Since hard water deposits can quickly reduce the efficiency of a dishwasher, most machines are fitted with an in-built water softener. This salt helps to keep the machine's softener working so that limescale and watermarks are less likely to form. Finish dishwasher salt is specially granulated for dishwashers - the particles are larger than ordinary table salt so they dissolve more slowly and don't clog the machine. Other salts can contain impurities like iron and carbonates that damage, rather than protect, machines, while Finish dishwasher salt is 99.9% pure for maximum effect