Sterling Silver oxidation

Answer 1

When you oxidize sterling silver, you are intentionally darkening (or tarnishing) it in a controlled way to increase detail and the appearance of depth in a design.

The most popular way to oxidize sterling silver is to immerse it in a solution of water and a chemical called liver of sulfur.

You can find liver of sulfur at most jewelry supply stores.

Source:
http://www.bellaonline.com/articles/art28859.asp
http://www.b2cjewels.com/education/silverguide.aspx