Black colouring with copper nitrate, also known as browning is particularly suited to obtain a very dark and very resistant surface coating, which is required, for example, in the optical industry, and, in general, when coloured objects must withstand prolonged, although moderate, heating.
This type of technology can be applied to both copper and brass , but the surface hardness resulting from the cold working of the surface is lost when heated. Achieving a solid black colour on tin bronze with this technique is difficult, as it does not get darker than brown. Moreover, this colouring process does not work for alpaca, which is rich in nickel, and for aluminium bronzes.
The most classical and primitive “recipe” for black coating made use of a solution of 120 g of copper nitrate, dissolved in 40 cubic centimetres of water (the complete dissolution is obtained by heating), with the addition of 0.5 g of silver nitrate dissolved in little water. The addition of silver nitrate is not always indispensable. The need for a certain amount of methanol to wet the surface better and improve the action of the colouring solution is also dubious.
The operating modes include degreasing and etching of the surface in hydrochloric acid or dilute sulphuric acid for a few minutes. Then the colouring process starts, by immersion in the case of small objects, or by blotting or brushing in the case of large surfaces. For the immersion, the solution must be less concentrated than as mentioned above and it must be heated at 40-50 °C; the immersed objects must be dried in a dry place and they take on a grey colour. Finally, they are slowly and evenly heated in a drying stove until the grey colour becomes black. The resulting surface powder is brushed away at the end of the heating process.
The workers assigned to the brushing must be extremely careful and protect their mouth and nose (wearing respirators with filters, or at least wet handkerchiefs), as this powder is extremely harmful. If the resulting colour is not dark black after the first application, or if it is not uniform, the process must be repeated. The coloured coating can be polished by brushing and also lacquered. On the other end, immersion or brushing with nitric acid is not recommended, as this acid is very aggressive towards both operators and brushes.
To darken the black colour further, a subsequent immersion in a suspension of hepar sulphur (10 g per litre of water) is recommended. This post-treatment after the application of a colouring bath with bismuth nitrate provides a nice dark black colour.
For matt black, the surface must be matted before colouring. The black colour obtained with this type of bath lasts long, much longer than the one obtained with persulphate-based processes or with ammonium black coating on brass.
The cost of the chemical reagents is very low, since their consumption is also low.
The black colouring is due to the formation of copper oxide, which is formed only in the presence of oxygen. For this reason, it is necessary to stir the bath in order to facilitate the presence of oxygen and its contact with the solution.