Abstract
Black manganese-rich crusts are found worldwide on the façades of historical buildings. In this study, they were studied exemplarily on the façade of the Freiburger Münster (Freiburg Minster), Germany, and measured in-situ by portable X-ray fluorescence (XRF). The XRF was calibrated to allow the conversion from apparent mass fractions to Mn surface density (Mn mass per area), to compensate for the fact that portable XRF mass fraction measurements from thin layers violate the assumption of a homogeneous measurement volume. Additionally, 200-nm femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS) measurements, scanning transmission X-ray microscopy-near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS), Raman spectroscopy, and imaging by light microscopy were conducted to obtain further insight into the crust material, such as potential biogenic contributions, element distributions, trace element compositions, and organic functional groups.While black crusts of various types are present at many places on the minster's facade, crusts rich in Mn (with a Mn surface density >150 μg cm−2) are restricted to a maximum height of about 7 m. The only exceptions are those developed on the Renaissance-Vorhalle (Renaissance Portico) at a height of about 8 m. This part of the façade had been cleaned and treated with a silicon resin as recently as 2003. These crusts thus accumulated over a period of only 12 years. Yet, they are exceptionally Mn-rich with a surface density of 1200 μg cm−2, and therefore require an accumulation rate of about 100 μg cm−2 Mn per year.Trace element analyses support the theory that vehicle emissions are responsible for most of the Mn supply. Lead, barium, and zinc correlate with manganese, indicating that tire material, brake pads, and resuspended road dust are likely to be the element sources. Microscopic investigations show no organisms on or in the Mn-rich crusts. In contrast, Mn-free black crusts sampled at greater heights (>8 m) exhibited fungal and cyanobacterial encrustation. Carbon-rich spots were found by STXM-NEXAFS underneath one of the Mn-rich crusts. However, these carbon occurrences originate from soot and polycyclic aromatic hydrocarbons (PAHs) deposited on top of the crust, rather than from organisms responsible for the crust's formation, as shown by STXM-NEXAFS and Raman spectroscopic measurements. Our results suggest that the crusts develop abiogenically, with vehicle emissions as dominant element sources.
Highlights
Historic buildings and monuments in urban areas often show extensive dark or black discolorations, which affect their aesthetic appeal and whose restoration can have considerable economic implications (Newby et al, 1991)
Our results suggest that the crusts develop abiogenically, with vehicle emissions as dominant element sources
This study aims to investigate the chemical composition of these coatings, which belong into the category Type IV of rock varnish (Macholdt et al, 2017), their distribution on the cathedral surfaces, their formation mechanisms, and the source of the Mn at the example of the Freiburger Münster
Summary
Historic buildings and monuments in urban areas often show extensive dark or black discolorations, which affect their aesthetic appeal and whose restoration can have considerable economic implications (Newby et al, 1991). Most of these blackened areas were thought to be formed by the accumulation of soot, which on carbonate building materials is often incorporated into a matrix of gypsum. The gypsum layer develops as a result of the sulfur dioxide (SO2) released in the course of fossil fuel combustion, which can either react directly with the carbonate material or after its oxidation to sulfuric acid in the atmosphere (Bonazza et al, 2005; Brimblecombe and Grossi, 2009; Grossi and Brimblecombe, 2002; Ruffolo et al, 2015). The growth of cyanobacteria (Bonazza et al, 2007b; Uchida et al, 2016), bacteria (Miller et al, 2012), and fungi (De Oliveira et al, 2011; Gorbushina et al, 1993; Saiz-Jimenez et al, 2012; Viles and Gorbushina, 2003) was implicated in the discoloration of exposed stone materials
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