Roman brick production technologies in Padua (Northern Italy) along the Late Antiquity and Medieval Times: Durable bricks on high humid environs

Production technologies of Roman to Medieval times bricks on the city of Padua were addressed by means of a multi-analytical approach, consisting in Spectrophotometry, X-Ray Fluorescence (XRF), Powder X-Ray Diffraction (PXRD), Polarized Optical Microscopy (POM) and Scanning Electron Microscopy (SEM-EDS). The Early-Christian (5-6th centuries) and Romanesque (12-13th centuries) areas of the Basilica of Saint Justine of Padua (5/6-16th centuries), and the remains of the Roman necropolis (1-3/4th centuries) still preserved under the basilica, were selected to collect the bricks in order to define possible differences between the materials used for the various building phases. The walls are mainly shaped by yellow (with pale and dark hue) and beige colored bricks with an overall good conservation state. The ceramic bodies of this type of bricks showed the development of high-temperature phases but a low sintering degree was achieved. Moreover, secondary phases such as zeolites and calcite were formed, within almost the pale-yellow bodies and intensively precipitated through the groundmass of the beige bricks, respectively. Mg-rich calcareous clays and chloritic-illitic clays were used, firing temperatures of or over 900°C were reached and more porous ceramic bodies were produced when higher was the carbonate content on the raw clays. A lesser carbonate content of the base clays and/or the decrease in the firing temperatures were the main technological modifications progressively accomplished, leading to color changes on the ceramic bodies from yellow to beige hue. The Roman production technologies might be largely inherited by the brick makers during the Late Antiquity and Medieval times in the city and the reuse of more ancient bricks during the Medieval Times was confirmed. Such reuse operations have allowed to observe that under high humid conditions the yellow hue bricks have been rather good preserved, while when exposed to insolation and fluctuations of the environmental conditions a significant granular disaggregation -with the concomitance darkening of the color pastes- is developed. The color of bricks may entail an identifier of a specific construction period of the city and durable bricks from local clays, especially suitable for high humidity areas and that may preserve the aesthetical values of the city of Padua, may be currently produced