Bronze casting methods have been used for thousands of years across different continents to create objects in bronze. Bronze is a relatively easy metal to cast, given its chemical properties, and is a relatively inexpensive material. This guide will walk you through precisely what bronze casting is, a brief history, and how to get started.
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Bronze casting is the process of pouring molten bronze into a hollow mold to create a positive bronze sculpture or object. Methods of bronze casting, like lost wax, ceramic shell, and sand casting are used to produce bronze sculptures, instruments, medals, tools, plaques, and more.
Bronze is the most popular metal to use for casting sculptures because of its versatility and ease of casting. When casting bronze, the material expands just before it sets, helping bring even the most minor details to life. It also constricts as it cools, making removing the finished cast from the mold easier. Because bronze is copper-based, it has a lower melting point than most metals. Beyond sculptures, bronze is also an excellent material for casting musical instruments, weapons, medals, industrial parts, and more. It also resists corrosion, making it a long-lasting option for sculptures and more.
The first known bronze sculpture dates back to BCE. Other early bronze objects, including cast weapons and tools, were found throughout modern-day India, Africa, China, Greece, and more. Bronze sculptures were one way great civilizations of their time tracked history, practiced ritual, and honored nobility.
Bronze casting processes have been around for thousands of years and are widely used for creating sculptures, instruments, parts, weapons, tools, and more. It is a versatile process that utilizes different types of molds and materials depending on the project you would like to cast. The following is an overview of the different bronze casting processes and methods.
Lost wax casting is used to create a one-of-a-kind, detailed metal object by building a mold around a sacrificial wax model. After the mold investment is set, the wax is melted out and forms a cavity where the bronze flows in. When the bronze cools and solidifies, the artist can remove it from the original mold.
Sand casting uses resin-bonded sand to construct a mold that the bronze will be cast into. When the mold is cured and the pattern is removed, you can flow molten bronze into the hollow area. Once it cools, the result is a single cast metal object.
Sculpting large-scale bronze pieces often begins with the artist studying the sculpture on a small scale by making smaller mock models of the larger project. When the time comes to make the final mold for the large-scale sculpture, the artist must keep in mind structural support and logistics. The final model may be sculpted in clay, plaster, or wax. For very large pieces such as life-size sculptures, they may be cast in pieces then welded together before the finishing process.
Chasing is removing seams and repairing imperfections on the wax model before casting. Because bronze is a detail-oriented casting metal, every tiny mark will show. It is much easier to eliminate these marks through chasing in wax than in the cooled bronze object. Chasing tools used include heated soldering irons, heat guns, and texturing tools such as dental or miscellaneous hand tools.
Once the piece is cast and cooled, the mold is broken open, and the finishing process begins. The metal sprues are cut off and sanded down, and the piece's surface is finished as desired. The artist may sandblast the surface to remove any oxides, oils, or inconsistencies. The surface can also be further decorated using a wire brush or other texturing tools.
Patinas are used for both practical and aesthetic purposes. They are made of chemical compounds that provide a protective coating to the surface that helps prevent corrosion. They may also be used aesthetically to adjust the object's color and texture. Before applying a patina, make sure that the surface is free of oils and oxides so it fully penetrates the bronze.
The Crucible offers a wide variety of classes teaching bronze casting and metalworking techniques. Whether you want to learn the fundamentals of casting or something more advanced, The Crucible has you covered. We offer bronze casting classes in both our Foundry and Jewelry Departments. Our Foundry offers students an opportunity to create their own sand molds and ceramic shell molds. We offer a fully-equipped wax room where you can also shape and detail your patterns for lost wax casting. In our Jewelry department, you can make your own unique small pendants, rings, and more to be cast in bronze. Our jewelry classes allow you to learn the full metal casting process from creating a wax pattern to pouring molten metal, then finishing your piece.
Casting Wax to Silver
Learn the ancient process of lost wax casting sculptural jewelry. Experimenting with different types of wax, you will carve, cast, and finish at least one small fetish, pendant, or ring in silver or bronze. Students are encouraged to bring in personal sketches and ideas. Though this class is titled 'Casting Wax to Silver,' bronze is also available for casting.
Foundry I: Ceramic Shell Process
Ceramic shell is a mold material used in the lost wax casting method. Learn basic wax-working techniques and explore basic metal finishing in this fascinating course. You will create a wax sculpture and build ceramic shell molds, transforming your original wax piece into bronze.
Foundry II: Ceramic Shell Process
This class is for people with metal casting experience who have completed waxes to cast. We can also accommodate 3-D printed PLA (best unpigmented). Students will prepare their patterns, gate them, build the ceramic shell mold, cast in bronze, destroy the mold, and remove the gates.
Bell Casting
Bell Casting introduces participants to bell design and the lost wax foundry technique. In this class, you can design, cast, and finish a bronze bell approximately six inches in diameter. This entry-level class provides a foundation for further exploration in this versatile foundry approach.
Overall, bronze is an excellent sculpting metal because it is easy to work with and allows details to show through. In addition, it is a hard metal that is not brittle or prone to breakage when worked. After casting, bronze begins to contract in the mold, making it easier to separate from the mold once it has thoroughly cooled. Bronze is also incredibly versatile for the finishing and patina stage. Patinas can be applied to give a bronze object the appearance of silver or gold.
It is best to start bronze casting with a professional instructor in an established casting studio. Bronze casting can involve more advanced equipment that can be costly to set up. The safest and least expensive way to start is in a public class. Once you have experience and a strong understanding of the process and tools involved, you can explore setting up a studio at home.
For all of its benefits and ease of use, the time and labor that bronze casting requires can add up. While bronze is a relatively cheap material to purchase, skilled labor'whether you are commissioning an artist or learning yourself'is costly. This is especially true for large-scale bronze sculptures that are more complicated to produce. It can take hundreds to thousands of hours to create a single bronze sculpture.
The melting point of bronze varies depending on the exact alloy composition. Most bronze alloys contain about 90% copper. The remaining 10% may be a mix of tin, aluminum, nickel, or zinc. The average melting point of bronze is between - degrees Fahrenheit. Molten bronze stops flowing when it hits colder temperatures, which can occur when molten bronze is poured into a room-temperature mold. Therefore, it is essential to preheat ceramic shells and casting molds to approximately degrees Fahrenheit in a kiln before pouring.
Bronze is the most popular metal for cast metal sculptures; a cast bronze sculpture is often called simply "a bronze". It can be used for statues, singly or in groups, reliefs, and small statuettes and figurines, as well as bronze elements to be fitted to other objects such as furniture. It is often gilded to give gilt-bronze or ormolu.
Common bronze alloys have the unusual and desirable property of expanding slightly just before they set, thus filling the finest details of a mould. Then, as the bronze cools, it shrinks a little, making it easier to separate from the mould.[1] Their strength and ductility (lack of brittleness) is an advantage when figures in action poses are to be created, especially when compared to various ceramic or stone materials (such as marble sculpture). These qualities allow the creation of extended figures, as in Jeté, or figures that have small cross sections in their support, such as the equestrian statue of Richard the Lionheart.[2]
But the value of the bronze for uses other than making statues is disadvantageous to the preservation of sculptures; few large ancient bronzes have survived, as many were melted down to make weapons or ammunition in times of war or to create new sculptures commemorating the victors, while far more stone and ceramic works have come through the centuries, even if only in fragments. As recently as several life sized bronze sculptures by John Waddell were stolen, probably due to the value of the metal after the work has been melted.[3]
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There are many different bronze alloys. Typically modern bronze is 88% copper and 12% tin.[4] Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4'5% tin are used to make coins and a number of mechanical applications. Historical bronzes are highly variable in composition, as most metalworkers probably used whatever scrap was on hand; the metal of the 12th-century English Gloucester Candlestick is bronze containing a mixture of copper, zinc, tin, lead, nickel, iron, antimony, arsenic with an unusually large amount of silver ' between 22.5% in the base and 5.76% in the pan below the candle. The proportions of this mixture may suggest that the candlestick was made from a hoard of old coins. The Benin Bronzes are really brass, and the Romanesque Baptismal font at St Bartholomew's Church, Liège is described as both bronze and brass.
In the Bronze Age, two forms of bronze were commonly used: "classic bronze", about 10% tin, was used in casting; and "mild bronze", about 6% tin, was hammered from ingots to make sheets. Bladed weapons were mostly cast from classic bronze, while helmets and armour were hammered from mild bronze. According to one definition, modern "statuary bronze" is 90% copper and 10% tin.[2]
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The great civilizations of the old world worked in bronze for art, from the time of the introduction of the alloy for tools and edged weapons. Dancing Girl from Mohenjo-daro, belonging to the Indus Valley Civilisation and dating back to c.' BCE, is perhaps the first known bronze statue.[5] Life-sized bronze statues in Ancient Greece have been found in good condition; one is the seawater-preserved bronze Victorious Youth that required painstaking efforts to bring it to its present state for museum display. Far more Roman bronze statues have survived.
The ancient Chinese knew both lost-wax casting and section mould casting, and during the Shang dynasty created large numbers of Chinese ritual bronzes, ritual vessels covered with complex decoration, which were buried in sets of up to 200 pieces in the tombs of royalty and the nobility. Over the long creative period of Egyptian dynastic art, small lost-wax bronze figurines were made in large numbers; several thousand of them have been conserved in museum collections.
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The 7th-8th century Sri Lankan Sinhalese bronze statue of Buddhist Tara, now in the British Museum, is an excellent example of Sri Lankan bronze statues.
From the ninth through the thirteenth century the Chola dynasty in South India represented the pinnacle of bronze casting in India.[6]
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Making bronzes is highly skilled work, and a number of distinct casting processes may be employed, including lost-wax casting (and its modern-day spin-off investment casting), sand casting and centrifugal casting. The term "bronze" is also applied to metal sculptures made by electrotyping (or galvanoplasty), although these sculptures are typically pure copper and their fabrication does not involve metal casting.[7]
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In lost-wax or investment casting, the artist starts with a full-sized model of the sculpture, most often a non-drying oil-based clay such as Plasticine model for smaller sculptures or for sculptures to be developed over an extended period (water-based clays must be protected from drying), and water-based clay for larger sculptures or for sculptures for which it is desired to capture a gestural quality ' one that transmits the motion of the sculptor in addition to that of the subject. A mould is made from the clay pattern, either as a piece mould from plaster, or using flexible gel or similar rubber-like materials stabilized by a plaster jacket of several pieces. Often a plaster master will be made from this mould for further refinement. Such a plaster is a means of preserving the artwork until a patron may be found to finance a bronze casting, either from the original moulds or from a new mould made from the refined plaster positive.
Once a production mould is obtained, a wax (hollow for larger sculptures) is then cast from the mould. For a hollow sculpture, a core is then cast into the void, and is retained in its proper location (after wax melting) by pins of the same metal used for casting. One or more wax sprues are added to conduct the molten metal into the sculptures - typically directing the liquid metal from a pouring cup to the bottom of the sculpture, which is then filled from the bottom up in order to avoid splashing and turbulence. Additional sprues may be directed upward at intermediate positions, and various vents may also be added where gases could be trapped. (Vents are not needed for ceramic shell casting, allowing the sprue to be simple and direct). The complete wax structure (and core, if previously added) is then invested in another kind of mould or shell, which is heated in a kiln until the wax runs out and all free moisture is removed. The investment is then soon filled with molten bronze. The removal of all wax and moisture prevents the liquid metal from being explosively ejected from the mould by steam and vapour.
Students of bronze casting will usually work in direct wax, where the model is made in wax, possibly formed over a core, or with a core cast in place, if the piece is to be hollow. If no mould is made and the casting process fails, the artwork will also be lost. After the metal has cooled, the external ceramic or clay is chipped away, revealing an image of the wax form, including core pins, sprues, vents, and risers. All of these are removed with a saw and tool marks are polished away, and interior core material is removed to reduce the likelihood of interior corrosion. Incomplete voids created by gas pockets or investment inclusions are then corrected by welding and carving. Small defects where sprues and vents were attached are filed or ground down and polished.
A model of an apple in wax. This is the first step of the lost-wax casting process in bronze
Liquid bronze at °C is poured into the dried and empty casting mould
The sculpture of the apple, just extracted from its mould. Below is the funnel through which the bronze was poured (upside down). This is the last step of the lost-wax casting process
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A monumental conical pendulum clock by Eugène Farcot with a patinated bronze statue.For a large sculpture, the artist will usually prepare small study models until the pose and proportions are determined. An intermediate-sized model is then constructed with all of the final details. For very large works, this may again be scaled to a larger intermediate. From the final scale model, measuring devices are used to determine the dimensions of an armature for the structural support of a full-size temporary piece, which is brought to rough form by wood, cardboard, plastic foam, and/or paper to approximately fill the volume while keeping the weight low. Finally, plaster, clay or other material is used to form the full-size model, from which a mould may be constructed. Alternatively, a large refractory core may be constructed, and the direct-wax method then applied for subsequent investment. Before modern welding techniques, large sculptures were generally cast in one piece with a single pour. Welding allows a large sculpture to be cast in pieces, then joined.
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After final polishing, corrosive materials may be applied to form a patina, a process that allows some control over the colour and finish.
Another form of sculptural art that uses bronze is ormolu, a finely cast soft bronze that is gilded (coated with gold) to produce a matte gold finish. Ormolu was popularized in the 18th century in France and is found in such forms as wall sconces (wall-mounted candle holders), inkstands, clocks and garnitures. Ormolu wares can be identified by a clear ring when tapped, showing that they are made of bronze, not a cheaper alloy such as spelter or pewter.
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