The Sant’Antioco di Bisarcio Romanesque Basilica (NE Sardinia, Italy): traces of ancient treatments on the stone materials of the façades

ABSTRACT: The Sant’Antioco di Bisarcio Basilica was built between the late XI and early XIII century. It was a diocese cathedral from the end of the XI to the beginning of the XVI century, when the episcopal see was suppressed. Longitudinal in plan, the church has a hall with an apse and three naves separated by pillars topped by Romanesque capitals. The original façade was overlain with a two-storey portico. The Romanesque fabric can be distinguished by the walls, built of medium to large ashlars, carefully hewn and lain using the technique adopted by builders of Tuscan tradition, employed in the ‘Giudicato di Torres’ since the XI century. The hewn stones were dry laid and carefully aligned, but did not achieve the refined features that distinguished Cistercian buildings. The materials used to construct the Basilica consist of pyroclastic rocks belonging to the volcanic cycle of calcalkaline affinity, which occurred in Sardinia between 32 and 11 My ago. Very small amounts of epiclastic products were also detected. Petrographic and geochemical data indicate that the volcanic rocks have composition mainly ranging from dacite to rhyolite. Traces of more or less continuous greyish, rosy or yellowish films are present on the ashlars and on the decorative architectonic elements. Their thickness is variable (10-50 μm) owing to the roughness of the stone surfaces. XRD analyses performed directly on the collected microsamples and on powders gently scraped from their surfaces revealed calcite+weddellite in some of them and calcite+weddellite+whewellite in others. Preliminary observations (the work is still in progress) of these Ca-oxalate films at the polarizing microscope (in thin and ultrathin sections) showed two different microstratigraphies. The first is a single film consisting of very fine grains of carbonate rock and even finer grains of yellow and red ochres in a micritic-weddellitic binder; in the second, a film similar to the preceding one is superimposed on another film made of pure whewellite. Ca-oxalate films, which represent the transformation in time of ancient finishes on many monumental surfaces, have never been found before in Sardinia. This case study and the one reported in a parallel work presented in the same meeting constitute a reference point for further studies to be conducted on other Sardinian monuments.