Provenance Study of Gypsum Black Crusts

Abstract

CaSO4- black crusts are the major cause of the deterioration of cultural heritage monuments. Gypsum black crusts are formed on marble or other carbonate rocks as a result of the reaction of atmospheric sulfur with the calcium of the stone causing erosion of its surface. Samples of CaSO4- crusts were collected by scraping off about 1 g of the layer of decayed carbonate rocks. A total of 18 samples from the archaeological site of Vryokastro at Kythnos island in Greece, were collected. These salts may have originated from different natural and anthropogenic sources. Understanding this deterioration process is crucial for the restoration/conservation of cultural heritage monuments. The natural sources include biological sources, construction materials, rainwater, marine spray, and pyrite oxidation within the rock substrate. Anthropogenic sources include pollution from fossil fuels. The oxygen and sulfur isotopes can help to discriminate the S-origin of black crusts. The isotopic analysis was conducted in the Stable Isotope Unit of the Institute of Nanoscience and Nanotechnology (NCSR Demokritos). For S isotope analyses, sulfate minerals were dissolved in deionized water and subsequently precipitated as BaSO4. Sulfur isotopic compositions were measured after the conversion of BaSΟ4 to SΟ2 using an elemental analyzer (Flash EA device) coupled with an isotope ratio mass spectrometer.