Iron patinas on alabaster surfaces (Santa Maria de Poblet Monastery, Tarragona, NE Spain)

Abstract Alabaster is a white and pure gypsum rock classically used in sculptures or for ornamental purposes, but its characteristic white colour is sometimes masked by the development of noticeable reddish stains over the surface of the rock. The main altarpiece of the Santa Maria de Poblet monastery (Tarragona, NE Spain; 16th century, Damia Forment) is the focus of this study. Red-to-ochre patinas with diameters of 2–20 cm are found on the alabaster surface of this altarpiece. The origin of such patinas are discussed, as they are the result of degradation processes of minerals present in alabaster rocks, contrary to what happens in the formation of most patinas. The patinas consist of two differentiated layers that may occur occasionally mixed. The Lower Layer contains iron compounds, which have precipitated around the gypsum crystals of the rocky support and have provided the characteristic red-to-ochre colour to the surface of the altarpiece. The formation of this layer was driven by the (bio)oxidation of the pyrite, which is disseminated over the alabaster surface. The formation of this film of iron-rich particles was conducted by a series of destructive and penetrative processes, promoting disaggregation and crystal reduction (mechanical and/or by dissolution) of the matrix minerals (gypsum, celestite, barite, calcite-dolomite…). The Upper Layer was grown by means of constructive (agglutination of particles by accretion) and destructive (destruction of the alabaster matrix and incorporation into the Lower Layer) mixed processes. Thus, the presence of small crystals (1–10 μm) of gypsum, quartz, calcite, celestite, barite, clay minerals and pyrite in this upper patina are mainly related to the residual products of the alabaster rocks. Moreover, some of the calcite, quartz and clay particles could also be considered atmospheric dust. Oxalates (weddellite and whewellite), portlandite and coal particles are not related to the formation of iron-rich patinas. Such studied patinas might date from the 19th century due to the partial destruction of the monastery after the approval of the Law of Confiscation of Religious Properties in Spain (1835) and no later than the end of the ninetieth century, when the monastery was abandoned. The entry of rainwater and presence of humidity inside the monastery would promote the pyrite oxidation processes. Currently the patinas seem to be stabilized, the pyrites are no longer in contact with water, which is needed for oxidation.