The incident happened around 10:00 a.m. at Alloy Services at 1515 Garnet Mine Road in Bethel, Pa.
The business produces anti-aircraft flares for the military.
Authorities say the mixing agents were in an oven that filled with gas. That gas ignited and caused an explosion.
Some of the resulting flames got into the duct work, but an employee managed to use a fire suppressor to put out the flames.
No one was injured.