Morphological and geochemical characterization of the particulate deposits and the black crust from the Triumphal Arch of Galerius in Thessaloniki, Greece: Implications for deterioration assessment

In the present study, 12 particulate deposits and one black crust sample were collected from the Triumphal Arch of Galerius in Thessaloniki, Greece and characterized by employing a multi-analytical approach including chemical analysis of trace elements and ionic species, as well as scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDS) to gain information about the micromorphology and the chemical composition in terms of major elements. In addition, one unaltered marble sample, e.g. the marble directly beneath the black crust, was examined by microscopic and isotopic methods to characterize its texture and origin. The particulate deposits consist mainly of calcite, quartz, aluminosilicate mineral phases, several metal oxides of Fe, Ti and FeCr with Mn and Cu. They also include bird droppings enriched in P and S, and plant residuals. The black crust has a similar mineral composition and is dominated by calcite with traces of quartz and halite, whereas P- and S-enriched particles are common. In both cases a coating on calcite crystals with a thin crust rich in Ca, Ba and S is commonly observed and is attributed to the previous conservation works. Concentrations of As, Zn, Pb, Cu, nitrate, sulfate, chloride and acetate were significantly higher in particle deposits than in the black crust as opposed to Fe, Co, Ni and formates that were at the same level. The traffic-related trace elements Pb, Zn and Cu and most ions were significantly higher in low-altitude deposition samples. The current marble deterioration is induced by a combination of factors, including mechanical, physico-chemical and biological processes.