Monthly Column by Dorothy Smith Jackson |
CAST IN BRONZE
Ever wonder how that lovely bronze equestrian
sculpture, you so admired in the gallery window, was made or why it costs almost as much
as a real horse does? Well, if you're interested and don't mind a long involved tale, read
on.
The "Lost Wax" method of casting is the most commonly used. Using this system an
artist can make a series of castings from the same original work. Most bronzes are done
this way and if you look at the base of a finished piece, you will often find a series
number. For instance, 4/20 would mean that this was the fourth casting in a series of no
more than 20. The following is a short version of what the start to finish on an
equestrian bronze would entail.
First the artist would create an "armature," which is the understructure of the
sculpture. This is usually supported by a metal pipe and made of bendable wire. The
armature allows the artist to build up their sculpture while maintaining an underlying
support (sort of a skeleton for the work). The next step would be to create the sculpture
over the armature. Different materials can be used for this, but an oil-based,
(nondrying), type of clay is typically used. The bendable nature of the wire underneath
allows the sculptor to change the position of his subject as she works. When this stage of
the work is done, it then goes to a "foundry" where the various moulds are made.
Foundries are businesses set up to deal with all the technical aspects of metal casting
and they take over at whatever stage the artist leaves off. If the artist brings a clay
sculpture to the foundry, the next step would be to make a reverse mould of the work. In
the case of a bronze, they would make a "master mould." This is generally made
by slowly layering one of the newer synthetic rubber compounds over the clay original.
This is painstaking work and takes time and considerable skill on the part of the
technician. After the rubber cures a layer of hard material is added to strengthen the
outside of the mould. Now you are left with a reusable negative impression of the
sculpture, which at this stage has been discarded. I might add that in order to make the
casting process easier, they often cut the sculpture up and make the mould from the pieces
(I can't watch that part.)
Now the sculpture goes on to it's next step, creating a wax. After the rubber mould is
cured, wax is then poured into it and a positive copy of the sculpture is produced. Once
the foundry reassembles this wax, some artists will come in and do fine touchup details on
the sculpture. Because the wax is a hard substance, it allows an artist to cut fine lines
and sharpen up details. Since each wax is destroyed in the following step, (hence the name
"Lost Wax,") any changes made in the wax are unique to that casting.
Now it's time to make another mould around the wax piece. This mould is made by building
up a concrete-like refractory material around the outside of the wax. "Gates" or
passages are included in the mould to allow the wax to be poured out and the metal to be
poured into the mould. The material that this mould is made out of must be able to
withstand heat up to 600 degrees Celsius. The mould is then heated and the wax is poured
out of the gates, leaving us once again with a negative impression of our original
sculpture.
Lastly, the bronze (an alloy made of copper with tin, zinc and lead added) is poured into
the now empty space in the mould. When cooled, the mould is broken away revealing the
metal sculpture.
The mechanics of making a bronze are complicated enough up to the point that I've
described them, but there is still more to do before the piece can go to the gallery or a
buyer. In fact, at this point it looks horrible and will take sandblasting, cutting off
the gates and grinding to even begin to look respectable. If you were to see a piece at
this stage you'd wonder about the dull brassy color and hope that somebody would remedy
the situation.
Enter the patina artist. Once the sculpture has been cleaned up from the bronzing process
and welded to it's base, the artist has to choose what kind of a finish would suit the
work. "Patina" is the coloration of a raw bronze through chemical reactivity to
certain compounds and applied heat. Basically you just paint on the proper chemicals and
heat them with a blow torch. But it's easier said than done and a good patina artist is
priceless.
Before the heat process of patination was discovered, metal would eventually form similar
patinas from exposure to the environment, (sea air tended to create a greenish patina, for
instance). Bronzes are even sometimes painted and I have seen several with jockey's silks
painted in the colors of their stable.
Well, that was quite a drawn-out story, but if you got this far, I'm sure from now on
whenever you see a bronze you'll think "Whew, that must have taken a long time to
do!!!"
