Weathering of gilding decorations investigated by SR: development and distribution of calcium oxalates in the case of Sant Benet de Bages (Barcelona, Spain)

Abstract

In this paper seventeenth century gilded paintings from the crypt of Sant Benet de Bages, a medieval monastery in the Catalonia region of Spain, near Barcelona, have been studied. Cross sections from two different gilded decorations were studied by means of optical microscopy and electron microscopy and EDS to determine the statigraphy and elemental composition, and by means of FTIR coupled to a microscope to determine the binding media associated to each layer. These preliminary results demonstrated that gilded decorations were made by the application of a gold foil on a mordant substrate on a gypsum base, while the mouldings of the vaults seem to be gilded on a bol with a glaze on top of the gold leaf. It is interesting to notice that the first remained unaltered, while the gilded vault mouldings look almost black, due to the darkening of the organic material. To elucidate the causes involved in the darkening of the sample from the vaults a set of synchrotron μXRD and μFTIR experiments have been carried out on these samples at the ESRF (ID18F and ID21, respectively). High brightness and small spot working conditions revealed the development and distribution of calcium oxalates in the binding media, which seem to be responsible for the darkening. Results point out the fact that weddellite (CaC₂O₄·2H₂O) is the phase formed in those layers where organic material has also been identified or at least it would be supposed to be by bibliographic sources and not necessarily those superficial as it would have been suggested due to the similarities with patinas formation.