The paper deals with a medieval subterranean interior situated in the centre of Vienna. The so-called Virgil chapel had originally
formed the crypt of a two-storeyed church dated around 1240, which had been demolished in 1781. Filled with demolition material,
the crypt survived and was excavated during the construction works for the underground railway in the 1970-ies.
At the time of uncovering, important parts of the original surfaces carrying a painted render were still in place, but since
then, the decay of the wall surfaces proceeded continuously. The problem was obviously caused by soluble salts migrating from
the soil of the surrounding bedrocks into the walls.
Since no technical solution to isolate the walls from the adjacent ground was found and occasional desalination of the walls
by poultices had no lasting effect, the present study aimed at reducing the damaging actions of the salts by an appropriate
climate control. The approach was based on a thorough documentation of all relevant types of decay, followed by careful monitoring
of all dynamic factors and processes driven by climate 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 continuously
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 rather low relative humidities.
It was understood, however, that reduced losses of paint layer and plaster were due to a consolidating effect of the salt
crystals, a mechanism likely to produce unstable conditions on a long term for a number of reasons. Periodical salt extraction
by poulticing will therefore be necessary as auxiliary means following the climate control.
Johannes Weber, R. Burszán, G. Pinar, E. Hottenroth, J. Riedel, K. Sterflinger