Conservation and restoration of metals is the activity devoted to the protection and preservation of historical (religious, artistic, technical and ethnographic) and archaeological objects made partly or entirely of metal. In it are included all activities aimed at preventing or slowing deterioration of items, as well as improving accessibility and readability of the objects of cultural heritage. Despite the fact that metals are generally considered as relatively permanent and stable materials, in contact with the environment they deteriorate gradually, some faster and some much slower. This applies especially to archaeological finds.
Metals and the agents of deterioration
An essential cause of deterioration is corrosion of metal objects or object deterioration by interaction with the environment. As the most influential factors of deterioration of historical objects should be pointed out as the relative humidity and air pollution while in archaeological objects a crucial role has composition, depth, humidity and amount of gasses in the soil. In cases of marine or fresh water finds the most important factors of decay are the amount and composition of soluble salts, water depth, amount of dissolved gases, the direction of water currents and the role of both microscopic and macroscopic living organisms. Deterioration of materials associated with metals Associated materials deteriorate depending on the origin whether they are organic or inorganic materials. Organic materials usually fail in a relatively short period of time, primarily due to biodegradation. With inorganic materials are these processes considerably longer and more complex. Amount of gases, humidity, depth and composition of soil are very important. In case of salty and sweet water finds essential are amount of gases dissolved in water, depth of water, direction of currents, and microscopic and macroscopic living organisms.
Agents of deterioration Physical Force Physical force is one of the most common means of damage to metal objects, which "are considered to be strong and resilient though exhibit weakness and brittleness under certain conditions." This includes breakage, dents, and scratches which occur in accidents, improper storage and mounting, mishandling, and over-polishing. Fire Low melting point alloys, such as pewter or lead-tin, are at risk of damage due to fire, though other metals are not at risk from the fire itself, but the pollutants caused by smoke. Water Contact with water, or a complete immersion in water, will lead to some degree of corrosion. The more oxygenated the water is, or the higher quantity of salt present in the water, will cause a more rapid and aggressive corrosion to metal. Short term exposure to water "can result in rapid surface corrosion, such as when flash rusting occurs on iron or steel objects that have been even momentarily wetted." Iron and steel are most affected by water. Pollutants Atmospheric pollutants are one of the more common agents of deterioration for metals, with tin and tin alloys most affected and resulting in corrosion. Most common pollutants include dirt, soot, dust, and chemicals. Fingerprints, salts, fatty acids, and polish residues can also cause corrosion. Incorrect temperature and relative humidity Higher temperatures increase the rate of chemical reactions and corrosion. Temperature also affects relative humidity, thus should be monitored and controlled. The higher the relative humidity (65% and above), the higher the risk of corrosion. Lead is the least affected by high humidity.
Metals conservation planning As with the conservation and restoration works on any other material, here are the basic tenets of conservation-restoration based on the quality of execution and the best possible preservation of cultural, historical and technological identity and integrity of objects. Minimal intervention, reversibility and repeatability of preferred treatment are essential, as well as the possibility of easy identification of restored parts. Recently, the non-toxic nature of materials and procedures used in conservation has become important too, both in relation to objects and conservator-restorer as a performer, but also in relation to the environment.
Research
Nowadays, scientific research is an integral part of conservation treatment of metals, in which different scientific methods and techniques help in determining what should be done in the preservation and care of the object. Conservators investigate the materials and techniques used in the creation of an object to better understand and diagnose an object's condition and make plans for effective treatment.
Identification of metals and alloys
Simple methods – visual examination, spot tests, specific gravity Scientific methods – X-ray fluorescence, X-ray diffraction (XRD), Particle-induced X-ray emission, LIBS, SEM, electrochemical techniques, metallography
Identification of corrosion processes and products Simple method – visual examination, spot tests The Oddy test – for copper, silver, and lead Scientific methods – X-ray Diffraction, SEM, metallography
Identification of materials associated with metals Simple methods – visual examination, spot tests, specific gravity Scientific methods – X-ray fluorescence, chromatography, Raman spectroscopy
Identification of technology used to produce objects Simple methods – visual examination Scientific methods – metallography, x-ray radiography, x-ray computed tomography
Decision making In preparing the strategy of a metals conservation project, an interdisciplinary approach is essential. This implies the participation and close collaboration between as many experts as is possible. At minimum, the curator (archaeologist, historian, or art historian), a scientist specialized in the corrosion of metallic objects of cultural heritage, and the conservator or restorer should be involved in the project.
Documentation Systematic and well-managed documentation is an essential prerequisite for quality conservation and restoration treatment as "it is no longer considered acceptable to undertake a conservation treatment without recording the object and the intervention." Conservation documentation should include the state of the object's condition before, during, and after treatment. Any technique used to assess the object's condition should be documented as well. Conservation "documentation can also be viewed as a 'surrogate object', and can therefore form part of preventive conservation strategies intended to enhance access to information while reducing the handling of objects." Although documentation requirements differ across institutions, most records follow the same general format, including:
Object data, such as location, ownership, and accession records Progress data, such as the date the object was received for treatment and when treatment is completed Technical data, such as examination results and analysis Object condition and treatment data, including materials and equipment used in treatment Recommendations, such as advice for storage and display or reexamination and further care of the object References, photographs, and diagrams
Ethics and ethical problems in metals conservation The ethical concept of conservation of metal objects in principle is the same as those in other fields of conservation-restoration of cultural heritage.
However, there are several specific problems that can only be found in the conservation of metals; problems of heat treatment of archaeological objects, and the problem of radical restoration of historic, technical, and architectonic objects too.
While the first case problem is primarily in the destruction of valuable scientific data, the problems in the case of technical, architectural, and historical objects are that radically restored items only simulate the original appearance of the object, thus that object can be considered more or less fake, which only superficially simulate long-lost or the never existing state of an object. Whenever possible the preservation of real historical substance is preferred.
Ethical problems connected with conservation of sacred metallic heritage objects can be included too.Thompson, J.C. (1998) On restoring sacred objects conservation-us.org
Preventive conservation
Preventive conservation, also known as collections care or risk management, encompasses all actions taken to prolong the life of an object."NPS (2012) National Park Service: Museum Handbook. Chapter 8: p.8:2 "Conservation Treatment" Retrieved 24 April 2020 and is an important element of museum policy. Members of the museum profession are entrusted to create and maintain a protective environment for the collections in their care. A good preventive conservation program minimizes the need for conservation treatment by blocking, avoiding, or minimizing the agents of deterioration.NPS (2012) National Park Service: Museum Handbook. Part One. Chapter 3 p.3:2 "Preservation: Getting Started" Retrieved 24 April 2020 Emergency planning, environmental safeguards and monitoring are all types of preventive conservation. Scientific research continues to discover new ways of safeguarding collections. Today various monitoring devices assist in the observation of changes in the Agents of Deterioration and other changes that may assist in diagnosing destructive activity before it is a disaster. In the image to the right a device is being attached to the Liberty Bell to monitor any changes in the crack. Metallic heritage objects are sensitive to environmental conditions such as exposure to light and ultraviolet light, temperature, relative humidity, water and moisture, and various pollutants especially chloride salts. Safeguards of protection against threats of natural disasters such as flood or fire need to be planned for and maintaining an environment that keep all Agents of Deterioration within safe limits and controlling their fluctuation will assist in the preservation of metals.
Whether in storage, on display, or in transit metals are best preserved in a "moderate climate that avoids extreme temperature and RH fluctuations and that excludes daylight and/or filters out ultraviolet light and infrared radiation and air pollution provides the appropriate environment for collection preservation."NPS, "Museum Collections Environments", NPS Handbook, Part 1, 4:9 A controlled environment can protect metals from polluted air, dust, ultraviolet radiation, and excessive relative humidity – ideal values are temperature of 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 are best stored in rooms (or plastic boxes) with very low relative humidity, except if they come from a bog or high water environment, in which case the right equilibrium with the environment needs to be found. Particularly valuable items can be placed in sealed micro-climate containers with an inert gas such as nitrogen or argon. Metals with active corrosion fare better with lower relative humidity: copper or copper alloy objects up to 35% RH and iron objects 12–15% RH.