Observations on the factors influencing stability of building stones following contour scaling: a case study of oolitic limestones from Budapest, Hungary

Salt-induced decay of porous, granular building stones is often manifested initially by contour scaling, followed by rapid surface retreat through granular disaggregation and/or multiple flaking. It has been hypothesised that rapid retreat in polluted environments could be avoided if newly scaled surfaces can be stabilised by, for example, the re-growth of gypsum crusts. In this paper, we investigate the conditions required to ‘switch on and off’ rapid decay through a case study of oolitic limestones in a city that continues to experience high levels of atmospheric pollution. In a locally humid location adjacent to the River Danube, positive feedbacks are seen to occur following scaling that favour continuous rapid retreat linked to the synergistic salt weathering effects of halite and gypsum. In contrast, the Castle Hill overlooking the Danube is less prone to winter and autumn fogs and there is evidence that stone retreat here may be more sporadic and that episodes of contour scaling can be interspersed with periods of stability marked by the re-growth of gypsum crusts. On individual stone blocks it is therefore possible to see fragmentary evidence of several crusts. The negative feedbacks that operate to stabilise newly scaled surfaces are thought to be associated with low chloride concentrations, and a relatively dry microclimate at the block surface that inhibits the frequency and depth of surface wetting. This is linked in turn to the continentality of the meteorological climate of Hungary. It is possible, however, that particular significance attaches to the rapid deposition and effective retention of particulate pollutants rich in water-soluble sulphate and coincident biological colonisation of scaled surfaces that aids the adherence of precipitated dust.