THE RATIONALE FOR MICROABRASIVE CLEANING: A CASE STUDY FOR HISTORIC GRANITE FROM THE PENNSYLVANIA CAPITOL
J. CHRISTOPHER FREY, & TIMOTHY NOBLE
This article summarizes the findings from the masonry cleaning component of a one-year conservation study for exterior granite from the 1906 Pennsylvania Capitol. The authors designed and administered the study in 1997–98 under a professional contract awarded to Perfido Weiskopf/Graves/Noble Joint Venture by the Department of General Services of the Commonwealth of Pennsylvania. The conservation study was broken down into two specific areas of research and testing: (1) the characterization of the stone and its deterioration mechanisms and (2) the identification of treatments to correct problems described in the initial portion of the study. The results of the study provided the basis for specifications that addressed a series of remedial procedures, including conservation cleaning, repointing, stone repair, removal of metallic stains, and removal of biological growth.
The capitol building is the architectural centerpiece of the government of the Commonwealth of Pennsylvania, a distinction that automatically imparts to it a great deal of significance and symbolism. Designed by architect Joseph M. Huston and completed in 1906, the exterior of the building (fig. 1) is constructed of Woodbury Gray granite that by the late 1990s was characterized by a series of problematic conditions that both compromised the stone's appearance and affected its performance characteristics. Natural weathering and a series of aggressive cleaning treatments had resulted in physicochemical surface alteration, color change, decreased surface permeability, exfoliation, atmospheric deposition, and metallic staining that visually and materially degraded the landmark building.