Metals for Special Uses

Step 2 - Activities

Activity description
The activity consists in testing and justifying different cleaning methods used to restore shine to two well known metals: silver and copper.

Materials

• a silver jewel or a small silver object that has to be cleaned
• 3 copper objects that have to be cleaned (also copper coins work well)
• 2 plastic cups
• vinegar
• lemon juice
• table salt
• ketchup sauce
• sodium hydroxide (caustic soda, available at the supermarket)
• aluminum foils
• a small plastic basin

Warnings:

• all materials are safe except caustic soda and its solution, that have to be handled with gloves;
• the activity has to be carried out far from flames or heat

Procedure

a. Salt and lemon to clean copper

1. Place 2 tablespoons of table salt in the plastic cup.
2. Add lemon juice till the level of the salt.
3. Stir for a while (without dissolving salt) and immediately rub the first copper object with the mixture.
4. Finally, rinse with water and dry with a soft cloth.

b Salt and vinegar to clean copper

1. Place 2 tablespoons of table salt in the plastic cup.
2. Add vinegar till the level of the salt.
3. Stir for a while (without dissolving salt) and immediately rub the second copper object with the mixture.
4. Finally, rinse with water and dry with a soft cloth.

c. Ketchup sauce to clean copper

1. Place a layer of ketchup on the surface of the third copper object.
2. Wait for half hour.
3. Finally, rinse with water and dry with a soft cloth.

d. Caustic soda and aluminum to clean silver

1. Completely cover the plastic basin with aluminum foils.
2. Place the silver object inside the basin.
3. Prepare a basic solution by dissolving 150 g of caustic soda in 2 L of water (distilled water works well).
4. Pour the solution into the basin till covering the silver object.
5. Wait for few minutes (no more than 10 minutes). During this time you will observe an effervescence (gas formation).
6. Finally, rinse with water and dry with a soft cloth.

Conclusions and teaching notes
Copper surface becomes matt and darker because of the formation of copper oxides, while the formation of copper carbonate is responsible for green patches. Restoring shine to copper is simple because its compounds are sensible to acid action (meaning that they dissolve in acid solutions). Many commercial products are efficacious, but the traditional method, that uses table salt in combination with lemon or vinegar, works really well too: both lemon and vinegar are acid substances, while salt is the abrasive component of the cleaning mixture. Ketchup sauce contains tomato, that is acid, and vinegar: its acidity dissolves the side copper compounds and its density allows it to remain on the object surface, thereby reducing labour and use of abrasive substances.

The black patina that forms on silver surface is silver sulfide. This compound can be converted to neat silver again by a chemical reaction: the reaction occurs between silver sulfide and aluminum, in basic conditions (in our case the basic conditions are due to caustic soda). The reaction also produces hydrogen gas, responsible for the effervescence. Hydrogen is a flammable gas, thus, even it is produced in low quantity as in this case, it is better to carry out the activity far from flames and heat.