A Scientific Question - What Are The Compounds Formed When A Copper Coin Develops A Blue Patina?

There are many blue/green copper salts, but that may not accurately describe blue toning on a predominantly copper coin. Examples: copper sulfide, CuSO4; copper chloride, CuCl2; copper nitrate, Cu(NO3)2; and many more.

The chemistry of copper interaction with the environment is complex. Part of the complexity is found in the fact that copper has two readily formed oxidation states: copper(I) and copper(II). These states can have different chemical reactions, and the resultant compounds can vary greatly in colour. For example, copper(II) oxide is black, but copper(I) oxide is dark red; the combination of both copper(I) oxide Cu2O and copper(II) oxide CuO on the surface of an oxidized circulated copper or bronze coin creates an overall brownish appearance, but a certain chemical environment can push the balance towards either (I) or (II), creating either a reddish or blackish coin. Green and blue, however, come from the copper reacting with other things normally found in the environment. Specifically, carbon dioxide, and water.

In the case of environmental corrosion of copper, let's consider the bright green coating that usually forms on copper-clad roofs of buildings - as is visible in the Canadian parliament buildings and perhaps most famously on the Statue of Liberty (which was a bright copper colour when first made but was designed and intended to rapidly turn green). As these copper objects are sitting outside in the weather, they are exposed to constant rainwater, so by definition these copper compounds cannot be water-soluble; copper(II) sulfate, copper(II) chloride, and copper(II) nitrate are all a rich blue or green colour but are all readily water-soluble so do not form a major component.

Instead, the copper compounds are typically a mixture of copper carbonates (either I or II), copper sesquicarbonate (an equal mixture of both I and II), and copper(II) hydroxide. There will be copper oxides and copper sulfides in the mix as well, but these are either black or red so they do not contribute to the green-ness.

Copper(II) carbonate CuCO3 forms when copper reacts with carbon dioxide and oxygen, but is very water-sensitive; when it gets wet it spontaneously reacts to form azurite: Cu3(CO3)2(OH)2, which is deep dark blue in colour. Over time, this deep blue fades as some of the carbonate ions are replaced with hydroxide ions, forming another copper carbonate hydroxide Cu2(CO3)(OH)2.

Another key component of patina is copper(I) chloride, which is white and water-insoluble; how much this contributes to the overall composition of the patina depends largely on the object's proximity to salt water. Being right by the ocean, the Statue of Liberty probably has a lot more copper(I) chloride in it than the buildings in Ottawa, and this would also explain why the Statue of Liberty is subjectively more of a pale green, compared to Ottawa's more vibrant green.

Of more significance to coin collectors, copper(I) chloride is the key causal agent of what we call bronze disease, as the chloride ions tend to autocatalyse the formation of further copper by-products. The chemical reactions of bronze disease are covered in the Wikipedia article on the subject.

Further reactions can occur slowly over time, as for example a copper coin buried in soil for 2000 years. The deep dark green colouration so beloved by ancient coin collectors is typically made from malachite, or crystalline copper(II) carbonate hydroxide - Cu2CO3(OH)2. Malachite is, of course, a naturally occurring copper ore which was mined and refined in ancient times - meaning that the coin is essentially on it's way to turning back into the copper ore from whence it came.

Adding to the complexity of copper chemistry is, well, complexation - the ability of certain metals (including copper) to undergo additional chemical reactions beyond the simple acid-base reactions to form compounds known as complexes. Most copper complexes are deep vivid blue or green in colour; you don't need a lot of them present to impact the appearance of the overall surface.

One thing to note: we often call this mixture of greenish copper compounds "verdigris", but do not confuse this naturally occurring verdigris with the mediaeval green paint pigment also known as "verdigris", which is composed of copper(II) acetate and was manufactured by exposing sheets of copper to vinegar (acetic acid) fumes. Natural verdigris contains no copper acetate as there isn't enough acetic acid in the atmosphere to make much of a difference.

wow! that is GREAT explanation of coper compounds and oxidation.

Good stuff.