study what you’ve done. Isn’t it pretty? The various
mold keys are all visible now: the hemispherical
keys in the outer ring of plaster, the dovetail keys
where the silicone meets the plaster, and the barrier
key around the model itself.
11. MAKE THE SECOND
Before applying a layer of silicone to the other side
of the model, you’ll have to clean it and prepare it
well — but be careful not to mess with it too much,
as you want as tight a registration as possible.
Use a brush and soft damp cloth to remove any
clay residue. Apply mold release (or a thin layer of
vaseline) to the silicone so that the next layer won’t
stick to it. Make sure you cover it all, or else you’ll
ruin your mold. Silicone loves to stick to itself.
Apply 2 layers of silicone exactly as you did in
Step 6. As before, use very little accelerator in the
first layer, but you can use more, or a higher mix of
the kicker, in the second. Remember: the goal is to
get a ¼" blanket all around the part.
As with the first blanket, place the silicone key
wedges along the center of the model before the
second layer of silicone hardens. Cut the dovetail
keys from the second layer, gingerly lifting up the
edge to avoid cutting into the layer below (Figure O).
Figure P shows the mold with the second silicone
blanket done and cut. Now it’s time to lay on the
plaster for the other half of the mother mold. You’re
12. MAKE THE SECOND HALF
OF THE HARD-SHELL MOLD.
To prep for the second plaster, apply mold release to
the first plaster half. Again, a thin layer of petroleum
jelly works great (Figure Q). Cover the inside of all
the hemispherical key indentations because, again,
if they don’t release, all your work is down the toilet.
Using aluminized tape, available in the plumbing or heating section of any hardware store, build
a simple mold dam to contain the wet plaster and
give it a nice crisp edge that matches the clay dam
from the first blanket (Figure R).
Slowly drip the plaster onto the second blanket
of silicone. Again, this first layer is for detail, and the
fewer bubbles it has, the better it will hold.
Use your hand to spread the plaster over the
114 Make: Volume