On a possible mechanism of sandstone alteration: Evidence from electric potential measurements

We present the results of electric potential measurements made on an external molasse sandstone wall of a XIVth century church. The measured electric signal is clearly driven by the variations of the atmospheric temperature. A comparison of the electric signals recorded at different depths in the wall allows us to conclude that the observed correlation between thermal and electric signals is not due to a direct effect of temperature on the electrodes. The most likely explanation is that the daily Variation of the atmospheric temperature dynamically drives the fluid flow from inside the wall towards its surface where water evaporates. The flow is fed by water drawn up from a superficial water table by capillary forces. Electric potentials are generated by electrokinetic effect. The mineral salts dissolved in water are permanently transported towards the surface. The process of mineral dissolution, transport and subsequent recrystallization accompanying the temperature driven fluid flow can cause continuous delamination of the sandstone surface.