CARBONATE PETROLOGY, CONSERVATION AND THE CHARACTERIZATION OF HISTORIC MORTARS

Modern guidelines for the conservation of historic buildings indicate that every attempt should be made to retain or restore as much of the original building fabric as possible so that the resource continues to carry its original history. In many historic buildings, natural stone or bricks are joined using mortar, and many of the old mortars are prone to weathering and disintegration with time. Restoring these mortars is just as important in maintaining the historic integrity of a building, as is appropriately replacing building stone and brick, but until recently maintaining mortar authenticity received little attention. Historic mortars are commonly composed of a binder, usually lime putty, and aggregate, some sort of sand-size material. Although lime putty was very standard, it was the amount and kinds of aggregate that made mortars distinctive. Analyzing mortars for the percentage of aggregate is typically done by acid digestion of the lime, and in older restorations any suitable sand was used as a replacement. Presently, though, preservationists prefer to use original sand sources, but examination of some original sources in the central Kentucky showed that many sands contain substantial calcareous components that were also being digested by the acid, thus giving erroneous results. It was also difficult to locate original sand sources.

Because lime mortars are very similar to carbonate rocks, they are easily examined via thin-section petrology, and point-counting provides estimates of component percentages. Component percentages, as well as discovery of rare trace components like coal and shale fragments, chert and fossil debris, are also critical in discriminating original sand sources. However, in addition to the usual insoluble components, acid digestion revealed that clay was also commonly present, and this unexpected component was not easily seen in a thin section, because the clays mixed imperceptibly into the lime-putty binder and register as lime matrix in point counting. Although acid digestion provides very general estimates of mortar components, a combination of acid digestion and point counting in thin section is far more accurate in characterizing lime mortars and may provide the added benefit of locating original aggregate sources, all of which enhances the authenticity of site restoration.