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While working at CNC engraving, I had at my disposal a small bottle of liquid used to stain brass for creating signs etc - engrave a writing, stain the engraving, then grind/polish the flat topmost layer to remove the colored layer and leave it only in engraved depressions.

The bottle didn't reveal much about composition of the liquid, beyond some warnings about toxicity. It was a brass staining liquid, period.

The coloration it created was dark brown, that seemed black by comparison with the surrounding metal. The reaction took only a few seconds until full pitch of coloration was achieved and the liquid itself was transparent with very mild cyan-blue hue. The coloration was quite persistent; even roughly 10 years afterwards it's still good as new on the pieces I kept and required rather deep brushing to remove from the topmost surfaces.

Can you tell me what substance it was?

(below a sample with dark parts achieved through that liquid) sample

Edit: I found a very similar liquid on sale, but the page is equally devoid of composition details. The instructions say: [the liquid] is to be used in room temperature. It allows the finish in tones between black and light brown. The time of reaction is almost immediate. The color and speed can be controlled through dilution in water up to 50%. the liquid

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  • $\begingroup$ Could you also tell us on which metals this is used? $\endgroup$
    – Martin - マーチン ♦
    Dec 9, 2014 at 10:22
  • $\begingroup$ @Martin: The label reads "for Brass and Copper". $\endgroup$
    – SF.
    Dec 9, 2014 at 10:24

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This site regarding patina forming solutions might help: http://www.sciencecompany.com/-W160.aspx#1

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    $\begingroup$ Due to link rot, it makes your answer better if you can make some general statements or summary comments, even if the page linked to is a full and detailed answer itself. $\endgroup$
    – Aesin
    Sep 24, 2013 at 19:04
  • $\begingroup$ It doesn't seem to be any of these. It was a cold process on cold metal, and the solution wasn't light-sensitive (plain transparent flask), didn't have any sediment and was kept in liquid form with years of shelf life, so the only cold "to brown" process on the list doesn't match - the rest require the solution to be hot, or produce other colors. $\endgroup$
    – SF.
    Oct 2, 2013 at 8:01
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Sulfurated potash (which is on that list under brown to black) does come in a gel form, it could be that.

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  • $\begingroup$ It was definitely not gel- it behaved completely like water, and it had pretty much indefinite or at least very long shelf life. Also, the color doesn't seem to match (I've updated the question with a photo of such a liquid.) $\endgroup$
    – SF.
    Dec 9, 2014 at 9:42
  • $\begingroup$ The colour could be a colouring agent rather than the natural colour of the active ingredient. $\endgroup$
    – David Richerby
    Nov 14, 2016 at 9:52

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