AIC's 46th Annual Meeting

Bluestone, the common name given to a form of blue-colored feldspathic sandstone or greywacke quarried in Pennsylvania, New York, and Newfoundland, is now and was historically a popular building stone in the northeastern United States and eastern Canada. While generally durable, its laminar structure makes it susceptible to delamination, or the separation of layers of stone along their natural bedding planes. Examples of delaminating bluestone are easily found among historic buildings and monuments where the material was used. The nature of this form of deterioration has been well studied. However, a much less ubiquitous and less understood form of surface loss may also occur. In certain conditions, the stone may develop a hardened outer surface crust, as thick as 1 to 2 cm, which runs both parallel and perpendicular to bedding planes. This outer crust invariably detaches from the body of stone in a phenomenon known as case hardening or contour scaling. Contour scaling has been attributed to a variety of processes, such as precipitation of secondary minerals, hygroscopic swelling of constituent clays, thermal cycling, etc. This paper will discuss the use of microscopy in exploring the chemical and mineralogical alterations contributing to contour scaling in bluestone. This information may also be applied to understand scaling in other clay-bearing or laminated stone types. Applications for the presented research include diagnosis of stone deterioration in the field and development of effective conservation treatments. Examples will be drawn from New York City’s Highbridge Park and presented with photomicrographs of affected stones in thin-section.