Inside this room you need to make a new cog so you can open up the door. To do this grab the mold off the shelf. Go up the stairs and place the mold inside of the furnace. Go back down stairs and charge your Green Hand. Start the furnace. Open the doors and grab the cog when it has been forged. Place the cog into the door mechanism and pull the lever to open the door.
Walk forward through the doors onto the walkway. As you walk Mommy Long Legs will appear. Run back to the forge. Charge your Green Hand and use it to ignite the forge will running inside of it. This will cause the flames to shoot up, protecting you from Mommy Long Legs. Once she is go walk across the walkway until you see a lever on your left. Pull it and Mommy Long Legs will appear again, triggering the final chase.