Florence City Centre Historical Mortars: Digital Image Analysis of thin sections in view of conservation actions

Abstract

Conservation actions in historical urban centres must consider compatibility criteria in the selection of repair materials to ensure effective long-term intervention. Along this, compatibility between old and new mortars employed in conservation action requires the characterization of the historical mortar to design a new mortar mix. Digital image analysis (DIA) of thin sections of historical mortars observed under a transmitted polarised light optical microscope appears as an effective methodology for the characterization of mortars. In this work, three samples of bedding mortar were collected from historical monumental buildings of the period between the 14th and 15th centuries, in the city centre of Florence, a UNESCO Heritage Site since 1982, and subjected to petrographic investigation through DIA. Quantitative data about the composition of the analysed historical bedding mortars were determined, such as binder/aggregate ratio, percentage of porosity, the type and the percentage of granulometric selection. The comparison of the results obtained has made it possible to highlight the weaknesses and strengths aspects of the construction technique.