Salt-induced decay of interior walls, and climate control. The case study of Virgil's Chapel, Vienna
Subterranean structures affected by soluble salts may yield specific problems of
conservation, since none of the solutions usually applied to isolate the walls from the migration of salt may be applicable.
Apart from sporadic measures such as e.g. poultice extractions of salts, the only approach to reduce the rate of decay may
consist in the passivation of the action of salts by maintaining an adequate indoor climate.
This contribution presents the case of the so-called Virgilius Chapel, a medieval subterranean interior situated in the very
centre of Vienna. Since its discovery and excavation in the 1970-ties, the interior walls, which carry remnants of painted
plaster, suffer continous losses caused by soluble salts migrating from the soil of the surrounding bedrocks. Occasional measures
taken to extract the salts and consolidate the renders were of no sustained success, so that climate control seemed to be
the only means to improve the situation. This approach was based on careful monitoring of all dynamic factors and processes
driven by climatic cycles. Thus, in parallel to the monitoring of the room climate, all types of salt crystals occurring during
the seasons were sampled and analysed, the size and visual aspect of efflorescing salt crusts in
reference areas was periodically photo-documented, and the amount of losses falling down the wall was regularly collected
and quantitatively evaluated. In this way, the impact of climatic cycles on the action of salts – predominantly sodium chlorides
– was studied and the most appropriate conditions, defined by a minimum of material losses, were established at intermediate