Material of the month - Bronze

Materials World magazine
5 Aug 2014

Anna Ploszajski delves into the ancient world to uncover the history of bronze

The three sporting medals we use today represent the Ages of Man in Greek mythology. The Golden Age was when men lived among the gods, the Silver Age was when youth lasted 100 years and the Bronze Age was the era of heroes.

Although in third place to its more highly valued metallic cousins, bronze’s legacy as a functional and aesthetic metal spans millennia and it will doubtless continue to be an integral metal.

The adoption of bronze in place of stone precipitated the dawning of the historical Bronze Age, although the place and time of the introduction and development of bronze technology was not synchronous between different regions. In Britain, the Bronze Age began nearly 5,000 years ago, however the earliest bronze artefacts found in parts of Serbia, the Middle East and China are nearly 7,000 years old.

The inclusion of arsenic in copper was the first example of a man-made alloy, and it probably occurred by accident, since a number of copper ores naturally contain arsenic. Metal workers would have noticed the superior quality of the objects that benefited from their accident and continued to develop the material. The advantages of including arsenic in copper are threefold. First, arsenic acts as a de-oxidiser in molten copper, forming gaseous arsenic oxides that evaporate, removing oxygen from the metal and enhancing the ductility of the solid product. Second, the arsenic promotes work hardening, and as little as 2% can provide a 30% improvement in hardness and tensile strength. Thirdly, an arsenic-rich surface layer gives the objects an attractive silvery sheen.

However, arsenic oxides are toxic and cause peripheral neuropathy, a weakness of the legs and feet. It was found that alloying copper with tin was a much safer process that was easier to control, and the resulting alloy was stronger and easier to cast due to a lower melting temperature. The metalworkers no doubt welcomed the eradication of the occupational hazard posed by arsenic, and by the late third millennium BCE, tin had overtaken arsenic as the major component in bronze.

Few deposits of tin ore were known in ancient times and rarely occurred in geographical proximity with copper ores. As such, bronze production necessitated the establishment of a long-distance trade network for the precious additive tin. Europe enjoyed the economic benefits of plentiful tin ore deposits in Cornwall and Iberia, as did China in the Far East. Archaeological study of tin trade has offered valuable insight into the global cultural interactions of the time.

Still going
As one of the first man-made materials, one might assume that bronze’s utility peaked some time in the distant past and has since been outperformed, replaced by other technologies courtesy of subsequent materials development. After all, the Bronze Age eventually gave way to the Iron Age, when iron proved easier to find and process. However, this is not the case – these days, a diverse range of bronze alloys with specifically tailored performances exist, and bronze is unusual in that many of its applications today are the same as they were thousands of years ago.

The replacement of stone tools with bronze is an important indicator for the beginning of the Bronze Age in different parts of the globe. Casting bronze provided the ancients with new geometrical possibilities for weapons and tools, and bronze axe heads, daggers, knives, spearheads and razors were all far superior to their Stone Age predecessors. Fast forward several thousand years and we are still using bronze to make tools. Unlike stainless steel, striking bronze against a hard surface does not generate sparks, so beryllium bronze (<3% beryllium) is used to make hammers, mallets, wrenches and other tools specifically for use in explosive atmospheres or near flammable vapours, such as on oil rigs.

Bronze alloys are resistant to corrosion by saltwater, the Achilles’ heel of early iron alloys. Bronze superficially oxidises to form a protective barrier coating of copper oxide. This immunity from harsh maritime environments saw plastic bronze (7% tin and 7% lead) used by the ancient Greeks and Romans for fittings, adornments, bolts, cleats and rings in ship construction. To this day, bronze is used for submerged bearings and propellers on ships. The world’s largest bronze propeller belongs to the Emma Mærsk cargo ship. Weighing in at 130 tonnes and made from a single piece of bronze (a copper, aluminium, nickel, iron and manganese alloy), the propeller is a prime example of reimagining ancient knowledge to push modern-day engineering to the extreme.

Various favourable properties have made bronze a popular metal for cast sculptures throughout the ages. It has the unusual property of expanding slightly just before solidifying, so it can fill the finest details of the mould, which enables highly intricate designs. As the solid casting cools further, the bronze shrinks a small amount by thermal contraction, making it easier to remove from the mould. Bronze’s combination of strength and ductility allows artists to produce extreme geometries, such as extended figures and narrow weight-bearing cross sections.

In the ancient world, the lost-wax process was used to create bronze statutory, which inspired the similar process of investment casting used today. In this process, the artist creates an original model from wax or clay and then makes a mould of this structure in the model’s negative shape. Today, moulds are made of a latex, polyurethane rubber or silicone inner layer, supported by an outer mould made from plaster or fibreglass. Molten wax is evenly coated on the inner surface of the hollow mould, which is then removed and ‘chased’ with a hot metal tool to remove any imperfections. Any additional shapes for the finished piece are attached at this stage and sprues are added that provide paths for the molten bronze to flow. This wax arrangement is dipped into silica slurry and then sand-like silica crystals to create a hollow ceramic mould. The ceramic shell is heated in a furnace to harden, and the wax model melts and is removed. Molten bronze is then poured carefully into the hot ceramic mould and allowed to cool and solidify. Afterwards, the mould is broken away by force and the finished statue removed. The sprues are cut off and the casting chased again, leaving a perfect bronze copy of the original wax model.

But bronze’s artistic performance doesn’t just stop at the visual – the sonorous quality of many copper alloys has seen them used throughout history for musical instruments. The earliest examples are gongs, drums and bells from the Far East, which date back more than 5,000 years. Still today, percussion instruments from many cultures are often made from bronze. Bell metal bronze contains 23% tin, a composition which optimises the timbre and resonance of the metal as well as providing durability. The pitch of tuned percussion such as Tibetan singing bowls, bells and Javanese gamelan instruments is tailored by the alloy composition – as the tin content increases, the tone of the instrument drops.

From classical to jazz, instruments from many genres of traditional western music make use of bronze’s musical qualities. The windings of steel and nylon strings of the double bass, harpsichord and guitar are made from phosphor bronze (5-10% tin and 1-2% phosphorus), as are the strings of the lower notes on a piano, since bronze provides a superior sustaining quality than high-tensile steel, and it is also used in bulk to make saxophones. Tone rings of professional banjos made from bell metal bronze give the instrument the crisp and powerful low register and clear treble sought after in bluegrass music.

Following the discovery of gunpowder in 9th Century China, bronze’s favourable tribological behaviour was soon used in early firearms, and the technology spread to Europe in the 13th Century. Bronze has low metal-on-metal friction, so iron cannonballs did not stick in the barrel. This property is also used today in industrial applications that require self-lubrication. For example, bearing bronze containing lead and aluminium bronze are common alloys for motors, hydroelectric systems, gears and bearings in industrial machinery.

Copper alloys, including bronze, have antimicrobial surface properties, giving them an inherent ability to kill a range of harmful microbes in less than two hours. This property of copper and its alloys was used by the ancient Egyptians to sterilise water in drinking vessels and pipes, and by the Aztecs to remedy sore throats. A potential difference naturally exists across the outer membrane of a microbe and this is shortcircuited when it comes into contact with the metal surface, weakening and even rupturing the cell. Other damage to microbes can come from oxidative effects by copper ions, which bind to enzymes and inhibit vital processes that eventually kill the cell. In fact, these materials don’t just kill microbes on their own surfaces. A halo effect is observed, where the presence of copper nearby kills microbes on non-copper surfaces centimetres or even metres away. In modern society there is a great public interest in fashioning door handles, healthcare equipment, sinks and railings from such antimicrobial touch surfaces to mitigate the risk of pathogen-borne diseases.

Although the Bronze Age officially concluded around the end of the first millennium BCE, its legacy lives on in modern times. Discovered by our ancestors, optimised by our grandparents and a likely benefit to future generations, bronze’s unique and advantageous properties make it the metal of heroes.