PSA: Please don't clean your copper/bronze prints with HCl

I’ve mentioned that a great way to clean your copper parts is to put them in HCl for a little while. Strips the oxide down in seconds leaving the bare crystal. Well…

Don’t do that. Just don’t. Any residual chloride will cause a runaway, nearly irreversible chemical reaction with oxygen over weeks/months/years. It’s different from regular patina in that it results in yucky white and green spots that flake off as opposed to providing a protective barrier. This is also known as bronze disease.

As an alternative, we just did some experiments today and citric acid (~0.05 - 1 M) works very well too, though it does take a bit longer and needs heat + agitation. Theoretically, though, the shine should last a lot longer.

I’ll try to remember to post some pictures comparing the two methods a couple of months after washing.

Thanks for the writeup. I started with citric but have been using regular distilled vinegar and then immediately going to cleaning with a wire brush and rotary tool (otherwise it’ll “green” up pretty quick). Last I looked it’s malachite formation, but that was a late night wiki search with minimal effort put in.

I haven’t tried vinegar but I heard it is specifically used for making the copper green up. I could always try to get them to shine by polishing but I’ve been avoiding it because to me the crystals are a part of the charm of the method :slight_smile: I just wish I could find a way to keep them nice and shiny for a long time like that.

The big problem is the surface area. If it’s smooth it’s okay to use hcl I think as long as you wash it off (they do this in cleaning commercial copper equipment all the time). But with the texture as it is some inevitably gets trapped (as I learned when I put a still-acidic whistle in my mouth — actually the exception to the bronze disease thing because I put so much care into washing it after that)

Do you think if you dunked the part in a solution of water & baking soda the hcl would neutralize?

The problem isn’t the acid, but remaining chloride ion. So it should still be there after neutralizing unfortunately.

Ahh. Yeah that’s a bummer…

I have been using Sparex #2. It’s what jewelers use to clean bronze, copper, and silver after oxidation by soldering, casting, etc. Mix a half-strength solution and use it warm but not boiling (I use a small crock-pot for this). 10 to 20 minutes in the solution should clear up all but the most stubborn oxidation (which can be scrubbed with a brush). Neutralize in a baking soda solution and rinse with tap water.

The solution can be stored safely and reused until it is no longer effective. Use only copper tongs in the solution as using iron or steel will make it a copper electroplating solution (not a problem with copper items, but probably not bronze).

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Good to know, thanks for the tip :+1:

Thanks for the tip! Do you have any idea what’s in it by chance?

Sodium bisulfate. Also, I tumble my items with SS shot and burnishing liquid to bring out the shine and burnish the surface so it doesn’t feel so grainy/porous.

That’s really helpful, I’ll have to try it. I rarely polish because I have a pathological desire to keep the crystals visible cause I love them so much :slight_smile:

I actually am ignoring my rule recently because sometimes you just want nice shiny copper, even if it’s temporary.



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