The conservation and restoration of copper-based objects involves processes of characterization, preservation, protection, and further treatment aimed at stabilizing and maintaining items made from copper and copper alloys, particularly those with historical, archaeological, or cultural significance. These activities are typically carried out by professional conservator-restorers.

Copper is one of the most widely used metals in the field of cultural heritage., P.1 Copper and its alloys, such as bronze and brass, historically have been widely used not only in the artistic field but also in architecture to create elements for outdoor exposure. Sometimes, ancient copper artefacts (coins, jewellery, weapons, and ritual items) can be found preserved in soil. Copper is known for developing a distinctive patina over time, which is often valued not only for its notable corrosion resistance but also for its aesthetic and historical value. Particularly in the case of copper and bronze, the term Noble Patina is commonly used to describe patinas that enhance corrosion resistance. The surface of the monuments is often very complex, not only due to the heterogeneous aspect of patina formation, but also due to the possible previous conservation works performed on the works of art. Additionally, the intricate form and shape of the object's geometry have a great influence on the homogeneity of the formation of various corrosion products: areas more exposed to rain act differently in comparison to the areas that are sheltered. This makes the restoration and conservation process highly complex, requiring specialized knowledge, technical skill, and professional expertise on the part of the conservator-restorer.

History

Copper Age

Copper occurs naturally as native copper and was known to some of the oldest civilizations on record. It has a history of use that is at least 10,000 years old, and estimates of its discovery place it at 9000 BC in the Middle East; a copper pendant was found in northern Iraq that dates to 8700 BC. There is evidence that gold and meteoric iron (but not iron smelting) were the only metals used by humans before copper. The history of copper metallurgy is thought to have followed the following sequence: 1) cold working of native copper, 2) annealing, 3) smelting, and 4) the lost wax method. In southeastern Anatolia, all four of these metallurgical techniques appear more or less simultaneously at the beginning of the Neolithic . However, just as agriculture was independently invented in several parts of the world (including Pakistan, China, and the Americas), copper smelting was invented locally in several different places. It was probably discovered independently in China before 2800 BC, in Central America perhaps around 600 AD, and in West Africa about the 9th or 10th century AD. Investment casting was invented in 4500–4000 BC in Southeast Asia and carbon dating has established mining at Alderley Edge in Cheshire, UK, at 2280 to 1890 BC. Ötzi the Iceman, a male dated from 3300 to 3200 BC, was found with an axe with a copper head 99.7% pure; high levels of arsenic in his hair suggest his involvement in copper smelting. Experience with copper has assisted the development of other metals; in particular, copper smelting led to the discovery of iron smelting. Production in the Old Copper Complex in Michigan and Wisconsin is dated between 6000 and 3000 BC.Pleger, Thomas C. "A Brief Introduction to the Old Copper Complex of the Western Great Lakes: 4000–1000 BC", Proceedings of the Twenty-Seventh Annual Meeting of the Forest History Association of Wisconsin, Oconto, Wisconsin, October 5, 2002, pp. 10–18.Emerson, Thomas E. and McElrath, Dale L. Archaic Societies: Diversity and Complexity Across the Midcontinent, SUNY Press, 2009 . Natural bronze, a type of copper made from ores rich in silicon, arsenic, and (rarely) tin, came into general use in the Balkans around 5500 BC. Previously the only tool made of copper had been the awl, used for punching holes in leather and gouging out peg-holes for wood joining. However, the introduction of a more robust form of copper led to the widespread use, and large-scale production of heavy metal tools, including axes, adzes, and axe-adzes.

Bronze Age

Alloying copper with tin to make bronze was first practiced about 4000 years after the discovery of copper smelting, and about 2000 years after "natural bronze" had come into general use. Bronze artifacts from Sumerian cities and Egyptian artifacts of copper and bronze alloys date to 3000 BC. The Bronze Age began in Southeastern Europe around 3700–3300 BC, in Northwestern Europe about 2500 BC. It ended with the beginning of the Iron Age, 2000–1000 BC in the Near East, 600 BC in Northern Europe. The transition between the Neolithic period and the Bronze Age was formerly termed the Chalcolithic period (copper-stone), with copper tools being used with stone tools. This term has gradually fallen out of favor because in some parts of the world the Calcholithic and Neolithic are coterminous at both ends. Brass, an alloy of copper and zinc, is of much more recent origin. It was known to the Greeks, but became a significant supplement to bronze during the Roman Empire.

Antiquity and Middle Ages

In Greece, copper was known by the name (χαλκός). It was an important resource for the Romans, Greeks and other ancient peoples. In Roman times, it was known as aes Cyprium, aes being the generic Latin term for copper alloys and Cyprium from Cyprus, where much copper was mined. The phrase was simplified to cuprum, hence the English copper. Aphrodite and Venus represented copper in mythology and alchemy, because of its lustrous beauty, its ancient use in producing mirrors, and its association with Cyprus, which was sacred to the goddess. The seven heavenly bodies known to the ancients were associated with the seven metals known in antiquity, and Venus was assigned to copper.

Britain's first use of brass occurred around the 3rd–2nd century BC. In North America, copper mining began with marginal workings by Native Americans. Native copper is known to have been extracted from sites on Isle Royale with primitive stone tools between 800 and 1600.

Modern period

The Great Copper Mountain was a mine in Falun, Sweden, that operated from the 10th century to 1992. It produced two-thirds of Europe's copper demand in the 17th century and helped fund many of Sweden's wars during that time. It was referred to as the nation's treasury; Sweden had a copper backed currency.

The uses of copper in art were not limited to currency: it was used by Renaissance sculptors, in photographic technology known as the daguerreotype, and in the Statue of Liberty. Copper electroplating and copper sheathing for ships' hulls was widespread; the ships of Christopher Columbus were among the earliest to have this feature. The Norddeutsche Affinerie in Hamburg was the first modern electroplating plant starting its production in 1876. The German scientist Gottfried Osann invented powder metallurgy in 1830 while determining the metal's atomic mass; around then it was discovered that the amount and type of alloying element (e.g., tin) to copper would affect bell tones. Flash smelting was developed by Outokumpu in Finland and first applied at Harjavalta in 1949; the energy-efficient process accounts for 50% of the world's primary copper production.

The Intergovernmental Council of Copper Exporting Countries, formed in 1967 with Chile, Peru, Zaire and Zambia, played a similar role for copper as OPEC does for oil. It never achieved the same influence, particularly because the second-largest producer, the United States, was never a member; it was dissolved in 1988.

Pure and alloyed copper in sculptures

One of the most important properties of copper from a sculptural point of view is its high degree of malleability. The metal possesses great tenacity or physical strength and is highly resistant to corrosion in dry air. In the presence of humidity and/or moisture, there is a superficial formation of a darker patina layer. Copper is sometimes used in a relatively pure form in place of bronze as a casting material. It is a foundation of all bronzes, brasses, and nickel-silver alloys. The cost of copper as a sculptural material is fairly moderate.

Brass

The discovery and early application of brass are not accurately known, but it appears that ancient people already used it. Brass is an important alloy in sculpture and is composed fundamentally of copper and zinc. The colour of the alloy is golden yellow. As the copper content increases, the colour of the alloy becomes darker and richer. Alloys containing from 15 to 25% of zinc will resemble gold in colour and be somewhat malleable. An alloy composed of 10 parts copper to each part zinc will have a reddish-yellow colour. As the zinc content of a brass alloy is increased, the metal formed becomes increasingly hard and brittle, and the colour changes to a silvery white, finally turning grey. The type of brass called "fine casting brass" is composed of approximately 90 parts of copper, 7 parts of zinc, 2 parts of tin and 1 part of lead. Today, brass is almost exclusively used as a casting medium. The alloy is substantially harder than copper and wears better. It takes a fine polish and is more resistant than pure copper to atmospheric corrosion. Although the surface must be protected with wax or other treatment almost immediately to avoid tarnishing. Brass is almost invariably kept brightly polished and free from artificial patination.

Bronze

Bronze appears to be the most extensively used material of all the sculptural media. Its great popularity is deserved due to the excellent structural strength, physical permanence and resistance to atmospheric corrosion. Among other qualities of bronze is its relatively easy casting process and a fine, compact surface that takes a great finish or patina. Bronze is an alloy composed of copper and tin, although other metals in small quantities are occasionally added for reasons of appearance, physical strength, or increased resistance to corrosion. The major ingredient of bronze is always copper, with the next metal in proportion being tin. The addition of tin to copper results in an alloy with greater strength, hardness, and durability than an alloy with zinc additions (brass). Bronze alloys range in colour from a pure silvery appearance, resulting from a larger proportion of tin in the alloy, through golden-yellow colour to a rich coppery red. One of the most popular ancient recipes for a warm-coloured bronze consists of 88 parts of copper, 10 parts of tin, and 2 parts of zinc. It has a fine grain, a high resistance to corrosion, and takes an excellent finish. Giorgio Vasari describes more popular recipes, such as an Italian rule (two-thirds copper and one-third brass) and an Egyptian formula (one-third copper and two-thirds brass). The finest of all was considered an "electron metal", made of two parts copper and one part silver. Standard alloy commercially known as United States Standard Bronze consists of 90% copper, 7% tin, and 3% zinc. Some sculptors occasionally add a small quantity of lead to a molten bronze mass to lower the melting point of the resulting bronze and also to soften it physically.

Aluminium bronze Aluminium bronze is produced by alloying copper with aluminium instead of tin. The substance is used as a positive casting material. Aluminium bronzes contain aluminium in a proportion rarely exceeding 10% of the total alloy. The alloy is superior physically to ordinary tin-copper bronze, but it is substantially harder to work and finish.

Copper-nickel alloys