There are two basic types of lost-wax bronze casting



A useful general discussion of basic direct and indirect processes + a welcome bit of historical context. A few questions and comments are noted within, particularly regarding the indirect process. -mrc

Bronze Casting:

from Ancient Rome to Present-Day

Mamei Sun

ME 122F – Spring 2005

Background: cast bronze statues in antiquity

Bronze casting has existed for millennia, and is still be used in the creation and reproduction of art. The ancient Egyptians, Greeks, Romans, and many others used it in the creation of art as well as the instruments of everyday life. [1] Bronze was the most important metal used from about 3000 BC to 1000 BC in ancient Rome. It was highly used in many industrial purposes, such as for pots and pans, furniture, belts and brooches, armor, and other military equipment. Furthermore, it was also used for decorative purposes such as in figurines and states of gods, athletes, heroes, and government officials.

In the first century AD, Pliny the Elder discusses use of bronze for statues. They were generally statues of gods – the first bronze statue in Rome, in fact, was that of Ceres. Statues of men were usually made for the lasting commemoration of some important event– such as victory in the sacred games. The ones held at Olympia especially facilitated (word choice?) this – when an individual won three times, exact replicas were made of him and they were called “portrait statues.”

These bronze statues actually became popular because the first portrait statues in public spaces in Athens were of the tyrannicides Harmodius and Aristogiton in the same year as the expulsion of the kings of Rome – statues were thus set up as a civilized form of rivalry. Thus, it became a tradition for statues to sit in town forums, with rolls of honor on the bases so their memories could be preserved.[2]

The bronze casting process

Bronze is an alloy made primarily out of copper but also of copper, tin, and lead. The proportions vary, but usually around 70-90% copper, 10-20% lead, and 5-10% tin.[3]

One surprising fact about bronze casting is that contrary to popular belief, the entire statue or object is not made of bronze, just the outside covering. This was because not only is bronze extremely heavy, but in the day when war was common, any large quantity of bronze would be melted down to make cannons or other weaponry. Thus, only bronze in small quantities would be preserved. Underneath the layer of bronze is usually clay or a similar substance.

Lost wax casting

Lost-wax bronze casting is one of the oldest and one of the most popular. There are two basic types of lost-wax bronze casting. Though these methods have changed slightly over the years, the basic concepts are similar to when the ancient Romans used it.

Direct method:

The direct method is fairly simple, yet however it has the limitation that it can only create one finished bronze product from the original. Supposing one wanted to create something out of bronze, the following steps are what would have to be done. First, use clay to form a basic shape of the object. Next, cover the core with wax, and add details to the wax. After the details, add wax sprues, gates, and vents. The sprues and gates are to allow the wax to escape after it melts, and the vents are for the gases to escape. The next step is to cover the wax with clay, but before doing that, paint the wax model with very thin clay to pick up fine details. Add a coarser clay mantle, and attach it to the inner clay core by iron or bronze pins, called chaplets. Then, proceed to bake the clay mold at 1350-1450 degrees Fahrenheit until the wax melts out, and then raise the temperature and bake until the clay hardens. At this point, there is space left from the lost wax in which to add liquid bronze. When the bronze cools for a couple of days, break the clay mantle and remove the chaplets, vents, and gates. Finish the surface through cool-working techniques. [4]

Indirect method:

The indirect method requires a few more steps, but has an advantage in that more than one bronze product can be created from the original model. Start with the original model sculpted out of plaster, clay, marble, stone, or wood. Coat the model with a special protective substance and put into fine elastic material (such as ??? – what did they use in Renaissance times? What do people use today?). After the model makes an impression, take it out to leave just the elastic. Place fire-proof clay into the impression, making a duplicate of the sculpted original. Remove this clay and scrape off the surface slightly. When this is returned to the elastic, there is a gap into which the hot wax is poured. Then, as mentioned before, add wax sprues, gates, and vents. Apply granulated ceramic (fine clay) to the surface and pipes until it is thick and coarse. This becomes the “investment layer.” Following this step, heat until the wax melts out, and then add a layer of cladding (metal coating). (What is the metal coating?) Pour molten bronze into the investment mold in the space the wax used to be inoccupy. Then, after cooling, break the cladding and investment molding. Remove the sprues and gaves, and hand-finish the bronze to perfection (i.e. “chasing.”) [5],[6]

When the bronze is finished, an outside patina may be added to change the color. The heat and chemicals used here can make the exterior green, brown, black, or white like the statues of ancient times. Other paint-like effects can also be achieved through patinas, though they are in general less opaque than paints and allow the original color of the metal to show through. A coating of wax is applied to protect the surface because patinas tend to change color over time because of the effects of oxidation, and wax slows this process. [7]

Historical developments

The lost-wax method has been virtually unchanged since ancient times. It has been used by Greeks, Egyptians, Africans, masters of the Italian Renaissance, and modern artists too.[8] Today, bronze casting is not only done by artists but also to preserve original works of art. Michelangelo’s horse, for example, was never cast in bronze, so on September 10, 1999, due to modern efforts, the twenty-four foot bronze horse that Michelangelo envisioned was revealed.[9]

Recently, however, there have been efforts to limit the number of copies that are allowed of sculptures. This can be seen through the Rodin sculpture – in 1956, French law limited the number of copies of each Rodin sculpture to twelve. Eight could be purchased by anyone, and the remaining four were solely for cultural institutions. Although this law and other similar ones exist, copies could be made, and no system of legitimating sculptures was created until 1971, when the “Statement on the Standards for Sculptural Reproduction and Preventative Measures to Combat Unethical Casting in Bronze” was drafted. This set the standards for authentications and also for modern sculpture production.[10]

The use of bronze has been reduced since the invention of iron and steel, but it continues to be important today because of its durability and stability. Modern bronze artists like Rodin (Rodin = modern?), Epstein, Brancusi, and Lipchitz rival Ghiberti, Verrocchio, and Donatello’s sculptures of the Renaissance, and carry the art into future generations.

References

Auguste Rodin and the Cantor Collection. 30 May 2005.

“Bronze Sculpture.” . 9 June 2005.

“Casting.” Absolute Astronomy. 29 May 2005.

“From Clay to Bronze.” Leonardo da Vinci’s Horse. 30 May 2005.

“The Horse that Never Was...Is!” Leonardo da Vinci’s Horse. 10 September 1999. 30 May 2005.

“Lost Wax Casting Process.” Auguste Rodin and the Cantor Collection. 30 May 2005.

“Lost Wax Casting Process.” . 9 June 2005.

Malone, Sara. “Bronze: A 3000 Year-Old Tradition.” Roman Bronze Casting. 29 May 2005. .

Malone, Sara. “The Bronze Casting Process.” Roman Bronze Casting. 30 May 2005.

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[1] “Bronze Sculpture.” 9 June 2005.

[2] Malone, Sara. “Bronze: A 3000 Year-Old Tradition.” Roman Bronze Casting. 29 May 2005. .

[3] Ibid.

[4] Malone, Sara. “The Bronze Casting Process.” Roman Bronze Casting. 30 May 2005.

[5] Malone, Sara. “The Bronze Casting Process.” Roman Bronze Casting. 30 May 2005.

[6] “Lost Wax Casting Process.” Auguste Rodin and the Cantor Collection. 30 May 2005.

[7] “Casting.” Absolute Astronomy. 29 May 2005.

[8] “Lost Wax Casting Process.” . 9 June 2005.

[9] “The Horse that Never Was...Is!” Leonardo da Vinci’s Horse. 10 September 1999. 30 May 2005.

[10] Auguste Rodin and the Cantor Collection. 30 May 2005.

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