I've tried this on copper a couple of times... it looks gorgeous
but as soon as you touch it it flakes off... How do you get the
amonia patina to stick or actually colour the copper and not just
sitting on the surface like an unbonded powder?
I have done some experimenting with this type of patina, though I
mostly did it on brass rather than copper. I would expect much the
same results on copper.
I concluded that the longer it takes to develop the patina, the
longer it stays on. When I did it by putting the metal object in a
closed plastic container (a Star Wars lunchbox, if you must know)
along with-- but not IN-- a small dish of ammonia, and left it for up
to a few days, the patina was so durable I could steel wool it
without removing the color.
It didn't occur to me before, but it may also have helped that I was
etching areas, leaving the resist on, and patina-ing the etched
areas, then removing the resist. You could try a light etch with
ferric chloride before treatment to see if it helps.
I also had good results burying pieces in wood chips moistened with
ammonia or ammonia and salt, but in the last few weeks I've been
trying this with students, and the results have been mixed (no, I
didn't bury the students in wood chips. Ah, English!) One thin piece
of copper left for a week dissolved into nothing but a bright blue
layer of oxide. Pretty blue, though!
Fortunately, all the materials for these trials-and-errors are