Dave Evans on tue 27 mar 01
Karen,
When making a plaster mold a few precautions should make this an =
easy process. The first requirement in one piece molds is that the form =
(in this case the glass bowl) not have any undercuts. An undercut is =
where a part protrudes or angles in a way that would stop you from =
pulling the finished plaster form from the mold. Any time the plaster =
fills in a place nearing 90 degrees, or especially if there is an =
overhang, breaking the form may be the only way to remove the plaster. =
Gentle slopes are easiest. Secondly, you need to use some sort of =
releasing agent to separate the plaster from the form it is filling. If =
you don't, suction will try to keep the plaster firmly in place as you =
try to pull the plaster from the form. Small imperfections in the form =
also tend to hold the plaster firmly in place. A releasing agent will =
help with this a little. Once the plaster is stiff you may try blowing =
some air along the sides of your form to help separate the pieces. An =
air compressor or canned air is ideal, but even a bike pump with =
something attached to direct a narrow stream of air between the two =
halves might help. The release agent can be a commercial one such as =
mold soap or a spray separator, or something as common as dish soap, W-D =
40 or Vaseline.
Best of luck,
John Evans in San Diego=20
Karen Lookenott on tue 27 mar 01
Another question about a hump mold. Can I use plaster of paris for the
mold, or do I need to use some other type of plaster? Again, this is going
in a glass bowl, which I now know to put Vaseline in. :-)
Bruce Girrell on tue 27 mar 01
Karen,
I used a large metal bowl for a similar project. The metal gives a little
and may be better than glass for this purpose. Prior to pouring plaster, I
coated the bowl with mold soap to aid in the release. Remove the plaster as
soon as it hardens. Don't wait for it to fully cure.
If you have access to compressed air (wear eye protection), you can direct a
stream of compressed air between the plaster and the bowl. Once the vacuum
between the two is broken, the plaster should come out easily.
Finally, you might consider giving yourself a handle on the plaster by
putting a chunk of wood into the center of the plaster as it is poured. This
will give you something to hang onto as you try to get the plaster free from
the bowl. Once the plaster is out, you can saw off the "handle" flush with
the plaster surface so that the plaster will rest on the table top properly.
Just some ideas
Bruce Girrell in northern Michigan
where Spring is supposed to arrive some time this year.
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