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The Bronze Casting Process
by Kelly Borsheim
On this page I tried to capture the gist of the bronze casting
process as I learned it from William Barnett, an Austin artist
who teaches at The Elisabet Ney Sculpture Conservatory in
Austin, Texas. Emphasis is on the bronze casting process itself,
so I would recommend a new sculptor take at least one sculpting
class first (since only about 3 weeks in 8 are spent creating
a sculpture in wax). The Ney offers classes that teach sculpting
in clay (figurative and abstract), plaster, concrete, and
stone (limestone and alabaster) as well as offering a life
drawing class. Prices are incredibly affordable, the atmosphere
is great, and everyone learns so much! In fact, many artists
in the bronze casting classes are repeats--it is a very affordable
way to create a work in bronze.
First a sculpture is made in wax. We used microcrystalline
wax mixed with paraffin (different ratios depending of hardness
desired). The wax should be soft enough to shape and yet hard
enough to stay put and hold detail. Any mixture will be softer
when heated and harder when cold, so an exact science did
not seem necessary. When I made my sculpture "Sea Turtles
I", I discovered the joys of sculpting in wax. The sculpture
was small and required little special handling. I sculpted
mostly with dental tools, which were also very portable. I
found myself sculpting any time I had to wait for any reason.
(Since I know how much I hate to wait, I usually carry something
to do with me at all times. This really helps to "chill
me out".)
Generally, it is best to have some idea of what you want
to create before you start--no matter how vague an image it
might be. I often start out with a very generalized notion
of something and, as I develop it, the piece seems to help
me determine how it will end up. The photo at the right is
an image I took of my two turtles and the kelp. I intended
to cast the three parts individually but needed to plan out
(before they were solidified in bronze) how they would work
together. Since the weather was getting warmer (this is Texas!)
and I had to be careful with my sculpture repositioning itself,
the photo was to help me recreate my intentions.
After the wax sculptures are completed, they are attached
to wax funnels. I forgot to photograph this stage, so bear
with my explanation please. The sculptures are attached to
this funnel by "welding" wax sticks (sprues) to
both the sculpture and the funnel. The tip of a metal tool
is heated up (over a torch). Then a wax stick is joined to
the wax funnel by using the hot metal to melt the two pieces
together. Sculptures are attached to the opposite ends of
these sprues in the same way. There is definitely a trick
to doing this well and it does take some practice to make
strong welds.
Sculptures are generally placed close to the bottom of the
funnel (since the bronze starts to cool the second it hits
the funnel). Thicker sprues are joined to the sculpture where
the bronze will enter (the bottom), while thinner sprues (vents)
are joined at the top edges of the sculpture and reach to
the top plane of the funnel. Sprue placement is very important,
since the air that is in the mold before the bronze is poured
must escape (otherwise it forms air pockets which the bronze
cannot fill). Also, since using this method creates sculptures
of solid bronze, our pieces should not be more than about
1-1.5 inches thick.
Our instructor, Bill, then takes the class's wax contraptions
to a foundry. An ceramic shell mold is made around the wax
and the wax is melted out of the mold. This is the scary part
because the original sculpture no longer exists. If the mold
is broken or damaged somehow, the artist must start over from
scratch. The molds are returned to Bill and we are ready for
our casting weekend.
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We are now at the home and foundry of artist Bill
Barnett. The image on the left is of one student removing
a mold from an oven (the big white cylinder in the top
center--lifted by another student via cable and pulley).
The bronze should not be poured into a cold mold. Bill
can be seen in the background (right of center) removing
the melted bronze from the furnace. The mold is then
set into a big metal pot filled with sand (which safely
supports the mold).
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The image on the right is of Bill supporting
the crucible filled with liquid bronze while two
students tip the container, pouring the hot metal
into the mold.
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The hot mold is carefully removed, kept in an upright
position, and allowed to cool. (This does not take long--15
minutes?.)
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Once the bronze has completely cooled, the investment
mold must be chipped away (most of it cracks fairly
easily, but the stuff that gets stuck in the cracks
and crevices is very difficult to remove). The sprues
(now solid bronze) can be cut away with a hack saw or
other tool and remelted later (although only once or
the quality of the bronze deteriorates). The final cleaning
and shaping of the sculpture is done with a Dremel tool
and/or a sandblaster. A patina is added to the sculpture
as desired.
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Please browse website www.borsheimarts.com for more bronze
casting lesson .
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