Hardness in soap making refers to the hardness of the bar of soap after it has finished the curing process. When crafting a soap recipe many calculators will offer a hardness range. This is typically a number between 29 to 54 with the higher number being a harder bar of soap. This number is a general guideline and does not mean that a number outside this range will not produce a perfectly fine bar of soap. Hardness is typically referred to a solid bar of soap made with NaOH -, Sodium Hydroxide, as any soap made with KOH – Potassium Hydroxide will result in a liquid or creamy product and offer no hardness at all.
The hardness of a bar of soap will be affected by the fatty acid profile of the oils used in the recipe. As a general rule hard oils will create a harder bar of soap. Palm, Lard, Tallow, Coconut Oils, Bees wax, Mango Butter, and Shea Butter will all produce a hard bar of soap. As an example commercial brands like Ivory, Dove, Dial, Irish Spring, Jergens, Nivea are all primarily made from tallow resulting in the hard waxy bars most people recognize.
Soft oils or liquid oils will produce a softer bar of soap that will create a softer and more moisturizing effect. Olive oil is an exception. Olive oil take a longer time to cure and may seem soft for a very long time upon its unmolding, but a bar of soap made primarily from olive oil will result in a very hard bar that will need to be sliced before it has fully cured.