Do environmental conditions determine whether salt driven decay leads to powdering or flaking in historic Reigate Stone masonry at the Tower of London?

In order to evaluate the potential for using environmental controls as a preventive conservation strategy for Reigate Stone masonry, this paper tests the hypothesis that powdering and flaking decay patterns are a direct consequence of climatic variables. Samples of masonry affected by each decay pattern are analysed for soluble salt content using ion chromatography. The results are used to build thermodynamic models in ECOS Runsalt. These identify NaCl and NaNO3 crystallisation in both environments, with NaCl dominating in powdering masonry and NaNO3 dominating in flaking masonry. Freshly quarried samples of Reigate Stone with a range of physical characteristics are contaminated with a mixture of these salts. Replicates of each sample are subjected to two distinct accelerated ageing regimes. These regimes are based on environmental monitoring and designed to reproduce the observed decay pathways. One set is subject to partial immersion and relatively stable, high RH, to stimulate a zone of NaCl crystallisation near the surface. The replicate set is subject to occasional wetting and frequent drops in RH, to stimulate NaNO3 crystallisation within the fabric of the stone. The results provide some indication that the intended decay mechanisms were achieved, supporting the hypothesis; however, physical variation in the stone was a far greater control on the rate of decay and the emergence of distinctive decay phenomena. This extended to both the mineralogical composition of the stone and to pre-existing decay features, such as crusts and cracks. The implication of these results is that the emergence of decay patterns in Reigate Stone cannot be explained solely by environmental mechanisms; baseline mineralogy and historical contingency can play a crucial role. Whilst environmental controls may provide a conservation strategy in some cases, it is likely that detailed assessment of the case specifics will be necessary.