Museum is a non-profit, permanent, institution in the service of society and of its development, open to the public which acquires, conserve, researches, communicates and exhibits for purpose of study, education and enjoyment, the tangible and intangible evidence of people and their environment (ICOM 2006). Museums are classified based on the types of object collection, the agent(s) who run the museum, the area the museums serve, the audience they serve and the way the museums exhibit their collections.
Museum’s collections include pottery, terracotta, wooden, leather, textiles, metals, ceramics and glass etc. However this piece will be focussing on metallic collections and how it can be preserved from corrosion as a major agent of deterioration in metallic objects. A variety of metals, metal alloys and metal finishes can be found in museum collections. Metals share many useful properties; namely, they are strong, are often lustrous, are good conductors of heat and electricity and they can be cast. As a group, metals share some common and unique properties. One such property is that, generally, they are susceptible to corrosion (gold and other noble metals being the exception). Each type of metal has its own degree of vulnerability to corrosion. In museum collections, the chief ones encountered in any quantity are aluminium, copper, gold, iron, lead, nickel, silver, tin and zinc.
Metal is a solid material which is typically hard, shiny, malleable, fusible and ductile with good electrical “a thermal conductivity” e,g. Iron, gold, silver, aluminium and alloys such as steel)
Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, carbonate or sulphide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment.
The presence of salts dissolved in the water can greatly accelerate corrosion. Among the most destructive are chloride salts, which, unfortunately, are also very prevalent. They are found, for example, in fingerprints left on a metal surface after manipulating it with bare hands. These salts are highly soluble, and it is extremely difficult, if not impossible, to render metal stable in their presence. Copper can combine with chloride ions to form certain corrosion products that are unstable in the presence of moisture. For iron contaminated with chloride salts, any moisture will cause a continuing chain of corrosion until the metal is consumed.
Metal objects need to be protected primarily from water and high RH as well as from pollutants and contaminants. These agents of deterioration can often be controlled together. Preventive conservation measures focus mainly on keeping the objects in as dry an environment as possible to prevent outbreaks of corrosion and keeping the air pollution-free and the metal surfaces free of contaminants. Identifying priority objects allows a multi-level preservation approach ranging from overall controls to object-specific microenvironments. Other agents of deterioration that affect metal objects are also reviewed.
Pollutants and contaminants: these include dust, fingerprints, salts, fatty acids, polish residues. Dust settling on metal objects can contain salts and particulates that are hygroscopic and cause local corrosion. In addition, the dust can obscure decorative details and act as an abrasive against a polished surface if rubbed off. Fingerprints and perspiration (or sweat) from hands contain salts, oils, moisture, etc., that easily transfer onto metal surfaces and can lead to noticeable corrosion, especially on polished metal surfaces.
A common problem observed on museum objects made of copper and copper alloys is a characteristic pale green corrosion caused by the reaction between the metal and polish residues.
Avoid all potential sources of water: water may come from burst pipes, melting ice, leaks because of heavy rain and wind, floods or condensation, raise objects off floors. In the case of heavier items, this can incur the use of lifting gear and specially constructed pallets, cover the top of storage shelves with a waterproof dust sheet (e.g. clear polyethylene plastic sheet or non-woven polyethylene sheet),enclose small metal objects in archival quality bags or water-resistant boxes (consult Enclosures) and avoid storing or displaying metal objects in areas where there is a risk of water leaking (e.g. near water pipes).
Others keep relative humidity levels moderate and avoid damp. The priority for metals is to avoid exposing them to high RH (damp, or RH >65%). A moderate RH range (40–55%), Eliminating all risks of high RH will benefit the whole collection and will improve the efficiency of localised measures that will need to be put in place for more sensitive metals, helping to keep their low RH microenvironments stable and effective for longer periods.
Temperature has a large impact on RH; a rapid decrease in the external temperature can cause the indoor RH to increase, which is conducive to corrosion.
Basic control strategies include the following: Keeping objects away from colder areas in a room (e.g. exterior walls with windows) because these will cause localised, damp pockets of air and result in higher corrosion risks for metal objects, improving building insulation to eliminate temperature gradients and cold spots, providing for air circulation, as this may prevent local concentrations of high humidity enough to at least slow down the rate of corrosion, monitoring the environment to identify cold or damp spots. Equip the area in which metals are stored and displayed with RH and temperature control monitors (data-loggers, hygrothermographs, RH indicator strips, etc.).
The following agents are risks to all objects in collections, including metal objects. Consult agents of deterioration for a broader selection of strategies that can be applied for each specific agent.
Physical forces – Physical forces can damage metals, causing dents, distortions and even breakage if excessive forces are applied. Archaeological metals are often brittle due to the deterioration they have undergone during burial, so they can be prone to cracks and fractures.
Preventive conservation strategies to prevent physical damage include:
Identify metal objects most at risk of physical damage and focus preventive actions on these, provide containers and padding as appropriate for the fragility of the objects. If several objects are placed together within a box, a tray or a drawer, separate them with padding to prevent their contact and the risk of nicks and scratches when they are moved or when the drawer is opened. Other options include using custom-made carved nests or immobilizing objects onto a padded surface using twill-tape ties.
Provide mounts or bases for objects that are unstable, unbalanced or top-heavy (objects with a high centre of gravity), handle objects in their boxes and storage trays carefully using appropriate techniques and equipment.
Thieves and vandals – Precious metals and valuable items (jewellery, coins and collectibles) are at risk of theft and so require enhanced measures to prevent this.
Dissociation – Small metal parts, including functional items such as screws or nuts and bolts, may sometimes be loose or insecurely attached to an object. When the object is handled or moved, these parts could be at risk of detaching and possibly becoming dissociated from the object.
Fire – Low melting point alloys—such as pewter or lead-tin solders—are especially at risk in case of a fire.
Incorrect temperature – Higher temperatures increase the rate of chemical reactions, including corrosion reactions (if other factors fostering corrosion are also present such as high RH). For example, for a given RH, iron will corrode twice as fast at 29°C than at 18°C.
As noted under Keep relative humidity levels moderate and avoid damp, temperature has a large impact on RH and needs to be controlled in order to properly manage RH levels thereby preventing corrosion and to prolong the life span of metals not just for the present but for its future utilization.
*Ogunbiyi is Principal Technical Officer (Conservation), National Museum of Unity, Ibadan.
Expectedly, the statue featuring a muscular, bare-chested father figure sweeping a scantily clad woman, revealing part of her breasts and thighs, behind him and holding aloft a nude child was seen as having contravened Islamic teachings by presenting the human form as an object of worship. “Look at the woman — half her body is uncovered. You can see her breasts and her bare thighs. That’s not good for a Muslim,” she says. “Really, this is not a good example for Africa — especially the way they’re dressed. The father, you can see his body. The child is completely naked. There’s too much nudity. The woman should be wearing something more proper, to show how Africans really dress,” a critic said of the monument.