Lead in black crust: quantification, localization and correlation to optimize risk assessment for cleaning

Built cultural heritages are exposed to environmental factors such as atmospheric pollution, especially in urban areas. Reaction between sulphur dioxide SO₂ and calcite CaCO₃ from calcareous stone forms a gypsum crust CaSO₄.2H₂O, commonly called “black crust”. This crust acts as a proxy of ancient atmospheric pollution due to the deposition of particles such as soot, organic compounds, heavy metals, etc. However, to prevent stone degradation, restore the initial aesthetic and the clarity of the architectural lines, black crust are usually removed by cleaning during a restoration project. Traditional techniques like mechanical cleaning are used in most cases. Yet, the inhalation of black crust particles may result in severe health issues. Lead, emitted from coal combustion and leaded gasoline during 20^th century and deposited in black crusts, may cause damage to the cardiovascular, nervous, renal and reproductive systems. According to previous studies, its bulk concentration ranges from a few dozen to thousands ppm depending on the city, the sampling location on the buildings and other factors. In France, prior to any black crust cleaning, the acid-soluble lead concentration must be measured by wipes rubbed on façade and must not exceed 1000 µg.m^-2. Otherwise, several measures must take place to ensure the safety of operators. However, these wipe tests were originally standardized for flat, horizontal and smooth surfaces. Moreover, wipes can only dissolve the top layer of a black crust, even though the distribution and behaviour of lead within black crust is not well-known in literature.

To address these questions, black crust samples collected in France were analysed using electron microprobe to determine the location and quantify lead particles. The morphology of these particles was further characterized using SEM-EDS. In addition, bulk analyses were also performed by ICP-MS to quantify the total lead concentrations. Results indicate that lead concentrations are high and that lead is mainly located in small particles and correlated with combustion metals such as iron. The results are key to optimize risk assessment and in situ measurements.