Why brass and bronze darken and turn green
The first stage of corrosion on brass and bronze is a dark tarnish, patina. More serious is green or blue-green corrosion, beneath which a red layer may still be hiding. Conservators point out that waxy green spots form where the metal touches leather or another source of organic acids, and that sulphur compounds from rubber speed up darkening. Weapons are full of such contact points: leather wraps, sheaths and wooden parts sit directly against the fittings.
Bronze disease shows up as light green powdery spots. It is caused by chloride salts and mainly affects archaeological objects that lay in the ground; according to conservators it is active especially above 55% humidity. Brass can suffer from dezincification, where zinc is lost from the alloy and the surface turns the salmon-pink colour of pure copper, later reddish to brownish, and becomes porous and weak. The cause is chlorides from sweat or seawater and acidic or ammonia-based cleaning agents.
Patina or shine: when not to clean
The National Park Service describes patina as a thin, chemically formed and fairly stable layer, brown, grey or black, often produced deliberately during manufacture and sometimes enhanced with wax, and advises against removing it. The Canadian Conservation Institute (CCI) recommends not polishing objects with a stable patina from historical use, deliberately patinated objects, objects with their original coloured lacquer, and explicitly East Asian bronzes, where patina is highly prized. Leave darker tarnish in the recesses of decoration, because it gives the pattern contrast. When in doubt, only clean the object and do not polish it.
Regular care
Do not touch fittings with a bare hand; fingerprints contain salts that, according to conservators, leave marks removable only by polishing. Use clean, laundered cotton gloves or nitrile gloves. Remove dust with a soft brush, for example a toothbrush or a bristle brush, and do not use feather dusters, because a trapped grain of grit will scratch the metal.
The Canadian Conservation Institute (CCI) removes grease with a mixture of equal parts methanol and water on cotton swabs, optionally with a little mild detergent added; the American source gives a few drops of detergent in distilled water. Do not soak objects in a water bath, and do not wet places where the fitting is joined to iron or steel, for example a guard directly on a blade. Cover non-metal parts, such as the grip or sheath, with clear polyethylene film. Scrape grey, white or green residues of old polish out of recesses with a wooden toothpick moistened with the same mixture. Always test each method first on an inconspicuous spot.
Polishing, when it is appropriate
If you do want to polish an object and neither the patina nor the original surface argues against it, conservators use precipitated calcium carbonate as the finest abrasive. Sources differ on the liquid: the Canadian one mixes it with water into a paste, the American one with denatured alcohol into a cream-like consistency. Apply it sparingly with soft flannel or cotton wool in circular motions, changing the cloth often so the lifted tarnish does not redeposit on the surface. The American source warns that ordinary ground or levigated chalk is no substitute for the precipitated kind.
Avoid commercial brass polishes. They often contain hard particles that scratch the soft alloy, ammonia that dissolves copper, or acids that trigger dezincification, and some leave an anti-tarnish film that complicates further treatment.
Wax or lacquer
Conservators protect clean metal with a microcrystalline paste wax without synthetic additives. The Canadian method dilutes it with an equal volume of mineral spirits, applies it thinly with a brush or cloth, warms objects without wood or ivory with a hair dryer so the wax flows into recesses, blots off the excess with paper, and, once set, buffs it with a lint-free cloth. The American source buffs the wax with silk or a nylon stocking and states that it protects for at least a year or two; renew it once tarnish reappears. Wax can be removed at any time with mineral spirits.
Conservators do not recommend lacquer for everyday use: it changes the appearance, is hard to apply in a thin, even layer, and is hard to remove. It suits cases where the object was originally lacquered. Professionally applied resin, according to the American source, protects for up to twenty years, but that is a job for a conservator.
Storage
For copper alloys, conservators give 30% relative humidity and below as ideal and 40 to 55% as an acceptable range. For objects affected by bronze disease, sources differ: one recommends keeping humidity below 35%, the other below 42%. Do not leave fittings in direct contact with untreated wood, acidic cardboard, rubber or freshly painted surfaces; separate a wooden shelf with a barrier film.
Inspection and when to call an expert
When inspecting, look for light green powdery spots, powder or flakes around the object, cracking of the surface, pink spots on brass, cracks, old repairs, and loose or missing parts. Entrust bronze disease, severe dezincification and compromised strength of fittings to a conservator.
Common mistakes
The most common causes of damage are polishing historical patina to a shine, commercial polishes with ammonia or acid, soaking in water, polish residue left in recesses, and fittings left damp in contact with leather or rubber.
Which weapons this applies to
This guide applies to Japanese tsuba guards, fuchi and kashira fittings, menuki and habaki, to brass and bronze guards and pommels of European swords and sabres, sheath fittings, Chinese jian and dao swords with brass fittings, and to bronze points and blades.