Viscose, which is a rayon fiber, loses strength when wet and sometimes shrinks. There are four major types of rayon, and different rayon types have differing degrees of wet strength. Regular rayon shrinks the most, while "high wet modulus" and "high tenacity" rayon have greater wet strength and shrink less.
Because regular rayon has a low wet strength, this fabric should be dry cleaned instead of machine washed. Regular viscose rayon is vulnerable to high temperatures, such as those from ironing, and it is also susceptible to mildew and acid. Materials made from high wet modulus rayon have better elastic recovery and can be machine washed.