1 INTRODUCTION

In 1987, the exhibition of the U.S. First Ladies gowns, a popular and important Smithsonian Institution collection, closed for renovation to allow the curatorial staff to reassess the collection's use and to give conservators a chance for long overdue examinations and treatments. This conservation project provided a unique opportunity for extensive collaborative research between scientists and conservators into the deterioration of silk, the material predominant in this collection.

The scientific research focused on, first, the effectiveness of several analytical approaches with minimal destructive sampling as a means of evaluating the object's state of preservation, and, second, the mechanisms involved in the degradation of silk. Since silk is a biopolymer, the effects of the environment on both the physical and the chemical properties over time are of interest to biochemists and bio-engineers as well as conservation scientists.

Traditional textile testing methods such as measuring color difference and tensile strength have a limited usefulness in evaluating naturally aged fabrics with unknown histories. Although color measurements can be made nondestructively, a color difference cannot be quantified because the original color is unknown. Tensile strength measurements require large samples by museum standards and then provide evidence of deterioration only after significant damage has occurred at the molecular level.

Previously, biochemical techniques were used to quantify the initial degradative changes found in artificially aged silk fabrics. Quantitative results can be obtained from very small samples. Becker and Tuross (1994) reported that the selective degradation mechanism progressed to random destruction along the molecular polymer chain with increasing length of exposure and increasing energy content of the incident radiation.

This paper discusses how biochemical investigations are used to relate naturally aged silk fabrics from a museum collection to artificially aged silk fabrics. The changes monitored in solubility and amino acid compositions are indicators of deterioration at the molecular level. The overall recovery of amino acids not only provides evidence of the extent of protein preservation but also serves as an indication of the state of preservation of the garment.

A comparison between the overall amino acid composition of naturally aged silk and that of new, degummed silk (essentially pure fibroin), provides insight into the presence or absence of sericin, silk's natural coating. The presence of sericin and the impact of this protein on the state of silk preservation were also investigated.