Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

FeCl would normally be a light or dark tan color. Black or green means
it's spent. The cheapest thing to try is to add some hydrogen peroxide
to it, which will convert some or all of the dissolved copper into a
cupric chloride etchant (emerald green). If that doesn't work, you'll
have to add some hydrochloric acid, effectively making an acid/peroxide
etchant that happens to have some spent FeCl in it.

I've used old etchant that wasn't spent and it works just fine; it's not
the age that counts it's how much you've used it.