfs polystyrene balls out of the
dried plaster mould.
Because the plaster shrinks my ball was in there pretty tight and I
had to break it to get it out.
I have 4 more balls bigger ones that the one I made before and wonder
if anyone here know what I can use to seperate my balls from the
plaster so that I can re-use the balls if anything goes wrong with the
mold.
Last time I have mixed dishwashing liquid with oil and little water
which didn't work too good and I think I will have more trouble with
the bigger balls.

If you have any ideas or suggestions please let me know.
Thanks in advance
Gabi

I have also had problems when trying to use dishwashing liquid,
which is actually a detergent. I think you really need a true soap,
which has always worked well for me. (Liquid hand "soaps" are often
really detergents, unfortunately, though it may not be obvious. Too
bad, since they are so readily available!)

Here in the US, the "old time" recommendation was Fels Naptha soap or
(I think) Murphy's Oil Soap. I have also used old Ivory soap flakes
with good results, but I don't know if they even make those any more.
(My box is about 30 years old!) If all else fails, and you don't want
to use commercially made parting compound, you could probably shave a
bar of hand soap into a powder and mix it up with water.

Plaster is generally not supposed to shrink. More likely is that the
plaster is just *really* close to the original, with no airspace
between. That means that atmospheric pressure (14.7 pounds per square
inch of surface) does a terrific job of holding the surfaces
together... in fact, many products sold as glues do nothing more than
fill up the air space between surfaces to produce this effect.

The most important thing is to make sure your parting line is
*exactly* at the diameter of the sphere, or you will never get it out
intact. Also, you mention that the balls are polystyrene. If this is
the kind with a pebbled surface like Styrofoam, you can also expect
problems because there are lots of little pockets that the plaster
will fill and lock things tight... it's really the same as an undercut
mold, on a smaller scale but multiplied many tmes over. You might
want to apply some sort of finish to the balls first to make them
glassy smooth. The question is *what*, since many things will dissolve
polystyrene. Maybe paste wax? Try on a scrap of the ball you already
destroyed.

Best regards,


Bob Masta

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