If you remove an iron kettle from its mold and bake it at a high temperature of 800 - 1,000 degrees Celsius, a film called an "oxide layer" will cover the surface. In Japan, we call the oxide layer by its popular name "Kuro-sabi(black rust)". Even though we call it "black," it is not pitch black. It is a variety of rust. When you start to use the kettle, it will be difficult for red rust to occur over the surface.
Scientifically, red rust is represented as "Fe2O3," and black rust is represented as "Fe3O4."
We advise you not to use a scrubbing brush or such to scrub the inside of an iron kettle. The reason is for you to use the kettle without removing the oxide layer.