Promethium is a general term used for any form of combustible fuel in the Imperium of Man, including the various isotopes of hydrogen used as both fuel and reaction mass in Imperial Plasma Generators and starship engines. The term promethium also refers more specifically to the substance that is the 41st Millennium's equivalent of napalm, a highly volatile incendiary gel derived in part from refined hydrocarbons which immediately ignites on contact with oxygen and is used in Imperial Flamer weapons to provide a lethal stream of fire. This form of promethium reaches super-hot temperatures. Similar to napalm, this gel sticks to an individual, continuing to burn on its own accord, even underwater and in the vacuum of space (igniting the substance it is sticking to). If properly blessed, it can be used as the fuel for Incinerators, and thus is often used by the Imperial Inquisition's Ordo Malleus and Ordo Hereticus to purge their enemies, particularly daemonhosts and rogue psykers. It is also the standard Imperial military fuel, used in main battle tanks, other armoured vehicles and aircraft. Since promethium can burn in space and underwater, it is extremely useful for boarding actions where the enemy might not expect to see flames in an airless environment. The term promethium is used to describe any petroleum product that fits this description. It is not, however, a substance that always possesses the exact same chemical composition due to variance from different worlds and processing methods. The variance is so small that one form of promethium usually cannot be distiguished from another, which is why the use of the single term "promethium" is used across the Imperium to refer to a wide variety of actual combustible fuels.
- Codex: Imperial Guard (3rd Edition, Revised Codex), p. 62