Petrographic characterization of historic mortar as a tool in archaeologic study: Examples from two medieval castles of Aosta Valley, Northwestern Italy

In the present work is reported the study of historical mortars sampled in the two medieval castles of Chˆatel Argent and Quart (Aosta Valley, NW Italy), which in recent years are subject to restoration projects. All the samples were subjected to a petrographic analyses with an optical microscope, in order to recognize and compare the minerals constituting the main aggregates with the geological formations surrounding the areas where the castles stand. Four mortar samples were analyzed for each castle with the scanning electron microscope (SEM-EDS system). This procedure allows us to know the composition of the binder (not distinguishable on the basis of a petrographic analyses under an optical microscope), to calculate indices related to the composition of the binder, to analyze the composition of the main minerals present in the aggregate. Then, compositional X-ray maps were performed, in order to investigate the relative abundances of the different elements and calculate other important information of the mortars such as the hydraulic index, the porosity distribution and the abundance of aggregate with respect to the binder. The analytical protocol allowed us to define not only the characteristics of the individual mortars, but also to detect the area of provenance of the raw materials used for their realization, guiding the choice of the best materials for future restoration operations.