High levels of the mineral calcium and magnesium is what makes water “hard”.
Groundwater is usually harder because it picks up calcium and magnesium from rocks such as limestone and dolomite before it reaches the surface. Other metals ions such as iron and manganese can cause hard water, but calcium and magnesium are the most common.
Hard water can cause deposits in piping, but when it comes to personal care it only really effects soap.
Soap falls under a family of chemicals known as surfactants, and specifically means when a fatty acid is reacted with an alkaline substance, usually lye. This includes traditional saponified oil soap. Other surfactants such as detergents (sodium lauryl sulfate, sodium laureth sulfate, cocoamidyl betaine) aren’t as affected by hard water.
Soap reacts with the ions in hard water and forms either soap scum, or scale. There are two types of hardness compounds, carbonate and noncarbonate. Carbonate hardness compounds are responsible for the hard scale that is deposited in pipes and around drains. Noncarbonate hardness compounds on the other hand create slimy soap scum, which can coat the skin and hair. Both will reduce the ability of a soap to lather and impede its ability to clean effectively.
Softening Hard Water
The minerals in hard water can be removed by many methods such as distillation and lime softening. These methods can be expensive to install and maintain. Magnetic water softeners, though cheaper, are not effective according to scientific studies.
Boiling water will remove carbonate hardness compounds which are responsible for lime scale, but not noncarbonate hardness compounds which are responsible for soap scum.
Products may also be formulated with ingredients that bind with calcium and magnesium minerals, allowing the soap or surfactant to function like it would in soft water. These ingredients include zeolite and chelators such as citric acid and tetrasodium EDTA. Look for these ingredients in the ingredient lists of soaps if hard water is an issue. You may also consider switching to a detergent based cleanser. Sodium Cocoyl Isethionate is a detergent that will function well in hard water and is very mild to the skin.
Most shaving soaps are traditionally made saponified oil soaps.
Since hard water impedes the ability for a soap to lather, using distilled water will allow your product to function as intended. Mineral or spring waters may contain enough magnesium and calcium to be considered hard water, so it’s best to use distilled water that hasn’t had minerals added back to it.
Shaving cremes are generally formulated with detergents and won’t be affected by hard water. Common detergents (that won’t be affected by hard water) are sodium lauryl sulfate, sodium laureth sulfate, sodium cocoyl isethionate and cocamidopropyl betaine.
However, soap scum formed by soap and noncarbonate hardness compound has actually been used in a product to enhance its ability to lubricate the skin and hair while shaving. The product Mitchell’s Wool Fat Shaving Soap contains magnesium sulfate (epsom salts) which is a noncarbonate hardness compound. This increases the formation of soap scum, which is slimy and lubricating. It’s also why many users of the product have diffculty in forming a rich lather with the product.
If you have hard water, you might achieve a similar benefit by boiling the water, since noncarbonate hardness compounds aren’t removed. This will leave behind the noncarbonate hardness compounds which will create a lubricating soap scum, the shaving soap will be harder to lather though.