Metal sculpture ranges from solid-cast statuettes of the ancient Near East to the massive steel public monuments of the late 20th century. In most instances, the deterioration of metal sculpture is due to the reversion of the metal to a more stable mineral state. In the case of iron, the process is most commonly known as “rusting” and results in a red-brown, powdery mineral iron oxide. Copper and its alloys most commonly alter to the green or blue carbonates of copper, malachite, or azurite or to the red-oxide mineral cuprite. Copper and its alloys may also quickly corrode in the presence of chloride by the cyclic process called “bronze disease,” during which copper is altered to copper chloride, a powdery white-blue product. Silver tarnishes rapidly even in the presence of minute amounts of sulfur, and lead will quickly corrode in the presence of acetic acid. Common to all of the processes is the presence of water, which is needed to initiate and complete the corrosion of the base metal to a more voluminous and less cohesive mineral product.

In the past, the treatment of metal sculptures often involved completely stripping the surface until it was free of all corrosion product or alteration. Abrasive techniques such as sandblasting or microbead blasting were regularly used, as was chemical stripping (which dissolved the mineral alteration products) and electrochemical reduction, which also stripped the surface of any corrosion products and of “patina,” the term usually given to corrosion products that are either naturally occurring or artificially formed on the metal surface. Patinas are valued for aesthetic beauty and for the authenticity that they lend the object. Today treatment of metal sculptures is far more conservative than in the past. Although sculpture may be polished (as in the case of silver sculpture that has been tarnished) or stripped of its alteration patina (as in the case of some monumental outdoor sculptures), alteration products are carefully evaluated for their importance and authenticity before their removal is considered, and patinas are far more often protected than removed. Any treatment that results in the reshaping of the metal or in any irreversible addition, such as soldering or welding to secure broken segments, is now considered with great caution.

At the turn of the 21st century, the conservator’s main intervention in the process of corrosion involved providing a more benign environment (usually meaning as dry as possible and as free of harmful pollutants as possible) and maintaining the sculpture’s stability through a series of preventive maintenance procedures, such as regular cleaning and the application of protective coatings. Regular maintenance has proved to be highly cost-effective and successful in the preservation of outdoor sculpture over the long term. Regular cleaning and coating (with waxes or synthetic polymers or both, which sometimes contain corrosion inhibitors) have kept corrosion processes in check, even in aggressive and polluted urban environments. In some cases, however, the conservator’s only option is to recommend that the sculptures be removed from the outdoor environment, placed in a protected area, and replaced by a replica made of a more-resistant material.

Although cleaning of metal sculpture can include the total removal of all corrosion products, including those termed and valued as patina, a more conservative approach continues to develop within the field, which recognizes the value of naturally occurring change to the metal surface. In the case of archaeological material and ethnographic sculpture, the corrosion products may hold remnants of original surface treatments or remains of associated materials or evidence of use. This evidence must be carefully studied, and a full understanding of the sculpture’s importance (now and in the future) must be weighed against its loss by cleaning.

Wood sculpture

Although relatively little wood sculpture survives from prehistorical and early historical periods, an enormous amount of sculpture was produced in the last millennium, particularly the polychrome sculptures of western European religious devotion and those of India, China, Japan, and other Asian nations. Wood is a very open and porous structure, the bulk of which is water, absorbed or chemically bound to its thin-walled structural cells. Like many plant materials, wood responds to changes in the humidity of its surrounding environment, taking up available water to reach equilibrium with the environment or, conversely, giving up water if the surrounding air is dryer. Dimensional changes to the wood occur when this exchange takes place. As wood takes up water, it will swell. As it loses water, it will shrink, sometimes dramatically. Both actions induce considerable stresses on the structure of the wood, resulting in irreversible warping or complete splitting of the wood section. Additionally, the physical strain placed on the structure by continual expansion and contraction weakens the wood or may cause further serious damage to wood already weakened by insect attack or age. When decorated with paint, wood will respond to heat and moisture with greater movement, destroying the bond between the wood and the less elastic paint and ground preparation, resulting in the painted decoration’s flaking away from the surface.

Wood can also be a food source or a nesting place for a variety of insects such as wood-boring beetles, termites, and grubs. Infestation can be so severe that the sculpture loses all of its structural strength and collapses. Wood can also be damaged by a variety of fungi and bacteria with similar results.

The predominant concern regarding the preservation of wood is the control of the environment. Exposure to light, particularly the ultraviolet and shorter wavelengths of the visible spectrum, results in both the chemical and physical alteration of all organic material, including wood. Wood can become darker or lighter or lose its structural integrity through the action of light energy acting as a catalyst for other chemical reactions.

Appropriate and stable temperature and humidity levels and an environment low in ultraviolet radiation, illumination, and pollutants can ensure the slowing of any deterioration. Regular dusting and general maintenance of the sculpture, as well as vigilant actions to keep damaging insects away, are also paramount. When intervention is necessary with wood sculpture, it normally involves some form of consolidation, either of the wood sculpture’s structure or of its decorative surface. The range of consolidants for each of these actions is broad, including synthetic acrylic polymers, organic-based natural resins, and animal glues.