Why doesn't soap lather in hard water?

Hard water is water that has a large amount of dissolved minerals, such as calcium and magnesium. When present in high concentrations, these minerals make lathering with soap very hard to do, which is how the term “hard water” actually originated. Soap is made from sodium byproducts, the result of combining fatty acids with lye during the saponification process, that easily bubble into a full lather when combined with pure water. When soap is combined with hard water, however, the minerals chemically react with the soap causing slimy and runny lather that is useless for shaving. Additional problems can arise from hard water as well. When hard water evaporates, it leaves behind deposits of minerals on your shaving brushes and razors. Over time hard water scale can damage the delicate hairs of a shaving brush and even make the bristles ineffective at creating suitable shaving lather. In the article Overlooked Nemesis: Hard Water, I shared my experiences with hard water and suggested using filtration products or bottled water to see how a change in water properties can improve your shaves.

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