introduction to and history of metals conservation

An introduction to the history of metals conservation

The Metals Conservation Summer Institute May 27- June 7, 2006, Worcester, MA, USAValentin Boissonnas, HEAA Arc, La Chaux-de-Fonds, Switzerland


1. Traditional techniques and materials



2. The first conservation research centres




3. Chronological survey of concepts and techniques




3. Chronological survey of concepts and techniques • Corrosion, patina, original surface





• Conservation treatments of bronzes





• Conservation treatments of bronzes

• Conservation treatments of iron

Traditional techniques and materials

Traditional products and techniques used on metal artefacts aimeTraditional products and techniques used on metal artefacts aimed d at maintaining their function and preventing their deteriorationat maintaining their function and preventing their deterioration


Traditional products and techniques used on metal artefacts aimeTraditional products and techniques used on metal artefacts aimed d at maintaining their function and preventing their deteriorationat maintaining their function and preventing their deterioration

• Mechanical treatments: Riveting, soldering, polishing

• Chemical treatments: dissolution of corrosion products (vinegar,lemon juice, oxalic acid, etc.)

• Protective coatings: prevention of atmospheric corrosion after cleaning (paints, lacquers, resins, metallic coatings)

Traditional techniques and materials• 1000 BC:

Egyptians use excellent varnishes, heated combinations of oils and natural resins



• Pliny, AD 75:Iron can be protected from rust by means of cerussa (lead oxide), gypsum and vegetable pitch. Bronze can be coated with turpentine and pitch.




• Theophilius, 12th century:To protect iron, boil it in linseed oil and pitch





• Cellini, 17th century:First description of mechanical cleaning of archaeological bronzes with small chisels and hammers






• Traditional recipe for knights armour, 1616:Pour olive oil in a heavy mortar, stir it well with a pestle until it warms itself, put white lead into it and rub it again until it becomes black. Pour a little oil of tartar into it and with Neat’s foot oil or old fat make a salve. Smear armour with the salve.






• Traditional recipe for knights armour, 1616:Pour olive oil in a heavy mortar, stir it well with a pestle until it warms itself, put white lead into it and rub it again until it becomes black. Pour a little oil of tartar into it and with Neat’s foot oil or old fat make a salve. Smear armour with the salve.

• Far East:Lacquers are used well before the Christian area, mainly made from Rhus vernicifera

The first major conservation research centres

Königliches Museum Berlininaugurated in 1888.

Otto Olhausen (1840-1922) establishes the first scientific conservation lab and hires Friedrich Rathgen (1862-1942) as assistant.Rathgen soon becomes head of the research department and will stay there for 40 years doing pioneering work.

‘Die Konservierung von Alterthumsfunden’ (1898)


National Museum of Denmark in Copehagen (1855)Gustav Rosenberg (1878-1941), heads the conservation department.

‘Antiquités en fer et en bronze. Leur transformation et leur conservation’ (1917)

Works mainly with electrolytic reduction techniques to stabilize bronze and iron finds.


The British Museum London

1920 the conservation research department is opened by Dr. Alexander Scott after many objects were damaged because of bad storage conditions during the 1st World War.

‘The Cleaning and Restoration of Museum Exhibits’ (first report 1921, second 1923, third 1926)


The British Museum London

1926 Harold Plenderleith is appointed conservation-scientist and heads the conservation & research department.

‘The Conservation of antiquities and works of art’ (1956, 1971)

Robert Organ, a physicist, joins the research lab in 1951 where he works for 14 years before heading for the Royal Ontario Museum and later for the Smithsonian Institution in Washington.


Columbia UniversityProfessor Colin Fink and his assistant Charles Eldridgeare working at Columbia University in the electro-chemical department. They publish in 1925 a manual on the conservation of bronze antiquities.

The Metropolitan Museum of Art installs its first conservation laboratories at Columbia University following the example of the British Museum.


Rutherford Gettens, chemist and metallurgist, works at the Freer Gallery of Art in Washington on the technology of Chinese bronzes.

Fogg Art Museum in Harward publishes ‘Technical Studies in the Field of the Fine Arts’.

After the war museums scientists will found in 1950 the International Institute for Conservation (IIC).

‘Studies in Conservation’ first appears in 1952


France-Lanord (1915-1993)A founding member of the Laboratoire d’Archéologie des métaux de Nancyin France.

Helps developing major concepts such as the integrity of the object and the location of the ‘primitive surface’ or ‘skin’.

1850-1900 Digging up problems

Schliemann

Schliemann

Milkau

Rathgen

‘During the 1860's it was possible to "excavate" as many as sixty-six gravebarrows from an Iron Age graveyard in Ringerike, Norway, in ten days, amassing thousands of iron objects in need of treatment once removed from the soil(Hansen 2001).’



Boissonnas

CorrosionCorrosion: First interpretations are rather confused. Some call only nicely preserved green surfaces Edelrost or Patina (Krause, Vanino / Seitter), others accept a variety of colours, as long as the surface is uniform and smooth (Hausding).

Various attempts are made to chemically describe the variations in colour from green to blue to red with the burial context and the composition of the artefacts.


CorrosionCorrosion: Many bronzes are excavated from peat bogs and preserve a metallic surface. Others, from lake dwellings, just have calcareous deposits. The acceptance of metallic surfaces and theapplication of stripping techniques might well have been influenced by those early finds from Northern Europe.


Active CorrosionActive Corrosion on on copper alloyscopper alloys is first described by Mond and Cuboni (1893) and then by Frazer and Nicols in 1898. Bacteria discovered on such items lead to the suspicion that they are responsible for this ‘bronze disease’.

Contaminated objects are systematicallysterilized at 120-150°C.

Ellis & Hermanis

Boissonnas

Boissonnas


Cleaning of Cleaning of copper alloyscopper alloys :

The total removal of corrosionproducts is considered important by certain to regain the aesthetically pleasing metallic shine of the metal (Bibra 1869).

Others are less convinced, asmany objects are first stripped and then patinated to satisfy an esthetical look (Petrie 1888).

Van Langh


Mechanical Cleaning of Mechanical Cleaning of copper alloyscopper alloys :

All traditional mechanical methods were in use, such as wire brushes, abrasive papers, grinding tools, hammers, heating and quenching. Fragments were regularly soft soldered as no adhesives existed that had the necessary strength to hold load bearing metal pieces.

Animal glue (fish bladder) was mostly used for smaller parts, drilling and doweling were standard operations.

Boissonnas


Electrochemical cleaning of Electrochemical cleaning of copper alloyscopper alloys :

Finkener (1886) advocates electrolytic reduction in potassium cyanide (Cyankalium) baths and stripping by hydrogen evolution before thorough washing. Removal of all accessible core material! After brushing with wire brush objects are coated with nitrocellulose lacquer in order to ‘give back a metallic look’.

Rathgen


Electrochemical cleaning of copper alloysElectrochemical cleaning of copper alloys:

Axel Krefting (1883), suggests to remove all corrosion layers, preventing further rusting. Bronzes are stripped for 24h by immersion in sodium hydroxide with zinc granules without external electrical source.

Other obscure methods exist at the time, such as dipping coins in liquid lead (Pb), followed by quenching in cold water and leaving over night in milk. Is supposed to give a beautiful olive coloured patina.

Rathgen


Protective coatings for copper alloysProtective coatings for copper alloys:

Protective coatings are already used to protect, consolidate, but also to improve the look of bronzes. Mostly linseed oil, bees wax orshellac.

The invention of cellulose nitrate coatings (Celluloid or Zapon lacquers) is considered a major step in improving surface coatings and is immediately adopted by many conservators.

Paraffin wax will replace bees wax and improve long term protection.

Ashmolean Museum Oxford


Cleaning & stabilization methods for ironCleaning & stabilization methods for iron:

Peter Petersen (pre 1870): boiling in linseed oil and coating with hot beeswax (later replaced with Copal varnish).

Axel Krefting (1883): electrolytic stripping techniques. Coating with paraffin wax. Other coatings such as stearin, shellac, linseed oil are frequently used.

Boissonnas

1900-1920 Towards a better understanding

MallowanBrittner

BrittnerWilliams


Corrosion & mechanical cleaning of copper alloysCorrosion & mechanical cleaning of copper alloys:

1905 Corrosion that contains detail is defined by Rathgen as a noble patina.1917 Rosenberg is the first to talk of an original surface and how it can be replaced by corrosion products. He also suggests that it is possible to find it by mechanical means.

Rathgen


An interesting method is described by Springerwho coats bronzes with hot and liquid animal glue. While drying it shrinks and pulls off mostly external corrosion products.

Mechanical cleaning of copper alloysMechanical cleaning of copper alloys:

Born


Active CorrosionActive Corrosion on copper alloyson copper alloys :

The debate about what influences the corrosion of copper alloys continues. Sometimes even chlorides are attributed to the good preservation of bronze surfaces!

It is Rathgen (1889), Krefting (1892) and then Petrie (1904) who suggest that chlorides are the reason for actively corroding bronzes. From then onwards chlorides are acknowledged by most to be the source of active corrosion.

Conservation treatments now aim at removing chlorides from objects in order to stabilize them. This has priority over the growing concept of preserved information within corrosion products.


Stabilization ofStabilization ofcopper alloyscopper alloys :

Rosenberg advocates a localized stabilization method by using aluminium foil and an Agar Agar / Glycerolelectrolyte to electro-chemically reduce copper chlorides into stable cuprite or tenorite.

Rathgen


Stabilization ofStabilization of ironiron :

1904 William Flinders Petrie uses on-site washing of iron in water, oven drying, waxing.

1905 Rathgen & Finkener propose boiling iron in molten potassium cyanide.

1917 Rosenberg suggests heating iron up to 800 oC, quenching in potassium carbonate. Subsequent boiling in frequent changes of distilled water until no more chlorides. Boiling in paraffin or microcrystalline wax.

Other impregnation methods use water glass, natural rubber, dammar resin, linseed oil, and animal fats.

1920-1950 New concepts and old recipes

CarterBritish Museum

Schorsch British Museum


CorrosionCorrosion:Total removal of corrosion products is being seriously questioned by Rathgen in his reedited book (1924) and Scott (1926), who now considers a corroded surface far more important than a shiny surface of bare metal. With the introduction of the Rosenberg method there is an alternative to stripping actively corroding bronzes.

However, the loss of such surfaces observed with some treatments is still accepted in order to stabilize objects.

Plenderleith


Cleaning copper alloysCleaning copper alloys:

Vigorous chemical cleaning methods are still used such as electrochemical cleaning, boiling in sodium phosphate, Alkaline Rochelle salt and citric andsulphuric acid.

Plenderleith

Born


Stabilizing copper alloysStabilizing copper alloys

Scott (1921) suggests long immersion in sodium sesquicarbonate, sometimes combined with Alkaline Rochelle salt.

Nichols (British Museum 1924) uses localized stabilization treatments such as silver nitrate (later modified to silver oxide, then zinc powder).

Protective coatingsProtective coatingscome from the automobile and aircraft industry.Cellulose acetates and Perspex are replacing traditional natural waxes.

Boi

sson

nas


Stabilizing ironStabilizing iron:

Scott (1921) suggests to use long soaking in sodiumsesquicarbonate. Dammar varnish as surface protection.

In 1933 he tries natural rubber in solution and advocates the use of chlorinated rubber to consolidate fragile metals. Soon abandoned, as unstable and most irreversible.

Boissonnas


Stabilizing ironStabilizing iron:

For both bronzes and iron finds the use of controlled storage in sealed cases with buffering materials (silica gel) are beginning to be used after Rathgens 1924 publication. The use of nitrogen filled storage containers are first used for finds of the Sutton Hoo treasure in 1946.

Rathgen

1950-Today

1950-Today

CorrosionCorrosion:

Increasing awareness of corrosion crusts and the concept of original original surfacesurface. For France-Lanord and Eichhorn preservation of original surface has priority over stabilization.

Organ

But only in 1990ies the original surface is defined as the surface that represents the surface that represents the object when abandonedobject when abandoned. Bertholon (2000)defines the original surface more precisely as limitoslimitos,, that can be located with the help of markersmarkers.

Boissonnas

1950-Today

Electrolytic treatments of copper alloysElectrolytic treatments of copper alloys:

Boissonnas

1956 Plenderleith still recommends the use of electrolytic reduction in sodium hydroxide.

Most unfortunately this harsh method is still used today on some excavations and in labs that lack trainedpersonnel.

1950-Today

Electrolytic treatments of copper alloysElectrolytic treatments of copper alloys:

In the 70ies France-Lanord proposes electrolytic reduction with electrolytic reduction with low currentlow current. Increases removal of chlorides without stripping corrosion products. EDF pushes technique further for treating marine bronzes under cathodic cathodic protectionprotection in sodium sesquicarbonate.This treatment has been adapted as standard treatment for marine bronzes. The measurement of the corrosion potentialcorrosion potential of the object with a reference electrodereference electrode allows a precise and controlled reduction of only un-wanted corrosion products (chlorides).

Pennec

1950-Today

Electrolytic treatments of lead and silver alloysElectrolytic treatments of lead and silver alloys:

Consolidative reduction of very corroded lead also uses a low current, preventing hydrogen bubbling and reducing acidifies corrosion products to metallic lead. In the 90ies potentiostatic reduction helps refining this technique.

After 2nd World War, mineralized silver is also converted into metallic particles by electrolytic reduction.

Arc’Antique

1950-Today

Chemical cleaning of copper alloysChemical cleaning of copper alloys:

Various complexing agents and acids were used, such as Alcaline Rochelle salt, Calgon, ammonium hydroxide, EDTA, sodium hydro-xide, citric acid, formic acid andsulfuric acid.

All these products can etch copper alloys and if not properly used. Most research went into improving these existing recipes and changing to less aggressive chemicals.

Sodium tripolyphosphate (replaced Calgon), EDTA, alcaline dithionite (for chlorinated marine bronzes), sodium dithiolate (for copper sulfates), formic acid.

Boi

sson

nas

1950-Today

Stabilization of copper alloysStabilization of copper alloys:

1967 Madsen introduces Benzotriazole (BTA), a corrosion inhibitor developed for the copper industry. Widely adopted by the conservation community. The most recent studies use it in combination with another corrosion inhibitor, AMT.

Madsen

1950-Today

Protective coatings for copper alloysProtective coatings for copper alloys:

New protective coatings appeared after the 2nd World War such as Frigilene and Ercalene (nitrocellulose lacquers). Bedacryl (butyral methacrylate) and soluble nylon both proved insoluble after a while.

All these polymers were were replaced by acrylic resins such as Paraloid (in US Arcyloid) as well as combined with stabilizing agents (Incralac).

On outdoor bronzes microcrystalline waxes are still used today.

J. Scott

1950-Today

Cleaning of ironCleaning of iron

The introduction and adaptation of air abrasive cleaning with various abrasive powders (aluminium oxide, sodium bicarbonate, walnut shells etc.) improved cleaning techniques tremendously.

X-ray radiography and tomography greatly improved assessing and cleaning of corroded iron.

Boissonnas

1950-Today

Stabilization of ironStabilization of iron

1964 Hydrogen reduction is first used for the conservation of iron finds from the Vasa. These heat treatment involves temperatures of up to 850°C.

Pearson

Successful for stabilization of cast iron artefacts, but much metallurgical information is destroyed.Replaced by controlled electro-lytic reduction techniques.

Pearson

1950-Today


Hydrogen gas plasma emerges in the 80ies as the ideal treatment for large amounts of artefacts. The created hydrogen plasma is highly reactive and reduces iron oxides.

Proved not effective enough to reduce allchlorides.Today it is still used in some labs as a pre-treatment before desalination and mechanical cleaning.

Boissonnas

1950-Today


1984 EDF sets up a research programme advocating electrolytic techniques. Cast and wrought iron is treated, but also non-conducting materials such as leather, wood, paper or porcelain.

1987 The Titanic is discovered and 1800 items are conserved at EDF using innovative electrolytic techniques.

Lacoudre

1950-Today


Electrolytic techniques have greatly improved and the use of reference electrodes has given us safe tools to work with. They are currently also used in situ to monitor and stabilize submerged iron artefacts.

Lacoudre

Arc’Antique

1950-Today


A new desalination method using alcaline sulfite baths was introduced in the late 70ies and has been widely adapted for chloride contaminated archaeological iron.

This method breaks down insoluble chloride containing products, such as akaganéite.

Boissonnas

1950-Today

Adhesives & protective coatings for ironAdhesives & protective coatings for iron

After 2nd World War, epoxy resins are widely used in conservation of all materials due to their strength and durability. They replace traditional soldering methods that often damaged the patina and were highly irreversible. However, epoxy resins once cured, cannot be dissolved.

Much iron was consolidated and glued with epoxies such as Araldite. Today we struggle with such objects that have not undergone desalination, as they continue to break up, but cannot be retreated.

Lacquers are still in use, mostly acrylic polymers such as Paraloid (Acryloid) for indoor iron artefacts. Outdoors epoxy resins or polyurethane resins are used for their durability.

1950-Today

Preventive conservation of metalsPreventive conservation of metals

1950-Today


Dessicants and barrier films: Silica gel, ESCAL foils

Wallert

1950-Today



Oxygen scavengers: Ageless, RP System

Boi

sson

nas

Wallert

1950-Today




Pollution scavengers:Activated charcoal, impregnated textiles,

Wallert

1950-Today




Pollution scavengers:Activated charcoal, impregnated textiles,

Pollutant identification:Oddy test, Beilstein test, ….. Boissonnas

Tomorrow?

Tomorrow?

• Achieve better understanding of corrosion mechanisms

Selwyn

Boissonnas

Tomorrow?


• Achieve better understanding of current treatments, develop new and safer methods

Boissonnas

Tomorrow?



• Deal with an ever increasing mass of previously conserved and freshly excavated objects

Boi

sson

nas

Tomorrow?



• Deal with an ever increasing mass of previously conserved and freshly excavated objects

• Improve standards and facilitate access to education and training in conservation

Boi

sson

nas

BibliographyBertholon R. 2000 La limite de la surface dÕorigine des objets mˇ talliques archˇologi ques : caractˇris ation, localisation et approche des mˇcani smes de conservation. Th¸se de doctorat, discipline archˇ ologie, prˇse ntˇe etsoutenue publiquement � Paris, Universitˇ de Paris -1 Panthˇo n Sorbonne, UFR dÕart et archˇ ologie. Born, H. 1993 Restaurierung antiker Bronzewaffen, Band II Sammlung Guttmann. Verlag Philipp von Zabern, Berlin Bradley, W & Burns, R., 1955. Protective coatings for metals. Third Edition 1955. New York: Reinhold Publishing Corporation. Evans, U.R. 1951 The Corrosion Situation: Past, Present and Future. In: Chemistry and Industry, Weekly publication of the Society of Chemical Industry London, Nr. 34, p. 706-711 Gilberg, M., 1987. Friedrich Rathgen: The father of modern archaeological conservation. AIC Journal vol 26. No. 2. 1987. The American Institute for Conservation of Historic and Artistic Works. Gilberg, M. 1988 The History of Bronze Disease and its Treatment. In: Early Advances in Conservation (V. Daniels Ed.), Dept. of Conservation, British Museum Occasional Paper No. 65, p. 59-70 Ihrig, B. 1992 Theorien und Konzepte zur Bronzekonservierung von 1860 bis zum Beginn der 40er Jahre. Diplomarbeit am Institut f�r Technologie der Malerei, Staatlichen Akademie der Bildenden K�nste Stuttgart. Jakobson, T., 1984. Iron conservation at the National Museum of Copenhagen - past and present. In: Preprints of the International Counsil of Museums Committee for Conservation. 7th Triennial Meeting Copenhagen 1984. London: James & James. Jakobson, T., 1988. Iron Corrosion Theories and the Conservation of Archaeological Iron Objects in the 19th Century with an Emphasis on Scandinavian and German Sources. In: Early advances in Conservation. British Museum occasional paper No 65. London: British Museum. Krause, E. 1882 Ein neues Verfahren zur Konservierung von Eisen-Alterth�mer. In: Zeitschrift f�r Ethnologie der Berliner gesellschaft f�r Antropologie, Ethnologie und Urgeschichte, Berlin Verhandlungen 8, p. 533 Krefting, A. 1892 Om nogle Metallers Oxydation ved noitrale Saltes Medvirken, Videnskabs-Selskabet, Kristiania, Vorhandlinger 16, p. 1-11 Brinch Madsen,H., 1987. Artefacts Conservation in Denmark at the Beginning of the Last Century. In: Recent Advances in the Conservation and Analysis of Artifacts. Preprints of the Jubilee conservation conference Papers. London: Summer school press. Oddy, W.A. 1993 The history of and prospects for the conservation of metals in Europe. In: Current problems in the conservation of metal antiquities. International Symposium on the Conservation and Restoration of Cultural Property, Tokyo, Oct. 1989. Tokyo National Research Institute of Cultural Properties, Japan. p. 1-26 Organ, R.M. 1963 Aspects of bronze patina and its treatment. Studies in Conservation 8, 1-9 Petrie, F. 1888 The treatment of small Antiquities. In: Archaeological Journal 45, p. 85-89

Plenderleith, H. 1956. The conservation of Antiquities and Works of Art. Treatment, Repair and R estoration. London: Oxford University Press. Rathgen, F. 1889 Uber eine neue Anwendung des elektrischen Strohmes zur Conservierung antiker Bronzen. In:. Prometheus, Illustrierte Wochenschrift �ber die Fortschritte in Gewerbe, Industrie und Wissenschaft, Berlin/Leipzig, Jg. I, Nr. 13, p.196-198 Rathgen, F. 1902 Zapon und seine Verwendung zur Conservierung von Sammlungsgegenst�nde. In: Prometheus, Illustrierte Wochenschrift �ber die Fortschritte in Gewerbe, Industrie und Wissenschaft, Berlin/Leipzig, Jg. XV, p.485-487 Rathgen, F. 1915 Die Konservierung von Altertumsfunden I. Teil; Stein und steinartige Stoffe, 2. Auflage, Berlin Rathgen, F. 1924 Die Konservierung von Altertumsfunden II & III. Teil; Metalle und Metalllegierungen, organische Stoffe, 2. Umgearbeitete Auflage, Berlin/Leipzig Rhousopoulos, O.A. 1911 †ber di e Reinigung und Aufbewahrung der Altertumsfunde. In: Neue Museumskunde, Theorie & Praxis der Museumsarbeit, Berlin Jg 7, Heft 2, p. 95 Riederer, J 1992 Zur historischen Entwicklung der Kenntnis von Korrosionsprodukten auf kulturgeschichtlichen Objekten aus Kupferlegierungen. Berliner Beitrage zur Archaometrie, Band 11, p. 93-111 Rosenberg, G.A., 1917 Antiquitˇ s en Fer et en Bronze. Leur Transformation et Leur Conservation. Copenhague: Gyldendalske boghandels sortiment. Scott, A. 1926 The Cleaning and Restoration of Museums Exhibits. Department of Scientific and Industrial Research, Third report upon Investigations conducted at The British Museum, London Scott, A. 1932 The Romance of Museum Restauration. In: Journal of the Royal Society of Arts, Vol. 80, No. 41, p. 498-499 Scott, A. 1933 Deterioration and Restauration, with Especial Reference to Metallic Exhibits, The Museum Journal, Vol. 33, p. 5-8 Scott, D. A. 2002 Copper and Bronze in Art; Corrosion, Colorants, Conservation. Getty Publications, Los Angeles Setlik, b. 1903 †b er nat�rliche und k�nstliche Patina, sowie Reinigung und Konservierung von Alterth�mern. Chemiker-Zeitung, Heidelberg, Nr. 38, p. 454-455 Stambolov. T. 1985 The Corrosion and Conservation of Metallic Antiquities and Works of Art. Amsterdam: Central Research Laboratory for Objects of Art and Science. Vanino I. & Seitter E. 1903 Die Patina. Ihre nat�rliche und k�nstliche Bildung auf Kupfer, M. HartlebenÕs Verlag, Wien, Pest, Leipzig

introduction to and history of metals conservation

Documents