With the slab divider mold, be sure to line the bottom of the mold with freezer paper. The dividers will come out fairly easily, but if you don't line the bottom of the mold with freezer paper, you'll have a very hard time getting the bars separated from the bottom of the mold.
With single cavity molds, you don't have to rush. Just let the bars set and cool over night. They should pop right out of the molds quite easily.
A few more tips...
- I generally just use regular, fine grain sea salt for my salt bars. You do not want to use epsom salts. The magnesium in the bars makes them gooey, sweaty messes.
- You can use pink, himalayan or other unique soaps as long as they don't have high "other" mineral content - like dead sea salt, for example.
- You can add colorants and additives to salt bars just like other soap, just remember that you don't have a lot of time to manipulate the soap. It's going to thicken quickly.
- Some folks find that the salt seems to lessen the fragrance oil effectiveness - and hence will lean towards a more generous helping of fragrance oil in the batches. I find that .7 ounce per pound of oils (a pretty general usage rate) of fragrance is fine.