Why you shouldn’t use cold‑process or hot‑process soap bars on your hair

Many people who want a plastic‑free hair routine are tempted to use handmade soap bars on their hair, but these bars are very different from pH‑balanced shampoo bars. Cold‑process and hot‑process soap bars are true soap: oils are mixed with sodium hydroxide (lye) and cured. The chemistry that makes soap so good at cleaning skin also makes it unsuitable for hair and scalp health.

Soap bars are far more alkaline than hair

Dermatology research shows that healthy skin has a pH around 5.4–5.9 the scalp is about 5.5 and the hair shaft itself is even more acidic at 3.67. This acidic “acid mantle” keeps cuticle scales flat, locks in moisture and protects against. True soap cannot be made at a low pH; its alkalinity typically sits between pH 9 and 10. Soap makers and cosmetic chemists note that handmade shampoo bars made via cold‑process always have a high pH of 8‑10, even adding citric acid doesn’t significantly lower. Because hair prefers a pH closer to 4.5‑5.5, washing with an alkaline bar disrupts the acid.

High pH lifts the cuticle, causing dryness, frizz and breakage

When the hair fibre is exposed to an alkaline pH, the cuticle scales lift and the fibre becomes negatively charged. This increases friction between hairs and makes the strands porous, leading to dryness, tangles and frizz. A high‑pH soap bar strips away the hair’s natural oils and leaves it “squeaky clean,” but the result is brittle, dull hair that is harder to. The Beauty Brains, a blog run by cosmetic chemists, explains that bar soap lacks conditioning agents and causes the cuticle to become . An Indian dermatology study found that most soaps have a pH of 9–10, while shampoos tend to be near neutral, and that high‑pH cleansers increase dehydration and irritation of the skin.

The International Journal of Trichology warns that an alkaline pH increases the negative charge on the hair fibre, leading to greater friction and cuticle damage. Head & Shoulders’ educational materials similarly note that alkaline products force the cuticle to open, increasing friction and causing frizzy hair and. That is why hair colouring treatments use alkaline chemicals to open the cuticle, and why a neutralising conditioner is always needed afterwards to close it