Monument Future. Siegfried Siegesmund

to the Castle using the same Seneca sandstone. Ten points each were measured on the varnish patch and a bare stone area at the SW corner, and five points each at the other locations. The pXRF counts data were converted to Mn layer thickness using the Fe/Fe ratio method for the Mn patch and bare stone.

      The results are presented in Table 2 along with estimated ages. The calculated growth rates for the east entrance and the gatepost are reasonably close, on the order of 90 nm/yr. This is significantly higher than the maximum rate of 40 nm/yr observed for desert varnish (Liu & Broecker, 2008). However, the growth rate for the southwest corner is only a third of this, if its estimated age is based on the assumption that the layer began to grow as soon as the stone was put in place. Although historic photographic documentation is mainly in black and white and insufficiently detailed for conclusive determination, it appears that growth actually started much later. Dividing the thickness of the layer, 4 microns, by the rate of 90 nm/yr gives an age of 45 years or a start date of 1970. This is consistent with the period of great population growth and associated increase in automobile traffic around the Washington urban area.

Table 2: Varnish growth rates on the Smithsonian Castle, based on Fe/Fe ratios measured on Mn patches and bare stone using pXRF.

Location	Thickness, µm	Time, years	Growth Rate, nm/yr
S.W. Corner	4.0 ± 0.57	155	26 ± 3.7
Gatepost	2.41 ± 0.06	29	83 ± 2.0
East Entrance	2.38 ± 0.06	25	95 ± 2.4

       Conclusions

      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.

      166Acknowledgements

      The authors would like to thank Bill Rebel of American Engineering Testing, Inc., for providing the XRF analysis of the James Hill House varnish sample.

       References

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      167

       MICRODRILLING RESISTANCE MEASUREMENTS SYSTEM AND MORTAR PENETROMETER: TWO METHODS FOR EVALUATING IN SITU MORTAR RESISTANCE

Barbara Sacchi¹, Emma Cantisani¹, Teresa Salvatici², Carlo Alberto Garzonio²

      IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

       – PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

       VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Institute for Heritage Science – National Research Council of Italy (CNR-ISPC), Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy

2 Department of Earth Sciences, University of Florence, 50121 Florence, Italy

       Abstract

      In