American Mineralogist, Volume 88, pages 1996–2006, 2003 0003-004X/03/1112–1996$05.00 1996 INTRODUCTION Glassmaking has been an industrial activity for more than four thousand years. Flat glass, however, has been only exten- sively produced and used for architectural purposes for the last one thousand years, following the building of cathedrals and religious buildings in Central Europe in the early Middle Ages. Thus, the remnants of medieval stained glass windows consti- tute a part of our cultural heritage that has been exposed to environmental damage over centuries. This provides an excep- tional opportunity to test glass durability and to understand long- term environmental corrosion processes on glass. In addition, in historic buildings, both the rock and the glass have been exposed to the same atmospheric conditions, which allows for comparison of the mechanisms of decay on both materials. The main purpose of this paper is to provide new insights into the biodegradation processes of stained glass and associated mineralization. To do so, several European Medi- terranean case studies have been chosen and studied, taking into account the fact that the decay of historic glass seems to appear at an earlier stage when compared with most Central European glasses. State of the art Glass is made up of several components: network formers, network modifiers (flux and stabilizers), and coloring elements * E-mail: Domingo@natura.geo.ub.es Medieval stained glass in a Mediterranean climate: Typology, weathering and glass decay, and associated biomineralization processes and products M. GARCIA-VALLÈS, 1 D. GIMENO-TORRENTE, 2, * S. MARTÍNEZ-MANENT, 1 AND J.L. FERNÁNDEZ-TURIEL 3 1 Department Cristallografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona, c/ Martí i Franquès s/n, 08028 Barcelona, Spain 2 Department Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona, c/ Martí i Franquès s/n, 08028 Barcelona, Spain 3 Institut de Ciències de la Terra Jaume Almera, CSIC, c/ Solé i Sabarís s/n, 08028 Barcelona, Spain ABSTRACT Medieval stained glass (13 th –15 th centuries) from three restoration works (Santa Maria del Mar and the Pedralbes Monastery church in Barcelona, and the Tarragona Cathedral in the northwestern Mediterranean area have been studied to characterize glass decay. Electron microprobe analysis gave evidence of two types of glass: an Na-rich type (stable, Mediterranean, of Roman-like tradi- tion), and a K-Ca-rich type, similar to coeval Central European medieval stained glasses. The latter shows glass decay in the form of destructive (micro- and mesopitting) and constructive (patina and microcrust) glass surfaces. Nevertheless, this type of decay in the K-Ca-rich group of glasses is in terms of thickness reduction in flat glass an order of magnitude less than that commonly found in Central European countries with a continental climate. Macroscopic and microstructural studies (SEM-EDS, XRD) allowed us to identify biodeterioration decay with chemical effects similar to that for pure chemical hydration leaching and corrosion, associated with biomineralization with unspecific sulfate (gypsum, syngenite) and calcite mineralization as well as bioinduced (weddellite, whewellite, etc.) mineralization. Medieval-stained glass biodeterioration decay in the Mediterra- nean area shows patterns and products comparable to those developed on stone in the same historic buildings. (Fernandez Navarro 1985; Newton and Davison 1989; Römich 1999). The main network former of medieval stained glass is silica. Its content determines the physical and chemical behav- ior and the stability of silicate glasses. Flux decreases the tem- perature at which the mixture (essentially the silica) melts and is made up of alkaline elements (Newton and Davison 1989; Römich 1999). In medieval times, a flux was introduced into the mixture as vegetal ash. Stabilizers (lime, magnesia, etc.) make the glass strong and more water resistant. Phosphorus is a common component of medieval glasses (Pérez-y-Jorba et al. 1984); it can also act as a network-former (Römich 1999) and can be attributed to the ashes used as raw material to ob- tain alkali, especially potash (Freestone 1993). Furthermore, several metals (Cu, Co, Mn, etc.) were used to provide color to the glass (Bamford 1977; Newton and Davison 1989). If we consider the main elements in the glassy mesostase, historic glass can be classified, from a chemical point of view, into several types (Newton and Fuchs 1988; Brill 1999, and refer- ences therein). Most European medieval stained glass produced between the 12 th and 15 th century can be considered potassic in composition. Recent research on coeval European Mediterranean glass shows the continuation of a Roman-like sodium glassmaking tradition. Despite this, there is a mixture of pieces of glass of both compositions (usually restricted to different colors) in the same Mediterranean stained-glass windows (Julià et al. 2001; Garcia- Vallès and Vendrell 2002; Gimeno and Pugès 2002 in Spain; Gimeno, unpublished data, for Siena Cathedral, Italy).