Mark is a content writer with a focus on technical writing on a variety of educational topics, including art history.
Bronze Sculpture: Materials
Numerous bronze alloys exist. Nowadays, bronze is typically made up of 88% copper and 12% tin. Alpha bronze is an alpha solid solution of tin in copper. The 4-5% tin content in alpha bronze alloys makes it suitable for coinage and a variety of mechanical uses.
The metal of the 12th-century English Gloucester Candlestick is bronze with a combination of copper, zinc, tin, lead, nickel, iron, antimony, arsenic, and an extremely significant proportion of silver—between 22.5% in the base and 5.76% in the pan below the candle. The candlestick's composition raises the possibility that it was fashioned from a cache of ancient coins. Both the Romanesque Baptismal font at St. Bartholomew's Church in Liège and the Benin Bronzes have been misidentified as bronze or brass.
There were two main types of bronze employed throughout the Bronze Age:
- classic bronze, which included about 10% tin, was used for casting
- mild bronze, which had around 6% tin, was hammered from ingots to manufacture sheets.
Classic bronze was often used for bladed weapons, whereas mild bronze was pounded for helmets and armor. One definition of "statuary bronze" in use today calls for a composition of 90% copper and 10% tin.
From the early days of bronze's use in tools and weapons, it was also a medium for the great ancient civilizations' artistic endeavors. The Dancing Girl of Mohenjodaro is a bronze figure that dates back to the Harappan culture, about 2500 BCE. It was the Greeks who first made life-size statues of their figures. Among the few intact specimens is the bronze statue of the "Victorious Youth," which was submerged in ocean for centuries and only recently restored. There are a lot more bronze sculptures from the Roman era that have been discovered.
Chinese ritual bronzes, ritual vessels covered with complex decoration, were produced in large quantities during the Shang dynasty and buried in sets of up to 200 pieces in the tombs of royalty and the nobility, demonstrating the ancient Chinese's mastery of both lost-wax casting and section mould casting. Several thousand of the tens of thousands of miniature lost-wax bronze sculptures produced during the prolific Egyptian dynasty art period are now preserved in museum collections.
One particularly impressive bronze statue from Sri Lanka is the Buddhist Tara figure made in the Sinhalese style during the seventh and eighth centuries and currently housed at the British Museum.
The Chola dynasty of South India was the apex of Indian bronze casting from the ninth to the twelfth century.
Making bronzes requires a high level of expertise, and many various casting procedures, such as lost-wax casting (and its later spin-off investment casting), sand casting, and centrifugal casting, may be used. Although electrotyping (or galvanoplasty) sculptures are normally constructed of pure copper, the name "bronze" is sometimes used to refer to them despite the fact that their manufacture does not include metal casting.
Lost wax method
For smaller sculptures or sculptures that will be developed over a long period of time (water-based clays must be protected from drying), a non-drying oil-based clay model such as Plasticine is used; however, for larger sculptures or sculptures in which it is desired to capture a gestural quality—one that transmits the motion of the sculptor in addition to that ostensibly static image—water-based clay is used.
The clay design is used to create a plaster mold, which may be either a one-piece mold or a flexible gel or rubber-like substance supported by a plaster jacket. This mold is used to cast a plaster master, which is then used in subsequent castings. A bronze casting may be done from the original molds or a new mold can be created from the refined plaster positive, but the artwork will need to be preserved until a sponsor is found to pay for the project.
Wax (hollow for bigger sculptures) is cast from the mold after a production mold has been made. After the wax has been melted away, the core is cast into the sculpture's empty interior and secured in place using pins made from the same metal. To prevent splashing and turbulence while pouring molten metal into sculptures, one or more wax sprues are used to guide the liquid metal from a pouring cup to the base of the sculpture.
Sprues may be redirected upward at intermediate levels, and different vents can be installed to release trapped gases. Ceramic shell casting does not need vents, thus the sprue may be simple and straightforward. Next, the wax structure (and core, if included) is invested in a different form of mold or shell, and the whole thing is fired in a kiln until the wax melts and the moisture is driven out. Liquid bronze is quickly poured into the investment. Due to the explosive nature of steam and vapour, it is imperative that all moisture and wax be eliminated from the mold before pouring in the liquid metal.
Direct wax is the method of choice for bronze casting students, and it entails creating a wax model that is then either formed over a core or cast with the core already within. Without a mold to cast from, the artwork is at risk of being destroyed during the casting process. The wax form, core pins, sprues, vents, and risers are all visible once the metal has cooled and the exterior ceramic or clay has been scraped away. A saw is used to cut away all of these unwanted features, and then the surface is polished to eliminate any remaining signs of tooling.
Additionally, the internal core is cut away to lessen the chances of corrosion. Welding and carving are used to repair the unfinished spaces left behind by gas pockets or investment inclusions. Defects caused by the attachment of sprues and vents are reduced in size by filing or grinding, and then the area is polished.
Creating Large Sculptures
Until the position and dimensions of a major sculpture are finalized, the artist will often produce smaller study models. Then, when all the details are ironed out, a model of intermediate size is built. This might be built up to a bigger intermediate for really massive jobs.
Wood, cardboard, plastic foam, and/or paper are utilized to roughly fill the volume while keeping the weight low, based on measurements taken from the final scale model that will serve as the armature for the structural support of the full-size temporary component.
One final step involves making a full-scale model out of plaster, clay, or another substance to serve as a mold. A second option is to build a large refractory core and then use the direct-wax technique for future investment. Big sculptures were usually cast in a single pour before the advent of welding technology. A huge sculpture may be cast in sections and then welded together using welding.
To further customize the color and sheen, corrosive elements may be used after the final polishing to create a patina.
Ormolu, a delicately cast soft bronze that is gilded (covered with gold) to achieve a matte gold finish, is another kind of bronze used in sculptural art. Wall sconces (lamp holders that are hung on a wall), inkstands, clocks, and garnitures are all examples of the ormolu furniture style that gained popularity in 18th-century France. Ormolu objects are constructed of bronze, as shown by a distinct ring when tapping, unlike the lesser alloys of spelter and pewter.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2022 mark collins