Removing Excessive Tarnish
If you have to remove extremely difficult and stubborn tarnishing then the item of jewellery may need to be pickled. One method is to dunk the item in a heated solution of water and pickle salts or 10% sulphuric acid. It is best to do this in a professional Acid bath, which maintains a constant, operational temperature to the required twenty minutes. (the solution must be kept warm). Pickling is often used to remove excess flux during the soldering process.
Sulphuric Acid or Pickle Salts
Sulphuric acid (it used to be referred to as oil of vitriol) is a mineral acid and its acidic strength vary's according top the concentration of acid to water. The most common place to find it is in a car battery where a concentration of 33% is used. For silver jewellery purposes we use sulphuric acid diluted to a concentration of between 10%. Some weaker Pickling Solutions consist of diluted Hydrogen Peroxide or "Sparex" solution.
Other Methods for Pickling Silver.
If dilute sulphuric acid is unavailable, you can try Acetic Acid, which is generally obtainable from most chemists or home-brew and wine stores. It is nevertheless much slower than Sulphuric Acid.
Another possibility for mild tarnishing is to soak the silver overnight in vinegar.