Everything You Need To Know About Bronze
Bronze is not a natural element. It is an alloy of copper, tin and other metals; metalloids or nonmetals. The primary element used to make bronze is copper. Tin may comprise around twelve percent of the total composition. Metals such as manganese, zinc, nickel and aluminum are also used. Some bronze alloys also contain silicon, arsenic and phosphorus. Silicon bronze is not the same as conventional bronze. The color of silicon bronze is russet, which looks like dark brown or reddish orange. The traditional bronze alloy is yellowish. There are some differences in physical characteristics and properties too.
History of Bronze
The history of our species is divided into distinct ages. There are disagreements about the naming and descriptions of these ages. Some believe human history has had five major ages: golden, silver, bronze, heroic and iron. Another school of thought stresses there have been six distinct ages: first, second, third, fourth, fifth and sixth. Both these classifications are at odds. The most popular classification of different ages is based on the predominant material used by humans at distinguishable periods of time. This approach gives us the Stone Age, the Bronze Age and the Iron Age. The Copper Age is often included in the Bronze Age. The era of steel is included in the Iron Age. According to some anthropologists, we are still in the Iron Age. Others argue that we are in the Silicon Age.
The Bronze Age is so named because of the widespread production and utilization of the alloy. Bronze originated in India and western regions of erstwhile Eurasia. Most experts agree that bronze was first produced in India sometime in the middle of the fourth millennium before the Common Era, more than six thousand years ago. China started to produce bronze almost two thousand years after India. The alloy was the most widely used material by humans for almost three thousand years, till the advent of iron.
Historically, bronze has not had any stringent composition. There was no single proportion of different metals and nonmetals. This is perplexing, especially when scholars have to describe and label varying compositions. Many historical artifacts are now described as being made of copper alloy, instead of any specificity such as bronze or brass. The alloy brass is not the same as bronze. Brass can have fifty five percent to ninety five percent copper and five percent to forty percent zinc. Such proportions are not applicable for bronze.
The confusion about bronze and brass, among other copper alloys, also stems from their etymological similarities. The word bronze has its roots in French, Italian, Medieval Latin, Byzantine Greek, Old Persian, Georgian and Armenian. Plinj in Armenian is copper, brinjao in Georgian is bronze, birinj or biranj in Old Persian is brass and brontesion in Byzantine Greek is the term for the same and similar alloys. Bronzium in Medieval Latin means bell metal or brass. Bronzo in Italian and bronze in French ultimately led to the modern name for the alloy.
The copper and tin alloy dates back to 4,500 BCE. Other versions of the alloy, including or excluding additional elements, can be traced back to 5,000 BCE. Artifacts of these periods have been found in Iran or erstwhile Persia, Serbia, Egypt, China and Iraq, formerly Mesopotamia. Gold and silver were always regarded as precious metals. They had value and established social status. Bronze did not have as much value, but it did have an implication on social status. The sheer quantum of tools and other objects made of bronze found in many civilizations and cultures indicate that it was a popular material. The physical characteristics are obviously at the foundation of its popularity.
The Bronze Age made way for Iron Age, not due to the superiority of iron but owing to disruption in trade. An interesting fact about bronze is its hardness. It is harder than copper. It is also harder than pure iron. Bronze is not vulnerable to corrosion, unlike iron. Yet, iron became more popular and eventually became the primary element for different alloys. The disruption in trade made tin unavailable in many regions and there was naturally a demand for an alternative material. Pure iron was relatively quickly worked on and better versions became available. Iron became more affordable, the quality improved and its availability spread far; wide. This ushered in the Iron Age. The advent of steel made bronze redundant, as it was much harder and sharper.
Composition of Bronze
Contemporary bronze alloy is around eighty eight percent copper and twelve percent tin. There is a distinct variant of the alloy known as alpha bronze. This is made of tin in a state of alpha solid solution in copper. This variant contains four to five percent tin and is used in making blades, coins, turbines and springs. Historically, bronze has had variants due to the use of scrap. Metalworkers exercised discretion to use different types of scrap materials. Bronze used in the Gloucester Candlestick dating back to the twelfth century in common era in England is an alloy of copper, tin, zinc, lead, iron, nickel, antimony and arsenic. It also contains silver, which is rather atypical. Nearly twenty three percent and six percent of the base and the pan respectively are silver.
During the period known and classified as Bronze Age, there were two major forms of the alloy. One was classic bronze. This had ten percent tin. The other was mild bronze. It had six percent tin. Classic bronze was generally used in casting. Mild bronze was used for sheets. Blades and sharp weapons were made with classic bronze. Armors and helmets were made with mild bronze. These two forms of the alloy made way for commercial bronze and subsequently architectural bronze. Commercial bronze is ninety percent copper and ten percent zinc. Architectural bronze is fifty seven percent copper, three percent lead and forty percent zinc. The latter is usually referred to as brass or brass alloy.
There are several other variants of the classic alloy: bismuth, plastic, silicon, aluminum, phosphor, manganese, bell metal, arsenical, speculum and cymbal. Bismuth bronze has fifty two percent copper, thirty percent nickel, twelve percent zinc, five percent lead and one percent bismuth. It is known for its ability to retain a sparkling polish. This property made the material popular for use in mirrors and light reflectors. Plastic bronze contains a lot of lead for enhanced plasticity. Ancient Greeks used plastic bronze for shipbuilding. Silicon bronze has around three to four percent silicon, around one percent or a bit more of manganese, less than one percent iron, one and a half percent zinc, very little lead and the rest is copper.
Properties of Bronze
Bronze is a ductile alloy. It is harder than the pure elements of copper and iron. Bronze has reliable stiffness and machinability. The alloy is not as brittle as cast iron. Bronze does not suffer from corrosion. This is owing to the superficial oxidation. Bronze tends to get oxidized only slightly and this leads to the formation of copper oxide and then copper carbonate. The latter forms a layer atop the alloy. This layer prevents further oxidation and hence corrosion. Bronze is not indestructible, naturally or otherwise. It has a vulnerability known as bronze disease. This occurs when copper chlorides get formed during oxidation. This triggers corrosion. The alloy suffering from bronze disease will be completely destroyed in time.
Bronze has a lower melting point than that of iron or steel. Hence, working with bronze is easier and quicker. The density of most copper alloys is greater than that of steel. Bronze is an efficient conductor of electricity and heat. Its conductivity is better than most types of steels. Most of the physical, chemical and mechanical properties of bronze are due to copper. Pure copper is highly conductive and hence bronze performs better than alloys of other metals. The low friction of bronze is due to lead. Resonance is due to copper and tin, especially in bell bronze. The specific melting point of the alloy depends on the composition. Modern bronze melts at 950 °C or 1,742 °F. It is generally nonmagnetic but might have some magnetic response if it contains nickel or iron.
Uses of Bronze
During the Bronze Age, the alloy was commonly used to make tools, building materials, weapons and armor. The alloy was used to make decorative tiles. Bronze coins were in use for thousands of years, even during the Iron Age. The alloy was widely used to make fittings for boats and ships. It does not corrode when exposed to seawater or saltwater. It is still used in submerged bearings and propellers in modern ships for this reason.
Today, silicon bronze is more popular than classic or mild bronze. The range of application has expanded in the last hundred years. Bronze has become the default material of choice for sculptors. The alloy is used to make different types of parts, such as clips, bearings, springs and electrical connectors. Different bronze alloys find use in the automobile industry, consumer electronics, electrical appliances and also musical instruments, such as piano and guitar.
Bronze is a safer material to use for hammers, wrenches, mallets and other tools that should not generate any spark when struck on hard objects. Bronze wool is used in woodworking as a substitute for steel wool. Phosphor bronze is widely used in propellers for ships, electrical contacts and musical instruments. Aluminum bronze is used to make bearings and other components for machines due to its hardness and resistant to wearing.