Using St Paul’s Erosion Data to Predict Future Stone Decay in Central London

In a recent blog I mentioned a research project just completed on a 30-year remeasurement of stone decay on St Paul’s cathedral in central London. A second paper looks at how this data might be used to model decay into the future (http://www.sciencedirect.com/science/article/pii/S1352231012007145  - you need to have an account to get access to the full paper in Atmospheric Environment). Modelling erosion rates into the future tends to use relationships derived from erosion data of small (50x50x10mm) stone tablets exposed in different environmental conditions. Using such data and regression analysis a statistical relationship can be derived between stone loss and changing environmental conditions. These relationships are often referred to as dose-response functions.


Two equations stand out – the Lipfert and the Tidblad et al. does response functions. Using these two equations for the decades 1980-2010, they predict an erosion rate of 15 and 12 microns per year as opposed to the measured losses on St Paul’s of 49 and 35 microns per year. The ratio between the measured and the dose-response erosion rates varies from 3.33 in the decade 1980-1990 to 2.75 in the decade 2000-2010, so fairly consistent. The difference between the two measures of decay may result from differences in what they are actually measuring. The dose-response function uses small stone tablets, exposed vertically in polluted environments. The weight loss of these tablets is measured and then converted to a loss across the whole surface of the tablet. The micro-erosion meter sites measure the loss of height of a number of points across the same surface on a decadal time scale. Both measures are changes in height but derived in different ways. What is important is that both methods indicate the same patterns of change in relation to declining sulphur dioxide levels. Both measures of erosion show a decline and both show it in the same direction and, by and large, in proportion to each other. Interestingly, when the dose-response functions are used to work out erosion on the cathedral since it was built the long-term erosion rate (as measured by lead fin heights relative to the stone surface) is only 2.5 times greater than that predicted by the does-response functions. This is a similar ratio, more or less, to those indicated over the last three decades.

The St Paul’s data does not imply that dose-response functions do not work – if anything it confirms the patterns in decay they indicate – but the St Paul’s data does suggest that using these dose-response functions to model decay into the future may require a correct function, equivalent to the ratio of about 2.5-2.75 to convert the losses to those that will be found on St Paul’s Cathedral.

Atmospheric Pollution and Stone Decay: St Paul’s Cathedral

I have recently published a paper with colleagues from Oxford, Cambridge, Sussex and York on a 30-year measurement of erosion rates on St Paul’s Cathedral, London (http://www.sciencedirect.com/science/article/pii/S1352231012008400  - you need to have an account with the journal to access the paper). Pleasingly, the academic work did get some press (http://www.independent.co.uk/news/uk/home-news/pollution-erosion-at-st-pauls-cathedral-in-record-300year-low-8205562.html , http://www.stpauls.co.uk/News-Press/Latest-News/St-Pauls-safer-from-pollution-than-at-any-time-in-its-history , ) and I even did a very short radio interview on local radio (distracted during it because they just started their afternoon quiz to which I knew the answer!)


The paper outlines how the rates of erosion (and the rates of surface change) of five micro-erosion meter sites around the cathedral have changed over the decades since 1980 and how this has mirrored a dramatic fall in pollution levels in central London.

Figure 1 Micro-erosion meter site with protective caps being removed for remeasurement

Figure 2 Erosion rates, rates of surface change and environmetnal variables for central London 1980-2010

The erosion rates have dropped since the closure of Bankside power station in the early 1980s with atmospheric pollution, as indicated by sulphur dioxide levels dropping from 80ppb in 1980 to about 3ppb in 2010. Erosion rates fell from 49 microns per year in the decade 1980-1990 to 35 microns per year in the decade 2000-2010. Erosion rates in the 1980-1990 were statistically significantly higher than erosion rates in both the decades 1990-2000 and 2000-2010. Erosion rate in the decade 1990-2000 and 2000-2010 were statistically similar. Although the decline in erosion rates was not as steep as the fall in pollution levels, erosion rates are now at a level that could be explained by the action of the acidity of ‘normal’ or ‘natural’ rainfall alone. ‘Normal’ rainfall is a weak carbonic acid, produced by the reaction of carbon dioxide and water in the atmosphere, meaning that ‘normal’ rainfall has an acidity of about pH5.6.

Erosion rates represent the loss of material from a surface but not all points’ measured lost material, some gained height over the measurement periods – this is surface change. Points can gain height for a number of reasons; slat in the stone could distort and push the surface up, lichens and bacteria could form crusts that raise the surface and eroded material might be deposited in depressions in the surface causing an apparent raising of the surface. The rates of surface change were 44 microns per year in the decade 1980-1990 but fell to 26 microns per year by the decade 2000-2010 (and were only 25 microns per year in the decade 2000-2010). This suggests that rats of surface change fell in a similar manner to rates of erosion and match the drop in sulphur dioxide levels as well.

Back in the 1980s the long-term rates of erosion, since the early 1700s were also determined using lead find. These lead fins were produced when the holes used to raise the stone blocks into the balustrade were filled with lead. Over time the fins became proud of the surface as the stone around them eroded. By measuring the height difference between the fins and the stone (and then dividing by the time of exposure) the long-term rates of erosion can be calculated. The long-term rates from 1690/1700 to 1980 were about 78 microns per year. This suggests that the cathedral has experienced much higher erosion in the decades before 1980. This does suggest that the erosion rates we have managed to measure from 1980 onwards, and the associated pollution levels, were not as damaging to the cathedral as those experienced in the years up to 1980.