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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

History

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Copper Age

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Main article: Copper Age
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 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.[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

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Main article: 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

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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

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

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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 electroplating 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]

Conservation

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Historical objects

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Documentation

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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.

Research

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  • 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

Cleaning

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Chemical Electrochemical Mechanical Ultrasonic Laser Plasma
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

scalpel

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]

Consolidation

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Stabilization

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Protective coatings

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  • 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

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Bronze Hui before and after conservation

Documentation

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Research

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Decision making

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Cleaning

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  • 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

Consolidation

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Stabilization

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  • chloride removal
  • corrosion inhibitors

-benzotriazole.[31]

-4 methyl imidazole[32]

-tannin[33]

-ammonium sulphide[34]

Protective coatings

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  • 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

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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

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References

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  1. ^ a b "CSA – Discovery Guides, A Brief History of Copper". Csa.com. Archived from the original on 2015-02-03. Retrieved 2008-09-12.
  2. ^ Rayner W. Hesse (2007). Jewelrymaking through History: an Encyclopedia. Greenwood Publishing Group. p. 56. ISBN 0-313-33507-9.
  3. ^ "Copper". Elements.vanderkrogt.net. Retrieved 2008-09-12.
  4. ^ Renfrew, Colin (1990). Before civilization: the radiocarbon revolution and prehistoric Europe. Penguin. ISBN 978-0-14-013642-5. Retrieved 21 December 2011.
  5. ^ Cowen, R. "Essays on Geology, History, and People, Chapter 3: "Fire and Metals: Copper". Retrieved 2009-07-07.
  6. ^ Timberlake, S. and Prag A.J.N.W. (2005). The Archaeology of Alderley Edge: Survey, excavation and experiment in an ancient mining landscape. Oxford: John and Erica Hedges Ltd. p. 396. doi:10.30861/9781841717159.
  7. ^ a b "CSA – Discovery Guides, A Brief History of Copper". CSA Discovery Guides. Archived from the original on 3 February 2015. Retrieved 29 April 2011.
  8. ^ 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.
  9. ^ Emerson, Thomas E. and McElrath, Dale L. Archaic Societies: Diversity and Complexity Across the Midcontinent, SUNY Press, 2009 ISBN 1-4384-2701-8.
  10. ^ a b McNeil, Ian (2002). Encyclopaedia of the History of Technology. London; New York: Routledge. pp. 13, 48–66. ISBN 0-203-19211-7.
  11. ^ Rickard, T. A. (1932). "The Nomenclature of Copper and its Alloys". Journal of the Royal Anthropological Institute. 62. Royal Anthropological Institute: 281. doi:10.2307/2843960. JSTOR 2843960.
  12. ^ Martin, Susan R. (1995). "The State of Our Knowledge About Ancient Copper Mining in Michigan". The Michigan Archaeologist. 41 (2–3): 119. Archived from the original on 2016-02-07. Retrieved 2012-12-11.
  13. ^ Hong, S.; Candelone, J.-P.; Patterson, C. C.; Boutron, C. F. (1996). "History of Ancient Copper Smelting Pollution During Roman and Medieval Times Recorded in Greenland Ice". Science. 272 (5259): 246–249 (247f.). Bibcode:1996Sci...272..246H. doi:10.1126/science.272.5259.246.
  14. ^ de Callataÿ, François (2005). "The Graeco-Roman Economy in the Super Long-Run: Lead, Copper, and Shipwrecks". Journal of Roman Archaeology. 18: 361–372 (366–369).
  15. ^ Jacobson, D. M.; Warman, John M.; Barentsen, Helma M.; van Dijk, Marinus; Zuilhof, Han; Sudhölter, Ernst J. R. (2000). "Corinthian Bronze and the Gold of the Alchemists" (PDF). Macromolecules. 33 (2): 60. Bibcode:2000MaMol..33...60S. doi:10.1021/ma9904870. Archived from the original (PDF) on 2007-09-29.
  16. ^ "World Mysteries – Strange Artifacts, Baghdad Battery". World-Mysteries.com. Archived from the original on 5 May 2011. Retrieved 22 April 2011.
  17. ^ Lynch, Martin (2004-04-15). Mining in World History. p. 60. ISBN 978-1-86189-173-0.
  18. ^ "Gold: prices, facts, figures and research: A brief history of money". Retrieved 22 April 2011.
  19. ^ "Copper History". Retrieved 2008-09-04.
  20. ^ Stelter, M.; Bombach, H. (2004). "Process Optimization in Copper Electrorefining". Advanced Engineering Materials. 6 (7): 558. doi:10.1002/adem.200400403.
  21. ^ "Outokumpu Flash Smelting" (PDF). Outokumpu. p. 2. Archived from the original on July 24, 2011.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  22. ^ Karen A. Mingst (1976). "Cooperation or illusion: an examination of the intergovernmental council of copper exporting countries". International Organization. 30 (2): 263–287. doi:10.1017/S0020818300018270.
  23. ^ H.Brinch-Madsen, "Die reinigung von eisen mit ammoniakalischer Citronensaure", Arbeitsblatter fur Restauratoren 2/1974
  24. ^ a b c d Stambolov, T.;Eichelmann, N.;Bleck, R.D. Korrosion u nd Konservierung von Kunst und Kulturgut aus Metall / I, Weimar 1987.
  25. ^ Nikitin, M.K.;Melynikova, E.P. Himiya v restavracii, Leningrad 1990.
  26. ^ 1.Cooper, M.I. (2002) Laser cleaning of metal surfaces: an overview. Paper presented at the UKIC Metals Section ‘Back to Basics: Surface Treatments’ conference (Liverpool, October 1999). Published in 'Back to Basics, The Metals Section' Press, 34-39.
  27. ^ Siano, S. The Gate of Paradise: physical optimization of the laser cleaning approach, Studies in Conservation 46/ 2001.
  28. ^ Drakaki, E. et al. Evaluation of laser cleaning of ancient Greek, Roman and Byzantine coins, Surface and Interface Analysis, 42(6-7), 671 - 674., 2010.
  29. ^ Saettone, E.A.O., Matta, J.A.S., Alva, W., Chubaci, J.F.O., Fantini, M.C.A., Galvão, R.M.O., Kiyohara, P. and Tabacniks, M.H., 2003. Plasma cleaning and analysis of archaeological artefacts from Sipán. Journal of Physics D: Applied Physics 36: 842-848. Accessed 13.02.2015.
  30. ^ http://www.plasmaconservation.cz/soubory/2012/prednaska-pppt-2012-krcma.ppt Accessed 13.02.2015.
  31. ^ Stambolov, T.; Bleck, R.D.; Eichelmann, N. Korrosion und Konservierung von Kunst und Kulturgut aus Metall, Weimar I/1987., II/1988
  32. ^ http://www.medal-project.eu/11-Copper_conservation.swf[permanent dead link‍]
  33. ^ Schemahanskaya, M.S.; Lemenovskiy, D.A.; Lomonosova, M.V.; Nesmeyanova, A.N.; Brusova, G.P Novie metodi v restavracii archeologicheskogo metala, Vestnik restavracii muzeinih cenostei 1/11, Moscow 2008.
  34. ^ Belkin A.P., Nackiy M.V. Metod obrabotki ochagov "bronzovoi bolezni" mednih splavov sulfidami amoniya//Restavracija pamjatnikov istorii i kulturi/GEL, Informkultura/Ekspres-informacija.Moscow,1987.Bp. 3. -S.6-8

Further reading

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Books

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  • 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.(online)
  • Scott, D.A. Ancient and Historic Metals - Conservation and Scientific Research, Santa Monica 1994.(online)
  • Scott, D.A. Copper and Bronze in Art - Corrosion, Colorants, Conservation, Los Angeles 2002.(online)
  • 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.
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