Preservation strategies for avoidance of salt crystallisation in El Paular Monastery cloister, Madrid, Spain

El Paular Monastery (eleventh century) is one of the most important Carthusian monasteries in Spain and is highly affected by crystallisation of Mg-sulphates, together with chlorides and nitrates. Urgent remediation of the decay process is needed to guarantee the stability of the building materials from the cloister and to make their hallways suitable for the exhibition of an important collection of seventeenth century paintings. This paper aims to characterise the building materials, salts and their interaction to suggest preservation strategies to minimise the impact of salts both in the short and the long term. These strategies include architectural solutions (such as a ventilation system to avoid increasing dampness and hence the dissolution, mobilisation and crystallisation of salts), petrophysical-based solutions (i.e. exploiting the porosity differences between building materials and poultices to maximise salt reduction) and strategies based on the physicochemical behaviour of salts and relative humidity transfer through the stone (to determine the most suitable environmental conditions to prevent crystallisation of the most harmful salt species). This research represents both a practical and experimental exercise that is useful for conservation scientists and restorers involved in the field of preservation of monuments, and for environmental control to avoid salt crystallisation.