From A Wiki of Ice and Fire
The Alchemists' Guild is an ancient society of learned men, sometimes called pyromancers, who have claimed many magical and arcane abilities through the years. In recent centuries the maesters of the Citadel have largely supplanted the alchemists almost everywhere in Westeros.
In its earlier years, the Guild was very powerful, but by the reign of Robert I the Guild's abilities and influence are greatly diminished. Now only a few of the old order remain, there are few true pyromancers left.  A full member of the Alchemists' Guild is called a wisdom. There are also apprentices and acolytes to assist them. 
At the height of their power, Alchemists claimed the ability to transmute metals and create living creatures of flame. In their later years, they still hint at possessing vast secret stores of knowledge - but the only skill they demonstrate is the creation of wildfire.
Wildfire is a highly volatile liquid similar to napalm and Greek fire. Alchemists typically refer to it as "the substance". Wildfire can be set aflame with the smallest spark, and sometimes even by the heat of sunlight. As it ages, it becomes even more volatile. Once wildfire is lit, it burns with green flames that are almost impossible to extinguish. Wildfire will seep into many materials, making them flammable as well. Great quantities of wildfire will combust with explosive force. It is typically commissioned for use as a weapon of war,
The Alchemists create wildfire in their Guildhall, which is set beneath Rhaenys's hill in King's Landing. The hall is an imposing labyrinth of cold, black stone. Acolytes of the Guild make wildfire in solitary cells.
The Alchemists claim that the creation of wildfire is a delicate and magical process. Should the wildfire explode, the cells' false ceilings are designed to collapse and fill the cell with sand, snuffing the fire as well as killing the acolyte. Wildfire is contained in clay jars that are textured to increase grip. These jars must be handled carefully to prevent fracture or combustion. The most volatile jars are housed in storerooms of water and transported in sand-filled wagons. At any one time, the Guild's store of wildfire could number in the thousands of jars. 
Increased influence during the reign of King Aerys II
Aerys appointed three of the Alchemist Wisdoms, Lord Rossart, Garigus and Belis, to create a vast quantity of wildfire to destroy King's Landing should he lose the War of the Usurper. When his Hand Lord Qarlton Chelsted opposed the plan and renounced his duties in disgust, Aerys had him burned to death with wildfire and made Rossart as his next and final Hand. This was one of the reasons why Ser Jaime Lannister slew both Rossart and King Aerys.
During the Sack of King's Landing, a great many wisdoms were killed or went missing and the acolytes were inexperienced in destroying the wildfire, so it remained below the city, some of it only being rediscovered during the War of the Five Kings.
A Clash of Kings
During the War of the Five Kings, Cersei Lannister commissioned a great quantity of wildfire for the city's defences. The Alchemists reported an unexplainable increase in the effectiveness of their spells, where it was quickly noted that it hadn't been as easy since the time of the dragons, which allowed them to create the substance more quickly. 
A Feast for Crows
|“||“Wildfire is but one of the dread secrets of our ancient order. Many and wondrous are the things we might show you.” ||”|
|“||“King Aery took a great interest in our work.” ||”|
|“||His brother Jaime had told him a few stories of the Mad King and his pet pyromancers. ||”|
References and Notes
This page uses content from the English Wikipedia. The original content was at Organizations in A Song of Ice and Fire.