For cotton and polyester: Scrape off encrusted clay with a dull knife. Apply a liquid dish soap, such as Dawn, to the stain and let it sit overnight. Machine wash according to garment care label. If stain persists, apply color safe bleach or rubbing alcohol to the stained area. Machine wash again.
For washable silk and wool: Scrape off encrusted clay with a dull knife. Flush stain with cool water. Rub glycerin into the stain and let sit for 2-8 hours. Flush with cool water again. If stain remains, take garment to a professional.