Description (from Recipaedia)
Incendiary keg is a gunpowder keg with added incendiary materials. It is a lot weaker than regular gunpowder keg but its debris set flammable objects on fire. Do not use near forests!
Crafting
You use gunpowder kegs and sulfur to craft an incendiary keg of the same size.
Use
Use these just like regular kegs. But remember, they are a lot weaker than those.
Notes
The three sizes of incendiary kegs have explosive powers of 50 / 250 / 1250. They will throw masses of flaming debris up to 30 blocks so it is recommended to stay far away when they explode.