THE RATIONALE FOR MICROABRASIVE CLEANING: A CASE STUDY FOR HISTORIC GRANITE FROM THE PENNSYLVANIA CAPITOL
J. CHRISTOPHER FREY, & TIMOTHY NOBLE
4 WOODBURY GRAY GRANITE
Woodbury Gray granite was quarried at Hardwick, Vermont, beginning in the late 19th century, competing with nearby Barre granite quarries that extracted rock from the same geological formations. Despite the lack of a proven track record in large-scale construction projects, the stone quarried at Hardwick was accepted for use, probably because it was similar to but less expensive than stone from the more established Barre quarry (Heritage Studies 1987, 103). Commissioners supervising construction of the capitol building found fault with the stone soon after it began arriving on-site, citing problems with cutting, finishing, and uniformity (Heritage Studies 1987, 176–77). After several heated discussions between the state commissioners and Huston, and the near firing of the architect, stonework was eventually completed, albeit with significant discrepancies between the original plans and the final product (Heritage Studies 1987, 181).
Petrographic analyses conducted as part of this study found Woodbury Gray granite to be a biotite-alkali feldspar granite composed of 35–45% quartz, 40–45% feldspar, 10–15% biotite, and 0–10% muscovite (Moses 1997).
Stones were originally tooled (probably by steam-driven pneumatic equipment) with a series of serrated channels and ridges similar to a patent-hammered finish. Although it is a common method of finishing, the impact necessary to create the desired appearance effected microstructural changes near the surface, which in turn exacerbated certain types of deterioration. Repeated strikes from finishing blades created a series of fine microcracks to a depth of 3–5 mm beneath the exposed surface. These extremely fine fissures provided avenues into the subsurface for the penetration of moisture, the migration of soluble salts, and enhanced interaction by chemicals and other contaminants, often resulting in erosion, sheet exfoliation, deep staining, and other surface degradation.