Dust of dryness is the go-to object for removing water

The dust of dryness (Dungeon Master's Guide 255) (850 gp; 0 lbs.) affects water and only water (although the description listing several kinds of water). It gathers up to 100 gallons of water and turns it into a bead. If you throw the bead later it breaks and all that water reforms in that spot.

Please note that the target in question must either have accessible water that is recognized as water (ie: I doubt that the water which composes the human body counts, maybe stomach acid at best, or blood if you break it inside of an open wound... which would be a very nasty way to die...), or be a body of water. In the case of the proposed ashes, you might need to mix it with water to make it "wet enough" to trigger. (DM call, I presume.)

The dust has famously been used against creatures vulnerable to holy water… absorbing holy water in advance and then breaking the bead and deluging the undead or other target creature with 100 gallons of holy water yields an absurd amount of damage, the ramifications of which are beyond this question's scope.

As the dust is specifically called out to only affect water, it leaves all else untouched, which should result in your bead sitting in a pile of now de-watered ashes. Any irony is a bonus.