Iron is a versatile and ubiquitous material. The term Architectural Ironwork can be used to describe a huge number of objects, ranging from the humblest fixing, cramp or pin, through gates, railings and sculptures, to engineering structures such as bridges and piers, to whole buildings like conservatories and railway stations.
The most common materials associated with Architectural Ironwork are wrought iron, cast iron and mild steel. Stainless steel, cor-ten steel and pure iron can also be found, particularly in more modern objects, which may also include galvanised or metal-sprayed surfaces. In addition, other metals such as gold, silver, bronze, copper or lead are commonly used for decorative or replacement elements on ornamental objects in conjunction with iron. Other materials such as wood, masonry or concrete are also often associated with iron structures.
The most frequently-occurring problems associated with Architectural Ironwork are corrosion and its effects. There are two categories of corrosion: chemical corrosion and stress corrosion; each one can lead to the other.
The usual form of chemical corrosion is oxidation, where the iron links with oxygen to form rust. Rust has a greater volume than iron and its formation can cause large pressures to build up where rusting takes place in confined spaces. These forces can easily be great enough to distort and fracture surrounding material. This in turn allows more moisture into confined areas and so the disintegration process continues apace. The rate of chemical corrosion can be affected by the presence of different metals in contact with iron. Copper or bronze for example can promote or accelerate corrosion, whereas zinc or aluminium can reduce it.
Chemical corrosion can also be made much worse by the presence of chloride ions (as found in common salt – particularly sea-side conditions) or some atmospheric pollutants.
Stress corrosion can stem from incorrect design or manufacturing process (an object being too weak to support itself) or from the results of vandalism or accidental damage. The integrity of any protective coating will be compromised once an object has become distorted or damaged; moisture will be able to gain access to the underlying surfaces and start off the rusting process.
Rusting stops if the relative humidity around an object is low enough. Consideration should always be given therefore to re-locating particularly important objects to a more controlled and drier environment. Professional help should be sought if an object does not appear to be strong enough to support itself. A vandalised or damaged iron object should be repaired as quickly as possible after the event, so that chemical corrosion does not have a chance to set in.
Generally, chemical corrosion can be avoided by preventing either oxygen or water coming into contact with the ironwork; this is usually (and traditionally) achieved by painting. Maintenance of the paint coating therefore is the best way of avoiding problems, since most Architectural Ironwork will have been painted when new. Care should be taken however, not to over-paint heavily-corroded surfaces; this can create air and moisture pockets, causing the creation of an aggressive micro-climate that will increase the rate of corrosion.
The degree of success possible in maintaining a painted iron surface in good condition is dependant on how well the surface underneath has been cleaned. Generally the length of life of a coat of paint will be determined by the quality of the cleaning of the substrate that the paint has to stick to. It is extremely easy to either over-clean or under-clean ironwork. Over-cleaning can damage decorative elements made from softer metals, or destroy protective coatings like galvanising. Under-cleaning will reduce the life expectancy of the coating and predispose the item to re-rusting. It is important therefore that the complete range of cleaning options is fully evaluated before work begins.
It is important that the treatments used on Architectural Ironwork are recorded carefully. This will ensure that future treatment is less frequent, easier, and much more cost-effective.
Various rust removers and converters are marketed but their use should be avoided. Some are acidic and it can be very difficult to ensure that all residues have been removed, particularly from old wrought iron. Any residues that remain are likely to promote further corrosion. Even where converters have fully transformed the rust, adhesion problems can arise because the new compounds tend not to stick well to the underlying iron.
Care should always be taken to understand the original manufacturing processes and to change these only when absolutely necessary.
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© Icon, the Institute of Conservation 2011.
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 the Institute of Conservation. The Institute of Conservation would like to acknowledge use of the MGC publication 'Ours for Keeps' in the preparation of this text. The Institute of Conservation and its partners accept no liability for any loss or damage which may arise if this guidance is followed.
The Institute of Conservation would like to acknowledge the support of The Royal Commission for the Exhibition of 1851 in the production of this guidance information. Further information on The Royal Commission for the Exhibition of 1851 and its work is available at www.royalcommission1851.org.uk.