This kind of technology can be applied either on the copper or the brass, considering during the heater we loss the superficial rigidity deriving from the area cold working. On the tinny bronze is difficult to obtain a good black coloring with this method, because we stop it to the brown.
The most classic and primitive formula for the black varnishing used a solution of 120 g of copper nitrate in 40 cm3 of water ( total dissolution is heating), with the addend of 0.5 g of silver nitrate released in a few water. The silver nitrate addend in not always necessary. The operative modalities contemplate area degreasing and pickling in chloridric acid or in sulfuric acid diluted for some minute. Then we pass to coloring to immersion for little objects case, for big areas bump or brush-work. Adoperating for immersion the solution must be less concentrates than those over cited and it goes heated to 40-50°C. The dipped objects dry in an arid place and become grey. Final let heat them, slowly and in a uniform way, into a drying heater until it becomes black . The pulverulent superficial stratum that bears we brush away at the end of the heat.
It needs to put the maximum attention protecting the mouth and the nose from the brush assigned operators ( filter inspirators or damp wipes), because the dust is extremely deleterious. If during the first application the colour don't result dark black, it needs to repeat the processing.
The coloured copping ca ne polish and laquer for brush-work. Less advisable is instead the immersion or painting with nitric acid, because this acid is very aggressive either for the operators or for the brushes.
To deepen the dark black is advisable a secondary immersion in a suspension of sulfur spunk ( 10 g for q water liter). This type of post-treatment after a coloring bath application to bismuth nitrate gives a good dark black. To obtain a mat black colour , it needs to mat the area before the coloring. The obtained black colour with this kind of bath is well kept , more than obtained by persulfate processes or with ammoniac black varnishing on brass. The cost of chemical reactives is very low, because low is their consumption.
The black coloring is due by copper oxide formation, which bears also in oxygen presence. For this reason needs to shake the bath to help the oxygen arrive and its contact with the solution.