Restoration of outdoor stone sculpture presents a particularly challenging area due to the uncontrollable forces of environment and human activity which demand treatment solutions that are chemically stable, durable and inconspicuous. Several case studies of past treatments of outdoor stone sculpture are presented to determine to what extent the three criteria listed above were followed and how this may have affected the success or failure of the treatments. Where the old treatment has not been successful, new treatment solutions or modifications are suggested. A brief overview of traditional stone repair techniques is also presented.
In New York City’s Central Park, the exquisitely carved sandstone panels of Bethesda Terrace depicting birds amidst seasonal foliage, designed by Jacob Wrey Mould in the 1870’s, are a frequent target for vandals. The damaged elements have been replaced over the years with finely carved heads and wings only to be frequently broken off again. It was clear that a contributing factor in the vulnerability of the panels lay in the different appearance of the newly carved stone against the original soiled stone and the distinct join-line between the two. Keim potassium silicate paints (Keim Farben GMBH and Co.) were found to be effective for staining the newly carved stone when diluted with straight potassium silicate solution. Nine months later, although the staining treatment is still a definite improvement, the color has faded and in some cases has even changed hue. This may be due to the dilute application of the paint which did not form a coherent, and therefore, stable film. The chemical characteristics of the silicate paints are discussed as well as the results of outdoor weathering tests of silicate and other types of stains on various stone substrates.
The current conservation campaign of the Carrara marble Giuseppe Verdi Monument ( 1906) by Pasquale Civiletti at West 72nd Street in Manhattan allows a close reexamination of the last major treatment carried out in 1945. In that restoration, the conservator chose to deal with the problem of the fragility of the open form hands by carving the replacements as more durable closed forms. We know from historical photographs that the fingers were originally carved in the round and in the current restoration, we are restoring them to their original articulated form. This approach is made possible by using casting materials which exceed the strength of the inherently brittle material of marble. We are currently experimenting with casting materials such as polyurethanes, polymer modified gypsum and polymer modified mortars. The problems of texture- and color-matching cast replacement elements to the original stone as well as the stability of casting materials in the outdoor environment are discussed.