Greek fire was an incendiary weapon system used by the Byzantine Empire from the seventh to the fourteenth centuries. The recipe for Greek fire was a closely-guarded state secret; historians have variously speculated that it was based on saltpeter, sulfur, or quicklime, but most modern scholars agree that it was based on petroleum mixed with resins, comparable in composition to modern napalm. Byzantine sailors would toss grenades loaded with Greek fire onto enemy ships or spray it from tubes. Its ability to burn on water made it an effective and destructive naval incendiary weapon, and rival powers tried unsuccessfully to copy the material.