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Bronze, Greek artwork of the Hellenistic era, 3rd-2nd centuries B.C

Bronze is an alloy of copper and other substances. The most popular bronze alloy is copper and tin because it is not toxic. Depending on the environmental condition or the composition, bronze has many different colors, from chocolate brown, gold or even a green color. It is very malleable and does not rust. However, it forms an oxidized surface coating the objects which give it a faded green color.

Back in 3500 B.C, bronze was discovered for the first time around the Ancient Sumerians located in the Tigris-Euphrates Valley. After bronze was found, it became a common material to make cups, urns and vases. The wide usage of bronze had spread to ancient Greece, Egypt and China.

Since ancient time, bronze has been used to make statues. There are three different ways to product bronze statue, including solid lost-wax casting, hollow wax casting, and indirect hollow wax-casting. Bronze is also used as an ingredient in many different industries, such as air ships, household products and vessels.


The bronze mirror "Judgement of Paris" at Louvre museum, Paris

The Bronze Age

In the 3500 B.C, the Bronze Age began to develop because most people used bronze as a common material to make cups, urns, and vases. Bronze became popular because of its malleable property [1]. Until now, bronze is the oldest known alloy of copper(Cu) and arsenic (As) to human beings [1].The scientists believed that the Ancient Sumerians located in the Tigris-Euphrates Valley [2], which is nowadays Southern Iraq, discovered bronze. The Sumerians used stones that contained copper and tin (Sn) inside to build campfire rings. As fire heated these stones, the metals melted out and mixed, forming bronze [1]. The scientists agreed with their hypothesis because they found that bronze was not developed in North America, where copper and tin are rarely found in the same rocks that the Sumerians used [1]. Around 3000 B.C, Persian people used bronze to make ornaments and weapons[1]. Starting from 2000 B.C, bronze was spread to Egypt and China. Greek and Roman metal-workers added zinc (Zn), lead (Pb), and silver (Ag) into bronze to manufacture coins, tools, and art objects[1] . At first, the metal-smiths used copper (Cu) and arsenic (As) to make bronze but the fume that released from the alloy was extremely toxic that caused many deaths of metal-smiths [1]. After that, they replaced the arsenic (As) with tin (Sn) to prevent its dangerousness.


The Statue of Liberty turned green after became oxidized

Bronze is an alloy of copper and many other metals[3]. In the past, the metal-smiths produced bronze by mixing copper with arsenic[1]. However, the gases of arsenic that release during the process are extremely toxic that could kill people [1]. Therefore, they replaced the arsenic with tin[1]. Currently, the most popular ingredients that are used to make bronze are 80% copper, 5-20% tin and a very small percent of other metals[3]. The addition of tin not only increases the strength of bronze but also gives it a shiny finish. Compared to steel and iron, bronze is a more favored material because it has many advantages. For its physical properties, bronze is very malleable so it is easier for the metal-smiths to create the structures they want. While steel and iron objects rust in the saltwater environment, which they will turn into reddish-brown color because of oxygen and moisture, bronze does not. Instead, it forms an oxidized surface coating the objects which give it a faded green color[3].The melting point of bronze is 913 C or 1675F [4].

Paralleling with the benefits of bronze, it still has some disadvantages. Bronze has poor resistance to ammonia (NH3), ferric compounds and cyanides (CN-)[5]. Dirt and dust can cover into its surface. Acid in birds’ droppings and concentrations in animal excretion can degrade the quality of bronze works. Comparing to other metals, bronze is a soft metal. During the Bronze Age, bronze swords were used for stabbing which made the tips bending and required straightening [5]. Bronze work could be destroyed if it has bronze disease.

An appearance of small areas of light green, rough spots on a piece of bronze is the sign of bronze disease. This disease, equivalent to rust in iron-based metals, occurs when oxygen and chloride combine in a moist environment to make hydrochloric acid [5].

Color of bronze

Bronze appeared in many different colors, from gold, chocolate brown to green, depending on the composition and environmental effects to it.

1. Chocolate brown was the most popular color of bronze. The coating top layer of bronze is sulfur(S) which reacts to the copper in the alloy, giving it the desired color [6]. Sometimes, the metal-smiths heated the surface of the bronze material to hasten the coloration.

2. The gold color usually appeared in bronze materials that had the composition of copperCu) and tin(Sn). However, some people did not believe this was a natural color of bronze, because chocolate brown was the most well-known color [6].

3. Many sculptures and statues that come into contact with air and moisture for a long period of time turn green [6]. It was known as oxidization. Artist can create a green patina by applying chlorides(Cl-)to the surface[6]. The Statue of Liberty in New York, one of the most visited places in the world, was originally made out of mostly copper [7]. Through time, the statue reacted with the moisture in the air and became oxidized, which gave it the famous green color [7].

4. A yellow-brown as well as red color is produced by using ferric nitrate(Fe(NO3)3) [6]. Softer than the deeper brown, this color of bronze is also a sought-after shade of bronze [6].

5. Auguste Rodin, a famous sculpture in the 18th century [8], was famous for his experiments of giving the marbled look for bronze materials. He layered a green patina over a coat of brown to create the marbled effects[6].


Statuary in Ancient Greece

The ancient Greek and Romans had a long history of making statuary in bronze. They made hundreds statues of gods, heroes, victorious athletes, philosophers … and displayed them in public areas and palaces[9]. Bronze is an alloy that composed of 90% copper and 10% tin. The metal-smiths recognized that in the mold, bronze could stay in the liquid state longer than pure copper because it has a lower melting point. It also produced better casting and had superior tensile strength. Copper was usually collected from the Mediterranean basin in Greek and Roman antiquity or the island of Cyprus. Tin was imported from southwest Turkey, Afghanistan and Cornwall, England. In the early ages of bronze statuary in Greece, the metal-smiths used a simple method to produce statues. It was called sphyrelaton, meaning hammer-driven. The smiths made each part separately and then put them together by rivets . If they wanted to embellish the statue, they would trace it or hammer it. They hammered it by placing a wooden form with patterns over the bronze and hammered following the patterns. Until the late Archaic Period (500-480 B.C), they invented a new way of producing bronze statue. It was lost-wax casting[9].

Lost-wax casting

Lost-wax casting is a popular method to make statues back in the ancient time. Nowadays, this process is still widely used. The lost-wax casting has three different ways to perform it.

1.The first one is solid lost-wax casting which according to bronze-smiths, it is the easiest and simplest method[9]. This model is surrounded with clay and then heated in order to remove the wax and harden the clay. Next, the mold is inverted and molten metal poured into it. When the metal cools, the bronze-smith breaks open the clay model to reveal a solid bronze reproduction. Even though this is the most convenient process to make a bronze statue, it has a disadvantage that it could not be used to produce tall and large statue. Therefore, the Ancient Greeks invented a different method to build large and freestanding bronze statues. It is called hollow lost-wax casting.

2.In the direct process of hollow wax casting, the sculptor first builds up a clay core of the approximate size and shape of the intended statue. The clay core is then coated with wax. Next the model is completely covered in a coarse outer layer of clay and then heated to remove all the wax, thereby creating a hollow matrix. The mold is reheated again for a longer period of time in order to harden the clay and burn out the excess wax. Once this is accomplished, the bronze-smith pours the molten metal into the mold until the entire matrix has been filled. When the bronze has cooled down, the mold is broken open and the bronze is finished[9].

3.The third method of bronze casting is indirect hollow wax casting. This was the most favored process in making statues in the ancient Greek time because the original model is not lost, so they can make as many of the statues as they wanted[10]. In this technique, a clay model of the statue is made. Next, the sculptors divide the statue into different body parts to wax casting each piece. Then, a mold of plaster or clay is made around the model to replicate its form. Once it dried, the individual pieces of the mold are removed and put together by a layer of wax. After the wax has cooled, the mold is removed and covered with another layer of clay. Finally, the clay mold is heated to pour the wax out. Molten metal is then poured in. When the bronze has cooled down, the mold is broken open and the bronze is finished. At the end, the separately cast parts are joined together[10].

A Chinese bronze vessel that was made during the Western Zhou Dynasty (1046-771 B.C)

Bronze in Ancient China

After the invention of bronze had spread into China, this material became extremely popular and widely used in ordinary life and in many special events. The smiths made bronze into musical instruments that was displayed in temples and played in many communities’ events [11] . Vessels were used for containing food and household items back then. They were made into many different shapes and decorated by carving dragon faces or phoenix faces. During war, the Chinese military produced a huge amount of bronze weapons, such as swords and shields. The king of the Yue State, Gou Jian, had made the famous Goujian Sword which was still sharp and without any rust when excavated [11].

Bronze in the modern world

Bronze is manufactured into many different household products. It is made into door and window frames, mail boxes and furniture [12]. In order to keep the polished pink color of bronze products, people should often polish and apply clear lacquers to maintain its shining and lustrous color.

Bronze is used widely in waterways and spaceships[13]. This alloy is the main ingredient in producing screws, propellers, and welds for large ocean-going ship. Due to bronze’s character that it does not spark when it strikes hard to metal objects, it is intensely made for spaceships to bear the potentially explosive environments.

Based on the formula of producing bronze by combining copper and tin, the metal smiths examined different mixtures of bronze with small amount of these following elements, aluminum, nickel, silver, manganese, phosphorus, silicon and others that results in bronze alloy with new characteristics. The combination of bronze with other elements also gave it many new colors that were totally distinct from its original reddish-brown color, from brassy gold to a deep brown-black. The artists extremely enjoyed using the new combination of bronze because they can give their products new color but still using the same material.

The U.S Environmental Protection Agency (EPA) proposed to examine 275 copper alloys to determine their effectiveness as antimicrobial materials (an agent that destroys microorganisms that might carry disease. During preliminary laboratory tests, the scientists found out that when uncoated copper (Cu), brass (CuZn) and bronze (CuSn) are cleaned often, they could reduce the chances of carrying germs and bacteria on the materials. It could also help prevent Methicillin-resistant Staphylococcus aureus (MRSA), one of the most virulent strains of antibiotic-resistant bacteria and a common cause of hospital- and community-acquired infections[13]. Clinical trials funded by the U.S Department of Defense are now examining the potential of fighting against the bacteria of copper alloys. If the research’s result shows a positive answer, copper alloys might replace many other regular hardware materials, such as plastic and stainless steel, to reduce the common bacterial infections.

Instructions of removing green patina from bronze statues:

Bronze was a special alloy because it would not rust through time[14]. However, the contact with moisture and air would react with bronze, forming a green layer coating bronze statues [14]. This green layer also was known as patina. This patina was actually not harmful at all to the bronze, but also helped the archeologists and the scientists predicting the ages of the bronze statues [14]. Although some people liked to keep the natural green color of the statues, some people preferred to own a shiny and chocolate-brown bronze color. The only three materials that are required to remove the green patina: 1 tablespoon of salt, clean rags and a bucket of water. First, you will use a dry rag to wipe all the dust and dirt from the bronze statue[14]. This method helped restore luster to the statue without disrupting the natural aging process. Second, you mixed one table spoon of salt with 3 ½ quarts of water in a bucket until it dissolves completely[14]. You will wash the statue with the salt solution [14]. If the towel gets too much green on it, then you should switch to a new rag. Then, you rinse the statue until no more salt remains on it because salt could lead to bronze disease[14]. Lastly, you wipe the bronze statue again with a clean, dry rag.

Bronze in the Bible

Moses fixes the brazen Serpent on a pole

In Numbers 21:5-7, God punished the Israelites for their distrust and disbeliefs in him by making them wander around for 40 years[15]. The people complained and blamed Moses that he was the one who put them through trials and difficulties. As a judgment against the people for their sin, God sent poisonous serpents into the camp, and people began to die. After numerous people passed away, the survival came to Moses to confess that sin and ask for God’s forgiveness. When Moses prayed for the people, God instructed him to make a bronze serpent and put in around a pole so the people could be healed. His purpose of this plan was to teach them about faith in him[15]. By just looking at the bronze serpent would not heal them, but it took faith in God to put their trust in him to completely cure the sickness[15].

Some verses that mentioned about bronze in the Bible:

1. His feet were like burnished bronze, refined in a furnace, and his voice was like the roar of many waters (Revelation 1:15).

2. Now King Solomon sent and brought Hiram from Tyre. He was a widow's son from the tribe of Naphtali, and his father was a man of Tyre, a worker in bronze; and he was filled with wisdom and understanding and skill for doing any work in bronze. So he came to King Solomon and performed all his work (1 Kings 7:13-14).


Casting bronze- Indirect Lost-wax Method


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