Clean badly stained porcelain with a paste of peroxide and cream of tartar. Simply scrub with a brush and rinse. If the stain persists, try again by covering stain with the paste and adding a few drops of ammonia. Let stand for one to two hours, then scrub and rinse.
Stains on old porcelain can also be helped with a a bar of Fels Naptha or Octagon soap dissolved in one gallon of hot water with half a cup of mineral spirits added. Brush vigorously on the surface. Note: Be sure that the "stain" isn't just the metal showing through a layer of porcelain worn thin before you brush with anything too coarse. To brighten old porcelain, try rubbing with a solution of salt and turpentine.
Source: Fingerhut Tips
By duckie-do from Cortez, CO