[JWP]

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?

Thanks

Ian