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Author Topic: How did I turn my pennies red?  (Read 963 times)

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someguy17

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How did I turn my pennies red?
« on: January 10, 2017, 10:22:30 AM »

I realize this was probably dangerous, especially as someone who doesn't know what he's doing. I did this in the open air with a particle mask and I kept my distance, added things slowly.

But I want to know a little bit of the science about what happened to my pennies. I had a hand full of pennies, many were nasty, there were a couple newish ones. As per internet advice, I put them in a solution of salt and apple-vinegar, because I didn't have regular vinegar, in an effort to clean them and it worked real well on some. It was so fast I was a little excited about it, so I took it out side and added bleach. Most of them developed this coat of dark red, which I could scrub off with a wire brush. Also, the salt and vinegar, which looked like really dark urine, turned bright blue green when I added the bleach.
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Babcock_Hall

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Re: How did I turn my pennies red?
« Reply #1 on: January 10, 2017, 11:52:11 AM »

This sort of chemistry is well outside my comfort zone, but using bleach strikes me as exactly what one would not want to do.  It's an oxidant, and it could potentially oxidize the copper or zinc in the penny.
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Enthalpy

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Re: How did I turn my pennies red?
« Reply #2 on: January 12, 2017, 12:37:59 PM »

Be careful with bleach. It's the No.1 cause of accidents at home with chemicals. Do not mix it with other compounds, especially not in a room, unless you're a chemist.

Blue and green can result from copper ions.
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Arkcon

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Re: How did I turn my pennies red?
« Reply #3 on: January 13, 2017, 01:21:08 AM »

As a kid, I used to do the "salt as abrasive, vinegar as an acid cleaning of pennies" trick.  The blackened copper pennies were no longer black, and it was cool.  However, they didn't always become shiny copper, but often turned dull red.  So, some other oxidation occurred,  modern US cent coins are only copper plate over zinc,so the chemistry rapidly becomes worse.  You can check wikipedia for the various copper compounds,if you want to know more.
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Intanjir

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Re: How did I turn my pennies red?
« Reply #4 on: January 13, 2017, 08:38:09 AM »

If a pure copper surface isn't smooth then it can look a dull pinkish-red. It is pretty easy to get this sort of surface finish when electroplating.

This page has a nice step-by-step of about how to electroplate copper on ABS and goes through one of the most reddish pure copper finishes that I have seen:
http://bryancera.blogspot.com/2014/09/copper-electroplatingforming-3d-prints.html


This probably isn't what is happening here since you added bleach and made conditions more oxidizing than normal. So I would expect a layer of oxidation and not just a microscopically pock-marked surface of pure copper.
Copper's two oxides are red and black, and this is very consistent with a dark red coating that could be easy scraped off.

This is a bit unusual though since oxidizing copper in an aqueous environment tends to yield bluish-greenish copper compounds like hydroxides or chlorides or in this case acetates.
It is possible that it was just that the conditions were right for the oxide to form instead of any of these, otherwise I am at a loss for what it could be.
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