Metals can be listed according to their reactivity. Some, like sodium, are so reactive that they explode on contact with water. Some, like gold, are so unreactive that they appear to be unchanged after being buried for thousands of years. Copper is low on this scale. It will react, but slowly. This low reactivity, combined with its malleability makes it ideal for use in architecture. It can be cut and hammered into ornate shapes and it will protect itself with a pale green coating of copper salts called a patina. Note: this is not to be confused with verdigris, the name given to the green finish obtained by treatment with acetic acid to form copper acetate. Verdigris is soluble and not as durable as natural patinas.
These are the two telescope domes of the Royal Observatory Edinburgh. The picture was taken before the restoration in 2010. The copper cladding has a layer of green “patina”, that has built up since the domes were built in 1894. The patina is a combination of copper salts that are unreactive and protect the copper from further corrosion.
The domes are quite lightweight structures that rotate on a circular rail to allow the telescopes to point in any direction through a panel that slides open. They are also exposed to high winds.
It was not corrosion of the copper that was the problem. The thin dome structures had been shaken and buffeted for over a century and were simply falling apart.
This famous statue has hardly corroded at all. It is almost exactly the same age as the observatory. The copper sheets visible in the picture are only 2.5 mm thick and since 1886 they have only lost 0.1 mm in thickness.
The 3000 m2 of copper sheet on the Copper Box in London’s Olympic Park is pre-oxidised in the copper factory. The chocolate brown film of copper oxide advances the patination process and provides architects with a different colour option to the bright new copper. Eventually a film of green copper salts will appear on top of the oxide layer. This will take up to twenty years or more.
The top of the July Column in the Place de la Bastille in Paris. The monument is made from bronze (copper and tin), but the bronze figure at the top (Spirit of Freedom) is gilded with gold foil.
The Chemistry of Patination
The first step in the development of a patina is oxidation to form copper (I) oxide (Cu2O), which has a red or pink colour (equation 1), when copper atoms initially react with oxygen molecules in the air. Copper (I) oxide is further oxidized to copper (II) oxide (CuO), which is black in color (equation 2). If air is polluted with sulphur, for example from the burning of fossil fuels, then black copper (II) sulphide (CuS) also forms (Equation 3).
Equation 1 4Cu + O2 → 2Cu2O
Equation 2 2Cu2O + O2 → 4CuO
Equation 3 Cu + S → CuS
Over the years, CuO and CuS slowly reacts with carbon dioxide (CO2) and hydroxide ions (OH-) in water from the air to eventually form Cu2CO3(OH)2 (equation 4) , Cu3(CO3)2(OH)2 (equation 5) and Cu4SO4(OH)6 (equation 6), which constitute the patina. The extent of humidity and the level of sulhur-related air pollution have a significant impact on how fast the patina develops, as well as the relative ratio of the three components.
Equation 4 2CuO + CO2 + H2O → Cu2CO3(OH)2
Equation 5 3CuO + 2CO2 + H2O → Cu3(CO3)2(OH)2
Equation 6 4CuO + SO3 + 3H2O → Cu4SO4(OH)6
The reactivity series lists metals in order of their reactivity. The top five are extremely dangerous. The wedge shapes indicate how reactive they are. Copper is the only metal apart from precious metals that will not react with water or dilute acids. It will react very slowly with oxygen.
Questions and Activities
1. Which metals will not react with oxygen or water to form patinas?
2. Is gold ever used on the outside of buildings?
3. Find out which is the most reactive metal that can be used for roofing material.
4. On buildings close to the sea, the patina contains copper chloride as well as copper carbonate and copper sulphate. What causes this?