Removal of iron crusts from sandstone sculptures in a fountain

Abstract

Three historic figures from fountains in a park in Neschwitz, Germany Upper Lusatia (Ober Lausitz) had to be dismounted for workshop restoration. The sculptures from G. Knöffler, an important saxonian sculptor, dated about 1740 and show mythical scenes: putti sitting on fish like creatures. Although the sculptures originally were worked out of Cotta Sandstone, their visual appearance prior to restoration was much more like an iron casting than weathered sandstone material. The surface colour showed the typical red brown of rust, the corrosion products of iron. Further inspection by microscopy and SEM verified a considerable 50 µm crust of iron rust deposited all over the surface. In the second part of the twentieth century, the fountains of the park were feeded with water from a small well put down on the castle ground. In that eastern region of Germany, the village of Neschwitz is situated not far away from a lignite mining area. The lowering of the groundwater table, necessary for work in the open coal pits, resulted into dissolution of iron in the groundwater. Such iron rich water was then pumped to the fountains. With contact to air oxidation, the iron precipitated and deposited on the stone surface. The sophisticated stone restoration of the historical monument figures required chemical reduction of the iron by oxalic acid, its dissolution and removal via poulticing.