Monument Future

Acknowledgements

The authors are grateful to Arch. Prof. Mario Piana, protomagister of the St. Mark’s Basilica for authorizing the sampling, and to Dr. Alberto Conventi of LAMA for his collaboration in the SEM-EDS analyses.

References

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METHODS FOR THE INVESTIGATION OF STONE DECAY; IN-SITU AND NON-DESTRUCTIVE TESTING

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161

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

Richard Livingston¹, Carol Grissom², Yuri Gorokhovich³

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.

1 University of Maryland, College Park, MD 20742 USA, rliving1@umd.edu

2 The Smithsonian Institution, Washington, DC 20560, USA

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

The Mn/Fe counts ratio is essentially a qualitative indicator of elevated Mn levels. For a quantitative result the most straightforward approach is to estimate the mass of Mn. To accomplish this, a suitable calibration standard is required. In this project we used a well-characterized set of mudrock (shales, sandstones) samples (HR_01 – HR_65) provided by the manufacturer of the pXRF instrument. To more closely simulate the desert layer structure, McNeill and Cecil prepared a standard consisting of a thin layer of Mn on a glass substrate (McNeil et al. 2009). In either case the result is the total mass of Mn in the beam. This can be normalized to an areal density by dividing the mass of Mn by the beam area. One drawback of this calibration method is that it is valid only for a specific set of instrument settings such as beam current, standoff distance, etc. Thus, these have to be replicated in the field.

Areal density to thickness conversion

Portable XRF is an elemental analysis technique, and consequently the result of the measurements of the varnish is the areal density of the element Mn. However, the conventional literature on desert varnish usually is in terms of layer thickness, because the measurement method is based on optical microscope analysis of cross sections through the varnish (Dorn 2007). Therefore, in order to make comparisons with this literature it is necessary to convert the areal density into an equivalent thickness. However, this gives a nominal or effectiveness thickness that assumes no other minerals are present. This is not directly comparable to desert varnish thicknesses, which usually contains interlayers of clay minerals (Dorn 2007), but it is useful for comparing urban varnish layers on different structures.

Fe/Fe ratio estimate of Mn layer thickness

An alternative approach to estimating the amount of Mn that does not require a calibration standard arises from the overlapping of the Fe and Mn X-ray peaks as illustrated in Fig. 2. A corollary to this relationship is that the two Fe K lines at 6.3 and 7.0 keV bracket the K absorption edge at 6.5 keV in the attenuation factor of Mn. This means that the attenuation of the Fe Kβ is 6 times greater than for the Kα line. This differential attenuation makes it possible to measure very thin layers of Mn on the order of microns on top of the sandstone substrate (Livingston et al. 2020). The Fe X-rays are generated primarily in the bulk of the sandstone, but they are attenuated mainly in the surface layer of Mn. The method requires two pXRF measurements: one on the varnish patch and the other on a nearby area of bare stone. The effective thickness of the Mn layer can be calculated from the decrease of the Fe Kβ/Kα ratio of the varnish point compared to that of the bare stone. This method requires the 164assumption that the Mn is in the form of birnessite. In theory the presence of Fe in the varnish could lead to underestimates of thickness. However, microanalyses of varnish samples have shown that the varnish has very low Fe content (Macholdt et al. 2017b, Sharps et al. 2020). Moreover, sensitivity calculations have shown that the Fe content would have to be greater than 10 % to produce significant error (Livingston et al. 2020).

Field survey design

In order to make the pXRF measurements of urban varnish, it is obviously necessary to find their occurrences. Some have been found simply by random sighting, but there are more systematic approaches. One factor is the type of stone substrate. Most cases have been found on Triassic red sandstone in the United States, occurring in the Newark Supergroup, which is a geological formation that extends from South Carolina to Massachusetts, as shown in the map in Fig. 4.

Figure 4: Map of Newark Supergroup (Grissom et al. 2018).

Triassic building stone can have different local trade names, for instance, Seneca sandstone in the Washington area and Belleville sandstone from New Jersey in the New York City area, but it is essentially the same rock. Lists of buildings constructed with a specific building stone can be found in state geological surveys reports (Merrill and Matthews 1898), histories of local quarries (Peck 2013), architects’ catalogues raisonnés (Ochsner 1982), and architectural preservation studies (Matero and Teutonico 1982). It may be possible to minimize time in field searching for varnish by doing preliminary viewing of candidate buildings online with images from Google Earth.

Preliminary geographical distribution

Selected occurrences of Mn-rich urban varnish identified to date are listed in Table 1. These are divided into two categories: probable, based only on visual appearance; and confirmed, based on XRF detection of elevated Mn levels. In the second category those sites measured by pXRF are indicated in plain font. Asterisk indicate cases where the varnish was sampled and measured using laboratory XRF instruments.

Table 1: Locations with urban rock varnish. * Analyzed by laboratory XRF

At this time, insufficient data points preclude statistical analyses, but some preliminary observations 165can be made. It is evident that urban varnish is a widespread phenomenon and not just a local Washington problem. Second, on structures built with more than one type of stone, the varnish occurs only on the Triassic sandstone. This suggests that some property of this stone encourages the growth of the varnish. An exception to this rule is the identification of varnish on the Carboniferous sandstone of the Central Park’s Bethesda Fountain Plaza in New York City. Finally, the occurrence of varnish appeared to be an anomaly on the James Hill House in St. Paul, Minnesota, since it is located far from the Newark Supergroup region. However, records show that the building stone was shipped by railroad to St. Paul from Triassic red sandstone quarries at East Longmeadow, Massachusetts, which is within the Newark Supergroup.

Estimated growth rates on the Smithsonian Castle

At the time of writing the analysis of the most recent pXRF data from New York and Massachusetts sites has not been completed. However, the results from the 165-year-old Smithsonian Castle, which is located on the National Mall in Washington, can serve as an example of the method for calculating growth rates.

Three locations around the Castle were measured by pXRF. In addition to the southwest corner (Fig. 1), a patch at the east entrance was measured, which historic photographs show was free of varnish as late as 1985. A third patch was measured on a gate post of the Enid Haupt Garden, built in 1987 adjacent to the Castle using the same Seneca sandstone. Ten points each were measured on the varnish patch and a bare stone area at the SW corner, and five points each at the other locations. The pXRF counts data were converted to Mn layer thickness using the Fe/Fe ratio method for the Mn patch and bare stone.

The results are presented in Table 2 along with estimated ages. The calculated growth rates for the east entrance and the gatepost are reasonably close, on the order of 90 nm/yr. This is significantly higher than the maximum rate of 40 nm/yr observed for desert varnish (Liu & Broecker, 2008). However, the growth rate for the southwest corner is only a third of this, if its estimated age is based on the assumption that the layer began to grow as soon as the stone was put in place. Although historic photographic documentation is mainly in black and white and insufficiently detailed for conclusive determination, it appears that growth actually started much later. Dividing the thickness of the layer, 4 microns, by the rate of 90 nm/yr gives an age of 45 years or a start date of 1970. This is consistent with the period of great population growth and associated increase in automobile traffic around the Washington urban area.

Table 2: Varnish growth rates on the Smithsonian Castle, based on Fe/Fe ratios measured on Mn patches and bare stone using pXRF.