The thing about salt bars is that you need a LOT of coconut oil to counteract the lather killing properties of salt, and because the CO is so high, you also need a high superfat to counteract the drying properties. Its a balancing act that when it comes together is wonderful. Check out the posts linked at the bottom of this page, or do a search for salt bars to learn more.
I've never heard of a salt bar needing preservatives - maybe the salt does help. However they can sweat if they are in humid conditions.