Assessment of residual effectiveness for Water-Repellent treatments for building stones. Water absorption tests on the monumental complex Cathedral of San Zeno and the Baptistery of San Giovanni in Corte in Pistoia

Preventive and temporal planning of conservation is rather difficult to perform, especially concerning monumental stone buildings; the choices are often made after the degradation phenomena have already started. Many studies are looking for the reasons for this ‘resistance’. In particular the present study, the result of a doctorate research, is focused on the most operational aspects: • quantifying the residual effectiveness of water-repellent protective agents used in past restorations; • providing elements in the drafting of maintenance plans for monumental heritage; • verifying where critical conservation elements reside. The test site has been the monumental complex formed by the Cathedral of San Zeno and the Baptistery of San Giovanni in Corte in Pistoia, where the Contact Sponge Water Absorption Test (UNI 11432:2011), typically used as treated/untreated test was employed on three lithological types (white marble, serpentine and Tuscan grey sandstone) as comparison test between treatments carried out at different knows times. The test campaign was conducted by Mila Martelli with the involvement of Superintendency of Archaeology, Fine Arts and Landscape; University of Florence DiDA, DST-LAM, Department of Chemistry and, in the phase of organization of the monitoring, with the involvement of Dr. Maria Jose’ Ybañez Worboys, art restorer. The results of this research must be read in consideration of the fact that it is an ex-post survey, for which not all factors have been kept under control from the beginning, but for which it was possible to make assumptions. This study illustrates the methods and the first test results obtained which show a rather rapid decline in the effectiveness of water-repellent treatments, already a few years after their application.