Rock varnish on architectural stone: microscopy and analysis of nanoscale manganese oxide deposits on the Smithsonian Castle, Washington, DC

Abstract

The Smithsonian Institution Building, commonly referred to as the Castle, is located on the National Mall in Washington, DC, and was constructed in the mid-nineteenth century for the purpose of housing all museum and scientific functions for the newly formed institution. Matching gateposts designed by the Castle's architect were erected more than a century later in the Enid A. Haupt Garden opposite the Castle. Black patches were recently noted on both structures, which are clad with locally quarried Seneca red sandstone. Portable x-ray fluorescence (XRF) spectrometry links the discoloration with elevated Mn concentrations. The discolored patches resemble rock varnish, a Mn-rich coating observed on rock surfaces formed in a variety of environments. Bulk rock and varnish chemistry, in addition to microscopy and microanalysis of the varnish, are presented here. On a bulk chemical basis, the Seneca sandstone is relatively poor in Mn, containing ~500 ppmw. In contrast, the rock varnish is greatly enriched in Mn relative to the stone and to a lesser degree in Pb, Ca, Zn, Cu and Ni. Cross sections of the black encrusted regions show that the stone's red coloration has been modified by black pigmentation from the surface down to ~250 μm. X-ray diffraction of blackened particles produced no discernable pattern, indicating concentrations below the detection limit, poor crystallinity, or both. Scanning electron microscopy and EDS-based x-ray microanalysis of the uppermost portion of the cross section reveal nanometer scale (<20-200 nm) Mn-rich and clay particles concentrated in a thin film (≪1 μm) at the surface. Additionally, Mn oxide particles decorate the surfaces of fine-grained minerals in sandstone pores within the discolored zone. Imaging and microanalysis of the rock surface reveal that the Mn-rich varnish is a discontinuous film ≪1 μm in thickness with an estimated composition of Na 0.2 Ca 0.1 Mg 0.1 Al 0.1 Si 0.5 Mn 1.9 Fe 0.5 O 6.7 . This composition most likely represents a nanoscale mixture of a Mn oxide (e.g., birnessite or todorokite) and an Al-rich silicate mineral. Seneca sandstone on the Smithsonian Castle and gateposts is discolored in patches owing to the Mnrich phase being deposited into two zones: (1) a vanishingly thin patina, and (2) nanoparticles coating grain boundaries and pores in the uppermost ~200-250 μm of the stone. While the mineralogy is similar to well-studied varnish formed in arid settings, rock varnish on the Smithsonian structures is significantly thinner. Because this architectural rock varnish is young, it may represent the earliest stages of formation of the more commonly described varnishes reported in the literature.

Black manganese-rich crusts on a Gothic cathedral

“…Furthermore, the patches are most abundant on the building blocks themselves, and only to a lesser extent on the mortar between them. The crusts seem to start growing on the sandstone and only cover the mortar in cases when the crusts span several blocks, an observation also made at the Smithsonian Institute (Vicenzi et al, 2016).…”

“…1) by Grüner et al (2011). They had also been observed on several other buildings of historical interest, such as at the church in Stödtlen, Germany (Grüner et al, 2011), the Smithsonian Institute in Washington D.C., USA (Grissom et al, 2014;Livingston et al, 2016;Vicenzi et al, 2016), and the Khmer temples in Cambodia (Uchida et al, 2016), indicating the worldwide abundance of these crusts. Manganese-rich crusts on historical buildings are not only found on sandstone building blocks, but also on laterites and bricks (Uchida et al, 2016).…”

“…However, in the last few years it was discovered that some black crusts on buildings in urban open areas are actually Mn-rich coatings (Grissom et al, 2014;Grüner et al, 2011;Livingston et al, 2016;Uchida et al, 2016;Vicenzi et al, 2016) with an as yet unknown genesis. These black crusts lack identifiable biogenic structures (Grissom et al, 2014;Livingston et al, 2016;Vicenzi et al, 2016) and are thus dissimilar to Mn-rich crusts found in cave environments, which had often been found to be of biogenic origin (Frierdich et al, 2011;Miller et al, 2012;Saiz-Jimenez et al, 2012;White et al, 2009). Manganese-rich coatings on facades of buildings of historical interest are often present on siliceous stones, such as quartz-based sandstones, in contrast to the soot-rich crusts on limestones, marbles, or calcareous sandstones (Bonazza et al, 2005(Bonazza et al, , 2007aBrimblecombe and Grossi, 2009;Brimblecombe, 2002, 2008).…”

“…Manganese-rich crusts on historical buildings are not only found on sandstone building blocks, but also on laterites and bricks (Uchida et al, 2016). No systematic association has been observed between the surface roughness of the building blocks and crust development, neither on the Freiburger Münster, nor on the Smithsonian Castle (Vicenzi et al, 2016). Furthermore, the patches are most abundant on the building blocks themselves, and only to a lesser extent on the mortar between them.…”

Optimization of a primary X‐ray filter for X‐ray fluorescence analysis of uranium and plutonium

“…In this study, a Tracer III‐SD handheld XRF spectrometer (Bruker AXS Handheld, Inc., Kennewick, USA) capable of rapid onsite analysis was used. This device is light and convenient to carry, and therefore, it has been widely used for analyzing historical cultural assets, archeological bricks, plants, and architectural stones . Our Tracer III‐SD was equipped with a Ag anode X‐ray tube, a silicon drift detector with energy resolution of 142 eV, and the original primary X‐ray filter consisting of 304.8‐μm (12‐mil) Al, 25.4‐μm (1‐mil) Ti, and 152.4‐μm (6‐mil) Cu (1 mil = 1/1,000 inch [25.4 μm]).…”