An ancient iron finishing process, widespread above all to treat the blades of swords and knives, not only to improve their aesthetics but also to protect them over time.
Dark etching is a chemical oxidation process in which iron or steel (not stainless steel), reacting with an acidic chemical product, transforms the lattice of its surface, making it become "brown". The blackish color is due to the type of iron oxide that is created on the surface, this oxide also gives greater protection to corrosion over time. To obtain a burnished blade it is sufficient to immerse a blade in a ferric chloride solution, the concentration is not important but only determines the reaction rate. The coloring nuances can change based on the mixtures used for the treatment, in fact together with or instead of ferric chloride other products or additives and / or specific catalysts can be used for the desired effect. The ferric chloride reported here, having low harmfulness compared to others, is recommended for DIY operations and is also easily available. Etching is a real art, we mention for example the splendid damask blades to which dark etching gives an even more elegant veined effect .
If you want to try the etching of old knives make sure before degreasing them well, they must not be touched by the fingers and must not appear halos, so you can only immerse the blade in the acid solution. Wait a few minutes and choose the degree of coloring you want to achieve. When you have decided to remove the blade and rinse it thoroughly with an alkaline solution, even the baking soda is fine. Pat dry as best you can verifying that the surface does not release more than black. It may be necessary to re-immerse it in the acid and repeat the operation. At the end we recommend to pass the blade with an oil to increase its shine and preserve it over time.
For home use, be careful with kitchen knives as if you do not have much practice in performing this ad hoc technique, you may continue to smell the oxide that would attack food.