(OOC: There may have been a bottle nearby. If she checks after Fuosala leaves for the moment, she does indeed find a long slender bottle with a stopper in it.
Bathing in the Cosmic Flame seems to be done in private, though it doesn't say, but Ashna assumes she would mention if it was public. Bathing directly in the energy source is fatal to any mortal or normal servants--even the beings of elemental fire overload. Only Ashna and Anzunda are in an altered state such that they can make use of the energy rather than be consumed by it, though apparently it helps Ashna a whole lot more than Anzunda, so the mephit's ability to use it is really more symbolic of loyalty. The specifics haven't been detailed, but it is clearly a power boost, useful but not vital, and symbolic of the superiority of the power Ashna possesses and can offer to those who are loyal to her)