Limescale, neither friend nor foe.

Open a kettle almost anywhere in limestone country: a white veil, sometimes a crust. Limescale is the only characteristic of water that everyone can see — which is probably why it gets credited with so many intentions. Let us look at it calmly, as a fact of geology rather than an adversary.

A matter of geology

Before reaching the tap, water has crossed rock. Along the way it takes on minerals — above all calcium and magnesium when it crosses limestone ground. That content is what we call hardness, expressed in French degrees (°fH): one degree corresponds to 10 mg/L — 10 ppm — of calcium-carbonate equivalent. Other countries state the same value directly in ppm CaCO₃; the conversion is a simple factor of ten.

Hardness is therefore a local signature. It varies from one town to the next, sometimes from one neighbourhood to the next, depending on the source being drawn. Limestone-rich ground tends to give moderately hard to hard water; granite regions give soft water [to be confirmed — by locality]. Your utility publishes the value for its network; an analysis at the tap pins it down at your home.

What limescale really does

In appliances that heat water, limescale precipitates: that is scale. On the element of a kettle, a coffee machine, a washing machine or a water heater, the layer insulates, slows the heating and increases energy consumption; descaled too late, appliances wear out faster [to be confirmed — order of magnitude]. It is a fact of upkeep, measurable — not a fate.

On surfaces, it leaves deposits: shower screens, taps, glasses out of the dishwasher. On skin and hair, very hard water combined with soap leaves a residue that can make sensitive skin feel tight — a real discomfort for some households, not a general rule.

The sensitive point is hot water: the higher the temperature, the more the carbonate precipitates. That is why the same water whitens the kettle yet leaves the glass of cold water untouched — limescale expresses itself mostly where you heat.

As for taste, calcium contributes to a water’s character. Hard water is not “bad” water: some great mineral waters are very calcic. Taste is a matter of balance and preference — not of morality.

What it is wrongly blamed for

Limescale is not a health problem. The World Health Organization sets no health-based guideline value for water hardness: calcium and magnesium are minerals our diet provides in any case.

No, limescale does not “clog your arteries”: that shortcut confuses plumbing with biology. The white deposit in the saucepan is calcium carbonate — the same mineral chalk is made of —, not a pollutant. And soft water is not automatically “better”: very soft, it can taste flat in the glass.

The reference figures

The usual scales read as follows [to be confirmed — reference scale]:

• below 15 °fH (≈ 150 ppm CaCO₃) — soft water.
• 15 to 25 °fH (≈ 150–250 ppm) — moderately hard water.
• 25 to 32 °fH (≈ 250–320 ppm) — fairly hard water.
• above 32 °fH (> 320 ppm) — hard water.

Where to find your value? In your utility’s annual sheet — or, more precisely still, in an analysis sampled at your own tap, indoor installation included.

Between the network value and the value at your tap, the gap can be real: a building softener, a water heater, long risers — the indoor installation has its say. That is the whole point of a measurement at home: it describes your water, not the neighbourhood’s.

When to act — and when to leave well alone

The telling signals are domestic: descaling sessions that come closer together, a water heater that labours, marks that return as soon as they are wiped. If your water is soft and your appliances serene, there is nothing to do — and we will tell you so with the same assurance.

If it is hard and the home suffers for it, the decision is made to measure: uses, equipment, skin sensitivity, taste. In every case the order stays the same: measure first, decide second. One sentence is enough here: the house’s analysis kit measures the hardness at your tap, at no cost.