Neat. The overall image is very good (although I would suggest cropping the top just enough to elminiate the shadows of whatever is just outside the frame), and there are a number of excellent images within this overall scene.
The whatever is a sight glass - there would have been a piece of high-temperature glass in the port that allowed operators to see what was happening inside the furnace.
Thank you Mono. I also would crop the bottom slightly (after I uploaded it I realized that I "overshot" the image slightly-i.e., the image doesn't fully cover the paper).