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.
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.