Damage to frescoes caused by sulphate-bearing salts: where does the sulphur come from?

Crystallization of sulphates in the form of gypsum and mirabilite/thenardite is one of the main causes of the accelerated disintegration of historic frescoes. The sulphur, which is necessary for these processes, may originate from a large number of different sources. Although the nature and velocity of the processes causing the damage, as well as the possible transport pathways of the saltforming ions, can be determined with reasonable accuracy, reliable information on the origin of the ions is seldom forthcoming. Sulphur isotope ratios were determined for various construction materials and salts from the church at Eilsum (Lower Saxony, Germany) where the romanesque frescoes have suffered serious damage. The studies showed clearly that sulphate on the walls inside the church, in contrast to that on the outside walls, originates mainly from the ground beneath the church from where capillary rise takes place within the walls. This information on sources and pathways of saltforming ions may contribute towards developing a programme of remedial measures.