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I have an object made of what I assume to be brass, i.e. an alloy of copper and zinc.

The curved parts have both a black layer and a white layer (also note the verdigris (I guess) on the left top, but this is the only spot on the whole piece) : enter image description here

If I understand correctly, the black layer is tarnish, or patina, and due to oxidation from sulfur dioxide in the air.

But I can't find reliable reports of white corrosion on brass. Could it be wet storage stain, a.k.a. "white rust" ?

Hints on how to remove it are welcome.

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  • $\begingroup$ As brass contains zinc and as most of zinc compounds are rather white or "colourless", it could be some zinc oxide, hydrated oxide, basic carbonate.. like that. Or some external dust ,or abrasive from past cleaning. $\endgroup$
    – Poutnik
    Commented Nov 14, 2020 at 19:08
  • $\begingroup$ The "white rust" in your reference refers to zinc or Zamak /die cast alloys; nothing to do with brass. I think the white ,black coatings are more related to paint or dirt. $\endgroup$
    – blacksmith37
    Commented Nov 14, 2020 at 22:37

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Judging from the location and colour, it is most likely residue from liquid polish that was not properly rubbed/polished off. Quite a few types of liquid polish for brass turn white when they dry, and any folds and creases in the design is where they would be most likely to accumulate if the person doing the polishing is not very thorough.

Depending on how long its been there, you may be able to shift it with just warm water and a soft brush or cloth, leaving the patina mostly in place. Otherwise you may need to use brass polish and polish the whole thing.

You could try asking advice from some of the better antique dealers or ideally someone who specialises in conserving antique brasses if the piece is a valuable antique.

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  • $\begingroup$ I just noticed a flat spot of the same white depot on a (non-metal) mineral part, so you must be right about it being some residuals of a cleaning product. And it's indeed surprisingly easy to remove, a damp cloth seems to be enough. $\endgroup$
    – Skippy le Grand Gourou
    Commented Nov 15, 2020 at 20:58

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