An Sonjae wrote:
The chapter on Brewing the Tea in Lu Yu's Classic of Tea says 'When
the boiling water is in its first stage, you may add a measure of salt
in accordance with the amount of water. You can tell when to stop by
sampling it.' ... describing the
different kinds of froth and scum that appear as the water boils, I
think he was not using the purest bottled mineral water. Black scum
seems to be especially bad, he says it spoils the taste.

Here's a datum that might be relevant: many organic materials are much
more soluble in ion-free water. Adding a tiny amount of ionizable
inorganics can cause "salting out" of the organics, an effect very often
used in purifying the results of an organic synthesis, as well as in
keeping pasta from sticking together in the pot.

Salt added to water containing a lot of marginally dissolved organics,
e.g. from plant decay, might knock a lot of them out into a separate
phase (solid or goopy) that would settle, froth off or stick to the
vessel walls. Salt might also enhance the complexation of tannins with
caffeine and the like, effectively removing bitter and astringent
components from solution.

Just a thought-

DM