Considerations on Complex Sequential Treatments of Gypsum Crusts: The Carrara Marble Capitals of the Philadelphia Merchants' Exchange

Abstract

William Strickland's Philadelphia Merchants' Exchange is notable for its early, innovative, neoclassical design, especially the colossal order colonnades surmounted by their Carrara marble capitals. These capitals are among the most exceptional examples of public architectural sculpture produced in the United States in the early 19th century. The mineralogic composition of the stone and the intricately carved geometry of the capitals render them highly susceptible to atmospheric weathering. Current conditions, including dimensional loss of volutes and other carved elements, prompted a year-long study to determine appropriate sequential methods for conservation, especially with regard to the compatibility of individual treatment phases. Laboratory analyses and field observations indicate a well-formed gypsum crust that retains the highly carved details of the stone while partially concealing highly friable subsurface conditions. Retention and stabilization of the external crust became the foremost objective of treatment. Passivation of the gypsum-encrusted surface was attempted with the application of barium hydroxide, followed by a series of cleaning techniques, including laser, chemical, and low-pressure microabrasive cleaning. Pretreatment with a commercial hydroxylating conversion treatment followed by ethyl silicate were applied to consolidate the friable stone. Spalling elements were micropinned with alumina ceramic pins, and visually significant losses were filled with hydraulic lime mortars. The following article discusses and evaluates the authors' conservation treatments applied in sequence in both laboratory and field tests.