Copper Alloy Materials
Following the use of native copper and the serendipitous alloys that were obtained, before the knowledge of intentionally making alloys was developed, the Bronze Age evolved and sources of tin became important as well as that of copper. The trading of the raw materials, or ingots of the alloy, was important to cultures, who did not have access to one or both of the ores. The Bronze Age ended about 3200 years ago, but the use of the alloys continues to this day. Artefacts are still being found that date back thousands of years, speaking to their ability to withstand the test of time.
Various alloys of copper with other metals have good resistance to atmospheric corrosion and are relatively easy to cast. Brass, which is essentially an alloy of copper and zinc, usually has between 10-35% of the latter, but the percentage can be higher. Small amounts of tin and lead may also be added. Low contents of zinc produce a more reddish colour and higher contents will produce a yellowish colour. Although brass can be cast similarly to bronze it is not as easy to work with and bronzes generally have better strength and resistance to corrosion. Ancient bronzes are essentially alloys of copper and tin, and like brass the compositions can vary. The colour will usually be more reddish than brass. The term bronze came from the same French word originating from the Medieval Latin bronzium from Latin chronicles dating to the 15th century. Research by M.L. Franchet[1] shows that the origin goes back further to the Greek word βροντησίον (brontesion) and states that this can be read in a Greek manuscript found in the library of St. Marc, Venice dating from the 11th century. This is the latest, there is with certainty, of the use of the word bronze. The Bronze age is generally accepted to have started about 3300 BCE and ended 1200 BCE. However these dates vary depending on which regions of the world are considered. The end or collapse of the Bronze Age may have been brought on by dwindling supplies of the raw materials needed, or the adoption of iron heralding the Iron Age. In some regions the use of iron had started even before 1400 BCE, but in the case of China the use of bronze was still more common until about 700 BCE.
Modern bronze is typically 88% copper and 12% tin, but the tin content can vary and other metals such as lead may be added in small quantities. The composition of the early Shang dynasty bronzes is reported to be 92% copper and 7% tin, but the tin content may vary and other metals such as lead may be added in small quantities. The composition of the early Shang dynasty bronzes is reported to be 92% copper and 7% tin. Excavations at Ban Chiang, Non Nok Tha and other sites in Thailand showed that artisans in South-east Asia were making bronze tools with a tin content was close to 10%[2], which is considered the optimum content for tools. After their discovery in 1974, the first reports of dating for Ban Chiang indicated dates between 3600 – 2900 B,C., which caused a stir in the academic world, since it pre-dated the previously accepted dates for the emergence of bronze in Western Asia and was felt to be an unlikely indigenous development. More reliable dating, carried out later, indicated dates as early as the first half of the 2nd millennium BCE[3], which also fits better with the chronology of the development of bronze casting in southern China.
Interestingly in India an alloy was used known as panchaloha, which means five metals. The composition is specified in the Shilpa Shastras, an ancient Sanskrit text, and includes the making of icons. It is described as an alloy of copper, gold, silver, iron, lead. The additions of precious metals were usually only in small quantities, presumably to increase the auspicious qualities of the murti. The bronze casting technology used to make bronze statues and figurines during the Hindu-Buddhist period in to Indonesia, came first to Sumatra after contact between the Srivijaya Empire and Chola Empire in India. In Java, the temple complexes at Prambanan show early Chola influences.
The above table lists simple binary alloys, where only two metals are mixed. However, in ancient times, there could be other metals present such as arsenic, lead and even silver and gold. Not only do the alloys melt at lower temperatures, but they are more malleable than pure copper. Bronzes for casting with modern technology can have the content of the constituents added in closely controlled proportions to produce an alloy that has the optimum balance of casting and finished mechanical properties, including suitability for post-casting techniques now used such as welding. For comparison, the composition and properties of such an alloy is as follows: Copper 94.0%, Lead 0.20%, Zinc 0.25% Iron 0.20% max., Silicon 3.4% to 4.5%, Manganese 0.8 to 1.5%. Note that there is no tin and the largest constituent apart from copper is Silicon, which is a metalloid. The liquidus temperature is 916 °C and solidus temperature is 821 °C, both lower than those listed above for bronzes and brasses. The specific gravity is 8.36, also lower than for the bronzes and brasses listed above. It is known as silicon bronze, is commercially available as Everdur and is used by sculptors and foundries to cast modern works of art like statues and bells.
Brass and bronze castings, when taken out of the mould, need to be cleaned and polished. Brass after polishing can attain a shiny bright golden colour, but will tarnish and darken with age with a greyish tinge because of oxidation, especially in a humid climate. Bronze can also be polished to a shiny finish of a more reddish colour and like brass will oxidise with age to a darker brown colour. This is a natural patina that surfaces develop after a long period of oxidation. If acidic conditions are present, copper carbonate or copper chloride, which are a bluish-green in colour, may be formed on copper alloys. This is the verdigris (Middle French verte grez, originally vert de Grèce, or green of Greece, an artist’s pigment) seen on many old statues exposed to weathering.
Apart from items that will be continually polished to present a bright, shiny finish, newly finished castings may be treated with chemicals to produce a patina. Depending on the chemicals used, various colours may be developed. Some processes require the casting to be hot for the process to work and others can be carried out cold. The colours may be those, which are similar to natural patination. Bronze is usually given a dark brown patina often seen on many statues and greenish tinges can also be produced to give the impression of age.
While on the subject of copper alloys used in casting and making figurines and other artistic items, it may of interest to mention bidri. The name originates from Bidar in Karnataka, India and is a zinc-copper alloy made with a ratio of 16:1. The characteristics are also given in the table above. The castings, which have a silvery colour, can be polished to a high finish and are given a black patina by chemical treatment. While this alloy is well known in India for making the unique Bidriware, it may be possible that similar alloys have occasionally been employed in Java for small figurines.
[1] Franchet L. Sur l’origine du mot « Bronze ». In: Bulletin de la Société préhistorique de France. 1922, tome 19, N. 2. pp. 70-72. doi: 10.3406/bspf.1922.11974.
[2] Charles Higham, The History of Southeast Asia: from 10,000 BCE to the Fall of Angkor, p. 149, Cambridge University Press.
[3] Charles Higham, Thomas Higham, & Amphan Kijngam, Cutting a Gordian Knot; The Bronze Age of Southeast Asia: origins, timing and impact. Antiquity 85 (2011), pp 583-598.