As cellulosic textiles age, hydroxyl groups (-OH) are converted to carbonyl groups (=O), which contribute to a dingy brown or yellow color. The chemical process known as reduction adds electrons to the cellulose fibers, stabilizing their molecular weight and returning carbonyl groups back to colorless hydroxyl groups. The combination of dissolved soils and cellulosic degradation often turn the wash bath the color of strong tea. The reaction of the sodium borohydride with water is also produces hydrogen gas bubbles, and the bath may give off a smell reminiscent of sulfur or chlorine. It is important to agitate the wash bath regularly to allow all sides of the textile to come in contact with the surface of the water where the chemical reaction is taking place.
If you are a textile conservator with experience in wet cleaning and bleaching historic artifacts, you may be interested in our MTS Handout, Bleaching Textiles with Sodium Borohydride, available on the MTS website.