Beginning Level Activity
1. Divide students into groups of three or four. Pass out pennies. Explain that the metal in a penny is an alloy of steel, nickel, and copper. Have students draw their penny and then answer the following questions in their journals, using adjectives to describe the characteristics of the metal in the coin.
What color is the coin?
Is it shiny or dull?
How would you describe the texture of the surface?
Is the metal hard or soft?
2. Have the students place a penny in the container and cover it with water. Then add a few teaspoons of salt to the water. Allow the coin to soak for a week. Have groups observe the coin after the week. They should draw the penny again and answer the questions above again in their journal. Compare these observations to their answers to the questions before they placed the coin in the saltwater. What has changed? What has stayed the same?
3. Display the pre-conservation view of the Victorious Youth. Ask groups to discuss what they see in the image and to make a guess about what they may be looking at. Then explain to students that they are looking at an image of an ancient Greek statue after it had been left in the ocean for over 2,000 years. Share the Background Information about the statue, pointing out that it is made of bronze. Explain to the class that over time the penny would start to look like this statue, because the metal in the penny would have a similar reaction to the salt water that this statue had. Explain that the statue has been affected by Bronze Disease (caused when chlorides and oxygen combine with metal in a damp environment). The disease takes the form of a sudden outbreak of small patches of corrosion and is distinguished by rough, light green spots. In this case, the damp environment was the ocean, and the chlorides and oxygen came from the salt water. Mineral deposits from the water and incrustations left by corals, shells, and other sea animals, caused the rough layer on the surface.
4. Display the post-conservation view of the Victorious Youth. Explain that scientists and conservators were able to stop the corrosion and remove the incrustations by cleaning the object. They used various tools and treatments. Over a three-month period, conservators scraped off the incrustations using mechanical tools to reveal the original surface. They also used several other methods to clean the object, such as extensive washing of the bronze with chemicals that neutralized the corrosion. Finally they placed the statue in a relatively low-humidity environment. As a result, we are now able to get a better sense of what the object looked like originally.
Intermediate Level Activity
Note: The intermediate and advanced activities below assume students have some prior knowledge of the periodic table and have had some exposure to basic chemistry.
5. Explain to students that bronze is a substance usually made of the elements copper, tin, and lead. The statue of the Victorious Youth also had patinas on its surface that contained these elements. Have students identify the elements copper, tin, and lead on the periodic table. Ask students to identify the characteristics of these elements based on their placement in the periodic table. Ask the students to research and record the electrical- and thermal-conductive properties of copper and tin. Explain that the warm temperature of the seawater where the statue was found sped up the corrosion of the metal. However, the combination of the corrosion layer and incrustations of sea life might have formed a protective exterior cocoon that preserved the interior and allowed the statue to last for over 2,000 years. Ask students to identify which of the metals in the statue would be most affected by this heat based on its thermal conductive properties.
6. Explain to the class that the metalscopper and tinin the statue exist as compounds, or alloys. The tin is really tin oxide (SnO2), and copper is really cuprous oxide (CuO2). Do not share the chemical formulas with the class. Instead, students should use a periodic table to identify the various elements and write the chemical formula for each compound. Explain that the two metals are melted and mixed together to form a compound that we call bronze. The bronze is created when the high temperatures release the oxygen. Explain that bronze has its own properties that are distinct from those of copper and tin. Bronze oxidizes very slowly when exposed to air. It is less malleable than copper but has the same conductivity. Overall it has little ductility.
Advanced Level Activity
7. Explain to the class that some of the corrosion of the Victorious Youth was caused by a chemical reaction between the bronze and the oxygen in the salt water. This chemical reaction resulted in an electron transfer otherwise know as oxidation. This means that the metal compound of bronze was combined chemically with the oxygen in the water to create a new compound of an oxide. In this process, electrons are released at the anode (salt water) and taken up at the cathode (statue).
8. Have the class balance the reduction and oxidation reactions, which occurred in the bronze when the statue was submerged in the ocean.
a. Electrochemical corrosion of copper alloy led to the production of cuprous ions. Cuprous ions combine with the chloride in the seawater to form cuprous chloride, a major component of the corrosion layer:
Cu e →
Answer: Cu e → Cu+
Cu+ + Cl→
Answer: Cu+ + Cl → CuCl
b. The statue now contains cuprous chlorides and is recovered and exposed to air. It continues to corrode when cuprous chlorides are combined with moisture and oxygen to hydrolyze and form hydrochloric acid and basic cupric chloride:
4CuCl + 4H2O →
Answer: 4CuCl + 4H2O + O2 → CuCl20 + 3Cu(OH)2 + 2HCl
c. The hydrochloric acid attacks the un-corroded metal to form more cuprous chloride:
2Cu + 2HCl →
Answer: 2Cu + 2HCl → 2CuCl + H2
This process would continue until no metal remained. Conservators had to scrape off the incrustation layers and find a way to stop the corrosion process.
9. Divide the class into working groups of four students. Explain that the conservators could stop the statue from corroding further through the following methods: electrolytic method, electrochemical method, sodium-sesquicarbonate method, and sodium-sesquicarbonate/vacuum method. Students can explore these treatments in detail by doing research on the Methods of Conserving Archaeological Material from Underwater Sites Web site. You may wish to note that the methods mentioned above are used to enhance salt extraction in archaeological metals, and if the salt extraction is not completed, the reaction outlined in number 8 would still occur. Conservators typically use benzotriazole or silver oxide in spot methods to treat local eruptions of bronze disease. Silver oxide seals areas of corrosion and prevents moisture.
10. Have groups come up with a combination of the methods to conserve the bronze statue. Students should record findings in their journals. Groups will share their findings with the class. Explain that the Getty conservators decided to use the sodium-sesquicarbonate/vacuum method to preserve the artwork. This method guaranteed that all unstable elements would regain stability without any loss to the original bronze and artistic qualities of the statues surface. In addition, they now keep the statue in a humidityand temperaturecontrolled room to keep the statue from corroding further. Compare the students recommendations to the actual treatment.