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Weighted silk is silk which has been treated to restore or increase the weight lost during the process of degumming. This processing started in the 19th century with vegetable-based solutions such as tannins or sugar. Chemical solutions based upon salts of lead or tin were then used, as well as silicate, phosphate of soda, and astringent extracts. These increased the weight considerably but led to accusations of adulteration as the properties of the silk were impaired.
Unlike most fabric/yard goods which are sold by the yard (or metre), silk is sold to the wholesaler by weight. The first step in processing the silk fiber is "degumming the fibre," the gum being a byproduct of the production of the silk fibre by the worm. Approximately one-fifth of the weight of the silk fibre is lost in the degumming process, and manufacturers felt they had the "right" to replace this lost weight with a filler of some sort. Silk has an affinity for several metallic salts, the most common of which being iron, lead, and tin. It was discovered to be an easy process to return this weight lost in the degumming process by soaking the fibre in a bath of these metallic salts. This process was called weighting, and by increasing the weight of the raw silk, the merchant increased his profit. Weighting with some metallic salts did improve the drapeability of the fibre; however greedy merchants soon began adding more weight than the lost one-fifth; sometimes the final weight was increased tenfold.
In 1938 the U.S. Federal Trade Commission ruled that silk that weighted more than 10% and black silk that weighted more than 15%, must be labeled Weighted Silk.
While silk is a strong and durable fibre, the weighting process is highly damaging to the finished goods. If the garment is worn, it wears out quickly and is highly susceptible to perspiration, salt, and tears; if stored away it becomes brittle and breaks. The method of storage—cold/dark, etc., has no influence on these factors. One readily sees examples of this damage in antique American "Crazy Quilts" where the silk fibres have disintegrated while the cotton and wool fibres remain in good condition, even after 100 years.
This practice was widespread in the 19th century and decreased somewhat in the 20th, but is still used to some extent. An alternative is "organic silk."