Surface hardness as a proxy for weathering behaviour of limestone heritage: a case study on dated headstones on the Isle of Portland, UK

This study estimates stone weathering rates using in situ surface hardness testing. Surface hardness changes are precursors to erosion and may be utilized to describe stone weathering behaviour. The method proposed here complements previous approaches to determining stone weathering rates by surface loss/change. A time series covering 1–248 years of exposure is investigated using a sample of 12 headstones in two nearby cemeteries. Using an Equotip D surface hardness tester, rates of change in surface hardness for top and bottom sections of the headstones were determined and the data evaluated using robust, nonparametric statistical methods. When considering all headstones as one time series, nonlinear behaviour is observed with rates of decline in surface hardness slowing over time. However, breakpoint analysis shows a breakpoint at c. 100 years, with higher rates of surface hardness decline (as measured by QC₅₀—the regression coefficient for 0.50 quantile regression) up to that point and lower rates thereafter. Up to c. 100 years, surface hardness declines more rapidly in the top versus bottom sections. Possible explanations for the differing rates in surface hardness changes are: (a) inherent natural stone variability and/or different weathering-stress history; (b) the use of two different Portland limestone varieties; (c) synergistic effects of microclimates and lichen cover. In order to gain a deeper insight into stone weathering behaviour, future studies could combine surface hardness measurements with surface change methods such as micro-erosion meter and lead plug index over short- and long-term time series on architectural heritage under real-world conditions.