Siegfried Siegesmund - Monument Future

At the time of writing the analysis of the most recent pXRF data from New York and Massachusetts sites has not been completed. However, the results from the 165-year-old Smithsonian Castle, which is located on the National Mall in Washington, can serve as an example of the method for calculating growth rates.

Three locations around the Castle were measured by pXRF. In addition to the southwest corner (Fig. 1), a patch at the east entrance was measured, which historic photographs show was free of varnish as late as 1985. A third patch was measured on a gate post of the Enid Haupt Garden, built in 1987 adjacent 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 2along 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.

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.

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.

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Barbara Sacchi 1 , Emma Cantisani 1 , Teresa Salvatici 2 , Carlo Alberto Garzonio 2

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.

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

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

In situ evaluation of mortars’ resistance and consolidant treatments’ performance in ancient buildings is an essential argument in their conservation. Moreover, the mechanical properties of an ancient mortar are directly connected with the state of preservation of the building or the structure in which it is included, and it is not a parameter easy to be determined, especially for the masonry joints. Mechanical methods often request sampling and laboratory tests, while only few micro destructive techniques are known as penetrometric tests. They can be either static (with action of a pressure drills) or dynamic (with action of a percussion drills) and commonly usable in situ. Among them, Microdrilling Resistance Measurement System and Mortar Penetrometer are two important methods for determining the resistance of a mortar.