What was Greek fire, and why could nobody copy it?

Greek fire was the Byzantine Empire's secret weapon: a liquid incendiary that kept burning on water, thrown in grenades or sprayed from tubes onto enemy ships. Nobody could copy it because the recipe was a closely guarded state secret, and in the end it was lost altogether.

Byzantine sources called it by other names: "sea fire", "Roman fire", "liquid fire", "sticky fire". The name Greek fire has been common since the Crusades. According to the chronicler Theophanes, a craftsman named Kallinikos fled to the Romans from Heliopolis and had devised a sea fire that burned the Arab ships. The historian James Partington thinks it likely that it was not the creation of one person at all, but was invented by chemists in Constantinople who had inherited the discoveries of the Alexandrian chemical school.

The timing mattered. Syria, Palestine and Egypt fell to the Arabs within a generation, and around 672 they set out to conquer Constantinople. Greek fire was used to great effect against their fleets in the first and second Arab sieges. Later the empire used it against Rus' raids, in 941 and 1043, and against the Bulgarians.

Secrecy was part of the weapon. Emperor Constantine Porphyrogennetos told his son never to reveal the secret, saying the fire had been revealed to the first Christian emperor by an angel, who bound him not to prepare it except for Christians and only in the imperial city. Because the recipe is lost, its exact composition is still debated, but most modern scholars think it was based on petroleum, comparable to modern napalm. It could be put out only by a few substances, such as sand, strong vinegar or old urine.

In the end the secret itself faded: no report confirms the use of Greek fire when the Fourth Crusade attacked in 1203, perhaps because the empire had disarmed, had lost access to the ingredients, or had lost the secret over time.