The following is the most effective and easy to use recipe for
darkening copper without touching silver. In fact it seems to whiten
the colour of the silver. The colour ranges from dark brown to deep
black and the latter seems to depend on the quality of the copper you
are using, so if you get great results from colouring isolate the
sheet you got it from!! The colour is also very stable.
Dilute 35gms of sodium hydroxide (caustic soda) in 500ml distilled
water. Immerse very clean piece. Colour takes from two hours to 2-3
days to develop. Wash and wax. Solution must be stored in glass and
can be kept for ages. Its brilliant!
Christel in West Oz