- "The Substance burns so hot it melts wood, stone... even steel. And of course flesh. The Substance burns so hot it melts flesh... like tallow."
Wildfire is a dangerous liquid which is created and controlled by the Alchemists' Guild in the city of King's Landing. It is a highly volatile material which can explode with tremendous force and burns with a fire that water cannot extinguish, only large quantities of sand.
Wildfire is identifiable through the distinctive green hue of its flames. Even in its stored liquid state it gives off a sickly green color. Pyromancers refer to it as "The Substance", while in the lowest circles it is derisively referred to as pyromancer's piss.
Thanks to his reluctant agent, Lancel Lannister, Tyrion is alerted to Joffrey's plan to use wildfire in the defense of King's Landing against the impending attack by Stannis Baratheon, throwing it from the walls with catapults. Wisdom Hallyne of the Alchemists' Guild shows Tyrion their stocks. Bronn derisively says he doesn't believe the pyromancers' claims about wildfire, as there are always charlatans on the side of battlefields selling "pig shit" to soldiers and claiming it has magical properties. Hallyne insists that the abilities of wildfire are indeed very potent. Bronn then points out that even if this is true, wildfire is too unstable to be used safely in such a manner by inexperienced troops: the moment one of the catapult teams drops a single jar, it could burn the whole city by accident. Tyrion agrees and commands Hallyne to begin producing wildfire for his purposes instead.
While discussing the defense of the city with Varys and Bronn, Tyrion identifies the Mud Gate as the city's most vulnerable spot and the most likely location for Stannis to attempt a landing. Tyrion resolves to make a stand there. When Varys asks him what can they use to defend the city, Tyrion replies "pig shit".When Stannis Baratheon's fleet arrives at Blackwater Bay, Ser Davos Seaworth (commanding the vanguard of the assault) is bemused to see only one ship from the royal fleet emerge to challenge them. He realizes the ship is unmanned so he halts his archers, but then sees the ship is emptying wildfire into the bay from pipes at its rear. Tyrion loaded all eight thousand jars of wildfire into the ship. At Tyrion's command, Bronn fires a burning arrow into the wildfire, which detonates the ship in a massive explosion that pulverizes all of Stannis' ships closest to the blast. The spreading fire then engulfs nearby ships. Hundreds, if not thousands, of sailors and soldiers under Stannis's command are burned alive. Stannis resolves to continue the assault regardless.
In the books
In the A Song of Ice and Fire novels, wildfire is a very unstable material similar to real life napalm or Greek fire. In real life, "Greek fire" was actually used by the Byzantine Empire in medieval times, and thus was actually a technology known to medieval people.
Wildfire's use under controlled conditions was popular with some of the members of House Targaryen and some worshipers of the eastern god of fire, the Lord of Light, such as Thoros, who used to coat his sword with a thin layer of wildfire and then set it ablaze to scare his enemies in combat.
The alchemists refuse to divulge how wildfire is created, claiming that it is done with magical spells (a claim dismissed by others, particularly the maesters). Wildfire is stored in clay pots in bare stone cells under the Alchemists' Guildhall in King's Landing. Each cell is built under a chamber full of sand, which can be emptied into the cell below to extinguish the fire in case of accidents. Wildfire grows more volatile with age and can only be disposed of through hazardous means. Wildfire is highly combustible and can be set alight even by bright sunlight. It is described as having an emerald green coloring once ablaze, which is so strong that it can turn the air around it green.
- Wildfire at A Wiki of Ice and Fire (spoilers from the books)