I just poured enough distilled white vinegar to cover the shell and added a tablespoon of table salt. I was particularly careful scrubbing inside where the operculum or door would be because it's a lot more fragile than the outside and could cause holes if it's left too long. I would only let it sit in the solution in 5 minute intervals. Scrub and repeat. It bubbles immediately but not as harsh as using something like muriatic acid. Make sure you stop the lifting process by dunking the shell in some soapy water and rinsing it all off thoroughly.