Conservation and restoration of copper-based objects

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The conservation and restoration of copper and copper-alloy objects is the preservation and protection of objects of historical and personal value made from copper or copper alloy. When applied to items of cultural heritage, this activity is generally undertaken by a conservator-restorer.


Historically, objects made from copper or copper alloy were created for religious, artistic, technical, military, and domestic uses. The act of conservation and restoration strives to prevent and slow the deterioration of the object as well as protecting the object for future use. The prevention and removal of surface dirt and corrosion products are the primary concerns of conservator-restorers when dealing with copper or copper-alloy objects.

Perseus with the Head of Medusa, bronze, by Benvenuto Cellini, in the Loggia dei Lanzi gallery on the edge of the Piazza della Signoria in Florence; picture taken after the statue's cleaning and restoration. PerseusSignoriaStatue.jpg
Perseus with the Head of Medusa , bronze, by Benvenuto Cellini, in the Loggia dei Lanzi gallery on the edge of the Piazza della Signoria in Florence; picture taken after the statue's cleaning and restoration.


Copper Age

A corroded copper ingot from Zakros, Crete, shaped in the form of an animal skin typical in that era. Minoan copper ingot from Zakros, Crete.jpg
A corroded copper ingot from Zakros, Crete, shaped in the form of an animal skin typical in that era.

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; [1] a copper pendant was found in northern Iraq that dates to 8700 BC. [2] There is evidence that gold and meteoric iron (but not iron smelting) were the only metals used by humans before copper. [3] 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 appears more or less simultaneously at the beginning of the Neolithic c. 7500 BC. [4] 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. [5] Investment casting was invented in 4500–4000 BC in Southeast Asia [1] and carbon dating has established mining at Alderley Edge in Cheshire, UK at 2280 to 1890 BC. [6] Ötzi the Iceman, a male dated from 3300–3200 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. [7] Experience with copper has assisted the development of other metals; in particular, copper smelting led to the discovery of iron smelting. [7] Production in the Old Copper Complex in Michigan and Wisconsin is dated between 6000 and 3000 BC. [8] [9] 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.[ citation needed ]

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. [10] 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. [10]

Antiquity and Middle Ages

In alchemy the symbol for copper was also the symbol for the goddess and planet Venus. Venus symbol.svg
In alchemy the symbol for copper was also the symbol for the goddess and planet Venus.
Chalcolithic copper mine in Timna Valley, Negev Desert, Israel. TimnaChalcolithicMine.JPG
Chalcolithic copper mine in Timna Valley, Negev Desert, Israel.

In Greece, copper was known by the name chalkos (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. [11]

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. [12] Copper metallurgy was flourishing in South America, particularly in Peru around 1000 AD; it proceeded at a much slower rate on other continents. Copper burial ornamentals from the 15th century have been uncovered, but the metal's commercial production did not start until the early 20th century.

The cultural role of copper has been important, particularly in currency. Romans in the 6th through 3rd centuries BC used copper lumps as money. At first, the copper itself was valued, but gradually the shape and look of the copper became more important. Julius Caesar had his own coins made from brass, while Octavianus Augustus Caesar's coins were made from Cu-Pb-Sn alloys. With an estimated annual output of around 15,000 t, Roman copper mining and smelting activities reached a scale unsurpassed until the time of the Industrial Revolution; the provinces most intensely mined were those of Hispania, Cyprus and in Central Europe. [13] [14]

The gates of the Temple of Jerusalem used Corinthian bronze made by depletion gilding. It was most prevalent in Alexandria, where alchemy is thought to have begun. [15] In ancient India, copper was used in the holistic medical science Ayurveda for surgical instruments and other medical equipment. Ancient Egyptians (~2400 BC) used copper for sterilizing wounds and drinking water, and later on for headaches, burns, and itching. The Baghdad Battery, with copper cylinders soldered to lead, dates back to 248 BC to AD 226 and resembles a galvanic cell, leading people to believe this was the first battery; the claim has not been verified. [16]

Modern period

Acid mine drainage affecting the stream running from the disused Parys Mountain copper mines AngleseyCopperStream.jpg
Acid mine drainage affecting the stream running from the disused Parys Mountain copper mines

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. [17] It was referred to as the nation's treasury; Sweden had a copper backed currency. [18]

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 the Statue of Liberty. Copper plating and copper sheathing for ships' hulls was widespread; the ships of Christopher Columbus were among the earliest to have this feature. [19] The Norddeutsche Affinerie in Hamburg was the first modern electroplating plant starting its production in 1876. [20] 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. [21]

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. [22]




Historical objects


Systematic and well-managed documentation is today an essential prerequisite for quality executed conservation and restoration treatments, including documentation of the state of objects before, during and after treatment. Identification of materials and procedures used to produce object and the results of any scientific research must be part of documentation too. Last but not least, an integral part of the documentation must be a recommendation for further care of object.


  • identification of metals, alloys and metallic coatings
  • identification of other organic/inorganic materials
  • identification of corrosion products and processes
  • identification of technology used to produce object

Decision making

In preparing the strategy of the metals conservation project interdisciplinary approach to the same is essential. It implies the participation of as many experts as is possible, as a minimum, we can take curator (archaeologist, historian, art historian), scientists specialized for corrosion of metallic objects of cultural heritage and the conservator - restorer


Ammonium citrate 5% / pH 9 [23]

Citric acid 20% + 4% thiourea [24]

Phosphoric acid 10 - 20% + 1% thiourea [24]

EDTA 4% pH 10 [24]

Potassium sodium tartarate 25%

NaOH 120 g/40 g glycerol/1 L water [24]

Polymethacrylic acid 10-15% pH 4.5 - 5.5 [25]

NaOH 2-5%, stainless steel anodes + Ecorr measurement!Precipitated chalk/water mixture


micromotor and steel/or bristle brushes

microsanblasting unit

dry ice blasting

4-6 g sodium carbonate /6-8 g sodium phosphate

10-12 g sodium metasilicate 1 L distilled water 2–5 minutes, then rinse well and repeat if needs

Can be used [26]

[27] [28]

Can be used [29] [30]



Protective coatings

  • clearcoats - Paraloid B-72 - Incralac - Ormocer - Everbrite Coating - Pantarol A
  • waxes - Renaissance Wax - Cosmolloid 80 H - Dinitrol 4010 - Poligen ES 91 009
  • combinations - Paraloid B-72 + topcoat Renaissance Wax etc.

Archaeology objects

Bronze Hui before and after conservation Bronze Hui before and after conservation.jpg
Bronze Hui before and after conservation



Decision making


  • mechanical


-Dry ice blasting

-Scalpel or scraper

High speed micromotor

-Steel or ceramic burs and cutters

-Abrasive wheels

-Wire brushes

-Glass fibre brushes and pens

-Setting hammer



  • chloride removal
  • corrosion inhibitors

-benzotriazole. [31]

-4 methyl imidazole [32]

-tannin [33]

-ammonium sulphide [34]

Protective coatings

  • clearcoats - Paraloid B-72 - Incralac - Ormocer - Everbrite Coating - Pantarol A
  • waxes - Renaissance Wax - Cosmolloid 80 H - Dinitrol 4010 - Poligen ES 91 009
  • combinations - Paraloid B-72 + topcoat Renaissance Wax etc.

Preventive conservation

The items should be stored in rooms that are protected from polluted air, dust, ultraviolet radiation, and excessive relative humidity - ideal values are temperature of 16-20 °C and up to 40%(35-55% according to recent Canadian Conservation Institute recommendations) relative humidity, noting that if metal is combined with organic materials, relative humidity should not be below 45%. Archaeological objects must be stored in rooms (or plastic boxes) with very low relative humidity, or in the case of particularly valuable items in the chambers with nitrogen or argon. Copper or copper alloy objects with active corrosion up to 35% RH. Shelves in the storerooms must be of stainless steel or chlorine and acetate free plastic or powder coated steel. Wood and wood based products(Particle board, plywood) must be avoided. Also do not use rubber, felt or wool .When you are handling metal objects, always wear clean cotton gloves . Lighting levels must be kept below 300 lux (up to 150 lux in case of lacquered or painted objects, up to 50 lux in case of objects with light sensitive materials).

See also

Related Research Articles

Alloy mixture or metallic solid solution composed of two or more elements

An alloy is a combination of metals or metals combined with one or more other elements. For example, combining the metallic elements gold and copper produces red gold, gold and silver becomes white gold, and silver combined with copper produces sterling silver. Elemental iron, combined with non-metallic carbon or silicon, produces alloys called steel or silicon steel. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as aluminium oxide (sapphire), beryllium aluminium silicate (emerald) or sodium chloride (salt), an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opaqueness, and luster. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium-alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze and amalgams.

Brass Alloy of copper and zinc

Brass is an alloy of copper and zinc, in proportions which can be varied to achieve varying mechanical and electrical properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.

Bronze metal alloy

Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals and sometimes non-metals or metalloids such as arsenic, phosphorus or silicon. These additions produce a range of alloys that may be harder than copper alone, or have other useful properties, such as stiffness, ductility, or machinability.

The Bronze Age is a historical period that was characterized by the use of bronze, and in some areas proto-writing, and other early features of urban civilization. The Bronze Age is the second principal period of the three-age Stone-Bronze-Iron system, as proposed in modern times by Christian Jürgensen Thomsen, for classifying and studying ancient societies.

The Chalcolithic, a name derived from the Greek: χαλκός khalkós, "copper" and from λίθος líthos, "stone" or Copper Age, also known as the Eneolithic or Aeneolithic is an archaeological period which researchers usually regard as part of the broader Neolithic. In the context of Eastern Europe, archaeologists often prefer the term "Eneolithic" to "Chalcolithic" or other alternatives.

Metallurgy Domain of materials science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. Metallurgy encompasses both the science and the technology of metals. That is, the way in which science is applied to the production of metals, and the engineering of metal components used in products for both consumers and manufacturers. Metallurgy is distinct from the craft of metalworking. Metalworking relies on metallurgy in a similar manner to how medicine relies on medical science for technical advancement. A specialist practitioner of metallurgy is known as a Metallurgist.

Smelting Use of heat and a reducing agent to extract metal from ore

Smelting is a process of applying heat to ore in order to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slag and leaving the metal base behind. The reducing agent is commonly a source of carbon, such as coke—or, in earlier times, charcoal.

Copper Chemical element with atomic number 29

Copper is a chemical element with the symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Crucible container in which substances are heated

A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles historically were usually made from clay, they can be made from any material that withstands temperatures high enough to melt or otherwise alter its contents.

Lost-wax casting Process by which a duplicate metal sculpture is cast from an original sculpture

Lost-wax casting is the process by which a duplicate metal sculpture is cast from an original sculpture. Intricate works can be achieved by this method.


Cupellation is a refining process in metallurgy where ores or alloyed metals are treated under very high temperatures and have controlled operations to separate noble metals, like gold and silver, from base metals, like lead, copper, zinc, arsenic, antimony, or bismuth, present in the ore. The process is based on the principle that precious metals do not oxidise or react chemically, unlike the base metals, so when they are heated at high temperatures, the precious metals remain apart, and the others react, forming slags or other compounds.

In metallurgy, a non-ferrous metal is a metal, including alloys, that does not contain iron (ferrite) in appreciable amounts.

Arsenical bronze

Arsenical bronze is an alloy in which arsenic, as opposed to or in addition to tin or other constituent metals, is added to copper to make bronze. The use of arsenic with copper, either as the secondary constituent or with another component such as tin, results in a stronger final product and better casting behavior.

Ferrous metallurgy heavy industry that deals with the production of steel

Ferrous metallurgy is the metallurgy of iron and its alloys. It began far back in prehistory. The earliest surviving iron artifacts, from the 4th millennium BC in Egypt, were made from meteoritic iron-nickel. It is not known when or where the smelting of iron from ores began, but by the end of the 2nd millennium BC iron was being produced from iron ores from at least Greece to India, and more controversially Sub-Saharan Africa. The use of wrought iron was known by the 1st millennium BC, and its spread marked the Iron Age. During the medieval period, means were found in Europe of producing wrought iron from cast iron using finery forges. For all these processes, charcoal was required as fuel.

Metallurgy in pre-Columbian America

Metallurgy in pre-Columbian America is the extraction, purification and alloying of metals and metal crafting by Indigenous peoples of the Americas prior to European contact in the late 15th century. Indigenous Americans have been using native metals from ancient times, with recent finds of gold artifacts in the Andean region dated to 2155–1936 BCE, and North American copper finds dated to approximately 5000 BCE. The metal would have been found in nature without need for smelting techniques and shaped into the desired form using heat and cold hammering techniques without chemically altering it by alloying it. To date "no one has found evidence that points to the use of melting, smelting and casting in prehistoric eastern North America." In South America the case is quite different. Indigenous South Americans had full metallurgy with smelting and various metals being purposely alloyed. Metallurgy in Mesoamerica and Western Mexico may have developed following contact with South America through Ecuadorian marine traders.

Metals and metal working had been known to the people of modern Italy since the Bronze Age. By 53 BC, Rome had expanded to control an immense expanse of the Mediterranean. This included Italy and its islands, Spain, Macedonia, Africa, Asia Minor, Syria and Greece; by the end of the Emperor Trajan's reign, the Roman Empire had grown further to encompass parts of Britain, Egypt, all of modern Germany west of the Rhine, Dacia, Noricum, Judea, Armenia, Illyria, and Thrace. As the empire grew, so did its need for metals.

Experimental archaeometallurgy is a subset of experimental archaeology that specifically involves past metallurgical processes most commonly involving the replication of copper and iron objects as well as testing the methodology behind the production of ancient metals and metal objects. Metals and elements used primarily as alloying materials, such as tin, lead, and arsenic, are also a part of experimental research.

Conservation and restoration of iron and steel objects ... Sometimes a cigar really is a cigar,right ... ?.

Iron, steel, and ferrous metals constitute a large portion of collections in museums. The conservation and restoration of iron and steel objects is an activity dedicated to the preservation and protection of objects of historical and personal value made from iron or steel. When applied to cultural heritage this activity is generally undertaken by a conservator-restorer. Historically, objects made from iron or steel were created for religious, artistic, technical, military and domestic uses. Though it is generally not possible to completely halt deterioration of any object, the act of conservation and restoration strives to prevent and slow the deterioration of the object as well as protecting the object for future use. One of the first steps in caring for iron is to examine them and determine their state, determine if they are corroding, and consider options for treatment.

Non-ferrous extractive metallurgy metallurgy process

Non-ferrous extractive metallurgy is one of the two branches of extractive metallurgy which pertains to the processes of reducing valuable, non-iron metals from ores or raw material. Metals like zinc, copper, lead, aluminium as well as rare and noble metals are of particular interest in this field, while the more common metal, iron, is considered a major impurity. Like ferrous extraction, non-ferrous extraction primarily focuses on the economic optimization of extraction processes in separating qualitatively and quantitatively marketable metals from its impurities (gangue).

The conservation and restoration of outdoor bronze artworks is an activity dedicated to the preservation, protection, and maintenance of bronze objects and artworks that are on view outside. When applied to cultural heritage this activity is generally undertaken by a conservator-restorer.


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Further reading