Verdigris, what some people refer to as green "gunk" on antiques, is a natural form of patina that accumulates over time on copper, bronze, and brass as it is exposed to air. In addition to varying shades of green, it may also be bluish green in color. It can also be induced using acids, and sometimes is artificially applied to new items made of these metals, especially copper.
Does Verdigris Damage Antiques and Collectibles?
It depends on the type of antique or collectible. For instance, when verdigris forms on the brass wire used to hold vintage jewelry components in place, this is considered to be detrimental to the piece.
Other antiques, take a bronze statue as an example, would not be considered damaged if a light amount of verdigris is present. Since this is considered to be patina on many antiques, it can actually preserve the value of rare pieces to leave them as is.
Should You Clean Verdigris?
Again, this depends on the type of antique or collectible you're evaluating. Carefully cleaning verdigris from vintage jewelry is preferable. Left unchecked, it will continue to eat away at delicate wiring and findings causing components to be lost and it may alter the color of affected metals irreparably.
Since verdigris on other metal objects with some age on them could be viewed as patina possibly adding value depending on the rarity of the piece, it is best to thoroughly research the impact cleaning will have on each type of metal antique or collectible before tackling those projects. Consult with an expert on the type of antique in question if you're not sure whether to clean verdigris from an antique.