Features of Roman plaster aggregates in Lombardy, Italy

Plaster making was reported in the seventh book of Vitruvius’ On Architecture describing two mortar coats: harenatum with sand for the bottom (render coat) and marmoratum with crushed marble for the top (finish coat). Petrographic analyses, with optical microscopy and X-ray diffraction, were made on plasters (2nd century BC - 4th century AD) coming from different roman sites of Lombardy: Milan, Brescia, Camonica valley, lake Garda, river Po plain. The analyses identified the plaster aggregates on the basis of composition, grain size and morphology. The render coats generally show river sand and/or crushed brick with some mineralogical differences due to the different supply areas. The finish coats show different types of aggregate: i) carbonate rocks (limestone, dolomite); ii) quartz crystals; iii) calcite crystals; iv) sand (quartz and silicates crystals and/or limestone clasts); v) sand and crushed brick. These differences can be useful to discriminate the changes among the time periods and among the sites.