Plastics are great for use as
master molds because they are water proof and smooth. The
smoother the surface, the easier it is to remove the cast mold
from the master. Plastics work well since they flex when
removing the mold from the plastic.
There are a wide variety of wood
products available. My favorite is medium density fiber board (MDF).
This material is is very consistent in its density and is dry so
shrinkage is not an issue as it can be with other wood products.
I can also glue (Titebond II) up two or more pieces of MDF,
rough cut a circle with my jigsaw and turn it on a wood lath for
A key to creating a master mold
with wood is water proofing it before casting over it. ANY
exposed wood will soak up water, swell and make the casting
around the exposed area crumbly.
Wood needs to be completely sealed
against water leaking into the wood. I primarily use Spar
Varnish from Ace Hardware. The gloss marine type soaks into the
wood very well and creates an excellent seal. It does take a
long time as each coat take 24 hrs to dry and MDF wood needs 3
to 5 coats. I then finish it off with green sealer.
Spray on lacquer also works well
and dries quickly. It does not work well with the green sealer,
so I do not use it as often.
We have used modeling clay carved
or molded to the desired shape as master molds. You need to make
sure that your finished shape doesn't have any undercuts that
would lock slumped glass into the mold, and also that the clay
is anchored so that it doesn't float up off the surface when
pouring the casting.
Molds made of pottery clay have
been problematic and are not recommended. The porous surfaces
are difficult to seal. Time, understanding and experience using
this kind of clay is strongly recommended.
For large or more hand-crafted
master molds, styrofoam can be a option. There are many
different densities of styrofoam available so care must be taken
in determining which kind will work the best for your
application. As with wood, styrofoam needs waterproofing.