The Walking Dead's writer and creator has furthered the mystery regarding what was in the barrels in Sunday's Episode 8x04.
During Talking Dead, Robert Kirkman discussed the barrels labeled "Dangerous Substance," by confirming what type of substance was inside them. Initially, fans suspected the substance may be weaponized after its effects were seen in on a Robocop tribute walker in Episode 8x02. While Kirkman doesn't tease the potential use of the substance, he does promise it is dangerous.
"It's probably paint thinner," Kirkman said. "It's corrosive material in the barrels which was effecting the zombies. They were trapped in that ravine and couldn't get out. It's always fun to think about it, not fun, but we really like to think about how the body would decay and how that would effect zombies and how you can put them in different situations that would result in very unique kinds of zombies."
In The Walking Dead comics, no one uses any man-made substances as weapons in the All Out War story. However, Negan does cover his weapons with a certain substance which would guarantee death. Rather than using any sort of corrosive or radioactive material, the villain orders his Saviors cover their weapons and bullets with walker guts to ensure a simple strike or wound inflicted on the enemies is enough to guarantee death.