When you change from conventional hair care products to a more natural hair care routine you often get the advice to do an acid rinse. Finishing the routine with an acid rinse is something that more and more people feel gives their hair both softness and shine. But what is an acid rinse? How does it work? And is an acid rinse something you should always do after washing your hair with natural soap?

What Is an Acid Rinse and When Do You Need One?

If your hair feels dull and heavy, even when newly washed, it might be suffering from product buildup, particularly if you washed your hair with natural soap. An acid rinse can then help you get rid of the soap residues. An acid rinse is a weak solution of mild acids like acetic, malic and citric acid that you rinse your hair with after washing. The acid rinse makes your hair feel cleaner, softer and shinier and usually, there is no need for a regular conditioner afterward. Other types of buildup, like anionic surfactants and oils from the scalp, can also be rinsed away with acidity. However, an acid rinse does not remove silicone buildup. To remove silicone you need to do a clay mask using rhassoul for example, or alternatively use a deep-cleansing, clarifying shampoo. If you live in an area with hard water, an acid rinse can work wonders for your hair even if you are using a conventional shampoo. Hard water makes hair enormously prone to tangle, something that an acid rinse can help with as well. But to understand how an acid rinse works you first need to understand a bit more about how hair is made up.

The Composition of Hair and Why It Gets Damaged

Hair is composed of three parts, the cortex, the cuticle layer and the medulla, a marrow only found in coarser hair. The cortex is the largest part and is formed by long fibers of keratin proteins. It is the fibers in the cortex that provide the hair with its strength and elasticity. A single hair strand can carry a weight of one hundred grams. However, the fibers in the hair are sensitive and can easily break or split. This is why they are surrounded by a protective layer of flat cells overlapping each other like the scales on a fresh pine cone. The scales are called cuticles and in healthy hair, they lay like flat shields, enveloping the hair shafts, forming a smooth surface that reflects light and gives luster and shine. The smooth surface also limits tangles and knots. If the hair is in good condition, the cuticles are covered by a thin oily layer consisting of 18-MEA (18-methyl Eicosanoic Acid) and free fatty acids that hinder water from entering inside the hair shafts. The oil is derived from sebum produced by the hair follicles on the scalp. By brushing the hair with a boar bristle brush you spread the oil over the hair shafts. The boar bristles carry the sebum from the scalp to the end of the hair shafts and coats each strand with a very thin conditioning layer of oil. It is very important though to keep the brush clean, otherwise, you just cover the strands with dirt.

A hair strand normally lives for about 6 years and grows about 14 centimeters per year. During this time, it is subjected to many types of stress. UV-light and various types of chemical treatments like chemical dyes, bleaching and perm treatments do most harm but also heat styling devices like hairdryers, and straightening and curling irons, as well as careless combing, put a strain on the cuticle layer that can result in chipped or lost cuticles. Over time the result is more porous hair that let in more water than is good for the hair, split ends and damaged fibers that break easily.

A large part of the strength and stability in a hair strand comes from special bridge bindings (cystine) between the fibers. It is, for example, these bindings that control if the hair is curly or not. When you perm your hair, you break the bridge bindings and create new ones in other positions to make your hair either curly or straight. UV-light and chemical bleaching of the hair damages (oxidizes) these bridge bindings and cystic acid is formed in their place. The cystic acid binds positive ions from the water like calcium, magnesium and copper. It is actually possible to measure an increasing amount of these ions from the hair roots to ends since the hair is more damaged at the ends.

An Acid Rinse Removes Soap Residues From the Hair

The first thing you notice when you do an acid rinse is that the hair feels cleaner since the acid rinse removes product buildup from the hair. Particularly if you live in an area with hard, calcareous water. The problem with hard water is that it is extra rich in calcium and magnesium ions that bind to the hair. The more damaged the hair, the more ions bind to the hair. In turn, anionic surfactants (negatively charged surfactants in shampoo and soap) bind to calcium and magnesium ions. Without question, the most prone to cling to the hair in this way are soap surfactants, the cleansing substances in natural soap. This is why hair might feel dull and lifeless when you wash your hair with natural soap, even more so in combination with hard water or if your hair is damaged from chemical treatments. This is where the acid rinse comes in. The acidity in the rinse dissolves the bound calcium and magnesium and by that the soap residues are rinsed away as well. Actually, if you live in an area with hard water, your hair might feel heavy even if you wash it with conventional shampoo. Hard water can also cause a buildup of other anionic surfactants than soap surfactants. An acid rinse can then make wonders for the hair, like a clarifying and deep conditioning treatment all-in-one, but without the heaviness that too much conditioning surfactants can cause.

An Acid Rinse Makes the Hair Soft and Glossy

Most of us with a little longer hair has unfortunately washed away nearly all of the natural oil layer that protects the hair shafts. Damaged hairs are also more porous. Together this makes it easier for water to enter into the shafts, causing them to swell. This phenomenon, that the hair shafts swell depends on if the water is acidic or alkaline. The harder the water, the more alkaline it is and the more alkaline the water, the more the hair swells. When the hair shafts swell, the cuticles rise like the scales on a dry pine cone. The scales then catch on each other and everything else like Velcro and the hair feels rough and tangled. Being exposed and unprotected like that, the cuticle scales easily chip and break or fall off entirely.

When coloring your hair with permanent dyes, this is exploited to get the color in under the cuticles. First using an alkaline solution for the cuticles to rise and to get the color underneath followed by an acidic post-treatment to close the cuticle layer.

The reason for hair swelling in alkaline water is that a base attracts and removes the positively charged protons in the hair and leaves behind a lot of unbalanced negative charges. The negative charges in the hair fibers then repel each other and that is what causes the hair strands to swell. If you restore the acidity by doing an acid rinse (that contains protons) you neutralize the charges, the hair shafts contract and the cuticles smooth down and lay nice and flat again. Hair conditioner works in a similar way. Positively charged surfactants bind to the negative charges, thereby calming down the hair.

In fact, the hair is so rich in groups that can be negatively charged that it takes a pH as low as 3,67 for the hair to reach a net zero-charge. So neutral pH is not enough for the swollen hair shafts to contract, it takes a pH below 6 for the shafts to even start to tighten up and the cuticles flatten. Many conditioning products are acidic but there are both alkaline and acidic shampoos. Natural, superfatted soaps have a pH that is slightly alkaline when concentrated but diluted in water, it is the pH of the water that prevails like all products that are rinsed out. So if your hair needs an acid rinse to flatten the cuticles depends more on the water where you live that if you use hair-care products with low or high pH.

Common Acid Rinses

Apple Cider Vinegar
Apple cider vinegar diluted in water is the most classic type of acid rinse that both nourish and removes soap residues and other buildup. Contains in addition to acetic acid also malic acid (1/4 -1/2 tbsp / 100 ml water*)

Lemon Juice
A lemon hair rinse is popular for blonde hair since it slightly bleaches the hair, adding highlights. Lemon juice contains citric acid and ascorbic acid (vitamin C), which helps get rid of soap residues and other buildup. Use diluted, otherwise, it can be drying to the hair. (1/2 tbsp / 100 ml water*)

Hibiscus Tea
Hibiscus flowers contain naturally mild acids like citrus acid, malic acid and tartaric acid that removes residues and buildup. Hibiscus tea is also nourishing since the flowers are extremely rich in antioxidants. Might give lighter hair tones a slight red touch and is therefore not appropriate for blonde hair. (1 tsp dried flowers / 100 ml water*)

Kombucha
Black and green tea are popular hair rinses for their antioxidant content but they are not acidic and you thereby miss the positive effects of acidity. Kombucha, fermented tea, on the other hand, is both naturally acidic and with plenty of antioxidants. (25 ml in 75 ml water)

The Effect of Clay
Although clay is not acidic, certain types of clay, like rhassoul, have a strong negative net charge and is therefore extremely prone to bind positively charged ions like calcium and magnesium. This applies to clays belonging to the smectite group. Besides rhassoul, also montmorillonite and bentonite clay belong to this group. A clay mask for the hair can consequently also work to remove soap residues and other buildup from the hair. Soap that contains these clay decreases the need for an acid rinse, in particular, if you live in an area with soft water. The effect is partly dependent on the pH and increases with alkaline pH. Other clays like kaolin and illite are structurally very different from the clays in the smectite group and do not at all have the same inclination to bind positive ions. On the contrary, kaolin has a certain capacity to bind negative ions.