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I am trying to change the color of the coins we have in Europe. I want to create patterns of certain colors. They are made from "nordic gold" (89% copper, 5% aluminum, 5% zinc, and 1% tin).

I put them into a kitchen oven at about 250°C and noticed that they tend to change color. The colors were irregular and uncontrolled. See the picture.

after oxidizing in a kitchen oven

I tried oxidizing it in a controlled way using electrolysis, like this guy on youtube. I used a two electrode setup with a soda solution as an electrolyte. The coin was my plus and had voltages up to 30V applied to it with no color change. It only started producing bubbles (probably hydrogen and oxygen). Low Voltages and low currents don't help as well. I scratched them with sandpaper and cleaned them with acetone before putting them into the electrolyte.

Do you know or have an idea on how to oxidize the coins in a controlled way?

EDIT: I couldn't believe that electrolysis had no effect, so I did it again; this time with salt-water as the electrolyte. at small voltages it just tarnished. At high voltages it becomes white, yellow, green and flakes are sticking to it when in solution. after electrolysis

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    $\begingroup$ Bronze will not anodize like aluminum ; aluminum forms a protective oxide layer , thicker with anodizing. I worked in corrosion and never saw a reference to anodizing building a protective oxide on copper. It is probably removing surface corrosion to leave a clean copper or gold color. I expect the aluminum is added for the strong hardening affect , not a color , The alloy is a first step toward a Manganese Bronze. $\endgroup$ Jun 26 at 16:44
  • $\begingroup$ @blacksmith37 I haven't thought about that I might be corroding the aluminum out of the solid solution! That explains why it became so white. I removed the coating and it had a strong copper color. So for the aluminum to stay inside or the copper just changing to a copper-oxide I suppose heating is my only option, as electrolysis seems to be too aggressive. I found an article on copper thin film oxidation: microlabgallery.com/gallery/InterferenceFilmsonCopper3.pdf It doesn't need to be the nordic gold, but it happens to conveniently have the right shape. I will try copper. $\endgroup$
    – RIJIK
    Jun 26 at 20:32
  • $\begingroup$ Copper only tarnished $\endgroup$
    – RIJIK
    Jun 27 at 5:41
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    $\begingroup$ Your reference is about very thin oxide film such as a few molecules of copper oxide. Much more commonly seen on stainless steels . For example look at stainless exhaust pipes on motorcycles ; gold at the cool end, to pink , to blur/purple at the hot end . A matter of the differencr of oxide layer a few molecules thick. And carbon steel , thin is gold ( tempered at 300 F) to thick is blue ( tempered at 700 F) ; (all in round numbers ). $\endgroup$ Jun 27 at 23:53
  • $\begingroup$ Dealloying does occur under special circumstances but I doubt very much that you have extracted aluminum from the bronze. $\endgroup$ Jun 27 at 23:55

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