Metals pervade historic buildings, most obviously in the form of decorative architectural metalwork such as balconies and railings adorning the façade. However, metals were also extensively used for roofing and structural support. Lead, iron, steel, and copper alloys can perform both decorative and functional needs, and often contribute in no small extent to the historic character of a building. Metals are hard and solid, giving a sense of strength and soundness, but metals can often be fragile, concealing weaknesses in joints and beneath layers of paint. Architectural metals can pose a wide range of maintenance and repair issues for homeowners which can be most easily grouped under two headings – failure of the metal itself, and the impact this failure has on the surrounding building and materials.
Different types of problems are found in different metals, and certain types of deterioration can be accepted without urgent repair in some metals. For example, both lead and copper form a coloured layer of corrosion products, or “patina”, when exposed to the atmosphere. The formation of different coloured corrosion layers in copper from the fresh salmon pink, to brown, to black and eventually green-blue is accepted as part of the normal ageing process, and even aesthetically appreciated. The corrosion products of iron, commonly known as rust, are not so appreciated and usually require some action to be taken.
Iron is found extensively in historic structures, as structural beams and columns, used to form verandas and conservatories, and especially for decorative elements to the main façade including gates and railings, balconies, boot-scrapers, window guards and the fanlight over the main door. Iron can be found in a number of different forms, as forged wrought iron worked by hammering, rolling and forming, or as moulded cast iron. How these metals behave over time, and how they deteriorate is intrinsically linked to their individual characteristics.
Once formed, most metals slowly change when exposed with atmospheric conditions to another more stable form, such as the mineral ore from which they were produced. This rate of chemical reversion, or corrosion, is affected by the amount of available water, oxygen, air pollutants, acid rain, salts, and the presence of dissimilar metals. Iron and iron alloys react rapidly on exposure to moisture and air, forming layers of rust which can appear red, black, yellow ochre, orange-red or dark-brown depending on the minerals formed. In some cases where corrosion is long-standing, all iron may have been lost and only the corrosion products remain. Corrosion leads not only to the loss of the metal feature, but can also damage the immediate surroundings. The development of rust from film, to layer, to crust causes expansion of the metal, exerting sufficient strength to break mortar joints, crack stones, and even to lift whole sections of masonry. Rainwater running over rusting metalwork deposits dark-coloured iron stains to the stone or brick masonry below, which are difficult and sometimes impossible to remove.
How then can ironwork be preserved? Periodic inspection for the early signs of rust, paying particular attention to vulnerable areas, will save a great deal of cost and effort in the long-run. Crevices are prone to corrosion as iron may deteriorate without exposure to the air due to different oxygen concentrations of the water trapped in the crevice. Joints with wood and stone, or metal-to-metal joints where iron shafts penetrate rails, and between the spearhard and the rail can be very damp allowing rust to form hidden from immediate view. Deformed areas such as bends and shaped areas are more vulnerable to corrosion as they have been previously stressed during manufacture.
Poorly maintained ironwork can give a bad first impression, showing rust, flaking paint, and fractured and missing metal components. However, much of the damage may be confined to the surface or to part of the metal feature, and whole-scale replacement is not usually necessary. Where whole sections of ironwork are missing, the original pattern can be found by comparison with neighbouring buildings, and noting the evidence of fixings in floors, pavements and walls for the spacing of uprights, columns and rails. Remnants of the original material may also be found on the site, and if only a few spearheads are missing for example, a mould can be taken from the surviving finials to reproduce them. Repairs should be carried out in situ wherever possible, as dismantling can cause further damage, and once taken apart, a complex object such as a gate can be very difficult to return to its original shape.
Once repairs have been undertaken, the existing rust must be removed as it provides a source of further corrosion beneath the paint unless it is stabilised. The preparation of the metal for painting through the removal of old failed paint, rust, loose particles, grease and any soluble salts is the single most important factor in ensuring the longevity of the metalwork. In many cases, full stripping is not necessary once the old paint is sound and not so thick as to obscure the detail of the ironwork. The areas showing rust and failed paint can be removed by hand by brushing and scraping or by a steel wire rotating brush on very low power. However, these mechanical methods may scour the surface and will not remove all rust particles. Flame cleaning is more effective in removing both paint and rust, but can warp sections of less than 2 mm, and care must be taken close to stone and brick.
Though the metal surface may appear burnished and clean after cleaning, rust particles will still remain, however these can be treated using a rust converter. A rust converter is a liquid which when applied to unstable iron corrosion products, converts them into stable compounds. These products are in common use for metal repairs, found in three main categories – grey acid-based, blue-black tannic-based, and silicate-based converters. The choice of converter depends on the particular situation and the strengths and limitations of each product, for example the blue-black tannic-based converters are very effective for metal conservation, but can stain any immediately adjacent stonework. The rust converter can be followed by a standard paint system.
The final part, and the key to the conservation of architectural metalwork, is isolation of the metal from the atmosphere in the form of an effective protective covering of paint. It is advisable to select a complete standard metal painting system using the recommended specific combinations of primers and finishing coats from one manufacturer, to avoid incompatibility between coats. Two coats of red lead primer, if available, should be applied, followed by two or more finishing coats. While black has been the favoured colour for railings since the late 19th century, earlier metalwork was also likely to have been painted red, green, blue or brown. The metalwork should then be reasonably stable for the foreseeable future, requiring only periodic inspection for signs of paint cracking or rust formation.