I started out in the structural steel fabrication business in 1976, and at that time the shop I worked in had a couple of home made ovens. They had made two boxes out of 1/8" thick steel that looked like they could hold a couple 50# boxes of rods. These "ovens" had one shelf each made from expanded metal, lined with tin foil and had a 100 watt light bulb in them. I never knew whether or not they reached the 250 degree minimum, but the rods were warm. Four years later when we applied for Category II Certification, the ovens were quickly converted to keeping food warm, and the guys could keep their food in them all day long. When the Company purchased rod ovens for the audit, the first day they were plugged in one of the guys took his small can of beef stew out of the old oven and put it in the new oven to keep it warm. At last break he opened the door and found that the can had exploded. I guess the old oven wasn't anywhere near 250 degrees.