Brass(?) corrosion

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    • #133426
      Laura Furman
      Member

      We have three items in our collection – two police badges and a leather pouch – that have evidence of oxidation, possibly indicative of bronze disease. The green discoloration is slightly powdery, and tiny bits will dislodge with a cotton Q-tip, but we fear that corrosion may have already set in. What is the best way to stabilize these artifacts and stop the spread of corrosion? The badge with the worst problem appears to be brass-plated? Thanks, Laura Furman, Curator of Collections, Midway Village Museum.

    • #133431
      Kory Berrett
      Member

      Thanks for the photos. Either badge may be a brass or copper alloy or may get its yellow color in another way such as surface plating or tinted lacquers.

      The Chief of Police badge looks as though the spotty corrosion is black (at least on my monitor). The parts that are still bright appear to be protected with a lacquer or some other kind of coating but the spots may be in locations where the coating has been damaged or has otherwise failed. The black color suggests a sulfide corrosion product. If all this supposition is accurate, an objects conservator can remove the corrosion and touch in the coating to stabilize the badge. Note though that the raw metal that will be found under the corrosion is likely to have been disturbed and may have a pitted texture or an off-color after cleaning.

      The star shaped badge is harder to figure out. It also looks like it has/had a coating, but this coating layer looks like it has been heat damaged or burned. Not sure what I can add here except that both are damaged, both are slowly getting worse, and there is no easy fix other than conservation. Keep them dry and cool until you can have them looked at.

    • #133430

      The best way to prevent the two badges getting worse it to keep them in a polythene box with a good seal, with a polythene bag of silica gel beads (prick small holes in the polythene bag to allow air interchange). Aim to have about 1/10 of the volume of the box filled with the silica gel. Use indicating (colored) silica gel so you can see when it needs changing, to maintain as low a relative humidity level as possible. Make sure the badges are padded with Plastazote (polythene foam) or acid-free tissue so they cannot move around and physically damage each other.
      Do not keep the leather bag with the badges as this may well be making the corrosion worse – it will have kept the humidity around the leather higher and may be exuding fats, acidic vapors and other chemicals which encourage corrosion. The leather itself would suffer if kept in the very dry environment which will help preserve the badges. Keep the bag in separate packaging but please make sure the association between the bag and its badge is kept – record them together in photographs, make sure their records state the association clearly and cross-reference their numbers if they don’t share the same number.
      A conservator with suitable training and experience would be able to minimise the corrosion, clean the surfaces to remove residues which might be causing problems, stabilise the copper alloys present with benzotriazole and coat the badges with a protective layer. As the previous poster has mentioned, the situation is complicated by the possible plating and lacquering, which will influence the types of treatments and materials which can be used. The American Institute for Conservation AIC can help you find a suitable conservator.

    • #133429

      Bronze disease is unlikely. You have common malachite copper corrosion products. It appears from the images (and this is only based on the visual) that at least the “star” badge, and possibly both. are brass plated iron objects. The sherrif’s badge will have some surface damage /plating loss once cleaned. The star appears to have lost much of its plating as the substrate iron base has corroded through the brass.

      The sherrif’s badge can be cleaned (by a conservator) with a very mild metal cleaner (no sulphides or ammonia content). The star will require more conservation attention but will not be “restored”.

      Store both in a neutral container in a climate controlled area. If necessary follow the previous suggestions regarding silica gel. Coating the objects after cleaning with a microcrystalline wax of a lacquer may slow down the reaction, but if stored properly this should not be necessary.

    • #133428
      Laura Furman
      Member

      Thank you to everyone who has chimed in–this helps alot!

    • #133427
      Kory Berrett
      Member

      I wanted to add that the possibilty of a ferrous alloy under the surface of the star badge had also occurred to me – you could try a magnet on this one to confirm if positive. Also, if you can show us a photo of the issue with the leather artifact mentioned in your original question there might be something to say about that one as well.

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