1 INTRODUCTION

Despite the popular belief in the immutable durability of stone as a building material, architects, sculptors, geologists, and conservators alike are well aware of the material's inevitable alteration over time. Improved examination and analysis methods have helped explain the susceptibility of many stones to a broad and complex array of decay mechanisms. These natural weathering processes have been further complicated—often reduced and sometimes accelerated—by a long history of conservation and repair techniques.

Correlation between the durability, performance, and weathering of building stone and its geochemical makeup can be traced to antiquity and the Renaissance through Vitruvius in the 1st century B.C. and and Leon Battista Alberti in the 15th century. Their observations were amplified in the late 19th century with the development of the geological sciences and the growth of the stone industries in America. With a deeper understanding came a broader audience.

Current conservation thinking assumes a methodological approach to the issues of stone deterioration and treatment. This assumption is based on an established system of principles, practices, and procedures as developed specifically for the preservation of stone buildings and monuments. Such an approach depends on a wide range of information and the contributions of various research methods, including an understanding of a structure's original and subsequent design goals and construction history, materials characterization and identification through scientific investigations, and objective treatment evaluation using laboratory and field assessment. Integrated thinking is necessary for the successful conservation of complex stone buildings and monuments; current and future use and maintenance must also be taken into account. The recent treatment of the exterior stonework of the Metropolitan Club in New York City affords an excellent opportunity to examine the value of this approach in some detail for North American building stone and to present new information on the mechanisms and large-scale treatment of iron-stained limestone and marble.