The hardness of your soap is directly related to the types, balance, and qualities of the oils you're using. Different oils make your soap harder or softer, depending on their fatty acid makeup.
However, many soap makers add a bit of salt to their soap to help increase the hardness. The real question is does this actually work? The answer is a little complicated. In short, the salt will create a harder bar of soap, though it may not be in the way you expect.
Why Add Salt?
The theory of adding salt to create harder homemade soap is one that has been around for some time. It does, however, come with controversy. While many soapmakers believe it works, others have refuted it as little more than a carryover from the days when lye was leached out of wood ashes. That's a step that most of us longer have to do because lye is commercially produced and readily available.
Through careful testing, we can say that adding salt does indeed increase the hardness of soap. Yet, it only makes soap harder while it's curing.
Adding salt does not result in an overall harder finished bar of soap, but it does make the bar get harder faster. There are a few benefits to this and the biggest is that you can use your soap sooner.
By adding a small amount of salt to your recipe, you can get your bars out of the mold faster and they're actually easier to remove. If you do not enjoy waiting three to four weeks for your soap to cure, add some salt and enjoy the speedier results of your labor.
How Much Salt?
You don't want to simply pour a bunch of salt into your normal soap recipe. The result will be disappointing. Instead, take the time to carefully measure out the appropriate amount of salt.
To do this, weigh out the water you are going to mix with your lye. Before you add the lye, add 1/2 teaspoon of salt per 1 pound of total oils used in your recipe. For example, if there are 32 ounces of oils in your recipe, add 1 teaspoon; for 48 ounces of oil, add 1 1/2 teaspoons.
Stir the mixture very well to make sure that all of the salt is dissolved. Add your lye to the salted water, making your lye solution and then resume your normal soap making procedure.