The sheen of silk is altered by exposure to water, especially hot water, and uneven drying creates watermarks.

"[Silk] may be dry-cleaned but should not be washed with water , as water can alter the sheen and crispness of the silk fibers and cause staining if dried unevenly." Banner, Bernadette. _Make, Sew and Mend_, p15 https://www.goodreads.com/book/show/57693333-make-sew-and-me...