Reinigen en retoucheren. De restauratie van de zandsteengevels van het Koninklijk Paleis Amsterdam

Abstract

The Royal Palace in Amsterdam has the largest sandstone-cladded façade of the Netherlands, made of Obernkirchen and Bentheim sandstone. The façades already showed black weathering three quarters of a century after their construction. The formation of thin black weathering layers occurs in many types of sandstone. The layers are made up by various components, including airborne particles and bio colonization. The current paper relates the results of a decade of scientific research into the possibilities of cleaning black-weathered Obernkirchen and Bentheim sandstone without causing any damage to the stone itself. In this case, two types of damage are distinguished: direct damage, such as loss of material, and indirect damage, i.e. possible damage that may result from changes in hygric behaviour. After initial research into the nature of the thin black weathering layers, the Government Buildings Agency commissioned several consecutive research projects into cleaning. A state of the art literature survey resulted in the initial selection of four cleaning methods, i.e. pastes containing EDTA as active component, turbulence water jet blasting with either pumice or Al-silicate, and Nd-YAG laser. Tests were done on both types of sandstone for each method. Samples were investigated by optical and scanning electron microscopy, hygric behaviour was evaluated, as were potential future colour changes. Only laser was considered effective in both cleaning and not causing damage, but affected the colour of the sandstone. With the upcoming restoration of the Royal Palace in Amsterdam in mind, and taking into account experiences elsewhere in the Netherlands, another method was considered, i.e. dry microblasting. The effects of dry micro blasting and dry micro blasting supplemented by laser cleaning on Obernkirchen and Bentheim sandstone (with special attention for the inscriptions on the sandstone, made with hematite) as well as on Carrara marble were studied in the laboratory first. Subsequently, a pilot was performed on the sandstone of the palace’s tower. Both methods were demonstrated to be effective and not causing damage. In practice, only dry micro blasting has been used. Besides cleaning, two additional surface treatments were investigated: the application of artificial patinas and anti-graffiti systems on the sandstone. In the first case, which involved artificial patinas with pigments fixated by spraying ethyl silicate, the effect on hygric behaviour, penetration depth and durability were studied. The second by using an acid rain test as worst case scenario. It was shown that the patinas did not significantly affect the hygric behaviour of the sandstone, whilst remaining present after 150 cycles of acid rain testing. In the case of the anti-graffiti systems, hygric behaviour, any visual effects, the possibility of effective cleaning, and effects on frost-thaw resistance were considered. A sacrificial anti-graffiti system based on bee wax was selected. Based on scientific material research, the Government Buildings Agency and the restoration architect made relevant choices for the restoration of the Royal Palace in Amsterdam.