6 CONCLUSIONS

The cleaning of buildings is propelled by both aesthetic and materials science considerations. In the case of the Brooklyn Historical Society, both apply: (1) soiling has altered the play of light on ornamented surfaces of the brick and terracotta and has disrupted coloristic effects by random distribution of gray and black on otherwise orange-red surfaces; (2) particulate components of soiling block pores, and hydrocarbon components add water repellency to brick surfaces. While these conditions reduce infiltration of rainwater through the external surfaces and provide some protective effect to the brick, they also engender the trapping of solutions of calcium sulfate derived from the interaction of rainwater with the mortar. These solutions later crystallize inside the brick as gypsum. In assessing conditions of the brick, it is found that the most advanced deterioration occurs only in locations of significant water infiltration. Even in areas where the brick is in better condition, the pattern of both soiling and deterioration points to a mechanism by which calcium sulfate–bearing solutions are drawn from the mortar into the brick. Since the vector of water movement that brings about deterioration is not from the outside in, i.e., through the external faces of the brick, the protective effect of the soiling is of little or no value. In fact, the soiling intensifies deterioration by concentrating calcium sulfate behind brick faces. For these reasons, it was decided to clean the brick at the Brooklyn Historical Society.

Several factors influenced the selection of cleaning materials and methods for the brick. First, the decision to use proprietary cleaners to ensure uniformity of composition narrowed the selection field. Second, the field was further narrowed in that only two of these proprietary cleaners were effective in removing soiling. One of these cleaners—the ammonium bifluoride–based Bac2Nu—left a disfiguring white haze, and the other, the hydrofluoric acid–based Restoration Cleaner, resulted in some loss of surface. A compromise was made, and the latter cleaner was selected, because it produced more uniformly clean brick without white deposits and because the minimal loss of surface did not have a negative impact on the intended appearance or on the physical properties of the brick. It is also speculated that the smoother brick surfaces will resoil less quickly.

The final element in developing the cleaning process for the building was the decision to remove the gypsum-based pointing and replace it with a lime-cement-sand–based mortar before cleaning commenced. This method reduces the risk of movement of gypsum from the mortar into the brick and reduces subsequent damage during the cleaning process and for the future of the building. Project specifications included the testing of brick after cleaning to determine if salts were mobilized despite the precautions taken, with provision for their removal by poulticing if necessary.

Masonry cleaning and repointing were performed during the spring, summer, and fall of 2002. At the completion of the exterior cleaning, a large percentage of soil had been removed from the brick, and the color of the brick was uniform and identical in color to the unweathered surfaces of the brick. Soil that was not removed after three applications of the specified cleaner was left on the brick (fig. 16, see page 74).

Fig. 16.

Portion of south elevation after repointing and cleaning