How Bronze
Sculpture is Cast
Mold Making - The first step in bronze casting
There are two separate molds required to cast
each bronze sculpture. The first mold is reusable and is made of rubber;
it is used to create a wax casting. The wax casting is then used to make a
second, one use, ceramic mold into which is poured molten
metal.
The first rubber mold is created over the original sculpture which can
exist in any material from wet clay to hard stone. Into this rubber mold, hot
wax will be poured which will be discussed in detail later. The ultimate success of the bronze casting will
depend on the accuracy of this rubber mold, so for that reason and for the
expertise and time required, most sculptors will hire a professional mold maker.
Typically a bronze casting will be poured in several pieces to facilitate the
flow of molten metal, so the original sculpture must also be divided into many pieces
and a separate rubber mold made for each piece. If the original sculpture is
still in clay, the mold maker may actually cut the sculpture into the
appropriate pieces thereby destroying the original in the process.
There are many different types of rubber used in
mold making; the exact type used will depend on many factors which include: the material of the original
sculpture, the desired speed of the rubber cure, the temperature and humidity of
the studio, the type of material to be poured into the mold, the number of pours
planned for the mold, and the personal tastes of the mold maker. Typical rubbers
used include: Latex, Poly-sulfides, Polyurethanes, Silicates. and Alginates,
with many varieties of hardness available in each
type. The most commonly used rubber is some form of polyurethane.
The first coat of rubber is the most important
for capturing the surface detail of the sculpture and must be carefully applied
and blown into all small surface details with compressed air. Next,
divisions, called fences or shims, are created for the mold by sticking three
inch wide strips
of firm, wax-covered paper ( made from paper cups) on edge into the surface of the
wet rubber. These divisions will allow each mold to be made in two halves
so that they can later be easily pulled apart to remove a wax casting. After the
first coat of rubber is cured and the fences are secure, more coats are
progressively applied to the sculpture and shims by brush, allowing each coat to
cure, until the desired
thickness is finally achieved, usually between one eighth and one quarter of an
inch. The final step of rubber application involves sticking pre-poured rubber bumps
about a half inch long at regular intervals into the wet rubber on each
side of the shims and at selected locations on the surface of the rubber coated
sculpture. These bumps called "Registration Keys" will allow the rubber to be held
securely and accurately in place inside the hard outer mother mold, which will
be made next.
After the rubber is cured and before it is
removed from the sculpture, a solid mother mold is made over it with either plaster
reinforced with burlap or with fiberglass; the mother mold will serve to later hold the
rubber in the proper position. The mother mold is also made in two
parts along the same shims which divided the rubber. The cured mother mold
halves are removed allowing the rubber to be separated at the shims. The
first coat of rubber must be cut through at the base of the shims to completely
remove it from the sculpture. After cleaning up the rubber halves they are
inserted back into the mother mold halves and are snapped into place by pushing
the rubber keys into holes in the mother mold which were made over the keys. The
mother mold is then reassembled and secured using clamps or
straps. Each half of the mother mold also has its own registration
keys along the division to ensure accurate alignment when reassembled.
Wax Pouring
The method of bronze casting by wax has been in use for
thousands of years. In the 18th century, French sculptors coined the
term "Cire Perdue" meaning "Lost Wax"; this term is still
used today to describe most art bronze castings.
Before the two halves of the mother mold containing the
rubber inserts are joined,
a hot liquid microcrystalline wax is carefully painted into the inside of the
rubber in order to capture all of the detail in the mold. Next the two mold
halves are joined and secured and hot wax is poured into the mold, sloshed
around, and then poured out. This step is repeated until the wax thickness
inside the mold has been built up to approximately one quarter of an inch.
After the wax has completely cooled, the mother mold is removed and the rubber
inserts are carefully peeled away to reveal
a hollow wax casting of the original sculpture. The wax casting is an
intermediate step which will in turn be used to create a more durable ceramic mold into
which molten bronze will eventually be poured. Each bronze casting in an edition will begin with
its own unique wax casting. If the bronze edition will have 35 casts, there must
be 35 wax casts made.
Wax Chasing
Chasing means finishing. The wax pour will create many
small imperfections which must be repaired and cleaned up. Examples of
imperfections include small bubbles in the wax which cause holes and the seam
lines where the mold was divided. Chasing is accomplished with hot wax and
delicate, heated tools to essentially re-sculpt damaged areas. At this point it
is sometimes possible to reattach some of the smaller separated pieces which
were molded separately for reasons of mold function; but the pieces which were
separated for metal pouring will be left and cast separate. It is also not uncommon for
the artist to add individual additions to each wax cast. The artist's signature
and edition number can also be inscribed into the wax.
Wax Spruing
When bronze is poured into a ceramic mold, it
will not cool evenly or flow to all parts of the mold evenly. This
limitation is overcome by creating a series of pathways called "Sprues"
which allow the molten bronze to reach the various areas of the casting smoothly
and evenly. In addition,
air that is compressed in the mold by the flow of bronze will either cause a void in the
cast or it will be compressed, superheat, and explode; so a network of vents must be provided to allow the air to escape. Both
Sprues and vents are created by rolling up wax into long rolls, the width of a
finger, attached between a wax funnel and selected positions on the surface of
the wax casting. The funnel will provide for the entry point of the poured
bronze. When this step is complete, the sprued wax casting will have a
network of wax, tree branches growing out of every surface.
Ceramic Shell Investment
The wax casting, complete with it's wax funnel and
branches of
sprues and vents, is fixed on a hanger and alternately dipped into
a large vat of a liquid ceramic slurry and then silica sand. This process
is repeated many times with each sand application progressing from fine to
course. Sixteen coats are not unusual and reinforcing wire mesh is applied half way through; each coat is allowed to
dry before another application. When complete the
shell is allowed to dry for about a week. This entire process takes a
couple of weeks.
Next, the shell is heated in an oven or autoclave to eliminate
the wax, which is why the process is called "Lost Wax." After the wax is thoroughly
removed, the shell is baked in a kiln at 1,700 degrees to temper the shell.
All wax is lost. In place of the wax funnel, sprues, and vents is now a
series of passageways leading to the cavity of the mold which used to contain
the wax casting of the sculpture. The molten bronze will
next be poured through the entry funnel, fulfilling the ancient prophesy: "where the was wax, there will be bronze."
Metal Pour
Pouring molten
bronze is the culmination of weeks of preparation and is the most exciting part
of the sculpture process. This step requires great skill, physical effort,
expensive equipment, and some degree of danger; so most artists will contract to a
bronze art foundry to do the actual pour while
the artist supervises the process.
The most commonly used metal is silicon bronze, which is
composed of 95% copper, 4% silicon, and 1% manganese. The bronze is
purchased in bars or ingots and is melted in a pot called a crucible in a
furnace at 2000 to 2,200 degrees. Prior to the pour, the ceramic shell
mold is buried in a sand pit with the pour cup or funnel facing up and
exposed. Typically several molds are buried in each sand pit. The
sand will support the molds as well as dissipate the heat of the bronze and help
prevent fracturing of the mold.
When the furnace lid is removed, the crucible and molten bronze
will be glowing red hot. On the top of the molten bronze will be a layer of
darker slag formed by impurities in the metal, which is skimmed off the
surface. The crucible is then removed by tongs and placed in a pouring
shank which is a round receptacle with two long handles on each side. The
shank holding the crucible is then lifted by two workers wearing helmets with
face plates and full body protection, and the crucible
is positioned over the mouth of each buried ceramic shell. The metal is
then carefully poured in without turbulence and slow enough to let air and gas
escape.
When the metal has cooled, the ceramic shell is
broken with a hammer, chipped
and sandblasted off revealing the metal casting complete with a bronze funnel, sprue
and vent tree. The tree is cut off and the raw blackened casting is
prepared for the final finishing. The thickness of the bronze casting is
equal to the thickness of the intermediate wax casting, about one eighth to one quarter of an
inch.
Metal Chasing
The first step in the metal finishing or chasing involves
welding. If the sculpture was cast in several pieces, the casting will be
welded together at this time. This is a critical step and requires the
participation of the artist because small errors in positioning of the pieces
can result in large changes to the posture and composition of a sculpture. Done
incorrectly the resulting cast may be radically different from the artists
original intent. Also any imperfections in the casting can be
repaired by spot welding bronze.
Next the chaser will use a variety of tools such as powered
grinders, rasps, hammers, chisels, files, and sandpaper to resculpt the surface
of all welds, seams, and imperfections. Once again either the artist will do
this himself or closely supervise the technician in the process. Extreme
care must be used to prevent changing the character of the sculpture's surface
texture.
The final finishing step is to thoroughly clean the surface by
sandblasting the entire piece. This clean surface can not be touched by a
bare hand prior to Patination or the oil in the skin will negatively affect how
the patina is accepted by the metal.
Patina
Patina describes the coloration of the bronze
surface. Contrary to common belief, the early Greek and Roman bronzes were
actually painted in bright, multiple colors. Unfortunately, most of these
bronzes were later melted down for sources of valuable bronze used in weapons of
war like cannons. The bronzes which survived the
millennia were hidden; buried by earthquakes, ship wrecks, and other disasters until
re-discovered. The resulting long exposure to the elements removed all
traces of color paint and created a natural oxidation patina derived
from the soil or sea water. The unique chemicals and minerals in each soil
composition created a different patina. Since the renaissance, these re-discovered
bronzes have become revered for their complex and natural patinas; and as a
result, for hundreds of years, artists and collectors have rejected as foreign the concept of
painting a bronze and have instead sought to duplicate these natural
forces in the patina of sculpture.
For the past several hundred years the accepted
patination of bronzes has been restricted to a very small palette of naturally occurring
oxidation tints, such as light green or brown. However, in the past several decades, thanks largely to the
progressive influence of western and wildlife sculpture, a new spirit of experimentation has
developed in the artistic community which has resulted in an exciting explosion of patina
colors. Fortunately for the artists of today, modern
technology does not require us to wait thousand of years to achieve a patina. Thanks to the processes of chemistry, a rich tapestry of oxidation
coloration can be created in a few hours. These chemical processes will actually
oxidize the surface of the bronze into the desired color. This color is actually
part of the metal and is not easily rubbed or scratched off allowing the feel
and texture of the bronze to remain intact and uncovered. Different colors
require different chemicals and methods of application. Some are applied cold
while some are applied to a surface heated with a blow torch.
The final step in patination is to seal the
sculpture. Large outdoor sculpture will typically be sealed in acrylic
varnishes while indoor sculpture is typically sealed in hot wax or oil which is
buffed out to a luxurious luster. In designing a patina for a sculpture, the
artist must take into account the setting and what the effects of age will do to
the patina. Typically a patina will darken somewhat after the first few
years, so the newly cast sculpture needs to have a lighter initial patina so
that time will mature it to desired result.
Basing
Creating the proper base for a sculpture is like
selecting the right frame for a painting. The base should compliment the
sculpture; not too big and not too small. A base can be made of bronze,
wood, or stone any any combination. The artist needs to have some idea of
basing when the sculpture reaches the metal chasing stage so that any necessary
nuts or threaded rods can be welded to the bottom to allow attachment to a
future base. Thankfully, for the artist, there are a number of expert basing
shops located near any major art foundry.
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