Metals on Cenotaph / Monuments

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A variety of metals have been used in the fabrication of Cenotaph/Monuments including: iron, tin, copper, zinc, aluminium, lead, with the most common being bronze. Typically, metals are featured as decorative elements such as plaques, sculpture, or lettering. Bronze is an alloy or fusion of the ores, tin and copper. Wrought iron, cast iron, and steel tend to be simple to identify, but alloys are more complicated, and their identification may require assistance from a conservator or conservation professional.

Most metals are chemically unstable, reacting with the atmosphere to form oxides over varying periods of time. Some metals, such as aluminum and stainless steel, form a protective oxide coating on their surface which cannot be penetrated by further oxygen molecules, permitting them to retain their shiny appearance with little or no maintenance for years. However, most metals require a coating or anodizing to prevent corrosion.

The long-term performance of metal components will depend on the their physical and chemical properties, the climate to which they are exposed, design details, and their proximity to other metallic and non-metallic components. In order to correct damage to a metal component, the cause of its deterioration must be understood. If the properties of the metal are not understood, inappropriate treatment may result in an adverse reaction that causes further deterioration. Generally, metal components tend to be durable, but components that are not suited for a particular location and function, or not receiving adequate maintenance, may become fragile.

The following describes typical forms of metal deterioration:

Corrosion occurs in most metals in reaction to oxygen, water, and other elements in the environment. As metals corrode, a chemical compound forms on their surfaces creating first a film, and later a crust. The expansion caused by the formation of this crust can break masonry where metal is bolted or embedded. In certain cases where the film of corrosion is stable and uniform, it actually forms a finish called a patina that protects the metal. Bronze statuary will turn a reddish-brown, but in the presence of pollutants, the surfaces may turn green and/or black. The end colour and period of time it takes the change to occur will depend on the atmosphere and the chemical pollutants present. Artificial patinas may be applied to metals for aesthetic reasons to achieve a particular colour.

Selective corrosion take place when only some of the metals in an alloy begin to corrode. A common example is when the zinc from a brass alloy corrodes, leaving white patches on the surface of the metal.

Galvanic corrosion occurs when dissimilar metals are in contact with each other causing an electrochemical reaction. The metal that is less resistant to corrosion will deteriorate at a faster rate. In the case of a bronze-clad aluminium door, the aluminium components will break down from corrosion while the bronze will remain intact.

Erosion or abrasion is the physical process whereby metal slowly wears away. This can be caused by natural factors such as water, wind or wind-driven sand, or by repeated human actions such as pushing doors open, or by acts of vandalism.

Plastic deformation or creep often affects lead components subjected to loads or high temperatures. This condition can occur with heavy statuary that sags under its own weight. Lead lettering, which is the most common use of lead on Cenotaphs, will not be affected by this problem.

Casting flaws in ironwork can be mistaken for corrosion. These flaws usually take the form of bubbles, holes, or cinders. Cracks may also form due to uneven cooling or flaws in the molten metal pour. The resulting flaws may become corrosion sites.

Cracks in metal can occur for a number of reasons: expansion of ice trapped between sections of metal; flaws in metal castings or extrusions; metal fatigue; no provision for thermal expansion and contraction of metal; and accidental impact or vandalism. How these cracks are repaired will depend largely on the cause of the damage and the type of metal. Some metals can be welded or brazed, while others, such as cast iron, have limited repair options.

Additional information can be found in the following sources:

Gayle, Margot, Look, David W., and Waite, John G. 1992. Metals in America's Historic Buildings. Uses and Preservation Treatments. U.S. Department of the Interior, National Park Service, Cultural Resources, Preservation Assistance. Washington, D.C.

Anson-Cartwright, Tamara. 1997. Landscapes of Memories: a guide for conserving historic cemeteries, repairing tombstones. Toronto: Ministry of Citizenship, Culture and Recreation. ISBN 0-7778-6339-1

Weaver, Martin E. 1993. Conserving Buildings: A Guide to Techniques and Materials. New York: John Wiley and Sons, Inc. ISBN 0-471-50945-0

Ashurst, John, Ashurst, Nicola, Wallis, Geoff, and Toner, Dennis. 1988. Metals (Practical Building Conservation, English Heritage Technical Handbook, Vol. 4). UK: Gower Technical Press. ISBN: 0470211075