Measurement of the Growth Rate of Urban Rock Varnish on the Smithsonian Castle by Portable X-ray Fluorescence

Patches of bluish-black manganese-rich rock varnish have been observed on the Smithsonian Castle and other red Triassic sandstone buildings in the Washington DC area. In order to estimate the growth rate of the varnish it is necessary to measure its thickness. A nondestructive method for thickness measurement has been developed based on portable X-ray fluorescence (XRF). This uses a standard-free approach to calculate the layer thickness from the differential attenuation of the iron (Fe) XRF peaks generated by the significant iron content of the underlying sandstone. This takes advantage of fact that the Fe K_α and Fe K_β peaks bracket the K absorption edge for Mn at 6.539 keV, which results in a factor of nearly sixty difference in the mass attenuation factor between the two Fe K peaks. This approach eliminates many of the factors of uncertainty such as surface roughness or X-ray beam variability. The estimated crust thickness is 4 ± 1 µm, assuming that it consists of pure manganese dioxide. This is consistent with the measured thickness of 5.4 ± 1.5 μm obtained by X-ray transmission scanning microscope on a cross-section of a sample of the layer. Given the age of the Castle of 155 years this corresponds to an average growth rate of 26 ± 6 nm/yr. This falls in the middle of the range of 1- 40 nm/yr. observed in desert varnish samples. However, similar measurements on a modern gatepost at the Castle made of the same sandstone yielded a growth rate of 930 ± 11 nm/yr. This suggests that growth rate on the Castle may not have been uniform over its lifetime.