The classic "lost wax" method of casting is used for bronze
sculpture, sterling silver and gold jewelry. Here's how it's done:
1. The original sculpture (or jewelry design) is created ,
usually in plastilene, oil clay, polymer clay or various kinds of wax.
Originals can also be wood, stone, marble, whatever, but I don't
usually work in those materials.
2. Once the piece is completed, for some sculptures, the piece is
readied for mold-making by having wax rods called "sprues"
attached. These rods allow the hot air to escape when the bronze is
poured, so there will be no air bubbles on the surface of the piece
(and so the nose, ears, etc., will all actually be there -- trapped
air can make an entire face disappear in a bubble!) Other sculptures
have the sprues added later after the wax has been cast in the mold.
The design of the original sculpture is what determines when sprues
are added. The process of adding sprues is called "gating up.
"
3. A rubber mold is made, either by me or by the foundry. If the
piece will be also cast in cold-cast porcelain or cold-cast bronze,
the mold making and casting process is different. I'll explain that
below.
4. The rubber mold has a exterior plaster or fiberglass
mold created (a "mother mold" ) to help support the rubber
when wax is poured into it. The original sculpture is removed (and
probably discarded unless it's stone or wood -- clay originals can
get quite damaged in the mold-making process.)
5. A mold release agent (like a silicon spray) is applied to the
interior of the mold before it's assembled, then the mold is put
together, ready for pouring.
6. A special wax is melted and poured into the mold. The mold is
turned to make sure all the surfaces are coated, then the wax is
poured out. The process is repeated several more times until the
correct thickness of wax is reached. (Small bronzes may be solid
bronze, as is "Lazy Afternoon." Larger bronzes are usually
hollow, with the bronze about 3/16" thick, depending on the size
of the piece.) The molds for monumental-sized bronzes are made in small, manageable pieces, and the bronze is welded back together after casting.
7. The wax is removed from the mold and cleaned
up (if the original was clay instead of wax, this is the point where
sprues and pour spouts are attached), removing mold marks and making
the piece as perfect as possible. (Bronze picks up every detail,
including the artist's fingerprints!) This process is called "chasing" the wax. Sometimes the wax casting has to be
cut in pieces for the bronze to pour better. The finished bronze will
be welded back together and finished so nicely that you'll never
know it started out in pieces.
8. The sprued wax casting is dipped in a liquid ceramic material
called slurry. The slurry changes color to indicate when it's dry.
Once the wax has been dipped in the slurry, it's coated with
ultra-fine zircon sand. This process is repeated several times,
with at least two coats of the finest sand as the first layers,
to capture all the surface detail. Coarser sand is used for the
final layers. The coated piece (which is said to be
"invested" -- the slurry and sand are the "investment")
is hung to dry.
9. Once thoroughly dry, the piece is put in a kiln
(oven) that heats to a very high temperature, melting out the wax
(thus the term "lost wax process"). The ceramic shell
("investment") is now a cleaned-out mold for bronze.
10. Melted bronze is poured into the heated ceramic mold
(still warm from the kiln, or heated so it won't crack when the molten
metal is poured into it).
11. When the bronze is cool, the ceramic shell (investment) is
broken away, revealing the bronze. The piece is sandblasted to remove
every speck of investment. The sprues and pour spout are cut off and
the nubs ground down, and any mold marks or other blemishes polished
away ("chasing" the bronze). If the sculpture was cut into pieces to be cast, it is welded
together and weld marks ground off. The welds create bumps and a dark rainbow hue on the bronze, and these nubs and color are also ground off. At this point, the bronze is a bright brassy color.
12. A mounting bar is
applied where the bolts will attach the piece to its base, or if
there is a solid section of base (like grass under a horse), holes
for screws or bolts are drilled and tapped.
13. Patina (acids which color the bronze and help define it) is applied and the high spots of the sculpture often rubbed with steel wool to keep the high spots light. The recesses are left with the majority of the patina and thus help define the shape of the sculpture. Molten clear paste wax is then applied, the piece is mounted
on its base, and you have a finished bronze.
People often wonder why bronze is so expensive. Each step of the above process requires an experienced artisan. Mold-making is an art in itself. So is wax pouring, wax chasing, bronze chasing and patina work. That's five different specialties right there. The wax is burned out of the investment, so the investment has to hold up to the bronze pour or the week or more of preparation spent in creating that investment is lost. Investments do blow out from time to time, with molten bronze running out everywhere, which is why foundry workers wear Kevlar suits to protect them. The artist may take anywhere from a month to 4 or 5 months or more to create an original work of table-sized art. Monumental work can take up to a year sometimes. If the mold-maker, wax chaser, bronze chaser or any of the others who work in this process don't do their jobs well, the entire original may be lost, with no reproduction possible. When you see a bronze that's beautiful in every way, it's the product of not only the artist, but of many talented artisans at the foundry, the skill of the basemaker and the marble supplier.
Cold-Cast Bronze, Cold-Cast Porcelain, Resin and Papercasting
These sculptures are usually foundry-cast, produced in a pressure
tank, as are their molds. The molds are silicon rubber with plaster
mother molds. The pressure tank is used to get bubbles out of the
mold (to ensure a smooth surface). The pressure tank is also used
when casting the sculptures to get bubbles out of the finished
sculpture and to be certain the casting material gets into every
part of the mold. Both are resin products. The cc bronze includes
bronze powder, the cc porcelain includes marble dust. The patina for the
cold-cast bronze is built into the material used for casting, but
there is a lot of hand-buffing, and additional patina work done to
get exactly the right look (I do this part). The cold-cast porcelain
is sandblasted to get the soft look of bisque porcelain. The finished
pieces have holes drilled in them for mounting (these pieces are
solid, not hollow). The finished pieces of both products are then
often sealed with clear polyurethane to protect the finish.
I plan to do some sculptures in fine art acrylic (this material
looks like ice sculpture, very elegant). The process for
producing this water-clear material is similar to that for the
cold-cast bronze or porcelain above, but with different material, and
the acrylic is cast in an autoclave (vacuum with heat) rather than
just a vacuum tank. Getting the acrylic to pour with absolutely no
bubbles is the main concern. The piece is then sandblasted (where you
want it to look frosted) or polished (where you want it to be clear).
The resins used in my reliefs (wall art) are cast in my home
studio instead of the foundry. I make the molds and cast those
myself because reliefs are simpler to mold and not as
much room is required, nor is there as much requirement for special
equipment as there is for the kind of resin poured at the foundry.
Papercastings are made with archival quality cotton linter. The paper comes as pulp with certain chemicals included to facilitate casting. The mold has to be at least 1" deep (or I have to build sides) to hold the pulp and water mixture. The mixture is poured in, magic words are said and art happens! :-) It takes practice and patience to get it right, believe me -- I threw away quite a few "mistakes" while I was learning the process. It takes several days for the papercasting to dry thoroughly, depending on how humid the weather is. The mold being out of use for so long while each casting dries is one of the reasons the papercastings sell for higher prices than the resins.
I've explained these processes to satisfy your curiosity (hey, I'm
a curious person myself!), but believe me, none of this is as simple
as it sounds! The first difficulty is in producing a good sculpture
(but that's also the fun part!). Mold making is a very difficult art
to master, and I do make my own molds for my wall art. Pouring wax without
turbulence showing is difficult, too. There's a lot more to making
sculpture than playing in clay! But it sure is FUN!
