Читать книгу Monument Future - Siegfried Siegesmund - Страница 230

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