The label is the only thing that tells you anything
Brand names, spring names, mountain imagery and the words natural, pure, mineral and spring on the front of a bottle carry no information whatsoever about brewing behaviour. Two waters from the same country with near-identical marketing can differ several times over in dissolved content, and one may make excellent tea while the other flattens it completely — which is why recommendations of the form use this brand travel so badly, since the same brand may be bottled from several sources for different markets. The analysis panel, usually on the back in small type and in milligrams per litre, is the whole of the useful information on the bottle. The same applies to tap water, and rather better: suppliers publish a zonal analysis, usually searchable by postcode, and it is considerably more detailed than anything printed on a label. Reading one takes a minute and replaces a great deal of guessing about why the tea is behaving as it is. It also converts the question from a matter of preference into a matter of fact, which is the main reason enthusiast disagreement about water is so unproductive: two people comparing brands are comparing labels, while two people comparing analyses are comparing water.
The three lines worth finding first
Calcium and magnesium together are hardness, and they are the ions most involved in how extracted material behaves once it is in solution — they predict mouthfeel and body more than they predict strength. Bicarbonate, sometimes printed as hydrogencarbonate and sometimes folded into a single figure for alkalinity, is the buffering capacity, and it is the line that predicts most about aroma, colour and the surface film. Total dissolved solids, which on many European labels appears as dry residue at 180 °C, is the sum of everything and is useful as a first sort rather than as a diagnosis: a very low figure and a very high one are both warnings, and everything in the middle needs the other two lines to interpret. The remaining entries on the panel — sulphate, chloride, nitrate, sodium, silica, potassium — matter at the margins and can be ignored until the first three have been looked at, with one exception: a high sodium figure on a water that also shows low hardness is the signature of a softened supply, and that is worth knowing before anything else is concluded.
Why bicarbonate is the one to read first
Hardness gets the attention because it is the number people already know about from kettles, washing machines and the scale in the shower, but alkalinity is what does most of the damage to tea. High bicarbonate holds the brew away from the acidity it would otherwise reach, and that single effect dulls aroma, darkens the liquor, flattens the perceived sweetness and encourages the metallic-looking film that appears on the surface of a cup of black tea within a few minutes. It also resolves a confusing and very common observation: two waters with similar hardness figures can behave completely differently in the pot, because one is high in calcium with modest alkalinity and the other is carbonate-dominated. The hardness number alone cannot distinguish them, and a great deal of contradictory advice about hard water for tea comes from exactly this conflation. If only one line can be checked, check this one. If the label gives alkalinity as calcium carbonate equivalent rather than as bicarbonate, that is the same property reported differently and the comparison still works, provided both figures being compared are on the same basis.
The units problem, which trips up nearly everyone
The same property is reported in incompatible ways across labels, countries and enthusiast writing. Hardness appears as milligrams per litre of calcium, or as milligrams per litre expressed as calcium carbonate, or in German, French or English degrees, or in milliequivalents per litre — and the numbers differ by factors of several between them, which is more than enough to turn a good recommendation into a bad one. Alkalinity is reported as bicarbonate or as calcium carbonate equivalent, and those are not the same figure either. Before comparing your water to any recommendation you have read anywhere, check that both are stated on the same basis, because a substantial share of the disagreement in enthusiast discussion turns out on inspection to be two people quoting different scales at each other with complete confidence. The label’s own units are always stated somewhere on the panel; the recommendation’s very often are not, which is a reasonable ground for discounting it. Where a source gives a target range without units, the safest assumption is that it cannot be used.
Your tap water has an analysis too, and it moves
Supply zones are not stable. Many networks blend sources seasonally, switch between reservoirs and boreholes as demand and rainfall change, or alter disinfection practice through the year, so the published figure is an average or a range rather than a constant — and the range can be wide. If tea that was reliable becomes unreliable without any change in leaf, equipment or method, a change at the supplier is a real candidate and is worth checking before rebuilding a technique that was not broken. The analysis will also tell you whether the supply is disinfected with free chlorine or with chloramine, and that decides whether standing water uncovered for an hour does anything useful: it helps appreciably with the first and very little with the second, which is why the traditional advice works for some people and not others. Some suppliers publish monthly figures rather than annual averages, which is more useful still. None of this requires any equipment, and it explains a class of problem that is otherwise invisible from inside the kitchen.
What an analysis cannot tell you
It describes the water leaving the treatment works, not the water leaving your tap. Pipework, storage tanks, a long unused run first thing in the morning, and the age and material of the plumbing all intervene between the two, and none of them appears on any report. Running the tap for a few seconds before drawing water for tea addresses part of that and costs nothing. The analysis also says nothing about your kettle, which after a few months of hard water is contributing a surface of scale to every boil and is a variable in its own right. And it cannot tell you what you will prefer: within the broad middle of the range there are genuine differences of taste, and different teas suit different points within it — the water that flatters a delicate green tea is not necessarily the one that flatters an aged dark tea. The honest use of an analysis is to rule things out, to explain a fault you are already tasting, and to make a comparison between two waters meaningful. It is not a specification to be matched.