Clean seashells in a solution that is one part water and one part bleach. Another method is to clean them in a dishwasher. When cleaning dead seashells with bleach and water, be sure to let them sit long enough.
There is no standard time limit for the cleaning process; the shells should stay in the solution until they look clean enough. Wear gloves to remove the shells from the solution and rinse them off while scrubbing them lightly with a small brush such as a toothbrush. When using a dishwasher, place the shells in a utensil basket to keep them from moving too much.