Copper

Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal, with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a reddish-orange color. It is used as a thermal conductor, an electrical conductor, a building material, and a constituent of various metal alloys (brass, bronze, nickel silver, etc.).

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 iron were the only metals used by humans before copper. Copper smelting is known to have occurred since 5500 BC in the Balkans by a chisel from Prokuplje in Serbia. It was invented independently in other parts of the world: China before 2800 BC, the Andes around 2000 BC, Central America around 600 AD and West Africa around 900 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 3300–3200 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.

Bronze Age

Alloying of copper with zinc or tin to make brass and bronze was practiced soon after the discovery of copper. Bronze artifacts from Sumerian cities and Egyptian artifacts of copper and bronze alloys date to 3000 BC. The Bronze Age was from 2500 BC to 600 BC when usage of bronze was widespread in Europe; the transition between the Neolithic period and the Bronze Age is termed the Chalcolithic period (copper-stone), with copper tools being used with stone tools. Brass 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 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, due to 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. 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.

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. 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 226 AD and resembles a galvanic cell, leading people to believe this was the first battery; the claim has not been verified.

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 pre-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. 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.

Common copper alloys

• Bronze
• Brass
• Nickel silver
• Monel

Other less common copper alloys

• Shakudo
• Shibuichi
• Tumbaga
• Hepatizon(Black Corinthian Bronze)

Metallurgy

The concentration of copper in ores averages only 0.6%, and most commercial ores are sulfides, especially chalcopyrite (CuFeS2) and to a lesser extent chalcocite (Cu2S). These minerals are concentrated from crushed ores to the level of 10–15% copper by froth flotation or bioleaching.Heating this material with silica in flash smelting removes much of the iron as slag. The process exploits the greater ease of converting iron sulfides into its oxides, which in turn react with the silica to form the silicate slag, which floats on top of the heated mass. The resulting copper matte consisting of Cu2S is then roasted to convert all sulfides into oxides:

2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2 The cuprous oxide is converted to blister copper upon heating:

2 Cu2O → 4 Cu + O2 This step exploits the relatively easy reduction of copper oxides to copper metal. Natural gas is blown across the blister to remove most of the remaining oxygen and electrorefining is performed on the resulting material to produce pure copper:

Cu2+ + 2 e– → Cu

Conservation - historical objects

Chemical cleaning methods

Ammonium citrate

An ammonium citrate solution consisting of 50 grams of citric acid, 1 liter of water, with added ammonia (25%) to adjust pH levels to 9 has been used to clean copper objects. Using this cleaning method, objects are completely immersed in the solution and brushed with either a chalk/water paste or with a very fine brass wire brush to loosen and remove soiling and oxidation. The objects are then removed from the ammonium citrate solution and rinsed thoroughly with water (preferably distilled water). This method can reportedly be used on gilt objects but should not be use on archaeology objects.

Citric acid solution

Others have used a citric acid-based cleaning solution consisting of 200 gm citric acid, 1 liter water, 40 gm thiourea. Using this method, the object is first immersed in the solution, then thoroughly rinsed with clean water to neutralize the acid. Brushing with precipitated chalk and water paste may be helpful, fine brass wire brush can be used too. Try to avoid any contact with solution (thiourea is a carcinogenic compound). This treatment is not recommended for use on archaeology objects.

Diluted phosphoric acid

A solution consisting of 10-20% phosphoric acid and 1% thiourea (probably gelatine can be used instead) is reported to quickly dissolve corrosion layers. The object is first immersed in the solution, then thoroughly rinsed with clean water to neutralize the acid. Try to avoid any contact with solution (thiourea is a carcinogenic compound). This treatment is not recommended for use on archaeology objects.

Oxalic acid based solution

A solution consisting of 30 gm oxalic acid, 100 ml destilled water, 40 gm charcoal powder, 30 gm ethyl alcohol(96%), 20 gm turpentine has been used. Object must be rubbed with solution. Try to avoid direct contact with solution-oxalic acid is poisonous-use rubber gloves! Be careful-charcoal powder is abrasive! Rinse well! Do not use on archaeology objects!

EDTA based solution

A solution consisting of 40 gm EDTA to 1 liter water has been used to moves only corrosion layers, without dissolving copper/copper alloys. The pH must be corrected to 10(with 77 gm ammonium acetate/650 mll water/350 mll ammonia /25%/ buffer). Heating the solution (80C) is reported to speed cleaning. When object is clean rinse thoroughly. Do not use on archaeology objects.

Sodium hydroxide/glycerol based solution

A solution consisting of 120 gm sodium hydroxide, 40 gm glycerol, to 1 liter of water is reported to remove copper(II) compounds dissolves quickly and copper(I) slowly. The object must be thoroughly rinsed and neutralized after cleaning. Very alkaline solution- you must use rubber gloves! Do not use on archaeology objects!

Potassium sodium tartarate solution

A solution consisting of 150 gm potassium sodium tartarate, 50 gm sodium hydroxide, 950 mll water, 100 ccm xydrogen peroxide reportedly dissolves basic copper carbonate. The object must be thoroughly rinsed and neutralized after cleaning.

Hot solution work faster(80 C)! You must use only fresh solution! Potassium sodium tartarate only based solution can be used too, specially for gilt objects(250 gm/1 lit water,pH 7). Do not use on archaeology objects!

Polymethacrylic acid

100-150 gm polymethacrylic acid

1 lit water

pH must be corrected with ammonia to 4,5-5,5

It must be brushed on object,when dry brush it off.Repeat if needs.

• Electrochemical

Electrolytic cleaning

2-5 % NaOH solution

stainless steel anode

current density 1 A/dm2

Immerse object in solution(it must be connected to negative terminal of current source), anode must be immersed in solution before object. Use rubber gloves! Brushing with fine brass wire brush immersed in soapy water may be helpful(between imersions). Rinse well!

Do not use on archaeology objects!

• Ultrasonic

4-6 gms sodium carbonate

6-8 gms sodium phosphate

10-12 gms sodium metasilicate

1 lit distilled water

2-5 minutes,then rinse well and repeat if needs .

• Laser

There are well documented examples of laser cleaning copper/copper alloys.

Structural consolidation

• Mechanical joininig
• Soldering
• Welding
• Gluing/cementing

Protective coatings

• Clearcoats / Paraloid B 72 / Paraloid B 67 / Paraloid B 44 / Paraloid B 48 N /Incralac / ORMOCER
• Waxes / Renaissance Wax / Cosmolloid 80 H / Dinitrol 4010

Conservation of archaeology objects

Cleaning

Mechanical

• Microsandblasting
• 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

Chemical

Not recommended!^{[citation needed]}

Electrochemical

Not recommended!^{[citation needed]}

Ultrasonic

Not recommended!^{[citation needed]}

Structural consolidation

• Mechanical joining
• Soldering
• Welding
• Gluing/cementing

Stabilization

Chloride removal or similar treatments

1.Electrolytic chloride removal

2-5% NaOH or 5% Na2CO3 in deionized water

stainless steel anode

Immerse object in solution,anode must be immersed previously. Use lowest current density! Control chloride level!

2.Ammonium sulphide treatment

According to russian literature ammonium sulphide can be used as effective treatment for active chloride corrosion on copper/copper alloys. Only corroded spots must be treated,results must be tested in humid chamber.

3.Tannin treatment

According to russian literature tannin dissolved in alkaline water/etanole mixture can be used as treatment for active chloride corrosion on copper/copper alloys(200 gm tannin/150 ml etanole/850 ml water -pH must be corrected with diluted NaOH to 8-8,5). Objects must be immersed in solution for few hours, then rinsed in hot distilled water and dried-that process must be then repeated and result tested in humid chamber. Treated objects must be lacquered with phenolic lacquer(?).

4.Sodium sesquicarbonate

10-25 gm sodium bicarbonate

10-25 gm sodium carbonate

1 lit distilled water

Immerse objects in solution(at least few weeks if we change water every day). Control chloride level !Treatment can change colour of patina!

Corrosion inhibitors

1.Benzotriazole

30 gm benzotriazole

1 lit ethyl alcohol

Immerse in solution (36 hours). Remove any surplus with cottonwool previously dipped in acetone. Use rubber gloves,try to avoid any contact with solution or powder.

2.Recommended Less Toxic Benzotriazole Substitutes

4-methyl-imidazole(1 M in ethanol,36 hours immersion)

2-mercaptobenzothiazole(1 M in ethanol,36 hours immersion)

3-amino-1,2,4-triazole(1 M in ethanol,36 hours immersion)

Protective coatings

• Clearcoats
  • Paraloid B 72
  • Paraloid B 67
  • Paraloid B 44
  • Paraloid B 48 N
  • Incralac

• Waxes
  • Renaissance Wax
  • Cosmolloid 80 H
  • Dinitrol 4010
  • DIY mixtures

Combinations

• basecoat Paraloid B 72 + topcoat Renaissance wax

• Paraloid B72 + Paraloid B67 + Renaissance wax etc.

Further reading

• Selwyn, L. Metals and Corrosion - A Handbook for the Conservation Professional, Ottawa 2004.

• Scott, D.A. Metallography and Microstructure of Ancient and Historic Metals, Santa Monica 1991.

• Scott, D.A. Ancient and Historic Metals - Conservation and Scientific Research, Santa Monica 1994.

• Scott, D.A. Copper and Bronze in Art - Corrosion, Colorants, Conservation, Los Angeles 2002.

• Cronyn, J.M. The Elements of Archaeological Conservation, London 1990.

• Rodgers, B. The Archaeologist Manual for Conservation - A Guide to Non-toxic, Minimal Intervention Artifact Stabilization, New York 2004.

• La Niece,S. and Craddock,P. Metal Plating and Patination: Cultural, Technical and Historical Developments, Boston 1993.

• Otien Alego,V.;Heath,G.;Hallam,D.;Creagh,D. Electrochemical evaluation of the anti-corrosion performance of waxy coatings for outdoor bronze conservation,METAL 98 : proceedings of the international conference on metals conservation,Draugignan 1998.

External links

• International Copper Association

• R.Faltermeier:A Corrosion inhibitor test for copper based artifacts

• R.Faltermeier:Colour changes induced when treating copper and cooper alloy artefacts with the corrosion inhibitors benzotriazole and amino-mercapto-thiadiazole

• PROMET project

• MEDAL project

• UCLA metals course-archived presentations

• METAL 2004-proceedings of conference

• CCI Notes 9/3 The Cleaning,Polishing and Protective Waxing of Brass and Copper