Green Crust on a Leather Bag’s Brass Hardware

Copper and its alloys, such as brass and bronze, react with the oils in leather and form waxy green corrosion products at their point of contact. The Canadian Conservation Institute describes this as common in museum leather objects, and it says that treating the corrosion is a job for an experienced conservator.

What the CCI says

• Metal components on leather objects, such as buttons, clasps, buckles, rivets and nails, often show signs of corrosion.
• Copper and copper alloys, including brass and bronze, react with leather oils and form waxy green corrosion products called copper stearate at the point of contact.
• Silver may show the same if it contains a small amount of copper.
• Where possible, prevent direct contact between leather and metal by inserting a sheet of Mylar or polyethylene.
• Copper corrosion on hardware can be removed with a wooden stick and a cotton swab dipped in mineral spirits, with Mylar strips under the metal to protect the leather.
• Further stabilisation is difficult and should only be done by an experienced conservator.

What you can do

1. Photograph the deposit.
2. Keep the bag dry and out of damp rooms.
3. Do not scrub or use household cleaners on it.
4. If the bag is valuable, ask a conservator or a repairer who works with leather and metal.

If you are choosing a bag, look at the hardware finish as well as the leather. Hardware that is plated or lacquered may react less, but the note does not test modern finishes, so keep any bag with brass fittings dry and check the contact points from time to time.

Where this answer stops

The CCI describes a conservator’s technique for museum objects, and it uses solvents that are not advisable at home. A modern bag has coatings and finishes that may not match the examples.