Analysis of the plasters of some relevant chapels of the Sacro Monte (Sacred Mountain) of Varallo Sesia, Piedmont, Italy

Varallo Sacred Mountain was realized at the end of the fifteenth century on initiative of the Franciscan Father Bernardino Caimi as a place of prayer, meditation and evocation of the Christian faith and has been included in the UNESCO World Heritage List since 2003. Some of its buildings show at the moment manifold maintenance, conservation and restoration problems. Therefore, in the frame of a detailed program of interventions, a research cooperation has been launched, aiming at singling out the existent materials and, as much as possible, their working techniques. In particular, this paper deals with the investigations carried out by means of X-Ray diffraction, scanning electron microscopy observations coupled with elemental analysis, as well as optical microscopy on the plasters of three relevant parts of the "Sacro Monte", precisely the complex of the Crucifixion, the Square of the Courts, the chapel of Christ's Transfiguration on Mount Tabor. Results from the investigated samples indicated that in all cases aerial binders were used together with local aggregates from the Sesia River. Some particular compositions were found in the three buildings, probably to obtain specific aesthetic effects. Several finishing layers used for the outside walls of the Chapel of Crucifixion were made of mixed gypsum-lime mortars, probably for aesthetical reasons, while typical lime mortars were found together with dolomitic ones in the Square of the Courts and in the Mount Tabor chapels. Finally, because of the use of dolomitic limes, hydromagnesite was also evidenced in some mortars.