Cleaning Limestone Masonry

For years preservationists have had to rely on invasive and labor intensive techniques to clean masonry structures. With the recent restoration projects at the Nebraska State Capitol, preservation staff looked for the latest developments in masonry cleaning and tested these techniques for their effectiveness to clean and preserve the Capitol. At the beginning of the Exterior Masonry Project, architectural consultants advised using a new cleaning technique with glass bead blasting to clean the tower facade rather than sand blasting. The premise being the glass beads would shatter as they removed the algae and dirt rather than clean and etch the surface as sand blasting does. Further testing revealed the soiling and algae on the exterior facade could be successfully removed by first applying a biocide treatment and then high pressure washing. The research and testing of cleaning techniques on the exterior limestone facade resulted in the most effective and least invasive technique being used to complete the cleaning of the Capitol’s base. The soiling on the interior walls of the Capitol results from a different set of circumstances. There are three kinds of soiling to be addressed. There is discoloration of the limestone as a result of mineral staining from leaks, soiling from indoor air pollution and soiling from human contact with surfaces. Humans excrete acids and oils which are absorbed by the porous limestone surfaces when touched by hands and other parts of the body. These three types of soiling have different chemical compositions. Capitol conservation staff have used different techniques to clean the interior walls, from soap and water scrubbing to clay poulticing. Both methods require protecting the surrounding surfaces and extensive use of water for rinsing and cleanup. During the planning of the 2008 West Chamber Restoration Project, Capitol Preservation staff and consultants looked for an interior cleaning product that would be effective and less invasive. They did not want water spray or clay dust spread throughout the highly finished legislative chamber. A new integrated latex/detergent poultice product was very successful in cleaning the stains and soiling in the George W. Norris Legislative Chamber with minimal impact. In 2009, the conservation consultant and Capitol conservation staff continued their testing of the latex/detergent product in two areas of significant mineral staining. Once the new copper roof was installed over the southwest quadrant, conditions which allowed the staining to occur were resolved, and cleaning began in the southwest elevator vestibule on third floor. This cleaning proved very successful and work moved to the third floor southeast stairwell. The latex/detergent poultice has successfully cleaned both the mineral deposits and hand soiling on the limestone. Capitol preservation staff now have another tool to use in their ongoing work to clean and preserve the Nebraska State Capitol.