In heating an ingot in the pit, particularly in an oxidizing flame, and in exposing the red-hot ingot to contact with the atmosphere, a film of iron oxide forms over its surface. This oxide is very brittle, and as soon as the ingot goes through the rolls, it drops off and falls through under the mill. This is known as mill scale or roll scale, and is valuable for use in wrought iron and steel furnaces, as has been mentioned.