THE DEGRADATION OF COUPLED STONES BY WET DEPOSITION PROCESSES

An investigation has been made of the influence of wet deposition of simulated acid rain, with subsequent dry/wet cycling, on the degradation of limestone, sandstone, marble and granite, interactively coupled by overlapping or butt joints to simulate architectural detail, and sloping at an angle so that run-off from the upper stone drains on to the lower one. Degradation was monitored by pH change and analysis of run-off solution and soluble extracts from the exposed stones, together with EPMA, SEM/EDX and X-ray diffraction examination of petrographic sections. Overlapping coupled stones show greater degradation of the lower stone for all stones, but particularly when it is granite and the upper stone is limestone. This is partly due to a longer time of wetness at the interface and closely adjacent areas (meniscus effect), and partly due to the deposition of ions from the upper stone and the buildup of damaging degradation products (mainly gypsum) on the lower stone and interaction with it. Butt-jointed couples scarcely show the effect due to no significant meniscus effect. This method of laboratory simulation for stones shaped and interactively coupled to reproduce protruding and re-entrant surfaces in typical architectural combinations, so that drainage from one to another occurs, can yield results similar to those found in practice, and be used to rank various stone combinations.