AIC's 47th Annual Meeting

When working on the conservation of large-scale masonry buildings, it is typically desirable to retain as much original material as possible. However, there is sometimes a need for more widespread replacement of original stone. In these cases, it is necessary to vet replacement materials for compatibility with the originals in terms of both performance and aesthetics. A diligent evaluation often involves a full gamut of physical property and durability testing. The research presented here focuses on a singular part of this testing program - the evaluation of replacement stone from an aesthetic perspective. It looks specifically at the alteration of stone appearance over time through environmental interactions, which is often referred to as "patina". Although the potential for visual change is only one of the properties that should be evaluated when considering stone sources, it can be argued that it has the most tangible impact and the greatest visibility to the public. This project involves subjecting samples of building stone to an accelerated chemical environment that is meant to recreate chemical conditions expected to occur in the field and potentially induce a patina on the stone. These methods are based on a century-old test described by William Parks in his "Report on the Building and Ornamental Stones of Canada", but this method has not been commonly employed in materials testing programs until it was recently developed by our laboratory. As part of the current research, patina testing has been performed on a variety of stone types and sources. Some materials, namely carbonate-based stones, are found to be more susceptible to visual changes through this test. Where other methods may be necessary to recreate a similar patina observed in more siliceous stones, these are also discussed. Still, the test has been demonstrated to act as a good predictor of actual stone patina on several different types of limestones tested from existing buildings. In addition to the demonstration of visual changes in the stone, the mechanisms of patina development are also investigated further using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results of this testing have given specifiers a fairly straightforward way to distinguish between replacement stone contenders, particularly when those stones were otherwise similar in terms of overall composition and physical properties. In some cases, stone sources that are essentially identical before testing develop markedly different patinas. The visual disparities between stones may be so dramatic as to completely rule out a source from consideration as a replacement material. These sometimes significant aesthetic changes are often attributed to only small differences in composition and would not be anticipated based on a general characterization of the stone alone