How to clean bronze without stripping the patina

Step by step: how to clean bronze without stripping the patina

1. 1

   Decide whether the piece should shine

   Look at the color first. A brown, dark or even green surface on a statue, sculpture or antique is often a patina applied on purpose, and polishing would remove it. If you are not sure, clean it without polishing, which is what museum conservators advise.

2. 2

   Put on gloves and brush off the dust

   Wear cotton or nitrile gloves so skin oils and salt do not mark the clean metal. Brush the whole piece with a soft natural-bristle brush, working into the recesses with a small paintbrush. Most of the dull gray film on indoor bronze is dust and comes off at this step.

3. 3

   Wash with mild soap and warm water

   Mix 2 or 3 drops of mild dish soap into a bowl of warm water. Dip a soft cloth or brush, squeeze it out, and wipe the bronze in small sections. On greasy fingerprints, go over the spot a second time instead of scrubbing harder.

4. 4

   Clear the crevices

   Gray, white or green crust in the folds of a casting is often dried polish from earlier cleanings. Loosen it gently with a wooden toothpick or a damp cotton swab. Never use a metal pick or steel wool, which will scratch the surface and the patina.

5. 5

   Rinse and dry completely

   Wipe the piece with a cloth dampened in plain water to remove the soap, then dry it right away with a clean, dry cloth. Let it sit in a warm room for an hour so water in the hollows evaporates. Water left in crevices can start new spots of corrosion.

6. 6

   Polish only if you want bright metal

   If the piece is meant to shine, such as bronze door hardware that has always been bright, use the mildest option first: a jeweler’s polishing cloth rubbed gently, then buffed with a soft cloth. Stop as soon as the color evens out. Each polish removes a little metal and softens fine detail.

7. 7

   Apply a thin coat of wax

   Rub a very thin layer of paste wax over the clean, dry bronze with a soft cloth or a soft brush for detailed areas. Let it haze for 10 to 15 minutes, then buff with a clean cloth until it no longer feels tacky. The wax slows tarnish and makes the next cleaning a quick dusting.

Should you remove the patina from bronze?

Usually not. On sculpture, plaques, medals and antiques, the brown, black or green surface is part of the finish, and once it is polished off you cannot get it back without a professional repatination.

The Canadian Conservation Institute’s care note for copper alloys points out that medals and statuary may have been artificially patinated when made, that the patina on Asian bronzes is highly prized and should never be polished, and that when in doubt it is safest to clean without polishing. It also warns that some “bronze” statues from the late 1800s and early 1900s are a soft white metal (often called spelter) coated with a tinted varnish. Polishing, or even a solvent wipe, can take that varnish off the high points. If a scratch on the base shows silvery gray metal underneath, treat it as spelter: dust it and wipe it with a barely damp cloth only.

Bronze that was always meant to be bright, like some cabinet pulls, door hardware and kitchen decor, can be polished. Check whether it has a clear lacquer first: lacquered metal stays bright for years and only needs soap and water. Polishing lacquered bronze cuts through the coating and leaves patchy tarnish.

What do green spots on bronze mean?

An even green layer on outdoor bronze is normal weathering, but small, light green, powdery spots that keep coming back are a warning sign called bronze disease.

The Canadian Conservation Institute’s note on active corrosion describes bronze disease as a light green powder erupting in spots on the surface, most common at relative humidity of 55% and above, and says the reaction is progressive and can cause extensive damage quickly. Airborne chlorides (common near the coast) and acetic acid fumes can also produce a powdery green layer.

• Brush the loose powder off with a dry soft brush, outdoors or over a paper towel.
• Move the piece to a drier room, away from a bathroom, a kitchen or an unheated porch.
• Do not store bronze in oak cabinets or cardboard boxes, which give off acidic vapors.
• If the spots return within weeks, or the piece is valuable, show it to a professional conservator rather than treating it at home.

What not to use on bronze

Skip anything abrasive, acidic or salty. Many popular home remedies remove patina and some leave behind exactly what causes new corrosion.

How to clean outdoor bronze statues and plaques

Outdoor bronze gets the same treatment, done on a cool, dry, overcast day: rinse with a garden hose, wash with mild soap and a soft brush, rinse again, dry, and wax.

Museums follow the same routine. The Krannert Art Museum at the University of Illinois washes its outdoor bronzes with water and a mild detergent to remove dirt and de-icing salts, then gives each one a yearly coat of paste wax, because rain alone does not keep bronze clean and can cause uneven streaking.

Work from the top down so dirty water does not run over areas you have finished. Bird droppings are acidic, so soften them with a wet cloth laid over the spot for a few minutes instead of scraping. Wax goes on best when the metal is dry and slightly warm to the touch; on a hot, sunny day it smears. For large monuments or anything with flaking, pitting or streaks of bright green running down the stone base, call a conservator who works on outdoor sculpture. For other metals in the house, see our guides to cleaning brass and cleaning chrome.

Mistakes to avoid

• Polishing a dark statue to “bring out the bronze” and losing a patina that cannot be put back.
• Using a vinegar and salt paste, which strips the finish and leaves chlorides in the crevices.
• Leaving the piece damp after washing, so water sits in hollows and starts new spots.
• Handling clean bronze with bare hands, which leaves fingerprints that later darken.
• Assuming a cheap “bronze” figurine is solid metal when it is varnished spelter that solvents will ruin.