Iron water is the one kind you can identify without being told. Not by the colour of the pool, which is usually clear, but by the ochre crust on the stone where the water runs, the rust-coloured rim on a fountain, the orange thread trailing from an overflow. Once you have noticed it you will see it at every bath built on this water in Europe.
The colour has a precise cause, and it is not a property of the water at all.
Why the stone is orange and the pool is not
Underground, without oxygen, iron dissolves as iron(II) — colourless, invisible, chemically content. A litre of it looks exactly like a litre of anything else. The moment that water reaches air, the oxygen oxidises it to iron(III), which is not soluble. It comes out of solution as a fine ochre precipitate and settles on whatever the water is touching.
So the orange is a record of contact with the atmosphere, not a measure of concentration. It builds up where water flows constantly and is scrubbed off where a pool is cleaned, which is why the pool you swim in is clear while the channel feeding it is stained. A bath can hold a great deal of iron and show you almost nothing; a trickle down a wall can look spectacular.
The same reaction explains a detail visitors notice and rarely connect: iron water tastes faintly metallic at the tap and less so an hour later, because by then some of the iron has already left the solution and is sitting on the bottom of the glass.
Where the 75 are
| Country | Baths |
|---|---|
| Germany | 40 |
| Hungary | 11 |
| Slovenia | 8 |
| Austria | 4 |
| Switzerland | 3 |
| Italy | 2 |
| France, Slovakia, Croatia, Poland, Spain, United Kingdom, Iceland | 1 each |
Germany holds more than half, and before reading geology into that it is worth reading record-keeping: German baths publish their water analyses far more consistently than most, so we simply know more about them. Twenty-four of the 75 hold an official medicinal-water certification — a status granted after testing, not a description chosen by a marketing department.
Iron alone, or iron in company
In 40 of the 75 baths iron is the only constituent the operator names — 53%, the highest share of any constituent in this catalogue. Sulphur stands alone in 43% of its cases, sodium in 4% and calcium in 8%. Iron is the one an operator will name on its own; the others are almost always part of a list. When iron does arrive with company, sulphur is the commonest companion.
| Also named in the analysis | Baths |
|---|---|
| Sulphur | 17 |
| Calcium | 15 |
| Sodium | 14 |
| Fluoride | 13 |
| Magnesium | 11 |
| Hydrogencarbonate | 11 |
| Chloride | 10 |
| Iodine | 8 |
| Radon | 1 |
Only three of the 75 publish a total mineralisation figure, which is too few to say anything about iron waters as a class. It is worth naming that gap rather than filling it: a number we do not have is not a number we can estimate.
Temperature and depth
Twenty-eight of the 75 publish a pool temperature. The median is 37 °C — body temperature — and the range runs from 21 °C to 45 °C. Twenty-three publish a well depth, with a median of 1,006 m and extremes at 120 m and 3,000 m.
Those two figures together describe two quite different kinds of place. At the shallow end, Waldfreibad Sahlenburg near Cuxhaven runs at 21 °C and Badesee at Bensheim at 25 °C — iron waters used cold, which most visitors would simply call a lake. At the other extreme, Mývatn Nature Baths in northern Iceland arrives at 45 °C and is used at 45 °C, needing no adjustment at all.
In between sit the baths where the gap between spring and pool tells the story: Emser Therme at Bad Ems takes water at 57 °C and holds its pools at 38 °C, while Tamási Termálfürdő in Hungary cools 55 °C down to 38. And two neighbours in Lüneburg — Salztherme and the Freibad Hagen — both run at 34 °C on water that arrives at 34 °C, which is what a well matched to its building looks like.
What the operators say it is for
| Stated use | Baths |
|---|---|
| Musculoskeletal complaints | 19 |
| Skin complaints | 12 |
| Circulatory complaints | 10 |
| Rheumatic complaints | 10 |
| Respiratory complaints | 6 |
| Recovery after injury | 4 |
| Digestive complaints | 3 |
| Gynaecological complaints | 2 |
Iron has an old association with anaemia and with drinking cures rather than bathing, and it is fair to say that these operators do not lean on it: only three name digestive uses at all. What they mostly claim is what almost every thermal bath claims — the musculoskeletal system.
What to look for when you are standing there
- Follow the water, not the pool. The inlet, the overflow channel and any drinking fountain carry the staining. The pool is filtered and cleaned and will tell you nothing.
- Read the crust, not the colour of the water. Heavy ochre on stone means constant contact with air over a long time; it says nothing about how much iron is in a litre.
- Expect a metallic taste at a tap and not in the pool. If a bath has a drinking fountain fed from the same source, that is where the water is closest to how it came out of the ground.
- Light swimwear will stain, and it is permanent. Iron(III) binds to fabric and does not wash out. This is the one practical consequence worth planning around.
- Ask when the analysis was last done. Composition changes slowly, but the sheet on the wall can be twenty years old and the well may have been redrilled since.
What this article does not tell you
Whether bathing in iron water does anything for the complaints these 75 operators list. We have examined nobody, and counting baths is not evidence about a body.
What we can tell you is which baths name iron, which hold a certificate an authority issued, what else the analysis contains, how hot the water arrives and how deep it came from. And we can tell you why the stone is orange, which is the part of the subject where the chemistry is settled and the marketing usually is not.