“Nitre Bluing” is a highly decorative process in which parts are dipped in a solution heated to 600 degrees. These parts are generally polished to a mirror like sheen to greatly enhance the beauty. Due to the high temperature used in Nitre Bluing parts that are subject to high pressure such as the barrel, bolt and receiver should never be Nitre Blued. I have seen nothing that can add more beauty to a nice old double barrel shotgun or similar firearm.