Recently my mother gave me the endowment she was given by her grandmother. It is not much because most was lost during the bombing of Dresden in 1945. A single porcelaine soup tureen from Meissen and a set of silverware survived. The silverware was rusty and tarnished, in this instructable I want to share the process that I used to clean it.
Step 1: Why Does Silver Tarnish?
Silver(Ag) was considered a noble metal by alchemists because it doesn't oxidise with air and moist. What they didn't know was that it reacts with sulfur(S). If they had, then the technology of photography had been found earlier. Apparently they also didn't eat cooked eggs with silver spoons because then the alchemists had seen how sulfur from ovalbumin could tarnish the silver.
Hydrogen(H) sulfide is a trace element in our air and creates a stable connection between the sulfur and silver atoms:
2Ag + H2S <=> Ag2S + 2H+ + 2e-
4Ag + 2H2S + O2 <=> 2Ag2S + 2H2O
The electrode potential of silver decreases from 0.8V to 0.1V and the surface becomes shabby. To reverse this low-energy state and reinstate a clean surface for our silverware, we need to invest energy into the silver.
Step 2: What Can We Do?
Whiting could clean the silver without energy, but this scratches the surface or removes a tiny part of the silver. We could do this with heat if the silverware was made purely of silver. If it was of lower quality (800) then enclosed copper oxidised and coloured it black again. A very good option is galvanising but most silver immersion baths contain thiourea which is teratogenic and carcinogenic. Leave this to experts, especially the waste disposal. Another reason is this procedure enables tarnishing afterwards much easier.
There is a cheaper and safer version of galvanising, with aluminium foil and an acid solution. The aluminium(Al) corrodes and delivers electrons for the corroded silver:
Al => Al3+ + 3e-
3Ag+ + 3 e- => 3Ag
The acid solution generates hydrogen on the surface of the silver:
2H+ + 2e- => 2[H]
Therefor we can redox the silver sulfide (tarnish) back to silver and hydrogen sulfide:
Ag2S + 2[H] => 2Ag + H2S
Of course silver oxide (crust) is reduced to silver and hydrogen oxide (yes, water):
Ag2O + 2[H] => 2Ag + H2O
The complete molecular formula looks like this:
3Ag2S + 2Al + 6H2O => 6Ag + 2Al(OH)3 + 3H2S
Step 3: Rust Removal
Actually silver is too soft to make cutlery of it, knife blades need a steel core for stabilisation. If the thin silver is gone, then the steel can rust its way to the surface. A soft acid like citric acid removes most of the rust, it creates a coordination complex with iron and releases carbon monoxide, water and hydrogen.
Step 4: Citric Acid
The best results can be achieved if the citric acid is heated to 80°C (176°F), use a glove if you do so. Spread some citric acid on a paper towel and rub the rust from the blade. The result is a shiny blade and you can already see the silver coating is gone in some places.
Step 5: Preparation
You need a plastic bowl, aluminium foil, boiling water, salt and baking soda/ sodium bicarbonate. Place the foil into the bowl and put the kettle on. The bowl should be outside because as I already explained this process will release hydrogen sulfide. It will smell like rotten eggs. You don't want this in your kitchen.
Step 6: Salt the Rust
Put the cutlery with the dull side on the aluminium foil. Although most of the rust was removed with citric acid and then rinsed with water, some can only bowdlerized with hot water and salt. Sprinkle salt on the rusty parts, sprinkle baking soda on the silver parts and add the boiling water. It should be around 6 tablespoons of salt and 1 teaspoon of sodium bicarbonate per liter of water.
Step 7: Rotten Eggs
The smell is pretty bad but it can take up to half an hour. Then take the silverware out and dry it with a dishtowel.
Step 8: Silver Care Afterwards
There are products for conservation of silver objects which slow down tarnishing because they absorb moisture and gases like a sponge. A piece of chalk next to the silverware can also absorb moisture and prevent tarnishing. Flannel is said to absorb sulfur and there are pretty flatware rolls made especially for silverware.
Step 9: Result:
In the pictures above you can see a side by side comparison of the final result and the initial condition. The spoons and forks are now in perfect condition. The knife blades have partially lost their silver coating of the steel cores and I will have to bring them to a specialist.
I hope this instructable was helpful and comprehensible. Enjoy your cutlery looking "as new".