One of the finest formulas for obtaing very high gloss finish is Swiss Black (Beutel) formula because it contains no Nitric or Hydrochloric Acid so it does not attack the metal surface too aggressively. Hence you end up with finer rust particles and only extremely fine surface pitting. Also you can adjust the aggressiveness of the formula by adjusting the amount of Alcohol used this allows for humidity and temperature variations. Though it takes a lot of passes to obtain a rich dark colour hence the high cost of having barrels blued this way.
