Blast Furnace is a Pack-a-Punched upgrade module in Call of Duty: Black Ops III. It gives the weapon a chance to shoot out a bullet that will make a group of zombies burst into flames and be killed. The zombie on which the effect initially triggered is killed instantly, while any nearby zombies within its effect radius will burn to death over 3-5 seconds.
Blast Furnace is a very powerful upgrade, able to kill zombies at any round while only needing one bullet to trigger. This upgrade can be used infinitely many times as long as ammo is present and it has a recharge of 15 seconds per use. It does not seem to have a cap on how many zombies it can affect, as an entire wave of zombies can be killed if they are bunched close enough together.
While Blast Furnace is arguably the best upgrade for taking out large groups of zombies, players should be wary that even after the effect is triggered zombies will take a few seconds to die, during which time they can still attack. This makes Blast Furnace an excellent perk for training zombies rather than getting a player out of a pinch.