CASE FURNISHING MATERIALS CAUSE SULPHIDE CORROSION

Abstract

In 1965, the Field Museum in Chicago opened an exhibit of its unique collections of Tibetan artifacts. All aspects of everyday and religious life were represented by objects made from a wide variety of materials. Of particular interest are the objects made of metal, especially silver. The exhibit involved three walk-in cases which were unusually tightly sealed. Great pride was taken in the modern appearance of the exhibit: innovative materials had been used to furnish the cases. The floor of the cases was covered with carpeting and there was vinyl wallpaper on all interior walls as well as on top of all horizontal surfaces of the tables inside the cases. Shortly after installation, a black bushy crystal growth appeared on some of the copper and silver objects. These crystals were brushed off, but reappeared within months. For the next 26 years, crystals continued to grow on the pieces and were periodically brushed off. In 1991, the affected objects were removed for conservation treatment and the source of the problem was sought. The carpet was immediately suspect, and a sample was taken for testing. The pile consisted of a mixture of nylon and wool fibers, backed with a coarse, loosely woven burlap adhered with a thick layer of a black, rubbery substance. Analysis showed this substance to be the synthetic rubber, poly-I-butene. Samples of the carpet were subjected to 'Oddy tests' [I] using silver blanks. The carpet was divided into its constituent parts and each was tested separately. Within 24 hours, all blanks were covered with a dense black layer of corrosion. While the pile fibers did produce sulphide corrosion on the blanks, the rubber backing produced the most corrosion and appeared to be the primary source of sulphur inside the cases. During 26 years of exposure to an extremely aggressive sulphurous atmosphere, the silver objects sustained severe damage. The corrosion took two forms: bushes made up of dense clusters of crystals, or a fine web of black lines. Underneath the bushy corrosion, the silver surfaces were severely pitted. On several objects, this pitting was so severe that the silver had lost its characteristic white lustre and appeared dull and gray. Underneath the web-like, or filiform, corrosion, deep channels had been eaten into the surface, badly disfiguring the appearance of the objects. On some objects, both forms of corrosion resulted in what appeared to be dc-alloying of the surface. When the corrosion was removed, the surfaces were reddish-brown in color, suggesting the preferential removal of silver. When polished, these surfaces regained the characteristic white lustre of silver. Corrosion also removed the very thin silver washes applied to the surface of many of the objects made of a copper alloy. This sulphide corrosion problem provides an extremely graphic example of the damage caused by the use of inappropriate case construction materials and furnishings. In this exhibit, seemingly innocent materials caused irreparable damage to this once splendid collection of silver objects. It is hoped that this example will convince museum personnel of the importance of testing and choosing all exhibit materials carefully and wisely. Author's address: Division of Conservation, Field Museum, Roosevelt Road at Lake Shore Drive, Chicago, IL 60605, USA.