There is a process involving Copper Nitrate to speed up the patina effect. It is best to use a contractor experienced with copper on this.

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Check out a wide variety of patinas at Sculpt Nouveau: http://www.sculptnouveau.com/
If they don't have it, it's probably not made. Nice folks too, I've used many of their products on my sculptures. Give them a call.

Excellent article on the topic here: http://www.copper.org/applications/architecture/finishes.html

Good luck!

(below is photo showing patina achieved with Sculpt Nouveau products)