Hair science · Water chemistry
Hard Water
What dissolved minerals do to hair, and what they do to a plant colour — which are two different problems.
This page started with letters from Florida.
Customers in one part of the country kept writing about the same two things: hair that squeaked and felt rough after washing, and a colour result that did not match what other customers were getting from the same box. Not one or the other. Both, from the same addresses.
We went looking for what those two complaints had in common, and the answer turned out to be underneath the state. Florida sits on limestone, and limestone dissolves into the water that runs through it.
One
What is actually in the water
Rainwater is soft. It picks up hardness on the way to your tap, by dissolving whatever rock it passes through. Hard water is not dirty water — it is water that has been somewhere.
| Ion | Where it comes from | What it does |
|---|---|---|
| Calcium Ca2+ |
Limestone and chalk — calcium carbonate | The main contributor to hardness. Deposits as scale |
| Magnesium Mg2+ |
Dolomite and other magnesium-bearing rock | The second contributor. Behaves much like calcium |
| Iron Fe2+ / Fe3+ |
Iron-bearing minerals, and older iron pipework. Common in well water | Not counted in hardness at all — and the one that matters most if you colour your hair with plants |
The one that gets left out of the measurement
Water hardness is defined as calcium and magnesium only. Iron is measured separately, and a water report can show perfectly moderate hardness while carrying enough iron to change what a plant dye does.
This is why the hardness map below is a starting point rather than an answer. It is also why two people with identical hardness readings can get different colour results.
How hard is hard
Hardness is given either in grains per gallon or in milligrams per litre of calcium carbonate equivalent. One grain per gallon is about 17 mg/L. Scales differ slightly between authorities; this is the one most commonly used in the United States.
| Description | Grains per gallon | mg/L as CaCO3 |
|---|---|---|
| Soft | Under 1 | Under 17 |
| Slightly hard | 1 – 3.5 | 17 – 60 |
| Moderately hard | 3.5 – 7 | 60 – 120 |
| Hard | 7 – 10.5 | 120 – 180 |
| Very hard | Over 10.5 | Over 180 |
And where it is
Why Florida in particular
Florida sits on a limestone aquifer — a karst landscape, which is what produces its springs and its sinkholes. Limestone is calcium carbonate, and groundwater moving through it dissolves calcium continuously. Much of the state draws its supply from that aquifer.
On top of which, a great many Florida households are on private wells, and Florida well water is frequently high in iron as well. Municipal supply is treated; a private well is whatever the ground gives you.
Calcium makes hair feel rough. Iron changes what a plant dye does. A customer drawing hard, iron-bearing well water gets both at once — which is exactly the pair of complaints that sent us looking.
You can look up your own numbers
If you are on municipal water in the United States, your utility is required to publish an annual water quality report — a Consumer Confidence Report. It is usually on the utility’s website, and it will give hardness and iron among other things.
If you are on a well, nobody is testing it for you. Home hardness test strips are inexpensive, and separate iron strips exist. We would rather point you at your actual numbers than at a map, for the same reason we publish our own pH readings: a figure you can check beats a figure you have to trust.
Two
Why minerals stay on hair
They are not simply left behind when the water dries. They are held there, by an electrical attraction between the mineral and the hair itself — and understanding that is what tells you how to get them off again.
Hair carries a negative charge
Keratin is a protein, and proteins carry charged side chains. At the pH at which people wash their hair, the surface of a hair fibre is negatively charged — principally from carboxylate groups on the amino acid side chains, and from sulphonate groups where the fibre has been oxidised.
This is ordinary protein chemistry rather than anything specific to hair, and it is the same fact that explains why anionic surfactants like sulfates interact with hair the way they do.
And calcium carries two positive charges
This is the part that matters. Calcium and magnesium are divalent — each ion carries two positive charges rather than one. Sodium, which is in soft water too, carries only one.
A singly charged ion attaches to one negative site and can be displaced fairly easily. A doubly charged ion can bind two sites at once, bridging between them, and that is a considerably more stable arrangement. It is why hardness is defined by calcium and magnesium and not by every dissolved salt, and it is why ordinary shampoo does not simply wash the deposit away.
A correction
Until recently this page explained it like this:
1. The micro-powder left by the natural hair dye is charged with electricity.
2. The electricity attracts minerals to your hair.
3. Minerals adhere to your hair and scalp.
The observation was right and the explanation was wrong. “Charged with electricity” describes static, which is a different phenomenon — and it puts the charge on the powder rather than on the hair, which is where it actually is.
Hair is negatively charged as a matter of protein chemistry, wet or dry, whether or not anything has been applied to it. The minerals are attracted to the hair. The powder is a separate issue, and we come to it below.
Some hair binds more than other hair
The number of negative sites on a fibre is not fixed. Oxidative damage creates more of them, and that changes how much mineral a head of hair will pick up from the same water.
When peroxide oxidises the disulphide bonds that hold keratin together, the cystine in them is converted to cysteic acid — which carries a sulphonate group, strongly acidic and negatively charged. Every one of those is a new binding site. We describe that conversion in more detail here.
Which produces an uncomfortable arithmetic
Bleached hair in hard water gets the worst of both. The bleaching added binding sites permanently; the water supplies ions to fill them. And because mineral deposit makes hair feel rough, it is easy to attribute the roughness entirely to the bleach and miss that half of it is coming out of the tap.
Our own colours do not oxidise the fibre — they are plant material in an acidic paste, and there is no peroxide step. So they should not be adding binding sites. But hair that was bleached or permed before you found us still has the ones it already had, and those do not go away.
The powder is a separate problem, and it is ours
The original page was pointing at something real when it mentioned residue. Milled plant material that has not been fully rinsed out is physically present on the hair, and it holds water, and water in hard-water country carries calcium.
This is not a charge effect. It is simply that a fine powder trapped near the scalp is one more thing for minerals to deposit into as it dries, and it is a reason to rinse a plant colour or a herb mask for longer than feels necessary — particularly at the nape and behind the ears, where powder settles.
And one of our products is worse in hard water than the others
Our Organic Shampoo Bar is a true soap, made by saponifying oils, and it reads pH 9.35. Soap is a fatty acid salt, and calcium reacts with fatty acid salts to form insoluble calcium soaps — the residue familiar as scum on a bath.
In soft water this does not arise. In hard water it is unavoidable chemistry, and the residue lands on your hair. If you are in a hard-water area and the bar leaves your hair feeling coated, that is what is happening, and it is not a fault in the bar.
Reetha and shikakai do not have this problem, because their saponins are not soaps. They are glycosides rather than fatty acid salts, and calcium does not precipitate them.
What it looks and feels like
| What you notice | What is behind it |
|---|---|
| Rough, squeaky, straw-like | Deposit roughens the surface, so hairs catch on one another instead of sliding |
| Dull, flat, no shine | A rough surface scatters light instead of reflecting it evenly |
| Heavy, limp, lacking body | Deposit has mass, and it accumulates |
| Never feels properly clean near the scalp | Deposit does not rinse off, so washing more does not remove it |
| Colour looks flatter than expected | Partly the dull surface — and partly something else, which is the next section |
Everything above concerns how hair feels and how light comes off it. None of it is damage in the sense the rest of this site uses the word: nothing has been broken, and the deposit can come off.
The next section is about something different. Not what minerals do to hair, but what one of them does to a plant dye — and that one is not calcium.
Three
What iron does to a plant colour
The second complaint from those letters was different in kind. Not how the hair felt — how the colour came out. Darker than expected, duller than the swatch, and in some cases with a grey or greenish cast that nobody had asked for.
Calcium does not do that. Iron does, and iron is not counted in the hardness figure at all.
Plant pigments are built to hold metal
Not deliberately, but structurally. Lawsone — the pigment in henna — is a hydroxynaphthoquinone: it carries a hydroxyl group and a carbonyl group on neighbouring positions. That arrangement of two oxygens side by side is precisely the geometry that grips a metal ion, one oxygen on each side.
The same is true of the polyphenols and tannins throughout the rest of the formula. A metal ion arriving in a plant dye paste does not sit there inertly. It becomes part of a new compound, and that compound is a different colour from the one you started with.
This is not obscure chemistry
Iron-gall ink was the standard writing ink of Europe for over a thousand years. It is made by mixing iron sulphate with tannins from oak galls. Neither ingredient is black. The complex they form is.
Every mediaeval manuscript, every Bach autograph score, the Constitution — all written in a metal-polyphenol complex. Iron plus plant tannin makes a dark, permanent colour, and it has been doing so reliably enough to build a civilisation’s record on. It will do the same on your head.
What that looks like on hair
| You expected | You may get |
|---|---|
| A clear red or copper | Brown, muddy, or noticeably duller |
| A light or ash blonde | Darker than the swatch, sometimes greyed |
| A warm brown | Cooler and flatter, occasionally with a green cast |
| The same result as last time | Something different, after moving house or after work on the mains |
The last row is the one that catches people. Nothing about the product changed. The water did.
Which brings us to something we chose not to do
When Radico was developing its colours, a partner in Japan — drawing on traditional Japanese dyeing practice, where iron mordants have been used for centuries — made a suggestion. A trace of iron, one part in ten thousand, would deepen the colour and cut the development time substantially.
The chemistry is exactly what this page has just described. It works. Iron is one of the most abundant elements on earth, it is not harmful at that concentration, and nobody would ever have noticed it on an ingredient list.
Sanjeev Bhatt said no, on the grounds that 100% organic either means every ingredient or it means nothing. It is why Radico asks for one to two hours on the hair rather than thirty minutes. The full story is on our About page.
And here is the part that took us a while to see. Hard water containing iron does to the colour precisely what we refused to do deliberately — without measurement, without consistency, and without anybody choosing it. We left the iron out of the box. Some of our customers are adding it from the tap.
Where the iron comes from
- Well water. The most common source, and the least regulated — a private well is whatever the ground gives you. Florida wells are frequently iron-bearing.
- Old pipework. Iron and galvanised steel plumbing sheds iron into the water it carries, particularly after standing overnight or after disturbance to the mains.
- Municipal supply. Treated, and usually low — but your utility’s annual water quality report will give you the actual figure rather than an assumption.
- Other hair products. Some products sold as henna contain metallic salts deliberately, to force a faster or darker result. If a previous colour behaved oddly with plant dye, that is worth considering. We say the same thing about “neutral henna” here.
And a second mechanism, less dramatic
Hard water is usually alkaline, because the calcium arrives as bicarbonate. Plant colour is mixed with water, and the water’s pH becomes part of the paste’s pH.
Our formula uses amla to hold the mix on the acid side, which is part of why amla is in 23 of our 24 shades. Mixing with strongly alkaline water works against that — a smaller effect than iron, but one more variable in the same direction.
The one change worth making, and it costs almost nothing
Mix the colour with distilled or filtered water. Not for washing — just for the paste.
A gallon of distilled water costs a couple of dollars and is enough for many applications. It removes iron, calcium and alkalinity from the one step where they do the most harm, which is the step where the pigment is actually forming.
If you take one thing from this page and you colour your hair with plants in a hard-water area, take this one. It addresses the mechanism directly, and we do not sell it.
What we have and have not established
The chemistry is not in dispute. Metal-polyphenol complexes are well characterised, iron-gall ink is a thousand-year demonstration, and lawsone has the structure to do it. That part is solid.
What we have not done is test our own shades in controlled water conditions. We have customer reports and we have the mechanism, and those two things pointing the same way is suggestive rather than conclusive. We have not run identical applications at graded iron concentrations and measured the colour difference, which is what it would take.
It is a straightforward experiment and we would like to run it. Until we do, this page is telling you a mechanism and an observation, and being clear about which is which.
Four
What takes it off
Mineral deposit is removable, which is the good news this page opened with. But the reason our own herbs help is not the reason this page used to give, and getting it right changes what you would actually reach for.
A correction
Until recently this page said:
These Indian herbs work so well in hard water because they contain biological chelating agents that help remove the mineral build-up.
We have not been able to support that, and we think it is the wrong mechanism.
A chelator grips a metal ion between two or more donor atoms — it needs charged or lone-pair-bearing groups arranged to coordinate the metal. The saponins in reetha and shikakai are non-ionic glycosides. Carrying no charge is precisely what our reetha page says makes them gentle on hair — and it is also what makes them poor candidates for chelation.
The claim was working in our favour and we could not stand it up, so it is gone. What follows is what we think is actually happening, which is a different and better-supported mechanism.
Acid dissolves scale
This is not subtle chemistry. Calcium carbonate reacts with acid to give a soluble calcium salt, water and carbon dioxide. It is the reaction behind every limescale remover in every kitchen cupboard, and it is why a kettle descaler is acidic.
Calcium soaps — the residue formed when soap meets hard water — dissolve in acid as well. Both of the deposits described earlier on this page come off in acid.
| Chelation | Acid dissolution | |
|---|---|---|
| How | A molecule grips the metal ion and carries it away in solution | The deposit is converted into a soluble salt |
| Typical agents | EDTA, phytic acid, citric acid | Any acid — citric, acetic, or a plant that happens to be acidic |
| Where you meet it | Clarifying and chelating shampoos | Limescale removers; vinegar rinses |
| What we have | Nothing, as far as we can establish | Shikakai, amla, and the Treatment Powder |
Citric acid appears in both columns, because it is an acid and a chelator — which is why it turns up in so many descaling and clarifying products. Not everything sorts neatly.
Our own numbers
We publish pH readings for our powders because they are figures you can check with an inexpensive meter. Here they are in the context of dissolving mineral scale.
| Product | Our reading | Relevance here |
|---|---|---|
| Shikakai | 2.1 – 3.0 | Genuinely in descaling territory, and the acidity is buffered rather than dilutable |
| Hair Treatment Powder | approx. 3.25 | Amla, reetha and shikakai together. Acidic by composition |
| Reetha | 4.54 – 4.90 | Inside hair’s own range — good for washing, weak as an acid |
| Colorless henna | 5.12 | Near hair’s own pH. Not doing anything to scale |
| Organic Shampoo Bar | 9.35 | Alkaline, and a true soap. The worst of our products in hard water |
And the gap in what we can claim
We have measured the pH. We have not measured mineral removal.
That shikakai sits at pH 2 to 3, and that calcium carbonate dissolves in acid, are both facts. Putting them together gives a plausible mechanism rather than a demonstrated result — we have not taken hair with a known mineral load, washed it, and measured what came off.
So: shikakai is acidic, acid dissolves scale, and we would expect it to help. We are not going to write “removes mineral buildup” on that basis, and if we run the measurement we will publish whatever it says.
Things that work that we do not sell
- A shower filter, or a water softener. The only thing on this list that addresses the cause rather than the consequence. If hard water is affecting both your hair and your colour results, this is the intervention with the largest effect, and it is a plumbing purchase rather than a cosmetic one.
- Distilled water for mixing colour. A couple of dollars a gallon, and it removes iron, calcium and alkalinity from the step where the pigment actually forms. Covered in the previous section, and the single most useful thing on this page.
- A chelating or clarifying shampoo. Formulated for exactly this, with agents that do genuinely chelate. Used occasionally rather than routinely, since they strip thoroughly. We do not make one.
- A dilute vinegar rinse. Acetic acid, and the oldest domestic answer to hard water there is. Inexpensive and effective at what it does — though note it will not undo an iron-shifted colour, because that colour is a compound rather than a deposit.
One thing acid will not do
An iron-shifted colour is not sitting on the hair. It has become part of the pigment. Where calcium deposit can be dissolved off, a metal-polyphenol complex is a new compound that formed during the colouring.
That one does not come off with an acid rinse, and it will grow out rather than wash out. Which is why the distilled water goes in before, not after.
If you are in a hard-water area
- Find out your actual numbers. Your utility’s annual water quality report, or a test strip if you are on a well. Hardness and iron are separate figures.
- Mix colour with distilled or filtered water, whatever you do about the shower.
- Rinse longer than feels necessary, particularly at the nape and behind the ears where milled powder settles.
- Consider whether the shampoo bar suits you. It is a true soap at pH 9.35 and it will form calcium soaps in hard water. Reetha and shikakai will not.
- If the colour result has shifted, look at iron before you look at the product. Particularly after moving house or after work on the mains.
A note on our sources
The water chemistry on this page is ordinary and uncontested. Hardness definitions follow the classification used by the US Geological Survey and the Water Quality Association. The behaviour of divalent cations at a protein surface, the acid dissolution of carbonate scale, and the formation of metal-polyphenol complexes are all standard chemistry rather than anything specific to hair or to us.
The pH figures are our own, taken on a handheld meter calibrated against pH 4.00 and 7.00 buffers, and published in full on the individual herb pages.
The connection between iron in water and altered plant-dye results rests on the chemistry plus reports from customers. We have said clearly in section three which part is which, and what we would need to do to settle it.
This page corrects two things it used to say — the static electricity explanation, and the claim that our herbs chelate. Both corrections are marked in place. If we have anything else wrong, write to laboratory@radicousa.com.
This page describes water chemistry and its effects on hair and on plant dyes, for general interest. Radico makes cosmetics; none of our products is intended to diagnose, treat, cure or prevent any condition, and nothing here is a claim that any of them removes mineral deposits. Water quality is a matter for your utility or a water treatment professional, and questions about drinking water safety should go to them rather than to a hair colour company.