To darken the copper I used liver of sulfur (LOS) gel and water. I don't put the clasp in the LOS water mix I use a paintbrush to paint it on, this just protects the inner workings of the clasp. Make sure your copper is clean (I use Penny Brite which is biodegradable, and a toothbrush to clean the entire chain). I use the LOS mix cold to avoid fumes but it works much faster if the piece or the water are warm. My mix is about 1/2 cup of water and a few drops of LOS gel. Once you have the color you like, rinse it in a cup of cold water with 1 tbsp of baking soda. This will neutralize the LOS on the chain. To dispose of the LOS add the cup of baking soda/water to the LOS mix.