Many 19th century brownstone buildings have deteriorated because builders did not understand the properties of this sedimentary stone. One such building if the recently restored French Library and Cultural Center in Boston. The library began life as an immense, yet elegant, residence. The organization retained Stahl Associates, Architects to design and manage the rehabilitation of the building's exterior. "In the natural course of events, brownstone--as a sedimentary stone--builds up in horizontal layers as it lies flat in the ocean," explains the company's president, Frederick Stahl. "What strength it has retained when the stone is kept flat by being placed horizontally on a structure--that is, when it is placed in the same dimension as its original bedding plane in the quarry." Frank St. Pierre, construction specialist at Stahl Associates, was assigned to manage the project's architectural aspects, so his immediate concern was replicating the missing and damaged brownstone. It was concluded that they needed a material with a portland-cement base for compatibility with the stone;acrylic polymers to enhance the repair mortar's ability to bond with substrate;and precise coloration to match the range of shades the brownstone had developed.