Technology of Medieval Mortars: An Investigation into the Use of Organic Additives

This work proposes a multi-analytical approach to determine the additives in historical mortars, the use of which is widely described in bibliographical sources, but has rarely been reported in the literature. A protocol to thoroughly analyse mortars was created (optical microscopy, X-ray diffraction, infrared spectroscopy, thermal analyses and gas chromatography - mass spectrometry). These techniques, which had already been carried out on samples from various sites from the Roman to the modern era, determined that additives had only been used in the mortars from the internal masonry at our sampling site: the medieval military shipyard of Amalfi (Italy). The investigations yielded information on the production technology, and FT-IR and GC-MS revealed a saccharide material-based additive in the mortars, of plant origin. The FT-IR spectra suggested the presence of a natural gum, which has been used since ancient times to strengthen the cohesion properties of mortars and their resistance to tensile stress.