Mineralogical-petrographic characterisation and provenance of "Porta Nuova" stones: a XVI century gate in Ravenna (Italy)

Abstract Porta Nuova in Ravenna (Northern Italy) is one of the most beautiful gates of ancient Ravenna walls, built in the XVI century (1580-1585). Through the centuries, the door underwent several restoration events, the most important in 1653. A recent architectural survey has revealed that the Door is currently undergoing both structural deterioration and bad conservation of stone materials. Therefore, the Door has been subjected to laboratory investigation for the purpose of collecting data useful for restoration. This work presents a detailed mineralogical-petrographic, diffractometric and electron microscopy investigation of Porta Nuova stones, which brought to a classification of both natural and artificial materials and to a preliminary evaluation of their deterioration. The results indicate the presence of different ornamental stones. The widespread material is Pietra d’Istria in the four varieties Orsera, “vacuolar”, “nodular” and “facies liburnica”. The use of red “marble” Rosso Ammonitico Veronese was saved for some elements (e.g. entablature frieze, columns pedestals) as to create a beautiful chromatic effect. The frame band of the entablature and the Composite capitals were built with Marmor Proconnesium, whereas Marmor Troadense was employed in the columns drums. Moreover, Pietra di Prun was identified in the capitals’ astragals and Trachite dei Colli Euganei in the small pillars supporting the upper hinges of the main door. All these ornamental stones belong to the historical materials imported since the Roman age in Ravenna from the Veneto-Istrian and Greek-Turkey areas. Taking into account the demolition of the Porta Aurea Arch (occurred in 1582) and of other buildings of Roman age, it is likely that most of the ornamental stones used to build Porta Nuova were re-employed from former buildings. The hanging wall façade is built with recovery bricks of different chromatic shades depending on the concentration of iron oxides. Characteristic secondary phases (e.g. gehlenite, analcime, cristobalite) detected by diffractometric analysis confirm that bricks derive from high temperature firing of the mixture in furnaces. Bricks surface grinding (“sagramatura”) was a common feature. Original lime bedding mortars and finishing plaster mortars with fine sand framework have been distinguished in the hanging wall. A late micro-conglomeratic finishing plaster mortar has also been detected, likely ascribed to one of the several restoration interventions.