Hardness is a measure of "hard like a brick". It is the sum of the percentages of myristic, lauric, palmitic, and stearic acids in your soap. Myristic and lauric come from coconut oil, palm kernel, babassu, etc. Palmitic and stearic come from lard, tallow, palm, etc.
Hardness is not the only thing to worry about if you want a long lasting soap. Water solubility is the other problem to consider. Soaps made from coconut and palm kernel are hard soaps, but they are very soluble. These soaps melt away quickly, so they clean well in hard or cold water and make lots of bubbles. Soaps high in palmitic and stearic acids are hard and fairly insoluble, so these soaps have a reputation for being long lasting. The downside of these soaps is that they don't make the abundant lather that many people like. Soaps made from the liquid oils (olive, rice bran, safflower, sunflower, etc.) tend to be less soluble than palm, tallow, and lard soaps. They clean the best in warm or hot water, and not so well in cold water. These soaps are soft, however, so they wear away quickly on the washcloth.
If you want a long lasting soap, you will want to make a soap that is ideally "hard like a brick" and fairly insoluble in water. Soapcalc does not calculate a "long lasting" number, but if you subtract the "cleansing" number from the "hardness" number, the answer will be the percentage of palmitic and stearic acids in your soap. The higher this number, the harder and more insoluble the soap will be. A soap with a high "long lasting" number will pay a price in less lather, so that's something to keep in mind.
I'm not sure if I have answered your question, Twiggy -- it has been a long day and I think I'm tired enough I'm rambling. I guess there is no one right answer to how much hardness (or insolubility) one should have in a soap. It depends on the soap maker.
Hardness is not the only thing to worry about if you want a long lasting soap. Water solubility is the other problem to consider. Soaps made from coconut and palm kernel are hard soaps, but they are very soluble. These soaps melt away quickly, so they clean well in hard or cold water and make lots of bubbles. Soaps high in palmitic and stearic acids are hard and fairly insoluble, so these soaps have a reputation for being long lasting. The downside of these soaps is that they don't make the abundant lather that many people like. Soaps made from the liquid oils (olive, rice bran, safflower, sunflower, etc.) tend to be less soluble than palm, tallow, and lard soaps. They clean the best in warm or hot water, and not so well in cold water. These soaps are soft, however, so they wear away quickly on the washcloth.
If you want a long lasting soap, you will want to make a soap that is ideally "hard like a brick" and fairly insoluble in water. Soapcalc does not calculate a "long lasting" number, but if you subtract the "cleansing" number from the "hardness" number, the answer will be the percentage of palmitic and stearic acids in your soap. The higher this number, the harder and more insoluble the soap will be. A soap with a high "long lasting" number will pay a price in less lather, so that's something to keep in mind.
I'm not sure if I have answered your question, Twiggy -- it has been a long day and I think I'm tired enough I'm rambling. I guess there is no one right answer to how much hardness (or insolubility) one should have in a soap. It depends on the soap maker.
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