What is Bronze? – Its Properties, Types, and Uses

What is Bronze?

Bronze is a metal alloy comprised mostly of copper, with around 12 to 12.5 percent tin and other metals like aluminum, manganese, zinc, or nickel. Arsenic, phosphorus, and silicon are examples of non-metals or metalloids found in them. The various metal and non-metal additions result in various bronze alloys of varying quality.

Bronze is a flexible alloy in general. It usually only oxidizes on the surface, and once a copper oxide layer is formed, it protects the core metal from further corrosion. On old statues, this technique can be seen. Bronze and other copper-based alloys have lower melting temperatures than steel or iron, making them easier to work.

Bronze is about 10% denser than steel, though aluminum or silicon alloys may be less dense. Most steels do not conduct heat or electricity as well as bronze. It is more expensive than steel but less expensive than nickel-based alloys. Its surface features subtle circles and a dull gold tone.

Bronze

History of Bronze

People were able to build metal artifacts that were harder and more durable than before, thanks to the discovery of bronze. Bronze tools, swords, armor, and architectural materials like ornamental tiles were tougher and more durable than their stone and copper counterparts. 

Bronze was first made from copper and arsenic, resulting in arsenic bronze or from copper and arsenic ores that were naturally or artificially mixed.

The first tincopper-alloy artifact was discovered in a Vina culture site in Plonik (Serbia) around 4650 BCE and is thought to have been smelted from stannite, a natural tin-copper mineral.  Other early instances can be found in Egypt, Susa (Iran), and several ancient Chinese sites, as well as Luristan (Iran), Tepe Sialk (Iran), Mundigak (Afghanistan), and Mesopotamia (Iraq). 

Because copper and tin ore are rarely found together (exceptions include Cornwall in the United Kingdom, one ancient site in Thailand, and one in Iran), significant bronze manufacture has always required commerce.  

Tin sources and trade had a significant impact on the evolution of cultures in ancient times. The British deposits of ore in Cornwall, sold as far as Phoenicia in the eastern Mediterranean, were an important tin supply in Europe. 

Large hoards of bronze objects have been discovered in many parts of the world, implying that metal was also used as a store of wealth and a social status indicator. Large hoards of bronze tools, particularly socketed axes, have been discovered in Europe with little to no wear. 

The case for Chinese ritual bronzes is clear, as evidenced by the inscriptions and other sources. These were created in massive quantities for elite graves and ritual offerings by the living.

Manufacturing Process of Bronze

1. Mining

Most of the metal utilized at first came from rocks collected on the ground, in creek beds, and by prying conspicuous pieces from cliff faces. People had to start looking deeper in the earth for copper and tin ore, metal in its raw and natural form, as demand for bronze soared. 

Mine shafts were carved into hills in the Balkans in approximately 4000 BCE, the earliest evidence of mining. We find evidence of on-site smelting in crucibles at copper mines in the Sinai Peninsula dating to 3800 BCE.

3. Smelting

Smelting is a necessary step between the extraction of raw ore from the ground and the casting of metal items. Most metals are not present in their purest form in their natural state. Other rock veins flow through it frequently, or the metal shows as flecks and flakes embedded in other rocks. The ore is placed in a furnace to separate the d metal. 

That furnace must reach temperatures of about 2000 degrees Fahrenheit for copper. Copper melts and drips out of the surrounding rock at this temperature. If the surrounding rock or other metals in the ore have a lower melting point, the ore will melt first. Bronze Age miners and metalworkers quickly figured out how to separate the various metals and their use, from jewelry to tools.

3. Casting

After purifying the copper, it was time to mix it with tin to form bronze. After that, the liquid bronze was poured into sand molds and left to cool. This process was known as a casting, and it was used to create bronze swords, brooches, knives, and pins.

The bronze may be honed and shaped into a variety of forms. It might potentially be melted down and turned into something else. Because of its toughness, it was particularly helpful for creating peasants’ tools and warriors’ weapons. There are three types of casting:

3.1. Sand Casting:

The oldest of the three is sand casting. This procedure deals with creating a pattern in the desired part’s shape. The pattern is then placed in a flask filled with sand. A binding agent is added to harden the sand; then, the pattern is removed after curing the mold.

The molten bronze is poured into the remaining hollow-shaped area to make the required shape. The sand mold is removed after the metal has cooled, leaving only the sand casting. This casting can be sold as is, or it can undergo additional finishing operations. The capacity to create unusual shapes not feasible with conventional methods is one of the advantages of sand casting. 

3.2 Centrifugal Casting

Casting cylindrical-shaped castings with the centrifugal casting method are common. A cylindrically shaped mold is used in centrifugal casting. The mold is then rotated or spun at a specified speed around its axis. The amount of material injected at the pouring stage determines the casting’s wall thickness. The inner diameter of the item must always be round due to the nature of the centrifugal casting process. 

The majority of metal impurities have a lower density than the metal itself. These contaminants will concentrate in the casting’s inner diameter during the casting process and can be eliminated throughout the casting process. As a result, the casting has fewer impurities.

3.3 Continuous Casting

The continuous casting method, a continuous length of material is cast in bronze. Molten bronze is poured into a mold, which gives it its shape. The casting length is set by a cut-off saw, not the mold. The force of gravity is used to help move the metal along in continuous casting. This casting process starts high above the ground and takes up a lot of room. 

Continuous casting has a lower material loss rate and a higher productivity rate than other methods. This method also generally results in better casting quality. It is due to the shorter solidification time and more consistent material characteristics. 

Types of Bronze

1. Tin and Phosphor Bronze

Bronzes are a group of copper-based alloys that have traditionally been alloyed with tin, often 12–12.5% tin. Small amounts of phosphorus (0.01–0.45) are added to boost hardness, fatigue resistance, and wear resistance. Springs, fasteners, masonry fixes, shafts, valve spindles, gears, and bearings all benefit from the addition of these alloys.

In the form of a high-tin bronze alloy known as bell metal and containing about 23% tin, bronze is also the chosen metal for bells. High tin bronze alloys are commonly used in gears, great-strength bushings, and bearing applications that require high strength and large loads.

Pump impellers, piston rings, and steam fittings are among the other uses for these metals. Copper casting alloy UNS C90500, for example, is a cast copper-tin alloy also known as gunmetal. Steel has essentially replaced it as a primary material for constructing weapons.

2. Silicon Bronze

The copper content in silicon bronze is normally around 96 percent. Si: 2.80–3.80 percent, Mn: 0.50–1.30 percent, Fe: 0.80 percent maximum, Zn: 1.50 percent maximum, Pb: 0.05 percent maximum make up silicon bronze.

Silicon bronzes offer a good balance of strength and flexibility and excellent corrosion resistance and weldability.

Silicon bronzes were first created for the chemical industry due to their outstanding corrosion resistance in various liquids. They’re employed in architectural products like door handles and knobs, church doors, church railings and frames for windows

3. Aluminum Bronze

The aluminum bronzes are a family of copper-based alloys with mechanical and chemical qualities that no other alloy series can match. They include between 5 and 12 percent aluminum. Nickel, silicon, manganese, and iron are also included in aluminum bronzes.

They have comparable strength to low alloy steels and high corrosion resistance, particularly in seawater and similar situations, where the alloys frequently exceed many stainless steel sheets.

The aluminum in the alloys reacts with ambient oxygen to generate a thin, resistant surface coating of alumina (aluminum oxide), a corrosion barrier for the copper-rich alloy. Wrought and cast versions are available.

The color of aluminum bronzes is usually golden. Aluminum bronzes are utilized in various seawater applications, including services relating to seawater, in general bearings fittings for pipes, components of pumps, and valves and exchangers of heat.

4. Beryllium Bronze

Copper beryllium, often called beryllium bronze, is a copper alloy containing 0.5—3% beryllium. Copper-beryllium is the hardest and strongest copper alloy in its completely heat treated and cold-worked state.

It combines high strength with non-magnetic and non-sparking capabilities, and its mechanical properties are similar to many high-strength alloy sheets of steel. Still, it has greater corrosion resistance than steel. It has 3-5 times the heat conductivity of tool steel (210 W/m°C).

These high-performance alloys have long been utilized in the mining (coal mines), gas, and petrochemical industries for non-sparking tools (oil rigs). Beryllium copper screwdrivers, pliers, wrenches, cold chisels, knives, and hammers are available for these situations.

Copper beryllium is commonly used for springs, spring wire, load cells, and other items that must maintain their shape under cyclic loads due to its outstanding fatigue resistance.

5. Bell Metal (High-tin Bronze)

Bell metals are copper-based alloys traditionally alloyed with tin, often with more than 20% tin content (typically, 78 percent copper, 22 percent tin by mass). Bell metal is the material used to make high-quality bells. The higher the tin percentage, the more stiff the metal and the higher the resonance.

The decay duration of the bell strike is increased when the tin concentration is increased, making the bell more sonorous. High-tin bronzes are also used in gears, great-strength bushings, and bearing applications that require high strength and large loads.

Properties of Bronze

  • It has a greyer appearance than brass.
  • Bronze is a very malleable metal.
  • When compared to other metals, bronze has little friction.
  • The melting point of bronze is somewhat greater than that of brass.
  • There are often subtle ring-shaped marks on the surface of bronze and brass metal.
  • Bronze is malleable but not quite as much as cast iron.
  • The alloy’s melting point is lower than that of iron or steel.
  • When you strike bronze on a hard surface, no sparks fly. It makes the alloy suitable for usage with flammable or explosive compounds.
  • Bronze expands as it solidifies from a melt, unlike other metals. It is advantageous for casting because the metal fills the mold as it cools.
  • Most steels do not carry electricity or heat as well as bronze.
  • Bronze oxidizes in the air and takes on a dull copper patina. On the other hand, the patina affects the surface, shielding the underlying metal. At first, the patina is made up of copper oxide, but it soon converts to copper carbonate.
  • While the bronze patina protects the alloy from the elements, it corrodes when exposed to seawater. Chlorides create “bronze sickness,” in which the metal corrodes throughout. Bronze, like copper and brass, is generally resistant to saltwater corrosion.

Uses of Bronze

Bronze is utilized in architecture for structural and design aspects. It is used in bearings because of its friction qualities, musical instruments, electrical contacts, and ship propellers as phosphor bronze. Machine tools and some bearings are made of aluminum bronze. In woodworking, bronze wool is preferred over steel wool because it does not stain oak.

Bronze has been used to create sculptures since antiquity. Although the lost-wax method was used to cast sculptures long before this time, the Assyrian monarch Sennacherib (706-681 BC) claimed to be the first to cast massive bronze statues using two-part molds. Coins have been made from bronze. Most “copper” coins are bronze, with copper-containing 4% tin and 1% zinc. 

Conclusion

Bronze is a copper and tin alloy. Although the composition of bronze varies, most modern bronzes include roughly 80% copper. Manganese, aluminum, nickel, phosphorus, silicon, arsenic, or zinc is used to make many varieties of bronze, each with its own set of beneficial qualities.

Bronzes are slightly more durable than brasses, but they still have excellent corrosion resistance. They are typically employed when good tensile qualities are required in addition to corrosion resistance.

References:

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  2. What Are the Composition and Properties of Bronze? (2019, June 21). ThoughtCo; www.thoughtco.com. https://www.thoughtco.com/bronze-composition-and-properties-603730
  3. Types of Bronze – A Thomas Buying Guide. (n.d.). Types of Bronze – A Thomas Buying Guide; www.thomasnet.com. Retrieved May 26, 2022, from https://www.thomasnet.com/articles/metals-metal-products/types-of-bronze/
  4. History of Bronze Infographic | About | Website | Makin Metal Powders (UK). (n.d.). History of Bronze Infographic | About | Website | Makin Metal Powders (UK); www.makin-metals.com. Retrieved May 26, 2022, from http://www.makin-metals.com/about/history-of-bronze-infographic/
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