I use copper in most of my work, often in conjunction with sterling silver (The "kupra" in the company name means "copper" in Esperanto). In time, copper will tarnish and darken as it reacts with compounds in the air.
It's inevitable. The copper will darken, even if it has a protective layer or patina applied. You may like this, or not. I've found that Sunshine Polishing Cloths do a good job of removing the tarnish and shining up the copper. I provide a free sample cloth with every direct sale of $50 or more.
There are other ways to clean your copper jewelry. Nunn Design, which produces jewelry components, tried and tested several methods using household items. I've used a few of these myself.
Lemon Juice and Salt
Vinegar and Salt
Baking Soda and Salt
White Vinegar and Salt - boiled. This was declared the winner
Ketchup and Water
Read about the testing on Nunn Design's blog.
Here's another technique I tried that produces a satin finish. Soak the piece in vinegar for several minutes, then clean with an old toothbrush and toothpaste.
One last thing. If you have purchased my jewelry and live in the Hampton Roads area, I will clean and polish the items for free. If you are out of the area, please contact me and I'll recommend the best way to clean your jewelry.
Lemons can clean your copper jewelry.