Читать книгу Monument Future - Siegfried Siegesmund - Страница 231

Our study of manganese-rich urban rock varnish initially focused on blue-black patches found on the Smithsonian Castle (1855), built of red Seneca sandstone (Fig. 1); microanalysis reveals that this Mn varnish occurs as the mineral birnessite: (Na,Ca)_0.5(Mn⁴⁺,Mn³⁺)₂O₄·1.5H₂O (Sharps et al. 2020). More recently we have observed Mn-rich rock varnish growing on sandstone buildings and monuments in the United States and Scotland, and Mn-rich rock varnish has also been reported in France and Germany (Gatuingt et al.; Macholdt et al. 2017a).

Figure 1: Patch of urban varnish on the southwest corner of the Smithsonian Castle. Note bluish appearance.

Research has indicated that this varnish has a biological origin (Livingston et al. 2016). Preliminary 162DNA studies have found fungi and bacteria growing in the varnish, but the role of Mn in their metabolic processes is still not clear. In order to gain a better understanding of the phenomenon it is necessary to increase knowledge of its origin, geographic distribution, rate of growth, and vulnerable types of stone. This could be used to develop a model to predict its spread and may also assist in developing treatments to control it. Obtaining this knowledge involves collecting data from actual cases of occurrence.

A crucial step in data collecting is the correct identification of an occurrence of the varnish as opposed to ordinary inorganic soiling or other types of biological growth such as cyanobacteria. A visual clue is the appearance, because the urban varnish tends to be slightly glossy and ranges from light blue to dark blue-black in color (Fig. 1) compared to the dark black matte appearance of other types. However, the essential diagnostic feature is the elevated level of Mn. This element can be measured very accurately on samples in the laboratory using X-ray fluorescence (Vicenzi et al. 2016; Sharps et al. 2020) or laser ablation mass spectroscopy (Macholdt et al. 2017b). However, taking samples can be problematic, because the varnish can be very thin and adherent to the stone. There is also the issue of the representativeness of the sample, since the varnish thickness can vary significantly on a local scale of mm (Macholdt et al. 2017b). Finally, taking samples is destructive and may not be acceptable on monuments for aesthetic reasons. The alternative is measurement on site using a portable XRF (pXRF). This is nondestructive, which makes it possible to measure multiple points on the varnish for a more representative characterization of the varnish layer.