Blackening techniques for metal 1. Hot Black Oxide - Hot black oxide is a chemical conversion coating formed by a reaction with the metal surface at high temperatures. * Process: 1. Cleaning: The metal is thoroughly cleaned to remove any oils, greases, and oxides. 2. Immersion: The clean metal is immersed in a heated alkaline solution of sodium hydroxide, nitrates, and nitrites (around 140-160°C). 3. Conversion: A black magnetite (Fe3O4) layer forms on the surface. 4. Rinsing: The part is rinsed in water to remove residual chemicals. 5. Sealing: An oil or wax is applied to seal the surface and enhance corrosion resistance. 6. Applications: Commonly used for tools, machine parts, firearms, and decorative items. 2. Cold Black Oxide - Cold black oxide is a room-temperature process that deposits a black coating on the metal. * Process: 1. Cleaning: Thorough cleaning to remove contaminants. 2. Immersion: The metal is immersed in a cold blackening solution containing selenium dioxide and copper compounds. 3. Reaction: A black copper selenide or similar compound forms on the surface. 4. Rinsing: The part is rinsed in water. 5. Sealing: A post-treatment oil or wax is applied for corrosion resistance. 6. Applications: Used for smaller parts and where high temperatures are undesirable.