North-Central Section - 42nd Annual Meeting (2425 April 2008)
Paper No. 20-6
Presentation Time: 9:40 AM-10:00 AM

VIEW FROM THE CONSERVATOR'S SCAFFOLD: OPPORTUNITIES FOR GEOLOGICAL DISCIPLINES TO INFORM PRESERVATION PRACTICE

LEE, John Greenwalt, John Greenwalt Lee Company, PO Box 724, Annapolis, MD 21404, jgl@johngreenwaltlee.com

After a decade working to conserve and replicate historic masonry around the country, the value of geologist input has been proven time and again. Historic buildings reflect regional geologic history through their mortars, a man-made stone of sorts. Using these same aggregates and clay fines for replica mortar-making ensures material and aesthetic compatibility.

Geologists can assist forensic investigations seeking to understand assaults a building has undergone through time. Knowledge of processes such as sulfate deposition on lime mortars leading to high solubility gypsum salts allows one to exploit the limits of the process on many buildings damaged during the era of coal burning. Specific forensic assistance case studies includes diagnosing subsurface water movement where creeks infilled before a building was constructed have been redirected beneath a 200-year-old building by subsequent construction nearby, leading to subsidence and cracking of the walls. On another, clay lenses were located just below the foundation of a 225-year-old building that had struggled with "rising damp" from early days, allowing us to assist in water diversion.

When replicating historic mortars, the easiest (although too infrequently used) method is to find the original builders' source on site. Buildings constructed before the railroads normally used aggregates from nearby whether creek-washed or pit dug and washed. Sometimes local restrictions make mining a site-specific source impossible today or the coastal water source no longer exists on the property. In these instances a geologist can point conservators to an alternate source. In the latter case, the sand was now downstream two miles and on the opposing bank. From corings geologists can locate the clay lens that matches site-fired bricks.

Their understanding of sedimentation processes allows geologists to assist in materials selection such as the optimum lime for carbonation of lime-sand mortars and guidance in stabilizing particularly unstable smectite and attapulgite clays faced on a project in Warm Springs, Georgia. The assistance of geologists has also lead to the development of new conservation techniques once we understood how to manipulate the transient but powerful calcium bicarbonate phase.

North-Central Section - 42nd Annual Meeting (2425 April 2008)
General Information for this Meeting
Session No. 20
Cultural Geology: Building Stones, Historic Cement and Mortar, and Archaeological Materials
Casino Aztar Conference Center: Walnut E
8:00 AM-12:00 PM, Friday, 25 April 2008

Geological Society of America Abstracts with Programs, Vol. 40, No. 5, p. 32

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