I am looking into the chemistry behind blackening brass and would like to know whether the black/gray colour observed on the brass is copper sulphide, oxide or is it perhaps a sulphide from the solution precipitating on the surface?
The solutions used are as follows:
1. Sodium thiosulphate + Lead Acetate
2. Sodium thiosulphate + Iron (III) Nitrate
Can anyone provide me with the chemistry for this process?