I've always gone by this site
http://www.intellweb.com/gcka/sterileplants.htm
Use 1 tablespoon of alum per gallon of water. Soak the plants a minimum of 20 minutes, up to a maximum of 2 hours. A two-day soak will kill snails and their eggs. Avoid using this method on extremely fragile plants.
http://www.intellweb.com/gcka/sterileplants.htm
Use 1 tablespoon of alum per gallon of water. Soak the plants a minimum of 20 minutes, up to a maximum of 2 hours. A two-day soak will kill snails and their eggs. Avoid using this method on extremely fragile plants.