Conservation of outdoor metal

While many metal types can be found in modern architectural applications, such as aluminium or even titanium, the most commonly used metals are lead and alloys of copper and iron. The masonry of many older buildings is often held together with iron cramps and lead is used for roofs and rainwater goods. Other features may include iron railings, glazing bars or 19th century mausoleum doors.
In terms of monumental sculptures, however, bronze is the favoured metal alloy, although lead, spelter, cast iron and sheet steel are also found.

Decay

Water is related to all the different types of decay processes that are found in metal. Chemical decay occurs naturally but at a greater rate in a polluted environment. Thermal stress, inherent defects and inappropriate intervention procedures, such as shotblasting also contribute to deterioration. Extensive organic growth or soiling are a result of neglect and can cause damage. Conservation will usually involve cleaning, stabilisation and repair. Cleaning is undertaken as soiling can hide decay, open joints, fractures or other damage. Cleaning procedures can be either chemical or mechanical and should aim to retain the ‘patina’, the chemically altered layer that is only microns thick, of the original surface.
Stabilisation is a procedure used to slow down or halt decay. It may simply involve moving an object from an unsuitable environment where this is possible or it may involve the application of a treatment directly to exposed metal.
Many metals are chemically unstable and unless protected, will revert to a more stable state by combining with oxygen, sulphur, chlorine and carbon in the process known as corrosion. At a very simple level, the requirements for corrosion are the presence of water and oxygen. Most massive metalwork is produced in sections which have then been jointed together. Joints are frequently points of corrosion attack. Hollow castings usually have an internal support structure, or armature, made up from iron, and this will corrode preferentially to the main body of metal if water is allowed in. Another feature which can greatly accelerate corrosion problems is casting flaws. Major flaws in hollow castings may permit the passage of water into the interior, thus allowing attack internally as well as on the outer surfaces.
Corrosion may result in slight discolouration through to the failure of a structural member. The situation may be complicated by the effect of two dissimilar metals being jointed together, which will accelerate the rate of decay. Atmospheric pollution will also accelerate deterioration. A state of equilibrium can occur when a metal will corrode no further and the thin layer of corrosion that has been produced at the surface, the patina, protects the underlying metal from further attack.

Treatment

When considering how to treat corroded metal it must always be remembered that what is now corrosion product was once metal. The key concept is stability and how to achieve this while maintaining respect for the original condition. Techniques for corrosion removal can involve the use of reagents specific for particular compounds or groups of compounds, or by mechanical means. These techniques should not be carried out by untrained personnel.
Once corrosion products have been removed, it is necessary to ensure the removal of any remaining soluble materials by washing in deionised water. Minor repairs can be carried out using a variety of materials. The final objective is to maintain as far as possible the stability of the metal while providing an acceptable surface finish. Additional physical and chemical protection can be applied in the form of specialist lacquers containing corrosion inhibitors. To enhance the weathering capabilities of the metal, at least two applications of microcrystalline wax which will dry to a hard and water-resistant finish are recommended. Wax finishing of ferrous (iron-based) metals is also highly desirable for both protective and aesthetic reasons. In either case, waxing aids maintenance as everyday airborne dirt and grime can be readily washed off with clean water and a minimum of effort.

Consulting a conservator

The decay of monumental metal is a complex area to which prescriptive rules do not apply. It is therefore important to get specialist advice. This need not be too detailed but an experienced metals conservator will be able to:
• Identify the metal type
• Ascertain what decay mechanism is going on and why it has happened
• Recommend and carry out preventive and remedial works as necessary
• Provide advice on future care, protection and maintenance
The materials and processes used at each of these stages will depend on the type of metal and the context - it is important that they are compatible and correct for each individual situation. Such work should be carried out using the skill and experience of a conservator.

Acknowledgments: Jason Ellis and Claudia Koehler
 

This article offers general guidance and is not intended to be a substitute for the professional advice of an accredited conservator. The views expressed are those of the author or authors, and do not necessarily represent the views of IPCRA. IPCRA accepts no liability for any loss or damage which may arise if this guidance is followed.