Micro FT-IR mapping of superficial layers on Septimius Severus Arch (Rome, Forum): evaluation of spatial distribution of calcium oxalate films on cross-sections.

The Infrared mapping spectroscopy is a new analytical technique able to obtain the characterization of both the composition and the spatial distribution on different layers of organic and inorganic compounds in multilayered samples. This technique is useful to study the alterations on the surfaces of monuments and aids the comprehension about the processes of growth of superficial layers due to alteration products of biological origin or residues of ancient conservation treatments. The aim of our work was to study the calcium oxalate films on the Arch of Septimius Severus (203 AD, Rome) in Roman Forum. Micro-FTIR mapping was conducted on cross-sections, acquiring infrared spectra with spatial information, rendered on chemical distribution maps for main constituents: calcium oxalates, silicates, calcium sulphate and calcite. The reflectance spectra were collected by a Bruker Optics VERTEX 70 Ft-IR Spectrofotometer, equipped with HYPERION 2000 microscope. The analytical approach highlights the strong adherence of oxalates on the carbonatic substrate. Moreover, the presence of past conservative treatments using silicatic compounds was investigated by the silicate/oxalate ratios, in each point of analysis. A stratification of layers based on oxalates, silicate compounds and gypsum was identified, clarifying different compound distribution and contributing the comprehension of past conservative practices on this monument.