How to Remove Rust and Green Corrosion from Costume Jewelry (and Does Cola Work?)

Costume jewelry is where most cleaning mistakes happen, because people assume there's nothing to lose. There is: a thin plating, a foil behind a rhinestone, a dab of old glue, a signature on the back.

It's part of our own history, too. In the early decades of the twentieth century, many immigrant Jews worked in the jewelry factories of Rhode Island, and some, such as the Lederer brothers and the Silverman brothers, became manufacturers themselves. Coro, one of the great American costume-jewelry houses, began with Emanuel Cohn and Carl Rosenberger. Their craft deserves a careful hand.

In this guide I'll show you what that green and that rust really are, how to deal with them without doing more harm, and what to make of the cola trick.

What you're cleaning

Under the shine, most costume jewelry is a base metal. Brass, for example, is copper plus zinc. The surface is often plated, and plating can be astonishingly thin. Under the FTC's Jewelry Guides, "gold electroplate" needs only 0.175 microns of fine gold, "approximately 7/1,000,000ths of an inch." The Canadian Conservation Institute (CCI) warns that a plating layer can be damaged "by cleaning with an abrasive polish or by excessive handling."

Two rules follow:

• Don't soak. A UK insurance broker's guide on vintage jewelry says "remember not to soak plated jewellery in water," and the U.S. National Park Service's advice for copper-alloy objects is stronger: "Never immerse the object in a bath of water."
• Keep water away from foil-backed stones. The same guide warns that "water behind the stone will ruin the foil and stones become dull, the setting beneath them can go green or they can fall out altogether."

Green corrosion

Green or blue-green on brass or copper is copper corrosion, and CCI calls it the more serious kind. Light-green spots that keep coming back can be "bronze disease." Organic acids react with copper to form these green corrosion products.

What to do: don't soak it, and don't scrub hard. Brush loose powder away with a soft, dry brush, keep the piece dry, and show anything valuable to a professional.

A green mark on your finger has the same cause. Dermatologists writing in the Indian Journal of Dermatology explain that acidic sweat leaches copper from the metal, forming green copper chloride and copper carbonate, and that it can happen with silver, gold and brass that contain copper. A health-system page calls it "not dangerous," and says soap and water usually removes it. The same dermatologists note that a coat of clear nail polish on the metal can prevent it.

Rust

Rust is iron corrosion: red, yellow or red-brown. On jewelry, look for it on any part made of iron or steel, such as a pin stem or a clasp spring. CCI explains that ongoing rusting comes from high humidity, and that "flash rusting" can appear soon after a piece gets wet. So the best rust prevention is dry storage, and drying every piece thoroughly after cleaning.

The cola trick, checked

The video above treats only true rust on steel parts. It dabs cola on with a swab in 5-minute rounds, or gives an all-metal piece a short soak checked every 5 minutes, then rinses, dries with a cool hair dryer and seals with microcrystalline wax. It keeps cola away from foil-backed rhinestones, real and imitation pearls, glued, painted or enameled pieces, and pot metal. And it says honestly that this is "not a trade secret."

The chemistry is real. Coca-Cola lists phosphoric acid among its ingredients. A 2016 study in the Journal of the American Dental Association measured about 380 drinks and found Coca-Cola Classic at pH 2.37, strongly acidic. Phosphoric acid is the main acid in the rust remover Naval Jelly, at 10 to 30 percent, and conservators note that it reacts with iron to form ferric phosphate.

The risk is real, too. The American Gem Society says plainly that "the acids in Coke can damage metals and softer stones." And an acid that attacks rust won't spare a plating 7 millionths of an inch thick.

What's unproven is cola on jewelry: we found no test of it by any gemological or conservation body. Our view: the video's limits are sensible. Treat only a rusty steel part, keep the cola on a swab and away from everything else, rinse, and dry at once. If a pin or clasp spring is badly rusted, a jeweler can replace it.

The safe everyday routine. For most costume pieces, that leaves a soft, dry brush, a barely damp cotton swab on the metal (never behind the stones), and a dry cloth. Then store each piece dry, on its own.

Volume V, Clean It Right, covers costume and plated jewelry in full, from rhinestones and vermeil to green fingers and rust, with a step-by-step method for cleaning a vintage brooch. Volume II, Grandmother's Jewelry Box, helps you read the signature on the back before you clean anything.