Calcium gets the blame for haze, but it is the trace metals that stain, catalyse and darken. Iron is detectable by taste at 40 µg/L in demineralised water, manganese coats pipework at 0.02 mg/L, and copper — the distiller's friend inside the still — becomes a liability the moment the spirit leaves it.
The world's best-selling spirit is roughly four-fifths water, Japan has a dedicated word for the water added before bottling, and China's largest spirits category treats dilution as a process step with its own quality problem. Asia's three biggest categories dilute harder than whisky does — which is exactly why their water is specified, filtered and, in one case, written into law.
Reduction makes a whisky visibly paler, and the arithmetic is unforgiving — cutting 60% ABV spirit to 40% spreads the same cask-derived colour across half again as much liquid. That is why colour is judged after the water goes in, why plain caramel is the only additive whisky law permits, and why the one thing your blending water must never do is contribute a colour of its own.
Condenser water sits on the far side of a copper wall and never meets a drop of distillate. It is also the largest water stream on a distillery site, and its temperature is one of the few flavour levers a distiller can move without changing anything else. Warmer water condenses vapour more slowly, slow condensing means more copper contact, and more copper contact means a lighter spirit.
A bottler can specify its blending water to a decimal point and then serve the result in a glass rinsed with hard tap water and dried in the air. The mineral film, surfactant residue and chlorine left behind by that final rinse sit in the one place where every aroma compound has to pass. Sensory laboratories solved this decades ago by rinsing tasting glasses with deionised water — here is why, and what it means for a bar and a home shelf.
Cognac and Armagnac houses rarely proof down with water alone. They use petites eaux — demineralised or distilled water pre-blended with eau-de-vie to a low strength, sometimes aged in cask before it is used — and they add it a couple of degrees at a time over months. The water in that mixture is the same neutral blending water a whisky bottler buys. What differs is the pace, the carrier, and the reason.