These brittle, fragile exoskeletons can easily break apart if they're handled too roughly.  Don't scrub sand dollars too vigorously. If you do scrub a sand dollar, make sure that you don't push too hard on the shell. Don't soak sand dollars in chemical solvents--such as bleach or acids--for too long. The solvent can contribute to the decay of the already-fragile object. Get it clean, but don't dissolve it. If the sand dollar has recently washed ashore, you may encounter the soft tissue of the deceased animal. You can let the shell dry naturally in the sun, you can bury it in the ground to let the bugs eat the tissue, or you can remove the tissue by hand.  If there is an odor--salty and musky, like decaying seaweed--about the shell, there may be decaying tissue within. Leave the sand dollar outside in the sun for a few weeks, and the remaining tissue will naturally decay and disappear. The exoskeleton will begin to bleach, lightly, and harden in the sun. When the sand dollar looks like a shell--relatively smooth, free of tissue--it is ready to use. Consider burying the sand dollar in the earth of your yard or garden. Anything more than a few inches deep will do. Within a week or two, worms and other detritivores in the soil will consume the dead tissue and leave your sand dollar picked clean. Be sure to mark the burial site with a distinctive stone or a stake so that you don't forget. Be careful not to crush the sand dollar when you're burying it or digging it up. You can remove the tissue with a sharp penknife. Be aware that the tissue has grown into the exoskeleton, and it may be difficult to remove every last bit. If you use a knife, be careful not to cut yourself or scar the surface of the sand dollar. Even if you remove the tissue by hand, you may want to allow the exoskeleton a few days to dry. Soak the shell thoroughly in clean, fresh water to remove any sand or debris that is stuck to them.  Fill a bowl or bucket with fresh water. Soak the sand dollar until the water becomes brown and murky. Replace the water in the bowl with fresh, clear water, and continue to soak the sand dollar until the water is murky again. Repeat this process until the water remains clear. If the exoskeleton is particularly gunky, you can mix the water with a few squirts of dish soap for a slightly stronger cleaning agent. If you're pressed for time, you can continually and carefully flush the sand dollars with water until they're clear of sand. Consider using a stiff, bristled brush to remove any clinging sand from shell fissures, cracks, and crevices. Be very gentle--sand dollars are fragile, and they may not stand up to a vigorous brushing. When your sand dollars are clear of sand, place them in the sun for a few hours to dry. . Some beaches are known for the sticky black tar that washes ashore, coating the rocks and the sand and the feet of anyone who treads there. If your sand dollar is coated with tar, it may not come clean with a water rinse.  Chip off any significant tar coating with a sharp piece of metal--a paint scraper will do, or a knife. Be careful not to cut yourself, and be careful not to scratch or break the sand dollar. It is best to avoid using force with sand dollars, given their fragility. Use baby oil. Squirt a dollop of baby oil onto the sand dollar and rub it carefully into the tar. If you don't want to get tar on your fingers, use a tissue or the corner of an old dish towel. Gently rub the tar until the baby oil begins to dissolve it. Within a few minutes, you should be able to remove most of the tar from the sand dollar. Consider using any of the following tar softeners: Eucalyptus oil, lard, margarine, mineral oil, olive oil, peanut butter, petroleum jelly, suntan lotion, or vegetable oil. Consider adding these softeners to the bowl of water when you soak your sand dollars, and consider soaking the tar-covered shells in a bowl of pure tar softener.

Summary:
Be very gentle when cleaning sand dollars. Remove decaying tissue. Rinse the sand dollar. Remove tar from sand dollars