Incidentlaly, Low-Tide's Gift is absolutely Salt. No questions asked. When the tide goes out, whatever's trapped behind evaporates and leaves salt behind. Which is a absolutely one of the most valuable things that exist in an ancient to middle-aged society, given how it's critical in food preservation.
So, given that, here's what we know.
(Hardened Soft)Pottery meets (Black Man's End), (Force of Fire)Explosion is (Low-Tide's Gift) Salt
Could (Black Man's End) be referring to
Pitch by any chance? (Also referring to as Tar), it's a fairly common thing that both catches fire and would probably drag you in and drown you if you fell into it. It's also a fairly common siege weapon used, since burning pitch poured over the wall is one of the famous ways to make problems go away.
Remember, the Rune has to be
inked, if that's the case, the first phrase is telling you the Material (Pottery) and the Ink (Pitch), then the second half describes what it actually does. (Explodes)
Actually, given that, the whole phrase honestly describes the entire process of evaporating salt in the first place. You contain it within ceramic bowls, fuel it with peat (Which is one of the precursors to pitch), and burn it, evaporating the impurities and leaving you with salt in the end.