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 Na0.2Ca0.1Mg0.1Al0.1Si0.5Mn1.9Fe0.5O6.7. 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 Mn-rich 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.
Highlights
The Smithsonian Institution Building, more commonly referred to as the Castle, was designed by architect James Renwick Jr. in medieval revival style (Fig. 1)
Disposition of rock varnish Irregular black or blue–black patches, measuring as large as 80 cm in long dimension, can be documented on the Castle edifice beginning only in the mid-1990s based on limited photographic documentation of sufficient detail
In 2015 this rock varnish was documented on every face of the building, with the heaviest concentration on the south façade (Fig. 3)
Summary
The Smithsonian Institution Building, more commonly referred to as the Castle, was designed by architect James Renwick Jr. in medieval revival style (Fig. 1). As the first Smithsonian building, the Castle housed chemical laboratories, exhibit halls, specimen storage, and all other institution functions, as well as the residence and office for physicist Joseph Henry, the first Secretary of the institution [1, 2]. It is clad with Seneca red sandstone quarried from the Bull Run Quarry at Seneca, Maryland, located 40 km northwest of the city [3, 4]. Seneca sandstone is described as an arkosic micaceous sandstone It is part of the Poolesville Member of the Manassas Sandstone of late Triassic age. In an 1847 report on the quality of the recommended sandstone, geologist Dr David Dale Owen stated that the Seneca sandstone is resistant to “atmospheric vicissitudes, (and) even the most severe mechanical wear and tear.” [5]
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