There are a few ways to age copper or to put a patina on copper. The first is the most obvious. Just let it age. Copper tarnishes very quickly faster than most metals and in no time you will have a nice patina on it. If you are impatient and want the aged look right now you can use "easy off" oven cleaner on it. Sounds strange but it works. I am sure there are other ways but I always use the oven
. cleaner to do it. Make sure it is "easy off" because I have tried others and they don't work as well.