The Rationale for Microabrasive Cleaning: A Case Study for Historic Granite from the Pennsylvania Capitol

Abstract

Specialized microabrasive cleaning treatments used for historic masonry have seen widespread use in recent years—in both architecture and sculpture. As these technologies continue to gain acceptance, it is important to develop a greater understanding of their practical applications, capabilities, and limitations. This article presents a case study on the rationale for selecting the JOS/Quintek Microabrasive System to clean granite on the Pennsylvania Capitol in Harrisburg, Pennsylvania. The selection of this treatment resulted from a one-year conservation study that characterized problematic conditions and evaluated a series of potential treatments to address them.

Factors that included the nature of the stone, historic finishing patterns, previous chemical treatments that may have been applied inappropriately, and aggressive weathering had created problematic conditions such as heavy soiling, color change, metallic staining, and decreased surface permeability. The authors developed a two-tiered conservation study to evaluate the effects of several different cleaning treatments (pressure washing, JOS/Quintek microabrasives, Façade Gommage, and chemical restoration cleaners) on several critical physical properties. The first tier of testing assessed the effects of each treatment in terms of soil removal, surface topography, tooling patterns, safety, and efficiency. The second tier of testing was designed to further evaluate the effects of the JOS/Quintek System, which was established as the most successful treatment examined in the first tier. The rationale for completing a focused assessment of the JOS/Quintek System was the fact that a limited number of comparable cleaning programs had been completed with the system and that few case studies addressing its potential effects had been published at the time. Assessment techniques were designed to evaluate impact on both surface and microstructural levels and involved a series of in situ and laboratory investigations, including treatment mock-ups, permeability testing, scanning electron microscopy, polarized light microscopy, reflected light microscopy, and colorimetry.