The preservation of outdoor sculptures differs fundamentally from the preservation of other artworks in an important respect—they are on permanent exhibition without the protection of a building. Writing in the GCI Newsletter in 2007, conservators Derek Pullen and Jackie Heuman described the long tradition of outdoor sculptures, identifying bronze and stone as the best surviving materials and pointing out the diverse management and conservation problems associated with these works of art. To avoid extensive and invasive treatments, regular maintenance of outdoor sculpture is crucial. Both good maintenance and appropriate restoration need to be proceeded by an exploration of the production of these sculptures, the materials involved, and the artist's intent, taking into account the sculpture's location and the local climate.

While traditional bronze sculptures with either an applied patina or a naturally developed patina survive well, bronze and metal sculptures with a clear varnish or those painted in full color will last only as long as the coating stays intact. Discoloration and wear deface the appearance, while delamination of the coating induces corrosion and other damage of the metal underneath. A rather different material, in both production and appearance, is composite plastic fiberglass-reinforced polyester, also known as GRP. Although strong and lasting, this new twentieth-century material has its own issues of wear and deterioration and, when used for outdoor sculptures, its own particular conservation challenges.

Art Production with GRP

The industrial development of GRP and its commercial availability have prompted artists to work with this material. Artists have favored GRP for outdoor sculptures because it lasts outdoors, is strong, is easy to work with, and is available in any color. The material allows artists to actually produce the final sculptures themselves, and to create playful works on an impressively large scale. From about 1960 onward, artists such as Jean Dubuffet, Niki de Saint Phalle, and later Atelier Van Lieshout worked in GRP for their outdoor sculptures, colored either by mixing pigments into the polyester resin or by artistically painting the surface afterward.

The process of making an artwork in GRP is complex. Niki de Saint Phalle constructed her early works by alternating fiberglass and polyester resin layers on a wire-mesh framework, painting them afterward. Her later works were produced from her designs by her assistants. Atelier Van Lieshout applies colored GRP over large wooden constructions of human and anatomical shapes cut out in foamed plastic. The final polyester layer in these cases is called the top coat. A different procedure for making an object in GRP involves molds, enabling series production and very smooth surfaces—as, for example, with the Futuro houses designed by Matti Suuronen in 1968.1 Here the final surface coating is called the gel coat, being the first polyester layer that is applied in the mold. Early on, Jean Dubuffet experimented with reinforced plastic and transferred his painted polystyrene sculptures with the aid of molds into GRP that he painted afterward.

The molds are the negative form of the artwork's model, made in plaster or cut from foamed plastic—as, for example, expanded polystyrene (EPS). The molds, often produced in GRP themselves, serve as the negative shape to form the GRP for the final artwork or for parts of it. The inside of the mold is treated with paraffin wax. Next, the gel coat, translucent or colored, is applied, and when it is half set, several layers of polyester resin and glass fiber are applied. After the complete GRP package is cured, the elements are removed from the mold, to be assembled into the final sculpture over a supportive frame. Colors can be mixed into the top or gel coat, but the artist can also choose from a great variety of commercial paint and varnish systems, including opaque, translucent, luster, and metallic paints.

Maintenance, Prevention, and Conservation

Regular surface cleaning is the basic maintenance of outdoor sculptures. Cleaning can be performed by trained staff using suitable cloths, sponges, and soft brushes, water, and neutral surfactants. More advanced cleaning, such as rinsing with low-pressure water combined with cloth and brushes, should be done only if needed and only if the material is sufficiently durable. This approach should be carried out cautiously by a conservator, as inappropriate cleaning mediums and tools can cause severe damage.

To prevent the wearing down of a sculpture's surface, sacrificial wax, acrylic, or emulsion protection layers—with optional UV absorbers and fungicides added—can be brushed or sprayed on and then monitored on a yearly basis. This standard procedure for outdoor bronzes and painted metal sculptures also works well for artworks in GRP.2

The deterioration of GRP sculptures manifests itself in various ways, from the micro to the macro level. Sunlight causes discoloration and, combined with rain, produces a dull and chalky surface after a decade or so. When the polyester wears down, water can enter the fiberglass reinforcing layer, causing mold growth and further damage after a period of frost. Larger breaks in the material can result in corrosion of the metal inner construction or in the rotting of any wooden structure inside.

Actual damage, breaks, and tears or the flaking of the paint layer require repair. Localized repair involves clearing away worn material. Preparing the area for a lasting fill and a stable retouching often entails irreversible loss of original material. Such a loss should be considered secondary to saving the entire sculpture and its appearance, as delaying intervention or doing nothing facilitates further decay and, in the end, costs more.

Retouching in an aged paint layer, however, may stand out over time, as the original and repair layers age differently. No paint layer, protective coating, or varnish lasts forever outdoors, and recoating ultimately becomes inevitable. For a durable recoating of GRP, the best current coating system that most matches the original surface should be selected. Because with good preparation and priming of the surface, the original surface may not stay intact, the concept of reversibility should be reconsidered in light of the main aim—to restore the sculpture's original look and the artist's intent, particularly when the artwork was fabricated by industry in the first place.

Decisions on Treatment

Several examples of the conservation of outdoor sculptures in GRP illustrate some of the treatment issues involved.

Some of the many Niki de Saint Phalle painted GRP sculptures have sustained damage, fading, and delamination of paint and are in need of treatment. The Lifesaver Fountain (1993) in Duisburg, Germany (a joint work with her artist husband Jean Tinguely), has recently been restored. The joints of the inner structure of the sculpture were strengthened by additional stainless steel profiles in order for the fountain to again be operated properly in its public space. Acrylic fillings were applied, and because total repainting was not yet necessary, localized painting—with translucent and opaque acrylic paint containing the same pigments as originally used—was carried out, with good results. A polyurethane clear coat was then applied to mimic the original.3

Jean Dubuffet experimented in realizing his monumental sculptures with reinforced plastic. He used epoxy resin, fiberglass cloth, aluminum grate, and polyurethane paint for the tree in his Jardin d'émail (1974), a massive piece in the sculpture garden of the Kröller-Müller Museum in the Netherlands. The top part of the GRP tree has displayed good durability over almost forty years, as it still retains the original polyurethane paint layer from 1974. In contrast, the large concrete construction, upon which visitors can walk, always needs regular care. Eight different types of paint layers applied there during the same forty years are proof of the complexity of choice in modern paint technology and of decisions to repaint the surface time and again. The Dubuffet Foundation in Paris provides advice concerning repainting his works, explaining that the black lines on Dubuffet's monumental sculptures are always hand-painted.

Also at the Kröller-Müller Museum is Atelier Van Lieshout's Mobile Home for Kröller-Müller (1995). When this large piece suffered badly from a leak in the roof and replacement was necessary, it was decided to ask the artist's studio to replace the roof by a reconstruction in new GRP in an improved shape, while conservators executed local repairs on the kitchen unit and the bathroom unit with epoxy glues and retouched the sleeping unit with polyurethane paint. A wax layer was applied as a sacrificial protection layer for the GRP.4

Managing the Future

Preservation of GRP outdoor sculptures depends upon regular cleaning and the application of protective coatings as part of general maintenance. When conservation treatments eventually become necessary, they should be based on preserving the work's original look and the intent of the artist, and the materials used must be sustainable in the outdoors, rather than reversible. The treatment cases discussed here suggest that traditional standards in conservation are too limiting for outdoor sculptures and that new standards have to be agreed upon by conservation professionals—standards that give precedence to preserving an artwork's identity over saving original material. Artists, artists' foundations, and fabricators could be an enormous help in making, keeping, and providing materials and swatches of paint as reference for any future repair or repainting. This kind of physical reference material, in the long run, may be of greater help than the trade name of a paint system or material in preserving these sculptures in their outdoor settings, as moving the sculptures indoors can hardly be an option.

Lydia Beerkens is senior conservator of modern art at SRAL Maastricht in the Netherlands. Frederike Breder is conservator of modern art at Museum Folkwang in Essen, Germany.

1. Lydia Beerkens, "Matti Suuronen's Futuro House prototype, 1968: Back in business in the 21st century," in Future Talks 2011: Technology and Conservation of Modern Materials in Design: Papers from the Conference Held at the Pinakothek der Moderne, Munich, October 26 to 28, 2011, ed. Tim Bechthold (Munich: Die Neue Sammlung, The International Design Museum, in press).
2. Conserving Outdoor Sculpture: The Stark Collection at the Getty Center, by Brian Considine, Julie Wolfe, Katrina Posner, and Michel Marc Bouchard (Los Angeles: Getty Conservation Institute, 2010), provides good guidelines for the regular maintenance of bronzes and other outdoor sculptures.
3. For more information on the conservation of Niki de Saint Phalle's Lifesaver Fountain, contact Martin Kaufmann, head of conservation, Restaurierungsatelier "Die Schmiede" GmbH, Duisburg;
4. Sanneke Stigter, Lydia Beerkens, Henk L. Schellen, and Sara Kuperholc, "Joep van Lieshout's ‘Mobile Home for Kröller-Müller': Outdoor polyester sculpture in transit," in ICOM Committee for Conservation, 15th Triennial Conference, New Delhi, 22-26 September 2008: Preprints, ed. Janet Bridgland (New Delhi: Allied Publishers, 2008), vol. 1, 489-96.