Bronze Statuary

The construction methods, problems and treatment techniques associated with bronze statuary.

Bronze is generally more physically robust and more resistant to corrosion than other metals. Consequently most public statues, including war memorials and plaques, are likely to be made of cast bronze.

In the foundry, the pale yellow-pink surface of a bronze statue was (and still is) coloured using traditional methods and chemicals. This skilled process is called artificial patination. The resulting patina (typically a mid to dark brown for public monuments) has limited protective qualities, so it is always further protected from the elements with wax coatings or sometimes varnishes.

To ensure continued protection of the bronze and its artificial patina, waxes or lacquers need to be renewed periodically. The frequency depends on the environment: as a guide, reapply waxes annually. Lacquers are generally only used in hard-to-reach locations, so would be renewed less frequently.

Common problems

You may see a bronze statue or plaque that has changed colour over time, from brown to green. This happens when the protective coating has not been maintained and the surface of the bronze has been exposed to air and rainwater. This colour alteration is referred to as natural patination.

Technically, the overall green patina layer is a corrosion product. However, it is slow to form, and once the natural patination has developed, the corrosion rate dramatically reduces. The compact, uniform, green patina layer, therefore, acts as a semi-protective coating and should be waxed as part of regular maintenance. If this layer is not maintained, green staining will appear on any stonework (such as plinths or pedestals) that is subject to run-off from the bronze. This staining is often impossible to remove.

If the bronze is not routinely maintained, a more serious problem will appear in areas that are not regularly washed by rainwater or are slow to dry. Pollutants can gather in these areas, resulting in a severe form of corrosion (or ‘pitting’ corrosion) that appears as bright green spots. This active corrosion must be dealt with urgently, so consult a conservator if you see this.

Another common problem on bronze statues is black crusts or disfiguring dark streaks caused by pollution. Potentially, a metalwork conservator can remove these as part of the cleaning and repatination process.

Inappropriate coatings (including epoxy and vinyl-based formulations and paint) may deteriorate over time, trapping moisture and pollutants that will cause active corrosion. A metalwork conservator would remove these as part of cleaning.

Iron ‘core pins’ accidentally left in the non-ferrous metal after casting will rust, causing the sculpture to split. Look out for rust stains on the statue or on its stone pedestal.

Treatment

Before any intervention, the conservator should carefully examine the bronze for evidence of the original surface treatments. These could include artificial patinas in various colours, leaf gilding or even paint. Findings from close inspection and discrete cleaning tests should inform the conservator’s recommendations.

Cleaning

Cleaning should aim to remove dirt (soot deposits, dust and other particulates, and bird droppings), localised active corrosion, failed or inappropriate surface coatings and disfiguring stains. This is achieved using medium-pressure water or high-pressure steam (for surface coatings), and the conservator should select the appropriate pressure and flow rate based on trials on hidden areas. (The amount of water will vary greatly depending on the equipment used).

Avoid harsher cleaning methods, involving chemicals or abrasives, unless there are no alternatives. It's always better to use selective and localised treatments using the most sensitive materials possible. All decisions regarding cleaning methods, materials and dwell times should be based on test results.

Repair

Use traditional materials and methods for physical repairs where appropriate and safe to do so. Check the original foundry fixings used to join the cast sections together and replace them if necessary. Repair holes and failed patches. Many repairs can be made without moving the statue, unless the conservator has to address structural issues such as failed joints or replacing the armature.

The remains of corroded iron core pins can, if necessary, be drilled out, filled with bronze rod, tapped or tapper-plugged into the holes. They can then be hand-finished by filing and chasing to blend with the surrounding surface.

Repatination

Consider repatination of the statue when active corrosion has affected much of its surface, or where the original patina has been significantly damaged. However, if the statue has developed an even natural green patina, do not remove it. Where appropriate, a conservator can repatinate this layer using various patination chemicals to restore the surface to a rich, dark brown-green colour (over-patination). This technique avoids the need for aggressive cleaning, while reinstating the artist’s original intention.

Generally, the conservator would prepare and then wax a small test area of patination, so that the custodian can approve the intended final appearance of the surface of the bronze. Tinted waxes may also be used, although they are comparatively temporary and difficult to maintain consistently.

Reapplication of protective coatings

As a final step, reapply wax coatings, ideally to a warmed surface. Wax coatings need regular maintenance. If access is difficult (for example, a bronze on top of a building), a conservator will be able to advise on practical alternatives.