To coat or not to coat? The maintenance of Cor-Ten 1 sculptures P. Decker * , S. Bru ¨ ggerhoff and G. Eggert Since the mid sixties Cor-Ten 1 steel became one of the most famous materials for outdoor sculptures until today. This material was used by many artists because of its good weather resistance. Due to not always suitable ranges of application many significant problems in the field of conservation appeared. These were surveyed in a project of the study programme ‘Object Conservation’ at the State Academy of Art and Design Stuttgart with special focus on the material and its protective rust layers, the premises for its protection, different cases of significant damage and their causes, the potential of preventive conservation and practised interventive methods. Although it seems to be in some way a contradiction in itself to coat a weathering steel, this has often been done in hands-on conservation to overcome corrosion problems. Therefore different coating materials already in use for outdoor metal sculptures were tested on Cor-Ten coupons in the framework of the CONSIST project in the Deutsches Bergbau-Museum Bochum (DBM). Cor-Ten B samples were covered with different coating materials such as: microcrystalline waxes, waxes used in technical corrosion protection, an acrylic resin and a polyurethane resin. The sheets were then weathered for 2 months in a climate chamber with a relative high humidity (but also drying cycles) and a climate chamber with a periodic changing relative high rate of SO 2 gas and humidity using standard protocols developed at DBM in earlier projects. The effectiveness of the protective function of each coating materials is compared, consequences for practical applications discussed. 1 Introduction 1.1 History of weathering steels With the upcoming industrialization in the 19th century there was special interest by the industry to develop a type of steel resistant to the attack of atmospheric conditions. The aim was to produce steel in large scale that does not need to be coated and maintained regularly. The economic aspect was predominant for the development of weathering steel. The first experiments were performed by adding copper to the steel [1]. In 1916 the first long-term weathering experiment with steel sheets consisting of steel with different alloy components and different amounts of components such as copper and phosphorus were carried out by the American Society for Testing Materials (ASTM) [1]. So we can say that the development of weathering steel is based on the development of the so-called ‘copper steel’. Later in 1929 the U.S. Steel Corporation started to develop their own corrosion resistant low-alloy steel by adding elements such as copper, nickel, chromium, silicon and phosphorus to the alloy. Still continuing research in this field U.S. Steel presented in 1933 their products Cor-Ten A 1 (the name Cor-Ten is standing for: corrosion resistance-tensile strength) [www.werkstoffe-korrosion.de., 10.04.2005] and Cor-Ten B on the steel market. But there were also different kinds of weathering steel in Germany since 1926. The Vereinigte Stahlwerke AG Dortmund patented a chromium– copper steel product especially for bridges in 1928 [2]. After World War II in 1962 the U.S. Steel Corporation published the results of a 15.5 years long-term weathering of different steel sheets. The intention here was not just testing different alloying components with different mass proportions but also testing the corrosive influence of different locations like rural, coastal and industrial atmospheres especially on Cor-Ten steel. Since these early days of developing resistant weathering steel products until today the alloying compo- nents and their mass proportions of the elements have changed several times. The result was continuous improve- ment for the products we know today. 1.2 Weathering steel as a material for artists Since the mid sixties when steel sculptures found their way into art Cor-Ten has kept its dominance as the preferred steel material by artists. It is chosen because of its slow colour-changing surface and of course its weather resistance. An early famous example in architecture is the John-Deere administration building in Moline from Eero Saarinen. Famous artists using Cor-Ten sculptures are e.g. Eduardo Chillida, Richard Serra, Barnett Newman, Donald Judd, Horst Antes and many more. 1.3 Classification All weathering steel products with the name Cor-Ten are produced today under the license of the U.S. Steel Materials and Corrosion 2008, 59, No. 3 DOI: 10.1002/maco.200804099 239 * P. Decker MWD-Restaurierung, Am Dobelbach 11, 70184 Stuttgart (Germany) E-mail: decker@mwd-restaurierung.de S. Bru ¨ggerhoff Deutsches Bergbaumuseum, Am Bergbaumuseum 28, 44791 Bochum (Germany) G. Eggert Staatl. Akademie der Bildenden Ku ¨nste, Am Weißenhof 1, 70191 Stuttgart (Germany) www.wiley-vch.de/home/wuk ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim