The Fire of Orthanc was an explosive craft of the wizard Saruman that was used by his army to destroy large fortifications. It was referred to also as "Blasting Fire", when Aragorn explains it to King Théoden.
Saruman first used the Fire of Orthanc at the Battle of the Hornburg, at which the fortress' strong walls were successfully breached, and again at Isengard during the Last March of the Ents, as a weapon against the rampaging Ents. It burnt and killed Beechbone and made the other Ents double their effort to destroy the place. It is possible that the weapon, or something similar, was used by Sauron during the Battle of the Pelennor Fields, as the books state that by magic or technology the ammunition of his catapults caught fire while in mid-air.
The exact composition of the Fire of Orthanc is unknown.
Portrayal in adaptations
The Two Towers film
In Peter Jackson's The Two Towers, the Fires of Orthanc was a weapon in the form of great bombs, used at the Battle of the Hornburg to destroy the Deeping Wall. They were lit by a Berserker, a rare, elite and powerful race of Uruk-hai.