Many naturals report that their hair feels and even behaves differently when they live, move or vacation in an area with hard water. I now finally have scientific answers as to what could be happening!
1. The cuticle surface is the first point of call for both soft and hard water
Both soft and hard water contain metal salts including notably calcium and magnesium. Hair seems to have a preference for calcium ions and when hair is placed in hard water, calcium deposits at the cuticle edges and a little bit may travel inside the cuticle layers but not really into the cortex (BBA, pp 1-5, 2003).
2. It may be more about pH than water hardness
Interestingly experiments have shown that for unbleached natural hair, there is little difference in the calcium deposits whether soft, hard or very hard water is used (J Cosmet Sci, pp 383-391, 2011). However, if the pH of the water is changed e.g comparing hard water at pH 7 to hard water at pH 8, then there is an increase in the deposition of calcium. Naturally hard water does tend to have a higher pH so it stands to reason that a potential fix for hard water may be adjusting its pH (e.g with lemon juice or apple cider vinegar).
3. Hard water makes natural hair a little more rigid
The effect of hard water does change if hair is chemically bleached or not. For unbleached natural hair, hard water use results in hair becoming a little more rigid compared to using soft water (technical terminology is an increase in Young’s Modulus). The implication for hair is that when it is wet with hard water, it is resistant to extension and will therefore require a little more stress (5% more) to break (Int J Cosmet Sci, pp 477-482, 2011). If your hair is straight to wavy, there are some advantages to this as you can use less force to comb your hair, however for curlier hair, this means a loss in flexibility and therefore you need to be careful when wet detangling.
4. Hard water can help your twist out last longer in humidity
Humidity can wreak havoc on some hair styles such as twist outs or curl/rod sets. A surprising advantage here for hard water is that the extra deposits of metal salts on the surface of hard water can actually give natural hair some humidity resistance allowing a set style to last just a little bit longer. This is thought to be possibly because calcium and magnesium may occupy spots on the hair where water vapour may want to bind and therefore slow down water uptake (Int J Cosmet Sci, pp 477-482, 2011). The increased resistance to humidity lasts about 2 hours showing that moisture uptake in humidity does happen, just slower.
Is hard water prominent in your area? How does it affect you?