[Unclenick]

The red oxide is Fe2O3 and the black is Fe3O4. There is also FeO. The various colors, red, yellow, blue, and black come from combinations of these. If you heat a piece of bare iron, like a nail at one end, you can get a rainbow from yellow through blue to black, going from hottest end (black) to the least warm end (yellow). Red may be converted by enough heat to become black slag in air, while using boiling water facilitates that conversion at lower temperature to avoid getting the metal that hot.