Читать книгу Monument Future - Siegfried Siegesmund - Страница 241

Portable XRF provides a nondestructive method of acquiring data on its geographical distribution and rate of growth. The Mn/Fe counts ratio can be used to distinguish the Mn-rich varnish from other types of surface deposits. The direct Mn and indirect Fe/Fe ratio methods can be used to estimate the layer thickness and hence the growth rate. Patches of urban rock varnish have been identified by pXRF on buildings across the northern United States from Washington to New York City to Minneapolis. 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.