3 THE AMERICAN WING

During the late 1970s and through much of the 1980s, The Metropolitan Museum of Art undertook a series of capital building projects to rehouse both the Egyptian and the American Paintings, Sculpture and Decorative Arts collections, as well as to install the Rockefeller Collection of Primitive Art. Coincident with the expansions in physical spaces was a philosophical change in the way the collections were to be exhibited. Not only were the most significant objects in the collection to be rehoused, but also those pieces that had previously been inaccessible would now be made available to the general public in areas adjacent to the main galleries.

Of particular interest for this study were the two different systems established in the American Wing for displaying silver without a coating (Weintraub 1981, 1988) in response to the two approaches taken by the curators for displaying the collection. In 1981, the principal silver objects were organized thematically around the balcony overlooking the Englehard Court in freestanding vitrines fabricated by Glasbau Hahn. The bulk of the collection was then installed for study and storage in 1988 in a distinct area, The Henry R. Luce Center for the Study of American Art, within two large, floor-to-ceiling cases. In both installations, all structural elements were specified as glass and metal both for aesthetic reasons and to avoid wood and wood products and their associated acidic vapors.

The vitrines around the balcony were designed to restrict the influx of outside air. The three glass panels that defined the sides and back of each vitrine were joined to one another with epoxy. The joins along each side were strengthened and warpage inhibited by overlaying the vertical edges of each panel with strips of glass similarly adhered. A gasketed sliding glass panel across the front provided access to the interior. The top and bottom edges of the glass panels slotted into metal channel associated with the metal framework for the light attic and the metal base of the vitrine. Design concerns necessitated that the interior shelves, blocks, and decks in the balcony cases be fabricated from MDO, exterior grade plywood with phenol-formaldehyde adhesive, and covered with a dyed cotton fabric. To counteract the acidic gases that might off-gas from the plywood and to provide a buffer against the influx of outside air, 3M strips of powdered, activated carbon cast out on a vinyl base were attached to the underside of all the shelves. The manufacturer changed the formulation of the strips early on to eliminate the plastic substrate and introduced Tarni-Shield with the activated charcoal embedded in a paper matrix.

The capacity of activated carbon to adsorb gaseous pollutants harmful to metals was understood early on in the packaging industry (Rance and Cole 1958). Its use was advocated by the conservation community initially as filters to be inserted into HVAC systems as a means of reducing the levels of outdoor pollutants allowed into the ambient air of the gallery (Thomson 1965, 1978; Garver 1968). The introduction of activated carbon as a passive sorbent for gaseous pollutants into the interior of vitrines appears to have first been suggested by Padfield (1966) and became the standard approach to the reduction of acidic gases in storage and display (Oddy 1975; Leveque 1986; Gilberg and Cook 1987; Craddock 1988; Druzik 1991; Weintraub and Wolf 1995; Lee and Thickett 1996).

While the Luce Center was under construction, the silver not on display was stored, unlacquered, in interior metal and glass cabinets equipped with externally mounted, positive pressure systems to supply conditioned air to the interior of the cases (Weintraub 1981) at a flow rate sufficient to restrict the intrusion of outside air. The pump was installed adjacent to the case and pushed a steady supply of ambient air through Tygon tubing into the cases. Inline canisters of silica gel and Purafil, a potassium permanganate and alumina-based material, controlled the relative humidity and adsorbed gaseous pollutants. An advantage of the Purafil over the activatedcarbon was that it was self-indicating, changing color when its saturation point was reached.

When the Luce Center was completed and the silver moved from the storage cabinets to its permanent home, the silica gel component was eliminated from the system, and larger pumps were utilized to accommodate the more generous, open spaces. The silver was displayed both on glass shelves and on formed metal shelving with a baked-on powdercoated finish. Brush gaskets adhered to the edges of the adjoining glass panels allowed the pressure to equalize.

In evaluating the two systems, an obvious appeal of the use of a passive approach was its lack of dependence on a mechanical system, but it placed a premium on the relative exclusion of outside air, the creation of an interior vitrine environment that was non-reactive, and the regeneration or replacement of the adsorbent prior to its saturation point. Depending on the surface area of the sorbent and the presence of other surfaces that might scavenge pollutants, the exhibition silver can act as a sink for hydrogen sulfide (Parmar and Grosjean 1989, 1991; Druzik 1991) such that tarnishing may eventually initiate. The experience in the American Wing seems to bear this out in that the silver has shown visual indications of sulfide tarnish, albeit at a slower rate than if no adsorbent were present.

Several factors have inhibited the success of the positive pressure system. The constant cycling of the pumps, and their sizing relative to the resistance of the tubing surface and the sorbent, necessitated their replacement every two years. Of equal concern was that the noise generated by the pumps was considered to compromise the visitor's experience. Since there was no alternative space for the pumps' installation, their use has been discontinued and a system of passive adsorption with canisters of activated carbon substituted; however, the size of the cases, the relatively minimal surface area of the sorbent, and the ease with which ambient air can enter the display area have all limited the protection afforded the silver.