THE EXHIBITION OF UNLACQUERED SILVER AT THE METROPOLITAN MUSEUM OF ART
4 THE ATTAROUTHI TREASURE
In 1986, the museum acquired the Attarouthi Treasure, a group of sixth-century, Byzantine, partial gilt, silver, repoussť objects including chalices, censers, and a Eucharistic dove in such a state of preservation that they came close to representing the original surface quality intended by the goldsmiths (Dandridge 2000). As documents of the visual aesthetic current at the time, it was important to present them to the public and scholars in as unblemished a state as possible. Further, the fragility of the gilded surfaces in discrete areas on the chalices had necessitated localized consolidation with Paraloid B-72, an ethyl methacrylate and methyl acrylate copolymer, precluding their being lacquered and subsequently cleaned without potential disruption and loss of the reattached gold leaf.
In preparing the specifications for a vitrine to house the treasure, elements of both design schemes utilized in the American Wing were adopted while incorporating several new products and introducing an active, recirculating, filtration system. Hahn fabricated the vitrine with the display area a box of glass and metal with a volume of sixty-four cubic feet, supported by a wooden base, with light supplied externally from an attic overlaying the glass top. Apart from the front panel that provided access to the interior, all glass-to-glass and glass-to-metal joins were secured via aluminum channel with inset silicone gasketing. As supplied, the contact between the front panel and the glass sides was not gasketed, and due to the size of the glass, the panel exhibited a slight bow. To provide a gasket and to close the space created by the bow, a bead of clear, neutral cure silicone caulk was run into the gaps down either side of the panel. Once cured, each bead was sliced through its center to allow the door to be opened and closed. Interior decks and blocks were made from extruded, high-density polyethylene foam, a product utilized in the packaging industry and suggested by Blackshaw and Daniels (1978) as a material appropriate for the storage of objects. A distinct advantage of the material as supplied by Sentinel Foam Products was that it was produced in plank form in a variety of densities. The 9PCF weight was selected since it could be shaped with woodworking tools, joined mechanically with screws, and allowed for fabric coverings to be attached with staples, eliminating any need for glue. Depending on the thickness of the material, it can distort under the strain of stretched fabric; however, this characteristic can be offset by attaching the foam to more rigid materials such as aluminum channel, L-bracket, metal sheeting, or Plexiglas. In this instance, the full two-inch depth of the material could be accommodated for the backing board and required no auxiliary support. The interior label copy was adhered with wheat starch paste to eight-ply acid-free ragboard. All materials had either been approved for permanent exhibition by the Getty Conservation Institute (1989) or by means of the Oddy test (1975) at The Metropolitan Museum of Art.
Two different approaches were investigated to actively condition the air within the vitrine, the reduction of the relative humidity to levels sufficiently low to inhibit the production of silver sulfide, and the elimination of hydrogen sulfide and other acidic vapors with an adsorbent. Michalski (1982, 1985) and Toogood and Wilson (1985) had designed modules that constantly reconditioned the air within a vitrine to a predetermined relative humidity. While successful, both systems contained multiple mechanical elements and required an adjacent space for installation not available in the Byzantine gallery. Given the inert quality of all of the vitrine materials and an ambient relative humidity of 40–50% in the galleries, it seemed more appropriate and practical to implement the second approach and focus on purging the atmosphere within the vitrine of those organic acids known to be most deleterious to silver.
The research undertaken by Parmar and Grosjean (1989, Parmar and Grosjean 1991) to study the effectiveness of sorbents in removing atmospheric pollutants from the exhibition environment quantified for the first time the relative capacity of a variety of sorbents including Purafil and activated carbon to successfully adsorb hydrogen sulfide in both the passive and active mode. For adapting an active system of filtration within a vitrine, they suggested using a fish tank pump in conjunction with a canister of activated carbon. Their system was adapted and a fish tank pump was installed in the base to constantly circulate the air in the display area through Tygon tubing and an in-line canister filled with 200 grams of activated carbon. A 12-20 mesh size was selected for the sorbent to maximize the surface area for adsorption, and at the same time minimize both the drag on the flow of air through the canister and the migration of dust into the exhibition space. A loose packing of cotton wool in the supply side of the filter served as a catch for any particulates. The tubing was inserted through holes drilled in the aluminum base of the bonnet with the voids around the tubing filled with a neutral cure silicone caulk. The minimal noise and vibration of the pump were dampened by the plywood surround of the base and by setting it into a foam box.
Since there is no visual indication of when activated carbon has reached its saturation point, an alternate means of evaluating conditions within the vitrine needed to be found. No active or passive monitors were available for detecting hydrogen sulfide at the levels that might affect silver. Blackshaw and Daniels' (1979) analysis of the relative rates of corrosion of different silver alloys indicated that polished blanks of sterling silver were more reactive than pure silver as well as comparable alloys worked in a more traditional manner. Two polished blanks of sterling silver were placed on the deck adjacent to the ports for air supply and return with one half of each blank brush coated with two applications of Agateen #27 diluted in Agateen Thinner #1. An arbitrary time frame of one year was scheduled for the replacement of the sorbent. The condition of the silver surfaces was monitored on a regular basis and over the course of the next nine years no visual change was perceptible either on the blanks or on the surfaces of the objects. While no data log was recorded for the relative humidity levels within the vitrine, periodic measurements in the gallery confirmed the expected range in ambient conditions of relative humidity, broken only by slight fluctuations at points of seasonal transition.