Sulfur determination on stone monuments with a transportable EDXRF system

An important part of the world's cultural heritage is represented by stone monuments. A very dangerous weathering process that deteriorates stone monuments and is caused by human activity is air pollution. One of the worst pollutants for stone monuments is associated with sulfur compounds, especially with gypsum. Gypsum is a rather soluble compound that may be washed away by rain. It leaves the surface of the stone clean but eroded and open to new corrosive processes. With transportable energy dispersive x-ray fluorescence (EDXRF) instruments it is possible to conduct in situ investigations, mappings, registrations and evaluations of the stone-degradation phenomena related to sulfate formation on stone and stone-like materials by quantifying sulfur concentrations directly associated with the presence of gypsum. The EDXRF instrument used in this project is a commercial system comprising a miniaturized low-power, air-cooled tungsten x-ray tube and a silicon drift detector (SDD) Peltier-cooled detector. The EDXRF instrument was calibrated using a series of standard samples specifically developed for this project with a sulfur content ranging from 0.5 to 15%. Correlating the sulfur peak counts with sulfur concentrations, a straight calibration line (R2 = 0.9985) was obtained. Several high voltage levels of the x-ray tube were tried to define the best operating conditions. The silicon escape peak of calcium had to be accounted for owing to its interference with the sulfur peak. In this paper we shall discuss the working method adopted and the results achieved following the examination of a number of stone monuments such as the “Fanciulla di Anzio” and other ancient Roman statues housed in the Roman National Museum, and the Sarcophagus of Pope Boniface VIII sculpted by Arnolfo di Cambio and situated in the Crypt of the Vatican Basilica. Copyright © 2007 John Wiley & Sons, Ltd.