Artist Steven Dickey
Painting, Art and Imagination...
The Lost Wax
The '''lost-wax casting''' process is an ancient
practice that is still used for artwork today. The process varies from
foundry to foundry, but the steps which are usually used in casting
small bronze sculptures in a modern bronze foundry are as follows:
1. Sculpting. An artist creates an original artwork from wax,
clay, or another material. Wax and oil-based clay are often
preferred because these materials retain their softness.
2. Moldmaking. A molding (process) mold is made of the
original sculpture. Most molds are at least two pieces, and a shim
with keys is placed between the two halves during construction so that
the mold can be put back together accurately. Most molds of small
sculptures are made from plaster, but can also be made of
Glass-reinforced plastic, fiberglass or other materials. To preserve
the fine details on the original artwork's surface, there is usually an
inner mold made of latex, vinyl, or silicone which is supported
by the plaster part of the mold. Usually, the original artwork is
destroyed during the making and initial deconstruction of the plaster
mold. This is because the originals are solid, and do not easily bend as
the plaster mold is removed. Often long, thin pieces are cut off of the
original and molded separately. Sometimes, especially in the case of
large original (such as life-size) sculptures, many molds are needed to
recreate the original sculpture.
3. Wax. Once the plaster and latex mold is finished, molten
wax is poured into it and swished around until an even coating, usually
about 1/8 inches thick, covers the entire inner surface of the mold.
This must be done in several layers until desired thickness is reached.
4. Removal of wax. This new, hollow wax copy of the original
artwork is removed from the mold. The artist may reuse the mold to make
more wax copies, but wear and tear on the mold limit their number. For
small bronze artworks, a common number of copies today is around 25.
5. Chasing. Each hollow wax copy is then "chased": a heated metal
tool is used to rub out all the marks which show the "parting line" or
"flashing" where the pieces of the mold came together. The wax is then
"dressed" to hide any imperfections. The way the wax looks at this
stage, is what it will look like when it is cast. Wax pieces that were
molded separately can be heated and attached; foundries often use
"registration marks" to indicate exactly where they go.
6. Spruing. Once the wax copy looks just like the original
artwork, it is "sprued" with a treelike structure of wax that will
eventually provide paths for molten bronze to flow, while allowing air
to escape. The carefully-planned spruing usually begins at the top with
a wax "cup," which is attached by wax cylinders to various points on the
7. Slurry. A "sprued" wax copy is dipped into a ceramic slurry,
then into a mixture of powdered clay and sand. This is allowed to dry,
and the process is repeated until a half-inch thick or thicker surface
covers the entire piece. The bigger the piece, the thicker the shell
needs to be. Only the inside of the cup is not coated, and the cup's
flat top serves as the base upon which the piece stands during this
8. Burnout. The ceramic-coated piece is placed cup-down in a
kiln, whose heat hardens the ceramic coatings into a shell, and the
wax melts and runs out. The melted wax can be recovered and reused,
although often it is simply combusted by the burnout process. Now all
that remains of the original artwork is the negative space, formerly
occupied by the wax, inside the hardened ceramic shell. The feeder and
vent tubes and cup are now hollow, also.
9. Testing. The ceramic shell is allowed to cool, then is tested
to see if water will flow through the feeder and vent tubes as
necessary. Cracks or leaks can be patched with thick ceramic paste. To
test the thickness, holes can be drilled into the shell, then patched.
10. Pouring. The shell is reheated in the kiln to harden the
patches, then placed cup-upwards into a tub filled with sand. Bronze is
melted in a crucible in a furnace, then poured carefully into the shell.
If the shell were not hot, the temperature difference would shatter it.
The bronze-filled shells are allowed to cool.
11. Release. The shell is hammered or sand-blasted away,
releasing the rough bronze. The spruing, which are also faithfully
recreated in metal, are cut off, to be reused in another casting.
12. Metal-chasing. Just as the wax copies were "chased," the
bronze copies are worked until the telltale signs of casting are
removed, and the sculptures again look like the original artwork. Pits
left by air bubbles in the molten bronze are filled, and the stubs of
spruing filed down and polished.
13. Patinating. The bronze is colored to the artist's preference,
using chemicals applied to heated or cooled metal. Using heat is
probably the most predictable method, and allows the artist to have the
most control over the process. This coloring is called patina, and is
often green, black, white or brownish to simulate the surfaces of
ancient bronze sculptures. (Ancient bronzes gained their patinas from
oxidisation and other effects of being on Earth for many years.)
However, with current artistic trends in the United States, many artists
prefer that their bronzes have brighter, more stylized patinas. Patinas
can be applied to replicate marble or stone. Depending on how the metal
is prepared, either sandblasted or polished, the finish can be either
opaque or transparent. After the patina is applied, a coating of wax,
which is the most traditional type of sealer, is usually applied to
protect the surface. Many artists prefer to use lacquer as a sealer on
some of the more unstable patinas. This protects the piece more from
ultraviolet rays. Some patinas change color over time because of
oxidisation, and the wax layer slows this down somewhat.
The lost-wax process can also be used with any material that can burn,
melt, or evaporate to leave a mold cavity. Some automobile manufacturers
use a lost-foam technique to make engine blocks. The model in this case
is made of polystyrene foam, which is then placed into a casting
flask, consisting of a cope and drag, which is then filled with
molding sand casting sand. The foam supports the sand, allowing
shapes to be made which would not be possible if the process had to rely
on the sand alone to hold its shape. The metal is then poured in, and
the heat of the metal vaporizes the foam as the metal enters the mold.