A hypothesis on different technological solutions for outdoor and indoor Roman wall paintings

The determination of the chemical composition of different parts of wall paintings (pigments, mortars and binders) provides information about technology of preparation of an artefact. Herein, we present a multi-methodological characterisation of wall paintings from a Roman archaeological site in Cuma, focusing on differences between an indoor (domus) and outdoor fabrication (a temple, Tempio con Portico (TCP)). Both pigments, binders and mortars were studied via a combination of destructive/μ-destructive (mass spectrometry, ionic chromatography, ICP-based techniques) and non-destructive (Raman microscopy, small-angle neutron scattering (SANS) and X-ray diffraction) methodologies. Particularly, the systematic presence of dolomite only in mortars from TCP may suggest an intentional use of such limestone for the outdoor fabrication of public interest. Differences between TCP and domus are also related to the composition of the pigment binder. In particular, the detected binders (studied by LC-MS/MS and GC-MS) were proteinaceous in the case of domus (possibly egg and animal glue) and drying oils in the case of TCP. Ultimately, our multi-methodological study provides an overall picture of the material components of paintings from fabrications with different use, proposing a hypothesis on technological choices according to conservative and destination reasons.