As visitors have already witnessed during the past year, the Metropolitan Museum has embarked on the task of cleaning and restoring its exterior (Figure 1). The four-block-long facade along Fifth Avenue, constructed directly in front of the original building by Calvert Vaux and J. Wrey Mould (1880), was erected entirely out of Indiana limestone, but consists of four separate wings that were designed between 1902 and 1917 by the renowned firms of Richard Morris Hunt and McKim, Meade and White.

Over the course of several decades the exterior of the Museum has darkened considerably, as a result of fly ash deposits held in place by gypsum. Fly ash is an aerosol made up of carbon and alumino-silicate particles and results from the burning of fossil fuels, while the gypsum is formed by the interaction between calcium carbonate, the main component of limestone, and acid rain, also a byproduct of burning fossil fuels.

Cleaning of limestone buildings is usually done with one of three methods: chemical cleaning with bases and acids, abrasive cleaning, and water misting, each of which has its advantages and disadvantages. Chemical cleaning is efficient, but the acids employed can easily cause damage. Abrasive cleaning methods have improved significantly in the last three decades through the introduction of reduced pressures of delivery, softer abrasive materials, and finer particle sizes, with the result that even relatively delicate stone surfaces can be safely cleaned. Abrasives, however, are unable to safely remove gypsum that has formed below the surface of the stone. One of the oldest cleaning techniques for limestone and marble is water misting. This system uses the low-pressure delivery of nebulized water that dissolves the gypsum, allowing the fly ash to be carried away in the runoff. Based on the underlying principle that gypsum is 150 times more soluble in water than calcium carbonate, it is preferentially removed as the water flows across the soiled surface. Water misting usually cleans limestone uniformly and is able to remove subsurface gypsum.

Although thinking about cleaning the facade began several years ago, when a Japanese businessman offered the services of his company, serious impetus was given to the project when it became apparent that sections of the cornice were increasingly unstable. It was at this point that an architecture and engineering firm was commissioned to conduct a survey, which included an assessment of the condition of the limestone and its pointing, as well as the viability of all exterior water management systems. The resulting inventory was used to identify possible treatment methods, which were investigated through mock-ups. Cleaning tests involved, inter alia, two proprietary abrasive systems and a proprietary chemical cleaning method, but ultimately water misting became the technique of choice for the Museum's facade. Mock-ups were also prepared for pointing, crack repairs, composite patching, and dutchmen, in order to provide the best match of color, texture, and profile for all these replacement and compensation installations. The survey, mock-ups, and test results were used to create a bid package/request for proposal that was sent to several prequalified contractors.

Work on the south elevation at 80th Street began in October 2002 and has proceeded north along Fifth Avenue as far as the main entrance. Plans call for the completion of the equivalent elevation north of the front steps during the next construction season. The most complicated part of the project, both in terms of treatment and logistics, is the main entrance itself, which is slated for cleaning in 2005. This section includes four caryatid figures and six medallions by architectural sculptor Karl Bitter that require special attention (Figure 2). Their condition will be assessed by Objects Conservation staff in order to decide if in addition to mechanics from the contracting firm, conservators will need to play an active role in the treatment of these works.

The cleaning is bringing about a most striking change in the Museum's appearance, as the chaotic soiling patterns are replaced by the intended play of light and dark created by the architectural features (Figure 3). Although less visible, equally important are the stone repairs mentioned above, which ensure both public safety and the safety of the collections by making the building envelope watertight. A complete repointing with a proprietary hydraulic lime mortar, as well as the installation of Monel metal gutters and copings for parapet walls, ensures that rain and melted snow will drain properly. Finally, nearly invisible netting has been installed to discourage pigeons from roosting on the newly cleaned facade.

Many have contributed to the development, management, and execution of the facade project. Tim Allenbrook and Kyle Normandin of the architecture and engineering firm Wiss, Janney, Elstner Associates, Inc. carried out the initial survey, devised and supervised the preparation of mock-ups and other tests, and wrote the bid documents/request for proposal. They continue to serve the Museum as advisors and managers. Walter Sedovic, of Walter Sedovic Architects, has provided peer review of all phases. The contractor selected for the project is Nicholson and Galloway, Inc., and work on the building has been ably guided by their project manager Mark Haynes and foreman Fernando Fuentes. Cleaning of the facade is overseen by Philip T. Venturino, Vice President for Facilities Management, and Thomas Scally, Manager. Their department has worked closely with conservators and scientists, including the author, as well as curators Morrison Heckscher and Thayer Tolles of the American Wing, both of whom also deserve thanks for providing background information on the building and its sculpture for the purpose of this article.

Erratum: In the original print publication of this issue of met objectives the author neglected to mention John Honick of Facilities Management, who has had a vital part in the day-to-day running of the project.

George Wheeler has worked in the Metropolitan Museum since 1979, and is currently Research Scientist in the newly created Science Group. He specializes in the field of stone conservation, and has published extensively on related topics.