Pure viscose fabrics, which are also known as pure rayon, shrink when placed in the dryer. This material has a low wet-strength rating, making it vulnerable to shrinking during an average wash cycle. Most viscose fabrics are labeled as dry clean only.
Some rayon fabrics are mixed with other fabrics, such as polyester. When other fabrics in the cloth make up more than 30 percent of the composition, the fabric becomes safe to wash and dry. A type of rayon called polynosic features modifications that give it a high wet-strength rating, making it able to survive a typical wash cycle.