By Derek Pullen and Jackie Heuman

Conservators of outdoor sculpture can be forgiven for sometimes feeling that they are witnesses to a hopeless struggle between sculptures and their environment. The effects on both traditional and modern materials of exposure to weather, pollution, and neglect are relentless. Stone crumbles, metals corrode, wood rots, and paint peels or fades. While conservators have a wide choice of treatment options for stabilizing and restoring outdoor sculptures, ensuring that protection remains effective is a formidable task. The ideal of a balance between the sculpture and the outdoor environment is hard to achieve and extraordinarily difficult to sustain, and any overview of outdoor sculpture conservation quickly arrives at an overriding theme: maintenance.

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With so much open-air sculpture in the public realm embodying symbolic, historic, and aesthetic value, the process of conservation requires negotiation and collaboration—drawing in, for example, art historians, custodians, conservation scientists, engineers, fabricators, and the sculptor or the sculptor's estate. It can be hard to understand that large, apparently solid structures become vulnerable when placed outdoors. Even the option of removing iconic, at-risk sculptures to safer conditions indoors can raise alarm, although such a practice has a significant precedent: in 1873 Michelangelo's David was moved indoors, and a replica was placed in Florence's Piazza della Signoria in 1910. Strong views have characterized outdoor sculpture conservation since at least the nineteenth century, and it can be said that the passionate response to this issue has been a positive feature, leading to fresh ideas and new perspectives. Today advocacy, debate, negotiation, and resolution are essential components of the conservation process for outdoor sculpture, for which the implementation of a long-term conservation strategy depends on commitment from those responsible for maintenance. Unless maintenance is regular, the environment will quickly regain the upper hand.

All outdoor monuments and sculptures eventually acquire new meanings and functions because their cultural context—our society and its values—changes. For this overview, we will concentrate on those outdoor sculptures of the twentieth and current century for which we, as a society, wish to retain their original purpose. In contrast, Egyptian temple statuary or Neolithic stone circles have acquired different meanings through age. They need to be conserved, but the reason for doing so has little to do with their original functions and everything to do with what they represent for us now. For these older monuments and sculptures, Western society currently recognizes age as a value to be preserved. However, with modern sculpture, we value newness as part of its aesthetic function, to be preserved, or at least managed, by maintenance. The challenge has been to conserve modern and contemporary sculptures without losing the functions and meaning that we expect them to retain in an outdoor context.

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The traditional materials of stone and bronze have naturally dominated publications and conferences about outdoor sculpture, just as they do the sculptural landscape. Public perception of what constitutes damage for different materials varies. The physical erosion of stone carvings fronting medieval European cathedrals is widely deplored as involving a loss of historical information and aesthetic quality, yet reactions to comparable deterioration of bronzes have usually been less critical. Before a bronze left the foundry, it would normally receive a chemical patina—usually black or brown on early twentieth-century works. The damage to original delicate finishes was rarely regarded as a problem; in fact, the development of a green patina was often valued as an aesthetic enhancement. A prevalent misconception was that verdigris (to use the generic term for a variety of corrosion effects) was intentional rather than a corrosion layer that had replaced an original patina (see Phoebe Dent Weil's excellent discussion of patina, which is reprinted in the GCI's Readings in Conservation series volume Historical and Philosophical Issues in the Conservation of Cultural Heritage, 1996).

Early Developments

As early as the 1860s, the Berlin Patina Commission addressed the concerns of the changing appearance of outdoor bronze sculptures. In 1921 the Times of London drew attention to the "deplorable condition" of many of the statues in London. In 1929, "The Open-Air Corrosion of Copper: A Chemical Study of the Surface Patina" was published in the Journal of the Institute of Metals in London by W. H. J. Vernon and L. Whitby. The authors asserted that the green patinas were stable and protective, and their view may have encouraged complacency about the need for treatment of corroding bronzes. By 1951 J. F. S. Jack of the Ancient Monuments Branch of the Ministry of Works, London, observed: "Despite the relatively good corrosion-resisting properties of bronze, the heavily polluted atmospheres of industrial cities contain substances capable of causing corrosion." He recommended a maintenance program of twice-yearly applications of lanolin for the numerous bronze statues in central London. Even though the sculptures acquired a uniform black appearance, the underlying bronze remained in good condition. The program continued for nearly fifty years.

It was only in the early 1970s, when conservators and scientists collaborated to study the mechanics of bronze corrosion, that the relevant terminology changed from aged patina to corroded surfaces. The condition of the Horses of San Marco in Venice gave rise to scientific investigation on a scale that set the pattern for other big projects, such as the Statue of Liberty, Ghiberti's gilded bronze baptistery doors, the Gates of Paradise in the Piazza del Duomo in Florence, the Albert Memorial in London, and the Equestrian Statue of Marcus Aurelius in Rome. The revelation of the recent effects of pollution and acid rain on sculptures that had survived for two thousand years alerted the conservation community to the urgency of the problem. This finding ran counter to the popular belief that bronze was indestructible. Henry Moore (1898–1986), the best-known British sculptor of outdoor bronzes, said, "Bronze is a wonderful material, it weathers and lasts in all climates. One only has to look at the ancient bronzes, for example, the Marcus Aurelius equestrian statue in Rome. . . . Under the belly of the horse, the rain has left marks which emphasize the section where it has run down over the centuries. This statue is nearly two thousand years old, yet the bronze is in perfect condition. Bronze is really more impervious to the weather than most stone." A few years later this statue was removed for conservation and eventually replaced with a much-criticized replica. Moore had been unaware of the extensive structural deterioration due to outdoor temperature cycles and pollutants, later revealed by a thorough technical examination. The original sculpture, now conserved, is currently displayed indoors in controlled conditions.

At the conference "Dialogue/89: The Conservation of Bronze Sculpture in an Outdoor Environment," Arthur Beale of the Museum of Fine Arts, Boston, presented an overview that highlighted not only the scale of the problem of environmental pollution and acid rain but also the growing public awareness of the need to maintain these deteriorating sculptures. There was consequently a greater willingness on the part of custodians to prioritize maintenance of works found to be most in need. At the conference, there was a debate (which continues today) on the pros and cons of removal of corrosion products on bronzes. It was agreed, however, that whatever treatment was carried out, the key to long-term protection for bronzes was regular maintenance and the renewal of protective coatings of lacquer or waxes; at the same time, there was consensus that further research on the ways in which these coatings respond to weathering processes was needed. Studies in Canada have identified and correlated the location of corrosion samples to increase understanding of corrosion and weathering processes.

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Modern Sculpture

The ethics of repatination also invigorated debate. Some conservators preferred to clean and leave surfaces untouched, while others chemically repatinated or used more reversible methods, such as pigmented waxes or lacquers. Information from foundries about applied patinas and coatings can often inform a conservator's decision on how best to preserve sculptures. Problems with Henry Moore's bronzes illustrate common concerns faced by conservators of modern sculpture. His later sculptures left his West Berlin foundry patinated in a range of colors, from bright gold to dark browns, and sealed with a polyurethane coating. As the coating broke down in sunlight and weathering, the exposed bronze darkened and oxidized unevenly. This change in appearance dramatically altered the light and dark contrasts on the forms. At present, few believe that this is a desirable situation; even so, the confidence to repatinate resides with those few conservators and fabricators who knew Moore or have sufficient contemporary documentation of the sculptures' original condition to interpret his views. It is interesting to note that Moore admired the effects of aging on older statuary yet protected his own works with a coating. Clearly the topics of repatination and the characterization of new and aged patinas require further research. It is urgent that contemporary accounts from fabricators, sculptors, and their assistants be captured.

Moore's views about his patinas and several other papers on modern outdoor sculpture were discussed at the 1995 London conference "From Marble to Chocolate: The Conservation of Modern Sculpture." Among the newer materials mentioned were aluminum, cement, concrete, and reinforced resins. Perhaps the most widely used newer material, employed by sculptors as prominent as Picasso, David Smith, Caro, and Calder, is mild steel. This material presents a challenge to traditional conservation ethics. Steel rusts unless protected by plating or an effective multilayer paint system that is continually maintained and even renewed when necessary. However, to maintain these works, radical restorations, such as repainting, are often necessary. Conservators stress the importance of interdisciplinary discussions with custodians, curators, and artists to ensure that the goal of treatment is in keeping with ethical guidelines, the artist's views on materials, and contemporary attitudes regarding authenticity.

Sculptors are increasingly using a variety of media, and conservators do not yet have adequate solutions to arrest the deterioration of many modern materials displayed outdoors. Technology and materials science have already changed the expectations of artists and audiences regarding the aging of sculptures. For instance, rusted sculptures, once thought unappealing, are now more aesthetically acceptable following the adoption of weathering steels by the building industry. These materials were developed in the 1930s to provide structural steel alloys that could be left unpainted. The steels (widely known under the trade name Cor-Ten) develop a stable rusted patina on exposure to the atmosphere and cycles of wetting and drying; yet sculptures made from Cor-Ten in the last forty years have shown alarming signs of deterioration. The Canadian Conservation Institute's conference "Saving the Twentieth Century: The Conservation of Modern Material," held in Ottawa in 1992, was one of the first international meetings to address the conservation problems of Cor-Ten. When Naum Gabo's Head No. 2 (1966), made of Cor-Ten, was displayed outdoors, it quickly became apparent that water collecting in pockets was hastening corrosion, especially around welds. Gabo made another version in stainless steel for outside display, and the Cor-Ten sculpture was brought indoors.

There are still few published accounts of the problems associated with mixed-media sculptures. Conservation of these artworks tends to require expert input from beyond the conservation profession—from architects, engineers, health and safety specialists, fabricators, and materials scientists. A good example can be found in Paul Benson's account of the restoration of Claes Oldenburg and Coosje van Bruggen's Shuttlecocks (1994) at the Nelson-Atkins Museum of Art in Kansas City, Missouri. The artists had stipulated that the four giant shuttlecocks, made of aluminum and painted glass-reinforced plastic (GRP, also known as glass fiber or fiberglass), be repainted every two years—but after consultation, a more sustainable, long-term solution using a new paint system was found.

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Maintenance is the mantra of all outdoor sculpture conservators, and it is frequently declared to be "the only viable conservation strategy for outdoor sculpture." The title of the 1992 American Institute for Conservation of Historic and Artistic Works (AIC) symposium, "The Maintenance of Outdoor Sculpture: Whose Job Is It?" organized by Virginia Naudé, Martin Burke, and Glenn Wharton, focused on the challenges involved in determining what to do and in getting it done, year after year. The skills required go beyond conservation expertise: they embrace public relations, negotiation, and project management. The scale of the project required to maintain an outdoor sculpture is often commensurate with the size and prominence of the object itself. It can be difficult to convince custodians that treatment is desirable when corrosion is perceived as patina—but it can be even harder on new projects to secure maintenance funds before any change is apparent.


Founded in 1989, the Save Outdoor Sculpture! (SOS!) program in the United States and the Public Monument and Sculpture Association (PMSA) in the United Kingdom are nonprofit organizations that help local communities preserve and promote their sculpture. Both agencies have contributed to creating more awareness of the scale and urgency of the problem of preserving outdoor sculpture. The SOS! campaign has collected information on over 32,000 publicly accessible outdoor sculptures across the United States, of which 54 percent were determined to be in urgent or critical need of conservation to survive. They are registered on the Smithsonian American Art Museum's online database, the Inventory of American Sculpture. In Britain, the PMSA's online National Recording Project fulfills a similar function by assessing the scale of the outdoor sculpture challenge—9,316 outdoor sculptures are recorded so far. The heritage preservation community is learning that these kinds of public campaigns work. They raise public awareness and funds for priority projects.

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Many of the durability problems associated with contemporary outdoor sculptures can be anticipated before commissioned works are fabricated and sited, especially if conservators are consulted at an early stage. Online guides are available to help prepare specifications for new commissions and select conservators. One advance in recent years has been the establishment of good practice guidelines, written by conservators, which stress maintenance as the single most effective action to preserve outdoor works. In the future, most outdoor sculpture commissions will be initiated with contracts that specify conservator involvement, maintenance responsibilities, and ownership rights.

Portable technology and tools are particularly useful for outdoor work. Technological advances include handheld X-ray fluorescence (XRF) units created for onsite elemental analysis. Laser cleaning technology continues to develop and become more portable; although not a panacea, it has potential for both stone and metal cleaning. Various laser systems are under evaluation by conservators and conservation scientists, and laser cleaning is now being applied to large-scale outdoor sculpture conservation efforts, such as the 2006 project to protect the Alexander Milne Calder bronze sculptures on Philadelphia City Hall, a collaboration involving conservation oversight by Andrew Lins of the Philadelphia Museum of Art. Better methods of documentation are being explored through photogrammetry and laser scanning, and online inventories offer the prospect of linking photographic and conservation treatment records to individual sculptures.

While conservation science helps identify, test, and predict material behavior, along with this information, we need to understand why artists choose a particular material and how they feel about changes to their work resulting from weathering. The International Network for the Conservation of Contemporary Art (INCCA), another online project, aims to collect, share, and preserve knowledge needed for the conservation of contemporary art. It encourages conservators to document their own experiences and to gather artists' views about the materials and techniques they use. The artist's viewpoint can enlighten the discussion surrounding contemporary works, including ethical issues of refabrication and replica creation. Similarly, questions about treatment, maintenance, or refabrication can be dealt with effectively at the commissioning stage.

Conservation projects involving outdoor sculpture can be large and complex. Unfortunately, conservation training does not often include project planning and management training. Furthermore, the conservation of outdoor sculpture is insufficiently recognized as an area of specialization; for example, within the AIC, outdoor sculpture is grouped with the Architecture Specialty Group, presumably because of a shared concern with immovable cultural objects. There is a definite need for a specialist education program at the graduate level to encourage more conservators to work in this area. Custodians of outdoor sculptures unwittingly allow unqualified operatives to damage sculptures with inappropriate treatments. More internship opportunities for conservators to gain hands-on experience are needed. Continuing scientific research will provide a clearer understanding of the effects and causes of degradation, and working with industry should help us find better solutions to complex problems. The common practice of casting an edition of multiple versions of the same bronze sculpture provides a research opportunity. What is effectively the same work can be located in several places around the globe with different climates and maintenance histories. Some versions of the sculpture will have remained indoors and have undergone alteration at a much slower rate than those situated outdoors. Comparing like with like and analyzing the differences may tell us a great deal about how form, material, and the environment interact. The real-life aging lab that is outdoor sculpture is an excellent place to test ideas.

Outdoor sculpture conservation, a relatively new discipline, is continually evolving to meet new challenges. Sculptures are becoming increasingly complex in structure and in the variety of media used, such as fountains, landscape, plants, and electronics. As the field broadens, conservators have to consider the rights of the sculptor to approve even emergency interventions. They also need to know when not to intervene, since some artists choose to face the outdoor environment head on. Antony Gormley has sited one hundred cast-iron sculptures on a beach where corrosion is part of the process (Another Place, 2005). While these artworks will certainly require maintenance in the form of regular structural inspections, the artist expects the figures to eventually succumb to the sea or be buried in the sand.

Nothing is certain in the conservation of outdoor works of art, except that all materials change faster outdoors, and only regular maintenance can delay that process. For sculptures sited indoors—a relatively benign environment—we can often put off routine treatments without causing further harm. For outdoor sculptures, delay is not an option.

Derek Pullen is head of sculpture conservation at Tate in London. Jackie Heuman is the senior sculpture conservator at Tate.

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Outdoor Sculpture at the Getty Center

In June 2007 the Getty Center opened the Fran and Ray Stark Sculpture Garden, transforming the grounds with a major outdoor sculpture collection. The collection includes twenty-eight American and European sculptures donated from the collection of the late film producer Ray Stark and his wife Fran. When the Getty accepted the Ray Stark Revocable Trust's donation in 2005, an intense installation schedule was developed involving collaboration within the Getty, as well as with the trustees of the Stark Trust, Richard Meier & Partners Architects, Olin Partnership landscape designers, KPFF structural and civil engineers, Hathaway Dinwiddie Construction, living artists, and foundations.

The Decorative Arts and Sculpture Conservation Department at the Getty Museum—responsible for the care of this collection—became involved before the donation, advising on how a collection of this scale would impact operations. An initial step was to gather information about the outdoor environment at the Getty Center in order to understand the specific needs for protecting the collection. Getty staff—led by conservators Brian Considine, Julie Wolfe, and Katrina Posner—reviewed reports on urban pollutants, checked salt levels in the air, tested the water quality, and collected wind measurements for kinetic works. For each sculpture, the conservators specified criteria for installation, then presented these requirements to the installation team to ensure that potential damage could be minimized through careful placements, protective coatings, and seismic mounting.

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Care of the collection began with thorough condition assessments, which included examination for structural instability. In some cases, weak areas in bronze castings were examined using X-radiography. Analysis of corrosion and coatings gave more information about the surface stability and guided selection of cleaning and coating protocols. Treatments primarily included reducing years of thick, dull wax buildup by using solvents, steam, and/or pressure washing. Recoating frequently involved the use of a microcrystalline paste wax that was compatible with existing wax on the bronze sculptures. The painted steel or aluminum sculptures had varying degrees of deterioration, and in the case of Calder's Jousters, a complete repainting was carried out after consultation with the Calder Foundation to ensure that the new colors conformed to the artist's original intent. When interviewing the living artists, conservators carefully collected extensive information about materials, techniques, and original appearance, so that any treatment undertaken would incorporate the artists' concerns. Two artists—Ellsworth Kelly and Jack Zajac—recommended that their works be repatinated because of their altered condition. The conservators and curators chose to implement the treatments while ensuring that the process was well documented. In both cases, the artists were actively involved.

An overall vision for the long-term preservation of the collection informed the planning and the treatments. The installation of the Fran and Ray Stark Collection, however, is only a beginning. More research work remains. Several major treatments were postponed until proper investigation could be undertaken. The maintenance plan is seen as an ever-evolving process, allowing for review, unpredictability, and the inevitability of change.

Julie Wolfe
Associate Conservator
Decorative Arts and Sculpture Conservation
J. Paul Getty Museum