Roy Mustang is a prominent character in Fullmetal Alchemist: Brotherhood, since he is an ally of the Elrics' and an ambitious Colonel who aims to become the next president of Amestris. He is also a unique alchemist.
Most alchemists work with metal, stone or even wood for their alchemy, to create weapons or barriers in combat. Instead of that, Roy Mustang has a flame-based alchemy style that creates jets and blasts of fire, and it's among the series' most powerful types of alchemy.
The Origin of Mustang's Flame Alchemy
Roy Mustang didn't actually invent this brand of alchemy himself. It was the creation of Berthold Hawkeye, an independent alchemist who believed it to be the ultimate form of alchemy. He was paranoid about it falling into the wrong hands or being lost for good, so he tattooed the arcane formula for it onto his daughter Riza's back for safekeeping. Years passed, and Mustang, a friend of Riza's, was entrusted with flame alchemy at last. Mustang modified the transmutation circle somewhat, removing its text and basilisk symbols, and instead, using a circle with a flame symbol at the top and a salamander at the bottom, fitting as salamanders are referred to as creatures of fire in real-world alchemical texts.
Mustang also had some custom gloves made to enable this alchemy, specifically, flame-ignition cloth. Whenever Mustang snaps his fingers, the gloves create sparks. The alchemy doesn't work barehanded — the Colonel absolutely needs his gloves. For convenience's sake, they bear the necessary transmutation circles to create flames. Mustang takes care to keep his gloves dry, in good condition and easily accessible, because, without them, his unique brand of alchemy is nearly impossible to perform. In a pinch, he can carve the transmutation circle into his hand, but he'd have to be very desperate.
Mustang's Flame Alchemy In Practice
In battle, Roy Mustang uses his alchemy to manipulate the oxygen atoms in the air to provide fuel for his alchemy. Typically, he will concentrate most of the air's oxygen around his target and in a narrow line from himself to the target, and once he snaps his fingers, the spark will set the oxygen ablaze, and with great force. Mustang has a great deal of control over this technique, arranging the concentration of oxygen atoms exactly how he wants. As a result, his attacks are potent but also precise, and he can minimize collateral damage during battle. In one instance, Mustang rescues Edward and some allies from a crowd of enemies by snaking fire through the battlefield and into all hostiles, leaving Ed and his allies unscathed.
Mustang can even use this brand of alchemy in two different ways. His right hand creates powerful, heavy-duty attacks, his standard method of fighting best seen in his attack on Lust the homunculus, in which he destroyed her over and over with his right hand. By contrast, Mustang's left hand is in charge of creating small-scale, ultra-precise attacks. He first demonstrated this on Envy, targeting only Envy's eyeballs or tongue rather than his whole body. Mustang then took it a step further and used alchemy to evaporate the water in Envy's eyes and tongue, temporarily blinding him and causing him intense pain. In theory, Mustang could do this to cripple any enemy without killing them.
In general, water dampens Mustang's alchemy, as one might expect. During rain, the air is far too humid for his alchemy to ignite anything, meaning he has to bow out of a battle, usually to comedic effect. But not all water will block Mustang's alchemy, as Lust found out the hard way. If the air itself is dry, but there is a lot of water around, the Colonel can actually split the oxygen and hydrogen atoms in the water molecules to create fuel for his alchemy. He can then set it ablaze, taking his enemies by surprise as they're blown up.