The Casa del Fauno in Pompeii: Archaeometric investigation of ancient mortars as an attempt to clarify the architectural phases

The compositional study of the ancient mortars, carried out through macroscopic observations, X-ray diffraction (XRD), and transmitted polarized light optical microscopy (OM), combined with multivariate statistical analysis (PCA), has provided important new insights into the process of understanding the building history of the Casa del Fauno in Pompeii. This concerns above all the final phase of the house, designated CdF 3, carried out after the earthquake sequence of AD 62/63, together with the awareness that the Casa del Fauno remained largely intact throughout the long period from the Samnite age until the seismic events. This further confirms the hypothesis that the house was modified in a continuous and progressive manner not so much through small structural interventions, but rather through two major building campaigns which, with phase CdF 2, reflect on the one hand the intention of the patrons to assert and reinforce the prestige of the Casa del Fauno — already achieved in construction phase CdF 1 — within the competition among the urban elite, and on the other hand indicate that the earthquakes provided the impetus for profound architectural changes in construction phase CdF 3. In detail, three main construction phases are considered: CdF 1, the original Samnite-period house (second quarter of the 2nd century BCE); CdF 2, a radical refurbishment and enlargement with a complete redecoration (first quarter of the 1st century BCE); and CdF 3, the final and less uniform phase connected with the earthquakes and their aftermath. Optical microscopy and PCA allow the eleven bedding mortars to be grouped into four compositional groups (MGr 1–4), all produced with the same local Campanian raw materials (lime and pozzolana) but distinguishable by petrographic composition. The correlation of these groups with the masonry evidence largely confirms the chronology proposed by previous historical-architectural research, but it also refines it: in particular, it indicates that the enlargement of the eastern wing and several interventions previously dated to the Augustan period should instead be attributed to the post-seismic phase (AD 62/63).