Saponification Index This determines how much lye is needed to turn the oil into soap, a process known as saponification. Each oil has its own saponification index value. This is why you can't simply switch oils when following a soap making recipe. Iodine Index Number This allows you to determine if the soap is going to turn out to be a hard bar or a soft one. If an oil has a low iodine index number, it will produce a harder bar of soap. A higher number means you will have a softer bar of soap.
