Firefighters were called to a large fire at the Amtek Aluminium factory in Freebornes Road, Witham, this morning.
Workers were evacuated before firefighters began tackling the blaze at 4:35am.
The fire is believed to have started in an extraction unit in the metal working facility.
Three appliances were sent to the scene and an aerial ladder platform was used to deliver water onto the roof of the building.
The flames were finally extinguished at 6:30am and a high pressure fan was used to clear the factory of smoke.
A joint police and fire service investigation will be carried out to establish the cause.
It was the 36th incident dealt with by working Essex fire crews since members of the Fire Brigades Union went on strike at 9am yesterday.