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Hardness

Hardness is the calcium and magnesium dissolved in your water, reported as a single number in milligrams per litre of calcium carbonate even though most of what is actually there is not carbonate at all. It is the reason kettles fur up, soap will not lather and a used teapot gets a pale ring round the inside. It also measurably changes what a cup of tea does: harder water pulls less flavour out of the leaf and darkens the liquor while it is at it.

Where your water sits

hardness, mg/l as calcium carbonate

42, browning
68, SCA target
soft
medium
hard
0150250300

The bands are the German statutory ones, the only legally defined hardness bands this section could verify against a primary source. They are set in mmol/l and convert to 150 and 250 mg/l. Hard water runs well past the right-hand edge; the axis stops at 300 so the useful end stays readable. A German water report gives °dH instead: 1 °dH is 17,8 mg/l, putting the bands at roughly 8,4 and 14 °dH.

What it is

Hardness comes in two forms that behave differently once heat is involved. Temporary hardness is calcium and magnesium bicarbonate: dissolved while cold, but it turns insoluble and drops out of solution when the water is heated, which is the limescale furring a kettle. Permanent hardness is calcium and magnesium bound to sulfate or chloride instead, and those compounds stay dissolved regardless of temperature. Both count toward the single 'hardness' number on a water report, expressed as calcium carbonate equivalent, but only one of them is affected by boiling.

What it does to your tea

Cabrera et al. (2021) brewed real green tea across synthetic waters from 21 to 338 ppm hardness as CaCO₃ and watched total catechin yield fall 2.4-fold, from 164.6 to 67.5 mg per cup, as hardness rose. EGCG and EGC dropped hardest, 3.2-fold and 3.1-fold; ECG and EC fell far less, and GC and C not at all, and caffeine held steady in two of the three tea brands tested. Separately, Yin et al. (2014) found calcium raises astringency and dulls the bitterness of EGCG and caffeine and the umami of theanine, though the concentrations involved were not published clearly enough to repeat here. Visible browning needs hardness above 42 ppm as CaCO₃ as well as a high enough pH, together, not hardness alone. Hardness is also what puts the iridescent film on a cup of black tea and the pale ring inside the pot: tea compounds complex with the calcium and settle out at the surface, which is why the same tea leaves no film in soft water.

How to find out what yours is

Your water utility's annual quality report usually lists calcium hardness directly, in mg/L or as CaCO₃; it is the most reliable number you will get without buying anything. Cheap hardness test strips, sold for dishwashers and water softeners, give a fast reading at your own tap and are accurate enough to tell hard from soft. A TDS pen measures something related but different, total dissolved solids rather than calcium and magnesium specifically, and should not be read as a hardness number (see minerals).

What to aim for

The SCA's Water for Brewing Specialty Coffee standard (2009), the only widely cited numeric target near this subject, sets calcium hardness at 68 mg/L as CaCO₃, with 17 to 85 mg/L given as an acceptable range. Nothing equivalent exists for tea: ISO 3103, the international standard for preparing tea for sensory testing, declines to set a numeric water parameter at all, saying only that water should resemble local drinking water and warning that distilled water will not give a representative result. Treat the SCA figure as a reasonable starting point borrowed from coffee, not a tea rule.

What you can do about it

A basic carbon filter pitcher removes chlorine and odours but does little to hardness; look specifically for ion-exchange resin if softening is the goal. A whole-house water softener swaps calcium and magnesium for sodium and removes temporary and permanent hardness alike, which boiling alone cannot do. If you only need soft water for tea, the simplest fix is cutting hard tap water with distilled or reverse-osmosis water until you land on a ratio you like: dilution lowers both forms of hardness equally, since it does not rely on precipitation the way boiling does.

What people get wrong

You will sometimes read that boiling converts temporary hardness into permanent hardness. That has it backwards. Temporary hardness is the bicarbonate fraction, and boiling is what removes it: heat drives off dissolved CO₂, the calcium carbonate that was holding it in solution turns insoluble, and it drops out as the limescale furring your kettle. Permanent hardness, the sulfate and chloride fraction, has no carbonate to precipitate, so boiling leaves it exactly where it started. A well-boiled kettle should end up slightly softer than the water that went in, not harder.

teabert, the tealytics teapot, keeper of the kettle
Boiling does not swap one kind of hardness for the other, it removes the bicarbonate half and leaves the rest exactly where it was. The white crust in your kettle is the proof, not a sign the water's gone wrong.

Sources