Firefighters responded but did not know what the substance was so, the Environmental Protection Agency was called, said Seattle Fire spokesman Kyle Moore.
The tubs were filled with nickel which prompted the firefighters to fight the fire with protective masks--nickel fumes are dangerous and can burn lungs.
The department closed off roads around the area at first to protect public from breathing the fumes but they were reopened shortly.
The building was made to contain the nickel in case it ever leaked out of the containers it was no threat to Lake Union, Moore said.
Electrical arcing related to the industrial plating equipment caused the fire, officials said. About $75,000 in damage was done to the structure and $75,000 to the contents of the building.