Interesting, especially since the exact composition of Greek Fire has been lost for centuries. Are you sure that the shells weren't filled with tabasco sauce, and thus gave the south a staple for their 20th century cooking?
Most likely it was a figurative reference to shells containing explosive chemical mixtures that rain fire and shrapnel down on their targets (i.e. gunpowder).
LOL. Incidentally, there are ads for tobasco pepper sauce in the old New Orleans newpapers that predate the present Tobasco Company (and the bombardment of Charleston).
I imagine the Feds were firing some early incendiary shells. The version they fired in 1863 didn't work very well.
After a brief respite granted by General Gillmore, the 200-pounder Parrott rifle gun again began lobbing shells into the city. The Charleston Mercury stated, "From thirteen to fourteen eight-inch incendiary shells fell into the City. ... No damage was done. It is unnecessary to make any comment on this act."
The gun firing these shells was the famous Swamp Angel. The Feds were loading it with 20 pounds of powder instead of the regular 16-pound load in order for the shells to reach the city some 7,900 yards away. See: Swamp Angel. The extra powder may have caused the gun to move in its jacket, leading to the bursting of the gun on the 36th round.
The Confederates fired at the Swamp Angel using mortar shells with timed fuses. The timing on the fuses was poor, and the mortar shells buried themselves in the marsh surrounding the Swamp Angel before exploding. The explosions covered the Federal gunners with mud.