What Causes Copper to Tarnish?

The bright aqua-green color represents the final stage of copper oxidation.
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Copper tarnish occurs in a multi-stage process that first gives the metal a deep red color, then a black hue and finally the sea-green patina made famous by the Statue of Liberty and world landmarks such as the Berlin Cathedral, Vienna's Belvedere Palace and the Parliament Building in Victoria, British Columbia. Copper tarnish may be a feature on architectural structures, but it can be an eyesore on copper sinks and pots, and there are ways to remove it.

However, when you remove the patina, you expose the copper to further tarnishing. Although it may be unsightly to some, the copper patina protects the metal underneath, which is why copper lasts so long outdoors.

Copper Tarnish Isn't Rust

Copper tarnish, like rust, is caused by oxidation, but they aren't the same thing. Rust occurs in metals that contain iron, and the end result of oxidation is iron oxide, which is a flaky compound that falls away, exposing more of the metal to oxidation and degrading it.

None of the compounds produced by copper oxidation are as fragile. They remain on the surface, and while intermediate compounds continue to oxidize to produce the final patina, they protect the metal underneath. Copper corrosion can lead to deterioration, as occurs with copper water pipes, but that's usually because of an exacerbating factor, such as water with high very high or low pH or high levels of dissolved salts or oxygen.

The Stages of Copper Corrosion

In the first stage of the formation of a patina, copper reacts with oxygen in the air to form copper (I) oxide:

(Cu2O): 4Cu + O2 → 2Cu2O

This turns the copper a reddish color. In the second stage, this compound reacts with oxygen to form copper(II) oxide (CuO), which is black:

2Cu2O + O2 → 4CuO

You can observe these reactions for yourself by placing a sheet of copper on a gas stove burner. It first turns red, then black.

The final stage of patination occurs more slowly and usually takes several years. Copper (II) oxide and copper (II) sulfide (CuS), which is another black compound that forms when there is sulfur in the air, react with carbon dioxide (CO2) and hydroxide ions (OH-) from water in the air to form three compounds:

  • Cu2CO3(OH)2
  • Cu3(CO3)2(OH)2 
  • Cu4SO4(OH)6 

These are the compounds that form the patina.

A copper patina develops faster in polluted air that contains a lot of sulfur, which is a byproduct of burning fossil fuels.

How to Clean Copper Tarnish

A copper patina is desirable on a historic building, and some people like the look of tarnished copper sinks and pots, but others prefer the look of new copper. If you're one of these, you need a weak acid that will dissolve the copper compounds without harming the metal. You don't need to look any farther than your kitchen cupboards for a good copper cleaner that isn't toxic, as some commercial copper cleaners are.

Virtually every website that offers advice on cleaning copper recommends vinegar and salt. Sprinkle salt on the metal and rub with a cloth soaked with vinegar and let the reaction between sodium chloride and acetic acid do the work. For hard-to-remove tarnish, fill your copper pot with salt water, add vinegar and bring to a boil.

To prevent tarnish, you need to coat the copper with a film that prevents exposure to the atmosphere. Both copper polish and lacquer are effective for sinks and decorative items, but neither should be used on cooking implements.

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