Natural Chemical Reaction To Clean Silver

Note: This is purely elemental chemistry, I have no idea if this is accepted within the numismatic community, but if dipping is accepted I should see no reason this would not be acceptable. Use discretion and advice from other more experienced collectors!

Comments PLEASE


When silver objects are exposed to pollutants normally found in air, they can gradually become dull and/or discolored. This darkening of silver is called tarnishing.

It occurs when silver undergoes a chemical reaction, classified as an oxidation reaction, with sulfur-containing substances. These substances may be airborne, but they can also be introduced through use, such as drinking wine from a silver goblet. The silver combines with sulfur and forms silver sulfide.

The reflective silver surface can be recovered in two ways:
1. Removing or dissolving the silver sulfide oxidation.
2. Chemically reducing the silver sulfide.

The first method involves using a commercial product to remove the oxidation but it also removes some of the silver ("dipping"). The second option, to chemically reduce the silver sulfide to another element does not remove any silver, and it actually converts the black silver sulfide to another element.

Base metals have a hierarchy as to the ease of oxidation, and conversely an ease of reduction of oxidation. The following list of metals indicate the level of oxidation, moving left to right is the easiest to the most difficult to oxidize.

Aluminum -> Nickel -> Tin -> Lead -> Copper -> Silver -> Platinum -> Gold

This method will reduce Silver Sulfide to Aluminum Sulfide through a chemical reaction by having the Silver Sulfide react using Aluminum as the reducing agent. This will reduce silver sulfide to elemental silver while forming aluminum sulfide by transferring from silver to aluminum, freeing the silver metal and forming aluminum sulfide.

This type of reaction is called an electrochemical reaction because an electric current flows between the silver and aluminum.

When silver is in contact with aluminum this reaction occurs. Aluminum in pure water would not remove silver tarnish because there is a film of aluminum hydroxide that stops the oxidation reaction. Sodium bicarbonate will remove the surface film of aluminum hydroxide (by dissolving it) to expose fresh metallic aluminum, ready for oxidation.

Using aluminum foil, distilled water and Sodium Bicarbonate this reaction can be induced and accelerated.

Materials

* Gloves and safety glasses
* Tarnished piece of silver (Sl)
* Pan or dish, an aluminum pie plate is good
* Aluminum foil (Al)
* Distilled Water at 16/1 ratio to Sodium bicarbonate
* Sodium bicarbonate, NaHCO3 (baking soda) at 1/16 the volume of water
* Heat Source
* Tongs (plastic or wood)

Note: the waste from this may be washed down the sink.

* Put on gloves and safety glasses
* Place the aluminum foil in the pie plate
* Place silver object on aluminum foil in plate making sure the contact is good
* Boil enough water to cover the object
* Turn off heat
* Add baking soda to water, this will foam so make sure the water is in a large enough container
* Mix solution
* Pour the solution on the object


Almost immediately the tarnish will begin to disappear. If the silver is only lightly tarnished, all of the tarnish will disappear within several minutes. If object is highly tarnished repeat until you are satisfied.

Boiling water is used because the reaction is faster when the solution temperature is higher. The solution carries sulfur released from the silver to the aluminum, where a layer of aluminum sulfide is formed. The aluminum sulfide then hydrolyzes to form aluminum hydroxide and gaseous hydrogen sulfide.