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Caring for your Copper, Brass, Bronze & Other Alloys

Egyptian lamp holder, 1468-96. Museum no. 109-1888.

Egyptian lamp holder, 1468-96. Museum no. 109-1888.

Problematic copper corrosion tends to form in spots that create pits in the surface. Sometimes the surface of a bronze object may look intact, but will have a pimply appearance - if you remove the surface over the spot you might find a corrosion pit containing light green powder. If left untouched, the pit will gradually get deeper and larger, and may eventually burst, revealing the corrosion below.

The most active and rapid form of copper corrosion is known as bronze disease. This is caused by chlorides, which are particularly aggressive corrosive agents (e.g. salt, which is sodium chloride). This corrosion process, once initiated, will proceed quickly, forming a rash of spots which burst, spilling out light green copper chloride. These corrosion products will then cause more corrosion to occur. 

Bronze disease is rare and is mainly seen on archaeological copper alloy objects that were buried in salty soils or were in contact with sea water. If you notice spots with a bright greenish powder emerging, consult a conservator since, whether or not it is bronze disease, it is best dealt with by a professional.

Cleaning copper & brass

The current fashion is for a light patination on historical copper and brass objects. Problems occur when such objects develop a darker patch of tarnish or an area of green powdery corrosion. It is very difficult to remove such patches without leaving behind a bright polished surface.

Please refer to the Basic Guidelines page on this website before attempting to clean your object.

You can either accept the darker patches (however green powdery corrosion is likely to require removal) or remove the dark patch or green corrosion products. It is quite likely that this will leave a bright patch. If this happens you can either leave the patch to slowly tarnish (this may take a year or more, and the patch may never be the same colour as the rest of the surface) or polish the whole piece and leave it uncoated to tarnish evenly. A conservator may be able to remove corrosion products and minimise the difference in appearance after cleaning.

Lamp holder surface showing the spotty appearance of bronze disease. © Sophy Wills

Lamp holder surface showing the spotty appearance of bronze disease. © Sophy Wills

To remove dark patches:

  1. Swab surface with methylated or white spirit to remove grease and dirt - some tarnish may also be removed.
  2. If tarnish remains, try gently rubbing a silver cloth over the surface (go to the page on Caring for your Silver).
  3. If tarnish remains, try a mild abrasive cream. Make a swab and pick up a little of the cleaning product. Rub gently over the tarnished area in a circular motion, working from the centre outwards. Remove residues with a swab moistened with white spirit. They can be difficult to remove and you may need to repeat this step.
  4. Swab lightly with methylated spirit or acetone.

To remove powdery green corrosion:

  1. Swab surface with methylated or white spirit to remove grease and dirt - some tarnish may also be removed.
  2. Cut a chisel shape on the end a bamboo kebab stick and, with care, use it to push off the powdery corrosion product. This will probably leave behind a slightly green shiny surface, which is usually considered acceptable. If it reveals green powder filled pits, consult a conservator.
  3. Swab lightly with methylated spirit.

Patinated bronzes and other copper alloys

Bronze statuary and other patinated copper alloys should be approached with caution. The most that should be done by way of routine maintenance is to keep the object dust free by very gentle brushing or a gentle rub with a soft lint-free cloth. Solvents such as white spirit, acetone or methylated spirit can damage or remove original finishes that add historical and monetary value to such objects.

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