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Why Does Copper Turn Green?


Copper is an intriguing and versatile material that has been used in a wide variety of applications in human history.

But why exactly does copper turn green? In this article, we’ll explore the reason why we see this green color, along with outlining some of the benefits and drawbacks, whether the process is reversible and the most famous examples of its occurrence.

What is copper?

Copper (Cu), is a naturally occurring reddish-brown metal with an atomic number of 29. Copper was one of the first metals that humans discovered, with evidence of usage dating back as far as 10,000 years ago.

This soft, malleable and ductile metal boasts high thermal and electrical conductivity, surpassed only by silver among pure metals. Copper’s unique properties have made it a staple material in various industries and applications throughout history.

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Periodic table: Transition metals

Copper metal belongs to the group of transition metals in the periodic table, which are elements found between Groups 3 and 12. Transition metals are characterized by their multiple oxidation states and ability to form stable compounds with different elements. This unique property enables them to be used in a variety of applications, ranging from electronics to construction materials. Other transition metals include:

  • Iron (Fe)
  • Nickel (Ni)
  • Chromium (Cr)
  • Manganese (Mn)
  • Cobalt (Co)
  • Titanium (Ti)

How long does it take copper to turn green?

Copper turns green due to oxidation, this is commonly referred to as a ‘patina. It can sometimes take several years for the process to occur, as it depends on environmental conditions like temperature, humidity, and the copper’s exposure to air and water; you don’t see it turn green overnight.

If there are pollutants such as sulfur dioxide or chloride ions in the air, the rate at which copper turns green can be significantly accelerated. Typically, it can take anywhere between 5 and 30 years for copper to develop its distinct green patina, although it may begin to appear within a few months under severe environments, with the conditions discussed above.

Benefits of copper oxide (patina)

Copper oxide can in some cases be aesthetically pleasing. Above this, it has the advantage of providing a level of protection to the material. The green layer of patina forms a barrier between the copper underneath and the environment, therefore, preventing the copper from further corrosion and increases durability.

This key benefit is especially useful for outdoor applications such as roofing, where the patina protects against the elements.

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What is oxidization?

Oxidization is a series of chemical reactions that involve the loss of electrons from a substance, ultimately resulting in the formation of a new compound. The oxidization process plays an ever-so-important role in a variety of natural and industrial settings, from the rusting of iron to the combustion of fuel.

In copper’s case, it undergoes a series of oxidation reactions to form copper oxide (Cu20) when it’s exposed to both oxygen and moisture.

Over time copper dioxide interacts with carbon dioxide, sulfur dioxide and other atmospheric gasses to form the distinctive green patina of copper hydroxide, copper sulfate and copper carbonate. Simply put, copper reacts to the elements and forms a distinctive barrier.

Alongside its green patina surface, the copper now has an extra protective layer against additional oxidation and corrosion.

When is copper oxide a drawback?

Of course, like with any chemical reaction, not everything about this process is a positive.

The buildup of copper oxide can reduce copper’s conductivity and compromise the performance of components in electrical applications. Copper oxide is less conductive than pure copper, therefore, resulting in greater electrical resistance, and the potential for overheating or device failure.

Copper oxide can also be problematic in industrial settings where processes require copper with high purity (i.e., semiconductor manufacturing). Copper’s presence can cause contamination and defects in final products, which necessitate rigorous cleaning and polishing processes to ensure the copper’s purity.

Is the process reversible?

The formation of copper oxide can indeed be reversed through both chemical and mechanical processes. In chemical treatments, weak acids like vinegar or lemon juice can dissolve the patina, while a mild abrasive like salt helps to gently scrub away the remaining green layer. This combination effectively restores the original copper color without harming the copper surface.

Mechanical methods, such as sanding or polishing, can also remove the green patina, but they require more caution. Abrasive materials or tools must be used judiciously to prevent scratching or damaging the underlying copper.

In some cases, professional restoration services might be necessary to ensure the best results. Reversing the patina process is achievable, but it’s essential to choose the appropriate method and exercise caution to maintain the integrity of the copper.

Famous examples of copper turning green

One of the most iconic examples of copper oxidation is the Statue of Liberty. Originally a shiny, reddish-brown color, the copper skin of the statue began to turn green soon after its installation in 1886.

Over the years, the green patina has come to symbolize the statue and is now seen as an integral part of its appearance. The patina also protects the underlying copper from further corrosion, ensuring the longevity of this historic monument.

Another famous example of copper turning green is the historic copper-domed roof of the Canadian Parliament Buildings in Ottawa, Ontario. Constructed in the early 20th century, the buildings feature a distinctively green copper dome as a prominent architectural feature.

Over time, the copper used in the construction of the dome has developed a verdigris patina due to the oxidation process. This green patina not only adds a striking visual element to the buildings but also serves as a protective layer, shielding the underlying copper from rust.

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Buy your copper at Metal Supermarkets

The green patina that forms on copper surfaces is a natural phenomenon, with both benefits and drawbacks. Understanding the science behind it not only answers the question of why copper turns green but can also helps us appreciate its practical and aesthetic implications.

Whether you’re looking for copper with a green patina for an artistic project or need high-quality, untreated copper for electrical applications, Metal Supermarkets is your one-stop source.

With a vast selection of copper products and knowledgeable staff, you’ll find everything you need to bring your project to life. Visit your nearest Metal Supermarkets or browse their online store to find the perfect copper solution for your needs.


Metal Supermarkets

Metal Supermarkets is the world’s largest small-quantity metal supplier with 125 brick-and-mortar stores across the US, Canada, and United Kingdom. We are metal experts and have been providing quality customer service and products since 1985.

At Metal Supermarkets, we supply a wide range of metals for a variety of applications. Our stock includes: mild steel, stainless steel, aluminum, tool steel, alloy steel, brass, bronze and copper.

We stock a wide range of shapes including: bars, tubes, sheets, plates and more. And we can cut metal to your exact specifications.

Visit one of our 120+ locations across North America today.

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