DEGRADATION IN WEIGHTED AND UNWEIGHTED HISTORIC SILKS
Janet E. Miller, & Barbara M. Reagan
SILK FABRICS DATING FROM the late 19th and early 20th centuries often are severely degraded as a result of weighting agents that were used to increase or replace the weight lost during the degumming process. Weighted silk is usually heavier and thicker and has better draping qualities than unweighted degummed silk. However, both the process of weighting and the agents used may weaken the silk fiber, shorten its wear life, and make it more susceptible to degradation by acid, alkalies, high temperature, and light.3, 7 As a result, many antique silks exhibit lengthwise splits and cracks at folds and creases and are so weak that they must be supported to be handled or displayed.
Historically, the artificial weighting of silk has been practiced for hundreds of years using a variety of inorganic and organic compounds. In the 20th century, the most common weighting agents were tin salts, such as stannic or stannous chloride, which were used alone or in combination with phosphates and silicates.3, 13 Weighting agents usually were applied to silk during the dyeing operation. Because fabrics were often sold by weight, some dyers and finishers would overweight silks for greater profits, even though this practice severely weakened the fiber. The adverse effects of weighting agents on silk fabrics resulted in a 1938 Federal Trade Commission rule requiring manufacturers to designate on labels the percentage of weighting agents applied if greater than 10% by weight (or 15% for blacks).
The selection of appropriate conservation methods for historic silk textiles and costumes may be impeded by the lack of knowledge pertaining to the physical and chemical changes that occur in silk fibers during aging or to the deleterious effects of dyeing and finishing chemicals, such as weighting agents. Furthermore, research on fiber aging has focused primarily on cotton and wool, while few studies have been conducted on silk. Information characterizing degradation in historic silks has been extrapolated primarily from research conducted on new silks, which have been studied to a limited extent for quality control purposes (i.e., to minimize damage during manufacturing), or from artificial aging experiments. Bresee and Goodyear4 examined fiber fracture patterns in naturally aged silks that had been degraded by heat treatments and accelerated light exposure. Similarly, in artificial aging experiments on silk, Leene et al.9 evaluated changes in the degree of polymerization and fiber tenacity and elongation. These authors concluded that aging and deterioration resulted from a complex combination of degradation forces.
In a recent study by Kuruppillai8 et al., the extent to which a deacidifying agent and two antioxidants reduced degradation in new silk was evaluated after exposure to heat and light. These additives were not effective in retarding strength loss, but they did reduce the formation of amino groups. Needles et al.12 investigated the effects of natural dyes and mordants on degradation in silk and wool subjected to xenon light and soil burial tests. Results showed that the mordanting agents improved the lightfastness of the dyes but increased burial-induced color change.
This research was undertaken to characterize physical and chemical degradation and changes in proerties of naturally aged silk. This information will aid in developing and selecting appropriate methods of conservation. The samples evaluated were new, unweighted silk habutai and naturally aged, unweighted and weighted, historic silk fabrics. Habutai is a soft, light, closely woven Japanese silk fabric woven from raw silk filature with little or no twist.7 The silk habutai also was exposed to chemicals, boiling water, and heat to examine their effects on the physical and chemical structure of the fiber, but the results are not reported here.11 Methods of analysis included tests of yarn tenacity and elongation, dilute solution viscosity, amino acid content, photoacoustical infrared spectroscopy, and optical and scanning electron microscopy. The metallic weighting agents present on the weighted historic silks were identified using neutron activation analysis.