But many soap makers add a bit of salt to their soap to help increase the hardness. This has been refuted as just a carry over from when lye was leached out of wood ashes, but my tests have shown that adding salt does indeed increase the hardness at first.
It does not result in an overall harder finished product, but it does make the bar get harder quicker. The benefit of a harder bar quicker, is that it makes getting it out of the mold quicker and easier...and (if you're not a real stickler for a 3-4 week cure) let's you use it sooner!
Adding the Salt
Weigh out the water you are going to mix your lye with. Before you add the lye, add ½ tsp. of salt per pound of oil in your recipe*. Stir well to make sure that all of the salt is dissolved. Add your lye to the salted water, making your lye solution, and resume your normal soap making procedure.
* That is, if there are 32 oz. of oils in your recipe, add 1 tsp. For 48 oz., add 1 1/2 tsp.