How Cold Cast Bronze
Sculpture is Cast
This process of casting is relatively new
and unlike tradition bronze casting does not involve pouring molten metal.
The process which is also called "bonded bronze" is accomplished
by mixing a liquid plastic, typically a polyurethane resin, with a finely
ground bronze powder. In mass, the bronze powder is very heavy and a pint
can weighs ten pounds, yet it is so finely ground that if a person sneezed into
the container there would be a cloud of bronze power filling the room; so it
must be handled very carefully. This heavy mix of polyurethane and bronze
powder, which has consistency of honey, is then poured into a rubber mold with
an amount which only partially fills up the mold. The partially filled
mold is then slowly rotated either by hand or on a rotational casting
machine. This rotation will allow the mix to evenly coat the interior of
the mold and as it hardens it will form a perfect bubble free exterior face coat
on the casting. Subsequent pours can be used to either thicken the wall of
the casting or create a solid cast.
A second type of cold cast bronze is created
using bronze powder and a matrix material called Forton. Forton is a derivative
of industrial hydrocal plaster, but much harder, denser, and non toxic. It
also has an water based acrylic polymer in the dry mix which creates a very hard
and solid casting. When mixed with the bronze powder, Forton will
create a very realistic bronze feel and look. The advantages of Forton
over Resin is that the exothermic temperatures which buildup in the mold during
curing are much lower and therefore the rubber mold will last longer. In
addition since it is a more dense and heavier material than resin, the Forton
will accept a blowtorch used in some bronze patina applications. The only
drawback is that because of the lower temperature, the casting will take longer
to cure and fewer castings can be created per day from a single mold. Fro
this reason major production facilities will probably use resin and smaller art
supplies will probably use Forton. Both will create excellent
bronzes.
Once the casting has hardened it is removed
from the mold and all imperfections are chased and cleaned up.
Buffing the surface will remove all outer layers of the resin or Forton matrix and the
remaining surface will be pure bronze. This bronze surface has the look, feel,
and weight of a traditionally cast bronze and can be finished and patinaed in
exactly the same manner as a traditional bronze. Although not generally
appropriate for limited editions, a cold cast bronze is the perfect medium for
cabinet editions, open editions, figurines, and other decorative arts.
Most importantly the process is many times more economical and faster than a traditional
cast bronze.
The cold cast process is not limited to
bronze. Almost any finely ground material can be used to create a bonded
casting. Typical applications have included marble, pecan wood, and porcelain,
and result in a casting which closely resembles the parent material.
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