Geomaterials used for the construction and decoration of the House of Petronia (I 16, 5) in Pompeii: an archaeometric study

The House of Petronia (I 16, 5) is a dwelling built at the end of the 3rd century BCE with a testudinatum atrium. This architectural typology, rare in the roman period, is characteristic of the more ancient phases of Pompeii and is specific of the middle class. The dwelling was affected by several transformations over time. In particular, at the end of the 2nd century BCE, it underwent a renovation that included an extension and the creation of a garden at the rear, from which several reception rooms were opened, including a large exedra covered with an elegant pavement, which probably had the function of a triclinium. At the beginning of the 1st century CE, the exedra was divided in three spaces: a reception room (15), a cubiculum (14), and a corridor (13) and these rooms were decorated with wall paintings in the 3rd style. In order to obtain data on the workers’ practices and identify the geomaterials and techniques used for construction and decoration of these rooms over time, a multi-analytical study including in situ non-destructive analyses and in-lab minero-petrographic investigation was performed, revealing interesting insights on a) the type of pigments used for the wall paintings, b) the production technology used for making plasters and bedding mortars, and c) the origin of stones used for the pavement. Digital microscopy, X-ray fluorescence, Fourier Transform Infrared Spectroscopy and Raman spectroscopy performed on wall paintings allowed for the definition of the composition of pigments used for decorating the rooms overlooking the garden, revealing the use of a characteristic Roman palette. Different colours were obtained by using pure, natural, and synthetic pigments (calcite, red and yellow ochre, red lead, green earth, Egyptian blue, carbon black), also mixed together to obtain specific hues, such as mixture of red ochre and red lead for red and brownish tones, addition of Egyptian blue to red pigments for purple or brightening with calcite for pink, mixture of green earth and Egyptian blue for green shades. Moreover, vibrational spectroscopy disclosed the presence of synthetic resins, likely spread on the decorated surfaces during previous restoration works for the conservation of wall paintings, and weathering products, mainly consisting in Ca-oxalates and Ca-sulfates, deriving from the alteration of organic compounds and Ca-carbonate substrate, respectively. Spectroscopic techniques, in fact, also provided information on the composition of the underlying support, made of lime-based plasters. Observations via Polarised Light Microscopy confirmed the use of lime binder to make plasters. The latter was mixed to volcanic aggregate showing a mineralogical composition consistent with local volcanic raw materials. The occurrence of leucite-bearing scoriae, garnet, and olivine along with crystals of plagioclase, clinopyroxene, juveniles (scoriae and pumices), and traces of amphibole and biotite, in fact, suggested the use of a volcanic sand from Vesuvius’ environs. The bedding mortars were also prepared with the same raw materials, although with different mix-designs. Regarding the pavement decorating the exedra of the 2nd century BCE phase, it consists of a cement floor with geometric decorative motifs in white cubic tesserae, very common in this period; however, it has the peculiarity of being enriched with polychrome stone inlays. Archaeometric data carried out on white, yellow, green, grey, black and red stones used for the decoration revealed that, except for the green stones, they are mainly constituted by calcium carbonate, occurring along with carbonaceous material in the grey and black stones and iron oxides and hydroxides in the red and yellow rocks, respectively.