Calcareous stones, such as marble, are particularly sensitive to urban environments. The reaction between sulfur pollutants and the calcite of the marble often results in the formation of an alteration crust composed of calcium sulfate dihydrate (gypsum). Analyses performed on the external crust and bulk stone verified the presence of gypsum, especially on protected portions of the capitals where rain water cannot wash away surface pollutants. Gypsum is comprised of a network of platy grains in which atmospheric particles of dust and dirt are trapped, imparting the black appearance of the crust. Gypsum presents a threat to the structural integrity of the marble, exhibiting an increase in water sensitivity and different thermal stresses. The encrusted surface maintains the sculpted integrity of the carved ornament, but beneath the crust, the bulk of the material is completely friable and detaching from the exterior surface of the stone. Microcracking between marble grains is also visible.

Analysis of the conditions survey revealed that while the stone capitals on the north elevation remain relatively sound, the south and west elevations continue to actively deteriorate. The south elevation was found to exhibit the worst conditions, primarily attributed to the presence of vehicular traffic on Walnut Street. Results of the barium-hydroxide testing showed that while this treatment is potentially highly effective, repeated applications and long dwell times may be required to ensure conversion of the sulfates and consolidation. All volutes, where they remain, display thermal cracking and require pinning to reinforce their structural integrity.

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