In this experiment, students deliberately corrode copper, to produce a pigment called verdigris. Students can then explain their observations using their knowledge of redox reactions.

Student Sheet

In this practical I will be:

  • Learning about how verdigris copper is produced, using key terms such as alloys, redoxreactions, oxidationstates, corroded and verdigris.
  • Observing and analysing differences between the two groups of copper coins, when exposed to different experimental conditions. 
  • Identifying whether the changes seen are because of a chemical or physical change, using my scientific understanding to explain my prediction. 

Introduction:

Like nearly all other ancient Greek science-artists, you have seen and heard how corroded copper, and its alloys, are an interesting shade of green. Some artists have even started using this verdigris copper as the pigment in their paints. However, you have noticed that there are definite differences in colour between the paint pigments, despite being created from the same material.

You wonder if this is due to the conditions the copper alloys are exposed to. Like all good science-artists, you decide to investigate further…

Wear eye protection.

Equipment:

  • 2 beakers (250 cm3) 
  • 100 cm3 measuring cylinder
  • Sodium chloride (24g 12g)
  • Spatula
  • Balance
  • Weighing boat or galipot
  • Stirring rod
  • 1 M ethanoic acid or vinegar (100 cm3)
  • 1 M citric acid (100 cm3)
  • 6 new copper coins
  • Paper towels
  • Stopclock
  • Thermometer
  • Pen

Method:

  1. In a 250 cm3 beaker, mix 100 cm3 of ethanoic acid with 12g sodium chloride. Stir until the sodium chloride dissolves. 

Put 6 copper coins in the sodium chloride / ethanoic acid mixture.

  1. Leave for around 10 minutes. Use the stopclock to time the process.
  • Why do you think the coins need to be left for a length of time?
  • Are there any visible signs of reaction e.g. colour change, heat from the reaction (you might need to use a thermometer) etc.?
  1. Pour away the liquid, and then carefully tip the 6 coins onto a paper towel.
  2. Take three of the coins, wash them in clean water and dry them.
  3. Put these coins on another paper towel and mark them as A.
  4. Take the paper towel with the other three coins on and mark this B.
  5. Leave them for around 1–2 hours or left in a safe place for next lesson.
  • Why are the coins separated into two different groups?
  • Describe the appearance of the copper coins.
  • Is this a chemical or physical reaction?
  • Explain why you think that.

Going further:

Instead of ethanoic acid try using citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid).

  • Is the resulting colour change different with this acid?

Theory:

The vinegar mixture causes a chemical reaction between the copper and the air known as a redoxreaction. This is what a reaction is called when atoms change their oxidation state.

A form of copper oxide had formed on the penny, and the copper oxide looks green.

Copper domes and copper covered roofs are often green in colour because chemicals in the air from the burning of fossil fuels react in different ways with the copper. 

In the natural process there are formed a number of different verdigris such as:

  • pale green nantokite, CuCl
  • vitreous green atacamite, Cu2(OH)3Cl;
  • pale green paratacamite, Cu2(OH)3Cl;
  • pale bluish-green botallackite, Cu2(OH)3Cl;
  • pale green clinoatacamite, Cu2(OH)3Cl;
  • light green anarakite, (CuZn2)2(OH)3Cl.

In the mineral world the most common verdigris is nantokite (copper (1) chloride, CuCl) named after the mines in Chile where it was first identified.

 

2 Cu(s)  +  2H+(aq)  +  Cl−(aq)  →  2CuCl(s)  +  H2(g)
           

copper(I) chloride

(nantokite)

    


In a wet environment the copper (I) chloride crust reacts with the water to produce copper(I) oxide (Cu2O) and hydrochloric acid. The hydrochloric acid produced will go on to react with more copper to produce additional nantokite. 

CuCl(s)  +   H2O(l)  →  Cu2O(s)  +  2HCl(aq)

copper (I) chloride

(nantokite)

  +  water   →   copper(I) oxide  +  hydrochloric acid 

 

2HCl(aq)  +  Cu(s)  →  2CuCl(s)  +  H2(g)
hydrochloric acid  +  Copper  → 

copper (I) chloride

(nantokite)

  +  hydrogen

But if water and oxygen are both present in quantity then both copper(I) chloride (CuCl) and copper(I) oxide (Cu2O) will react to produce copper trihydroxychloride, which varies from emerald green to pale green to pale blue-green depending on the crystal structure.

The most common copper trihydroxychloride is atacamite named after the Atacama Desert in Chile. Its colour varies from blackish to emerald green.
Clinoatacamite is a copper trihydroxychloride that is pale green.

Botallackite is the least stable of the four copper trihydroxychloride structures and is pale bluish-green in colour. It was first found and identified in a mine in Cornwall, England and is a rare corrosion product on archaeological finds.

One other verdigris copper salt is the light green anarakite (CuZn2)2(OH)3Cl) named after the Anarak province in Iran. It contains zinc because many bronzes used in outdoor statues have been forged with some zinc.

The Greeks and Romans made verdigris by corroding copper. They mixed verdigris with oils to make green paints.

Teacher and Technician Sheet

In this practical students will:

  • Learn how verdigris copper is produced, using key terms such as alloys, redox reactions, oxidationstates, corroded and verdigris.
  • Observe and analyse differences between copper coins which have been exposed to different experimental conditions. 
  • Identify whether the changes seen are because of a chemical or physical change, using their scientific understanding to explain their prediction. 

Introduction for teachers:

(This topic could start with a group discussion about metals and the way they change when they corrode. During which the teachers introduce the following ideas, especially the words in bold.)

Copper and copper alloys like bronze will corrode with long exposure to the air. When it corrodes the copper takes on a blue-green colour. Corrosion is the oxidation of copper; a chemical reaction between copper and oxygen in which copper oxide can be produced. 

The Greeks and Romans deliberately corroded copper to make a pigment called verdigris. Pliny the Elder (23-79 CE) recorded a recipe for the making of verdigris in his Natural History. 

Because verdigris is made through a natural process and the environmental conditions can vary, the verdigris salts will have different hues of green and bluish green. These colours can be due to different chemical composition and crystalline structure.

(This investigation can be carried out by pairs of students and they could be asked to prepare an article for a science magazine on the production of verdigris.)

Curriculum range:

This is a good practical for upper secondary students. Younger pupils can carry out the experiment for experience, but the chemistry could be too difficult for the majority of the younger pupils. The aim is to gain some understanding of the way artists used scientific methods to make pigments. It links with:

  • setting up simple practical enquiries, comparative and fair tests;
  • reporting on findings from enquiries and observations, including oral and written explanations, displays or presentations of results and conclusions;
  • using straightforward scientific evidence to answer questions or to support their findings;
  • building a more systematic understanding of the chemistry of paint by exploring the way coloured materials can be used to make a paint;
  • asking questions and developing a line of enquiry based on observations of the real world, alongside prior knowledge and experience;
  • using appropriate techniques, apparatus, and materials during laboratory work, paying attention to health and safety;
  • making and recording observations using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements;
  • presenting observations using appropriate methods;
  • interpreting observations and identifying patterns using those observations to draw conclusions;
  • presenting reasoned explanations, including explaining data in relation to predictions and hypotheses; and
  • learning about the concept of oxidation.

Hazard warnings:

Ethanoic acid is an acid and so it is advisable to take care when handling it. (Low hazard at 1 M. IRRITANT at 2 M it is similar to household vinegar at this molarity.)

Plastic gloves and safety glasses would be advised. If spilt wash away with plenty of water.

Equipment:

  • 2 beakers (250 cm3) 
  • Sodium chloride (24 g)
  • Spatula 
  • Weighing boat or gallipot
  • Balance
  • Stirring rod
  • 1 M ethanoic acid or white wine vinegar (100 cm3)
  • 1 M citric acid (100cm3) IRRITANT
  • 1 measuring cylinder (100 cm3)
  • 6 new copper coins
  • Paper towels
  • Stop clock
  • Pen

Technical notes:

The coins need to have a high quantity of copper in them so if copper based coins are not available use disks of copper. (Use the newest coins available to give a good comparison in the results). Since the modern British coins have a reduced copper content if you can get them then old pre-decimal coinage would be good.)

Results:

The coins that have been rinsed off in water and dried (A) look shiny, not green.
The unwashed coins (B) will start to turn green.

Going further:

Instead of ethanoic acid try using citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid). This should make the citrate salt and this will have a different colour green.

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