setting up nicely. The silicone doesn’t go all the way
to the border on the left, but that’s OK — next we’ll
be cutting dovetail keys around the perimeter.
Brush a small amount of vaseline or other mold
release onto them to help remove them from the
first half of the plaster mother mold.
8. CUT THE DOVETAIL KEYS.
After the silicone has cured, trim the edge of
the blanket to get a smooth line, and discard the
trimmed pieces. Use a sharp X-Acto knife to cut
dovetail-shaped keys around the perimeter, gingerly
lifting up the edge of the blanket and slicing upward
so you don’t cut into the clay below (Figure J). This
serrated edge will help the silicone blanket register
to the mother mold. My blanket here is a wee bit thin
at the outer edge. I could probably have trimmed it
closer, like about an inch away from the model.
Figure K shows the finished blanket. I’ve probably
used the minimum number of dovetail keys necessary to keep the blanket stable in the mother mold,
but you should err on the side of caution and add
more than you think the mold might need. Too many
keys just makes the mold more stable, but too few
and you’ve wasted a lot of work.
10. PLASTER OVER THE SILICONE.
After the first blanket of silicone comes the stone-plaster mother mold. The plaster will go on in a
couple of layers. The first layer is a thin coat for
detail — apply it slowly to avoid creating bubbles.
The second is supported by a hemp strengthener.
Figure M shows the first layer of plaster applied
over the blanket, the clay, and the keys. It’s fairly
thick and will take somewhere around an hour to set.
Stone plaster is much stronger than regular plaster. You can get away with using less, which keeps
your mold lightweight, but it’s still brittle like regular
plaster. So the next step is to reinforce it with some
spun hemp, available from moldmaking supply
stores. The hemp works much like fiberglass, supplying a matrix that increases the plaster’s flexibility
and makes it shatter-resistant.
Add a layer of hemp, then apply the second and
final layer of plaster. Don’t wait more than a day
between plaster coats, or else the second layer
might not stick well to the first. Also note that the
first layer of set-up plaster will suck water from the
new layer, making it set faster than the first.
Once the second layer’s set, turn the whole thing
over and gently pull off the clay dam, keeping the
model inside the mold (Figure N). Take a moment to
9. ADD HEMISPHERICAL KEYS.
I’ve left space around the edges of the silicone so I
can place hemispherical keys (Figure L). These will
register the 2 halves of the mother mold together.
I’m using injection-molded ¾" plastic hemispheres,
available at any plastics supply store, placed lightly
on the clay every 5" or so, just inside the border.