Transmute: Elemental Fire is a sub-skill of Alchemy which allows the Alchemist to change one 
[Heart of Fire] into three 
[Elemental Fire]. In order to perform the transformation, the alchemist must have a 
[Philosopher's Stone] and a skill level of 300.
Transmuting elemental fire will trigger a 10 minute transmute cooldown on the Philosopher's Stone.
Source
The recipe is sold by Lokhtos Darkbargainer at the bar in Blackrock Depths. A Friendly reputation level with the Thorium Brotherhood is required to purchase this.