Before you put any towels, clay clothes, aprons, or other fabrics from the studio into a washing machine, always rinse them out first in a bucket of clean water. If your laundry has a lot of clay on it, use the three-bucket rinse system.
Remember not to dump rinse water out. Set the rinse buckets aside and allow the clay particles to settle several days. Then carefully pour off the clean water on top and add the clay sediment to your scrape bucket.
Even after rinsing, there may be clay particles left behind. Some of these will simply be from the rinse water. At other times, there may be tough spots of clay or glazes that won't come out even after a thorough rinse. After rinsing as much material out of the fabric as possible, hose the fabric to get the rinse residue and tough patches out.
Once the fabric has been rinsed and hosed off, it is safe for your plumbing to place them into the washing machine.