Oxidation of a piece of steel by heating it to give it a more or less dark blue color. Bluing is an industrial process that is commonly used in horology. When the steel cools down at the contact of air, a fine layer of calamine will form, which will protect it from oxidation. Blued steel is frequently used on the hands of steel watches. Blued steel inspired the hand of the TIEFENMESSER, following the codes of the world of submarines.