I think they may have used magnesium in a separate breakable container and any type of flammable liquid, like turpentine, coal oil, whale oil, etc... Magnesium explodes when mixed with a liquid substance even water, a petroleum distillate or other flammable liquid would add to the fire and help expand it while not allowing it to be put out by water,,,

What do you think?

General Forrest poured it on a wet tree stump and ignited it.