Metallic elements compose the largest proportion of materials that make up our planet. Metallic ores are refined by the application of energy to produce metals. The physical properties of metal include luster, hardness, strength, malleability and temperature sensitivity.
Different metals exhibit different physical properties. Historically, these various properties have been exploited in the construction and fabrication of metal objects and structures. Metals are frequently selected for applications in architecture, decorative arts, fine arts, and functional objects.
Among the metals found in museum objects are gold, silver, copper, tin, iron, lead, zinc, nickel, and aluminum. Metals are frequently combined in order to modify their properties or to obtain a metal that is most suitable to a given application. The process of combining two or more metals is termed alloying.
An alloy is created by melting one metal and then adding another metal to it. An alloy is a solid solution since the metal-elements remain distinct, one suspended in the other. Examples of common alloys are brass (a mixture of copper and zinc), bronze (a mixture of copper and tin and other metals), and sterling silver (a mixture of silver and copper). By varying the proportions of copper and zinc in brass alloys it is possible to obtain a range of brass alloys with differing properties including colour, strength, corrosion resistance, or working properties.
Surface treatment can be an important characteristic of a metal object. Frequently, metals are finished to enhance the appearance of the object, to improve the functional performance (for example, corrosion resistance), or a combination of both. Surface treatments include polishing, plating, patination, coatings, heat treatments, and chemical coloring treatments.
Proper identification of metals is important to housekeeping procedures, accurate interpretive program information, storage and exhibit techniques, climate control, handling methods, and the conservation treatment process.
You can identify metals by color, weight, function, magnetic properties,, or hardness. If you are unsure of the metal object’s identification, consult a conservator or your regional/curator. To identify a metal object properly, you need to have a basic knowledge of the properties of metals and have a few tools and supplies. If you cannot identify a metal, it is better to use more generic terms to describe it. A metal is a white metal instead of running the risk of misidentifying it as pewter, silver, or nickel. Chemical spot tests or spark tests should not be used to identify metals. This can damage or destroy the object.
Corrosion is the chemical or electrochemical reaction between a metal and its environment that produces deterioration of the material and its properties. It is the most commonly occurring problem of metal objects. Corrosion can take many forms depending on the metal(s) that compose the object, the manner in which they are fabricated, or the environment in which they are exhibited or stored.
At times, the corrosion source may be from fabrication in the case of stress corrosion cracking in thin turned brass objects or contamination from solder flux in joined metals. In other instances, the conditions responsible for corrosion may be an integral part of the environment, such as a seacoast or polluted urban air.
The presence of a corrosion product on a metal object does not necessarily indicate active corrosion. Corrosion, tarnish, and patination are all examples of metal compounds, some of which are unintentional and considered disfiguring, while others are deliberately created for an aesthetic effect. While some forms of corrosion are more protective and stable than others, & most all are subject to failure at some critical relative humidity level or in the presence of certain pollutants. When metals are cleaned the surface can oxidize and be chemically stable.
Active corrosion can be identified visually by the following characteristics:
Colour (wet or dry in appearance); powdery or flaky formations on the surface of the object and similar deposits around the base of the object; loosely adhering formations, frequently appearing in patches; surface as opposed to uniform, dense, well-adhered deposits, and continuing change or growth
Surface characteristics and colours of metals vary by alloy. Corrosion products also vary in colour, depending on the alloy and cause of corrosion.
Note! Lead acetate corrosion is a severe poison that can be fatal if swallowed, inhaled, or absorbed through the skin. If you see white, crystalline corrosion products on lead objects in your collection, assume that they are lead acetate and handle accordingly. Material Safety Data Sheets uniformly state that protective equipment for lead acetate should include goggles, lab coat, vent hood, and rubber or plastic gloves. This type of corrosion is often seen on lead bullets and toy soldiers.
Keep relative humidity in metal collections as low as possible. Steels will not rust and brass will not tarnish below 15% RH. This is not a practical solution for metal objects in the historic furnished structure, but it may be for objects in storage cabinets or exhibit cases. Ambient temperatures between 60° and 75°F are appropriate for most metals. To inhibit active corrosion in salt air environments, metals should be housed in spaces with relative humidity levels no greater than 35%.
It is generally a good idea to avoid low temperatures for most metal objects. Low temperatures usually result in higher levels of relative humidity and the possibility of condensation on metal surfaces.
Many forms of air pollution are corrosive. Polluted urban air and coastal environments are among the more severe areas. Dirt and dust may contain chemical compounds that will react with metals or trap moisture close to the metal surface. Sulfur and sulfur compounds are probably the strongest tarnishing agents. Sulfur is present in the air from burning of fossil fuels and is generated from products such as foam rubber, carpet padding, paints, wool, and felt.
Cleaning and polishing remove original metal. Over-cleaning often results from a desire to have metals bright and shiny, especially brass and silver objects on display in a historic furnished structure. Intense treatment often results in the loss of information from the object. In addition, metal cleaners may leave harmful chemical residues that can generate further corrosion.
Lamidi is the chief conservator, National Museum, Benin City
