Several outdoor sculptures at the Guggenheim Bilbao including Besarkada XI (1996) and Begirari IV (2000) by Eduardo Chillida have been damaged due to pollution. The damage, which is currently minor, is due to certain particles in the air preventing a protective layer from forming on the so-called weathering steel out of which the sculptures were created.

The material is popular with sculptors as well as civil engineers for its self-protecting properties. “When the steel comes into contact with oxygen, it develops a protective layer made up of various iron oxyhydroxides that act as a barrier. That way the metal protected by this layer is well preserved,” Julene Aramendia-Gutierrez, a researcher at the UPV/EHU’s Department of Analytical Chemistry, explained on the university’s website.

In the case of the Chillida sculptures, however, that protective layer has failed to form in certain places. This is due atmospheric conditions such as the presence of particulate matter from car exhaust, nitrates, iron sulphates, silicates, and natural dust in the air.

As a result Aramendia-Gutierrez says that the sculptures’, “surface is rougher than it should be, it has lost some small fragments of material and does not have the colour it is supposed to have in theory.”

It is unclear whether the steel will right itself over the long term or if this damage will cause major structural damage. However, the researchers suggest that future steel artworks in the area use a material called Eco steel rather than weathering steel. The material was used in Chillida’s Elogio del Hierro III (1991), which has performed much better in the same atmosphere.