Boston Public Library Facade Seals

The seals of the City of Boston, The Boston Public Library and The State of Massachusetts were fabricated in Tennessee Limestone, in by the renowned sculptor August Saint-Gaudens in 1895.  Saint Gaudens is considered by many to be the most accomplished American sculptor working at the turn of the 19^th century.  Over the last 130 years the limestone elements have been exposed to an outdoor urban environment(Copley SQ, Boston), this has resulted in a loss of carved details and surface covered with layers of accretions and soiling. The most disfiguring and potentially damaging condition commonly observed on limestone architectural ornament is the build-up of gypsum. Gypsum is a degradation product found in calcite containing rocks(such as limestone and marble) which have been exposed to weathering in a polluted urban environment. Acidic precipitation dissolves the calcite and calcium ions combine with sulfate ions from pollution to form calcium sulfate(gypsum).   This results in black crusts(usually over a layer of thinner yellow crust on most limestone). The gypsum on the surface is disfiguring as it covers the exquisite carved detailing blurring the site lines of the architecture. Extensive and untreated gypsum formations can cleave off the more stable stone substrate, this can cause losses and cracks in the original stone and can be a potential safety issue when the stone elements are sited on upper building courses.  

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The building facade was staged with scaffolding to access the facade in a way to allow for access through main entrance. The condition of the seals were inspected and documented. The surfaces were cleaned of accretions, including the tightly bonded gypsum using a variety of methods that included washing with cleaners appropriate for use on historic calcareous stone and mechanical removal with small hand tools. The majority of the tightly bonded crusts were removed using a conservation laser.  It was imperative to use tools and techniques which limited the loss and damage to the original surfaces. Micro-cracks observed on the surfaces were injected with adhesives, losses to the carved details were re-formed using custom repair mortar. The joints between the sections of carved stone were cleaned of degraded mortar and filled with a custom mortar mix