22 ANALYSIS OF MOISTURE MIGRATION IN THE WALLS OF THE S. MARIA DEI MIRACOLI CHURCH IN VENICE P. BAGGIO, C. BONACINA and A.G. STEVAN Istituto Fisica Tecnica Università di Padova, Italy V. FASSINA Laboratorio Scientifico, Soprintendenza ai Beni Artistici e Storici, Venice, Italy Abstract The walls of the S. Maria dei Miracoli church in Venice are suffering serious degradation due to the combined physical and chemical action of soluble salts. The church walls have a high moisture content due to rain penetration. Through labora tory tests and with field measurements of the moisture content in various areas of the walls, the thermal and hygrometric characteristics of the bricks constituting the wall and of the marble slab covering are analyzed. The data concerning the indoor climate and its influence on the phenomena are consid ered too. Knowing the boundary conditions (indoor climate) and the relevant data of the constituting materials (clay bricks and marble), a numerical model of the heat and mass transfer in porous media is employed to study the drying of the walls. Keywords: Moisture-induced damage, Bricks, Historical Build ings, Porous building Materials, Capillarity. 1 Introduction The church of S.Maria dei Miracoli is one of the most famous monuments of the early Italian Renaissance. It was built in 1489 following the project of Pietro Lombardo. A unique char acteristic of this church is that the brick masonry is com pletely lined with decorated marble facings. These marble slabs show massive salt efflorescence formation. The decay suffered by the structures and decorations of the building is due to the combined physical and chemical action of soluble salts added to the particular aggressiveness of the Venice lagoon environment (Fassina, 1978). The damage to the walls derived from salt attack, however, requires high moisture content to permit solution, migration and crystallization of salts. According to studies carried out ten years ago, salt efflorescences on marble surfaces were caused by water migration from the brick masonry (De Vecchi et al., 1981). The presence of moisture in the building's struc ture can originate from various sources including rain, atmos pheric humidity, capillary rising and surface condensation phenomena. A detailed survey that considered three vertical lines (Fassina and Stevan 1990) demonstrated that the mois ture distribution in the walls (increasing with height) cannot Conservation of Stone and Other Materials. Edited by M.- J. Thiel. © RILEM. Published by E & FN Spon, 2-6 Boundary Row, London SEI 8HN. ISBN 0419188401 (vol. 1). 170