Monument Future - Siegfried Siegesmund

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       PORTABLE XRF STUDY OF THE GEOGRAPHIC DISTRIBUTION AND GROWTH RATE OF MN-RICH ROCK VARNISH

      IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

       – PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

       VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

      3 City University of New York, Lehman College, Bronx, NY 10468, USA

       Abstract

      Manganese-rich surface layers of urban rock varnish have been observed growing on sandstone buildings and monuments. Portable X-ray fluorescence provides a nondestructive method of distinguishing this type of dark layer from ordinary soiling by the detection of elevated levels of Mn relative to the underlying stone. On certain iron-rich sandstones the pXRF method can also be used to estimate the Mn layer thickness by the differential attenuation of the Fe Kα and Fe Kβ lines. If the age of the layer is known, the growth rate can then be inferred. Patches of urban rock varnish have been identified by pXRF on buildings across the northern United States from Washington (DC) to New York City (NY), Boston (MA), and Minneapolis (MN). These patches have typically been observed on red Triassic sandstone. However, they have also been found growing on older Carboniferous sandstone in New York City’s Central Park. Growth rates estimated from datable patches on the Smithsonian Castle and nearby gate posts are in the range of 83 ± 2.0 to 95 ± 2.4 nm/yr. This is significantly higher than the maximum rate of 40 nm/yr observed for desert varnish.

      Keywords: Rock varnish, manganese, portable XRF, Triassic sandstone

       Introduction

      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.

       The pXRF technique

      Several companies market portable XRF instruments. These consist of an X-ray generating tube, typically with a rhodium target, a silicon semiconductor X-ray detector, and electronics for pulse height analysis and data storage. The output spectrum is a histogram of the X-ray photon counts per energy bin, which is roughly one eV wide. The individual elements are represented in the spectrum by their peaks at characteristics energies. For example, the Mn peaks at 5.90 keV and 6.49 keV are shown in Fig. 2. The mass of the element in the material is proportional to the number of photon counts in the peak. A suitable calibration standard is required to convert the counts data into mass, discussed in more detail below. The pXRF instrument is designed to be handheld, but for accurate measurement in the field it is preferable to mount it on a tripod to maintain a constant standoff distance.

      Figure 2: Mn and Fe peaks in a pXRF spectrum of urban varnish.

       Interpretation of pXRF data

      As discussed above, the raw counts data are total counts, or counts per second, in the peaks of the elements of interest. To be useful these data must be converted into physically meaningful quantities. There are several approaches to this.

       Mn/Fe counts ratio

      The simplest approach to quantification consists of calculating the ratio of Mn counts to Fe counts. This takes advantage of the fact that the Fe content of the bulk sandstone dominates over any Fe content in the varnish, at least in the case of red Triassic sandstone. Hence, the Fe can serve as a 163reference value for normalizing the Mn variations and thus confirms that the varnish has enriched Mn. As shown in Fig. 2, the Fe Ka peaks overlap the Mn peaks. Consequently, the attenuation factors are essentially the same for the two elements, and the counts ratio approximates the mass ratio. This avoids the need to convert the counts into masses. In practice, pXRF spectra are acquired for several points in the varnish along with a similar number of points on the adjacent bare stone for comparison. Fig. 3 presents an example of the application of the Mn/Fe ratio method to an area on the Smithsonian Castle, which shows that the points within varnish patches have significantly elevated levels of Mn.

      Figure 3: Plot of Mn/Fe ratios for SW corner of the Smithsonian Castle.

       Direct estimate of Mn areal density

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