Water: the 98 percent

A filter coffee is around 98.5 percent water, and that water is not a neutral bystander — it is the solvent doing the extracting. Two properties do nearly all the work. General hardness (GH) is the dissolved magnesium and calcium: charged ions that grab onto flavor compounds and pull them into solution. More hardness, more extraction power — up to a point. Alkalinity (KH) is the bicarbonate buffer: it neutralizes acids on contact.

Buffer is the one that ruins cups quietly. High-alkalinity water eats a coffee's acidity before you ever taste it — the bright Kenyan flattens into cardboard, and no grind setting will bring it back, because the problem was dissolved in the kettle before the coffee arrived.