Because wood was a primary building material for structures and boats in the ancient world, incendiary devices were often used in warfare. Perhaps the most famous of all these devices was Greek fire. It was first invented by Callinicus (c. 620-?), an Arab who fled to Constantinople, the capital of the Byzantine Empire. The composition of Greek fire remains unknown today. It is believed to have been a mixture containing a liquid derivative of petroleum; other ingredients may have included potassium nitrate to supply oxygen, and quicklime to provide additional heat through reaction with water. The liquid was forced out of a bronze tube by a pump and sprayed over the target. It even burned on water, thus making it capable of destroying wooden ships. Greek fire was first used around 671 a.d. by the Byzantines against an Arab naval attack on its capital city. Like so many closely guarded secrets, the method of creating Greek fire was not a monopoly held by the Byzantines for long. The Arabs created their own brand of fire and used it against the Crusaders, and it may have been used in England as late as 1304 by Edward I. The advent of gunpowder, a more destructive invention, soon revolutionized warfare, thus making Greek fire obsolete. However modern warfare has used a variety of similar substances, such as