WEATHERING, CONSERVATION AND RESTORATION OF THE SANTA MÓNICA CHURCH IN GUADALAJARA, MEXICO Karla Jáuregui Arreola 1 , Wanja Wedekind 2 and Siegfried Siegesmund 2 1 Memento Conservación y Restauración Guadalajara/Mexico 2 Geoscience Center of Georg-August-University Goettingen/Germany Abstract The Santa Mónica Church of Guadalajara is an outstanding example of the Mexican baroque because of its rich decoration. Typical regional building stones consisting of a yellow porphyritic tuff called the "Cantera Amarilla" were used to build the church. Similar to many other buildings constructed with this material, the cut stones of the church also show back-weathering concentrated in the basement area, which can be traced to wet and dry cycles and the accumulation of salt. Conservation by desalination of the basement area was undertaken by cyclical sprinkling and measuring of the electrical conductivity of the excess water runoff. Using this method around 3000 grams of soluble salts could be extracted. During the restoration, diagnostic investigations were done consisting of mapping the deterioration, determining the moisture content and measuring the temperature as well as evaluating the salt accumulation by drilling powder analysis. The mineralogical composition of the original stone material was determined by thin sections and X-ray diffraction (XRD) measurements. Investigations on the petrophysical properties focussed on the impact of wet conditions. Compressive and tensile strength tests were done under dry and water-saturated conditions as well as hygric and thermal dilatation, swelling pressure, water uptake, and sorption. To evaluate the impact of the inhomogenous internal structure, experimental testing of hygric swelling on a cubic stone block were performed. The results from the study show that a reduction of the mechanical properties up to 40% by water saturation could be detected. Back-weathering is probably due to the inhomogenous internal structure, resulting from the interaction of clastic material and the fine-grained ash-matrix induced by different swelling intensities and swelling pressure. Keywords: tuff stone, desalination, moisture expansion, critical values 1. Introduction The church of Santa Mónica is located in the historical centre of Guadalajara, the second largest city in Mexico. This building was once part of a monastary and was erected in the first half of the 17 th century. The church is an outstanding representation of Mexican baroque because of the richly decorated facades (Fig. 1) and is 12 x 64 meters in size and 14 meters in height. Two portals with Salomonic twisted columns decorate the baroque facade, where they are covered with rich and intricately-carved ornamentations, including grapes, cobs of maize, angels, double eagles and symbols of religious orders. As a model for the different decorations, the cathedral of Cajamarca in