The blast sent smoke billowing into the sky that could be seen for miles around the damaged factory about 15 miles (24 km) southeast of Cleveland.
The explosion at the I. Schumann & Co. metals plant in Bedford drew fire departments from throughout northeast Ohio.
Oakwood Fire Department Captain Brian DiRocco addressed the media on the scene, saying 13 people were taken to the hospital, many of them with burn wounds, and one more was being treated on site.
At least one was in critical condition, and one was pulled from the debris. All of those injured were on site, the falling debris having spared those at neighbouring businesses, DiRocco said.
"The people were mostly walking wounded. They were coming up to us," DiRocco said. "I'm sure there's a lot of people that work here that were in shock."
DiRocco said he had inspected the site before and found it a safe place "except for the fact that it's a foundry. You are dealing with molten metal, so there's always an inherent danger."
Representatives of the company, which produces copper, brass and bronze alloy, could not immediately be reached for comment. The business has operated since 1917, according to its website.
Matthew Wiggins, the owner of the neighbouring business Rose Colored Gaming, told WOIO he heard a large explosion and that "within a second or two, it sounded like large amounts of debris were hitting the roof."
The explosion was about 70 miles (112 km) northwest of East Palestine, Ohio, where earlier this month a train loaded with toxic chemicals derailed, causing a fire that sent a cloud of smoke over the town and forced thousands of people to evacuate.