The item description do not list information on mana requirement to use it.

as it was: I've started a game in the immediate vicinity of ruffians and an ogre (banished one probably), I've taken down the bandits and found a bottled fire item - "will do 16 damage" cool I thought. When ogre attacked in his turn, I tried to use the bottle. insufficinet mana was the result. next combat turn ogre got lucky and oneshotted me. 

frustrating...

moment later I've checked the hiergamemnon, there is no entry on the bottled fire.

Either the item should not use mana, or it should be clearly stated in the item description. In any case add it to the encyclopeadia