Micro-environmental change as a trigger for granite decay in offshore Irish lighthouses: implications for the long-term preservation of operational historic buildings

Abstract

Following automation of lighthouses around the coastline of Ireland, reports of accelerated deterioration of interior granite stonework have increased significantly with an associated deterioration in the historic structure and rise in related maintenance costs. Decay of granite stonework primarily occurs through granular disintegration with the effective grusification of granite surfaces. A decay gradient exists within the towers whereby the condition of granite in the lower levels is much worse than elsewhere. The lower tower levels are also regions with highest relative humidity values and greatest salt concentrations. Data indicate that post-automation decay may have been triggered by a change in micro-environmental conditions within the towers associated with increased episodes of condensation on stone surfaces. This in turn appears to have facilitated deposition and accumulation of hygroscopic salts (e.g. NaCl) giving rise to widespread evidence of deliquescence in the lower tower levels. Evidence indicates that the main factors contributing to accelerated deterioration of interior granite stonework are changes in micro-environmental conditions, salt weathering, chemical weathering through the corrosive effect of strongly alkaline conditions on alumino-silicate minerals within the granite and finally, the mica-rich characteristics of the granite itself which increases its structural and chemical susceptibility to subaerial weathering processes by creating points of weakness within the granite. This case study demonstrates how seemingly minor changes in micro-environmental conditions can unintentionally trigger the rapid and extensive deterioration of a previously stable rock type and threaten the long-term future of nationally iconic operational historic structures.

Could marine aerosol contribute to deteriorate building materials from interior areas of lighthouses? An answer from the analytical chemistry point of view

“…Since many years, lighthouses are considered important constructions from architectural, historical and cultural point of view. In the scientific literature, few references can be found about the characterization of original composition of the building materials from lighthouse, and even less regarding their deterioration processes …”

Approaches to the Study of Salt Weathering of Geological Materials

“…In the case of field studies we found some more balance between the studied rocks (of course excluding limestones and sandstones). Nonetheless, there is some predominance of igneous rocks, clearly dominated by granites (Sousa et al [22]; Rivas et al [40]; Silva Hermo et al [43]; Warke et al [44]; Costa [45]; Pozo-Antonio et al [46]), followed by volcanic rocks (Seiki et al [3]; Columbu et al [37]; Antonelli et al [47]). However, we also found studies of gabbros (Matović et al [48]) and diorites (Bader et al [49]).…”