Memory of Historic Stone

charles-bridge3Stone has a memory. When geomorphologists talk about ‘memory’ they are talking about a physical memory within a natural system. So, stone can ‘remember’ if it experienced a very harsh frost event, and that ‘memory’ will manifest itself physically – through a change in the stone’s physical characteristics.

What is true of landscapes is also true of stone in a building. Historic sandstone structures carry an inheritance, or a ‘memory’, of every past stress that the stone has undergone since its placement in a façade. This stress inheritance, which conditions how the stone performs in the present day, may be made up of long-term exposure to a combination of low magnitude background environmental factors (for example, salt weathering, temperature and moisture cycling) and, superimposed upon these, less frequent but potentially high magnitude events (for example, lime rendering, severe frost events, fire). The impact of complex histories on the decay pathways of historic sandstone is not clearly understood, but work demonstrated in the research linked to above seeks to improve that understanding through the use of a laboratory ‘process combination’ study.

In the study, blocks of quartz sandstone were divided into subsets that experienced different simulated histories. These subsets were then subject to salt weathering cycles. The results of the study illustrated the complexity of the stone decay system, showing that seemingly small variations in stress history can produce a divergent decay response over time (even in the same stone type).

Applied to real-world historic structures, this concept helps to explain the spatial and temporal variability of response to background environmental factors on a single façade, and encourage conservators to include the role of stress inheritance when selecting and implementing conservation strategies.

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