(the natural patina formed with the oxidization of copper) is not only a pain to clean, especially from intricate antique costume jewelry, but it is also a toxic substance. Verdigris is also a corrosive agent, meaning that this naturally-formed coating is actually eating away small bits of metal as long as it is allowed to remain untended for long periods of time. Verdigris forms as a result of oxygen, moisture and other pollutants the metal has come in contact with over the years. The pollutants also include body sweat and oils, make-up, perfumes, hair products, and lotions. This allows dust and small dirt particles to coat the jewelry, adding even more fertilizer to the formula. I think of this substance as a contagious disease to vintage jewelry because yes, it can spread amongst pieces that are stored together.
Verdigris grows on pure and alloy forms of copper, brass, and bronze, including items that may have once been plated with silver or gold. When a small amount of the plating has been removed or worn very thin, the surface is prepared for verdigris to germinate, especially if there is already a bit of it near the scar, chip, or nick, such as being stored with other items already infected.
From about age 9, I have been collecting vintage rhinestone costume jewelry and I have always used cheap toothpaste and a soft brush to clean any verdigis from the metals. Now, you cannot get this abrasive cleaner (most toothpastes contain pearlite - a form of volcanic glass) near any stones, especially if they are foil backed, or pearls - natural or manmade, and you need to keep both toothpaste and water away from any parts that may be glued. I begin by using a dental pic, toothpick, straight pin, and small scrap pieces of twisted jewelry wire to get as much of the crud off as carefully as possible. Then I have a go with a child-size soft toothbrush and toothpaste, always brushing in the direction of the metal. I rinse the piece off bit by bit using water dampened scraps of t-shirt and cotton swabs and when it has been cleaned enough for me (all of the verdigris gone but leaving some nondestructive patina), I store these pieces in a velvet lined box, each individually bagged in plastic - unless it has a natural pearl, those pieces are in little velvet bags. To clean the tops of any glass stones, I use a cotton swab dipped in ammonia, and then rinse with a clean swab and dry with another. No, none of the stones in the vintage jewelry that I personally collect are either foil-backed or glued.
There are many other ways that other folks clean antique costume jewelry. Some use vinegar, others swear by lemon juice, still others like to use either ketchup or Worcestershire sauce! Whatever you do, remember that after spending hours fastidiously cleaning these special pieces, make sure they are completely dry before storing them.
Answer contributed by Dale "Cougar" Armstrong