Pielorinho
Iron Fist of Pelor
Umbran said:
Spellcasters are not experts at their magic, in general. They are experts in their magic if they have the Knowedge(Arcana) and/or Spellcraft skills. Being able to cast a spell is not the same as knowing the details of it's function and operation.
I disagree with this, but that's more from a metagame perspective than from a rules-perspective (this is not, I think, something delineated in the rules themselves).
As a player, I know it's no fun in a game like D&D to have my magic work in a weird fashion. D&D sets up magic to work like technology: spells are standardized and specific in their effect. When your DM rules that purify magic works in a way that's different from how you understand it to work, it's frustrating.
Mystery and wonder are great elements of a game. But they should come from other places, I think: the motives of the enigmatic cult, the unearthly singing from the abandoned city, the powers of the strange demon, even the effects of the novel spells cast by enemy (or even friendly) NPCs.
Having one's own spells operate strangely is just frustrating. YM, natch, MV.
In this case, though, it shouldn't vary TOO much. The spellcaster has almost certainly seen purify food & water in action before: either she's cast it herself, or one of her superiors cast it. She ought to know whether it can remove big chunks of stuff, or whether it separates impurities from the water, or what.
But basically, this is a fun-issue. I find it more fun to let players be fully-informed on their own powers, unless their ignorance is an interesting plot-point.
Daniel