US Coins

Watch what you're doing

In my last column (Dec. 30, 2013, issue of Coin World), I reported that recent conservation research showed that using silver dips to remove toning (i.e., silver sulfide) increases the likelihood of recorroding in the future. Based on their results, I cannot recommend their use. This month, I will review the alternative cleaning techniques.

Before you reach for a cleaning product, lightly wipe the surface of the coin with a soft, clean cloth or makeup sponge. If the layer of silver sulfide is in its early stages, it can be removed this way.

This technique works best with coins with a smooth surface as the toning is less likely to adhere to a smooth surface than to a rough one.

Galvanic cleaning

Galvanic cleaning entails immersing a coin and a piece of aluminum foil in a dilute solution of baking soda (sodium bicarbonate) or washing soda (sodium carbonate) in water. This technique works by setting up a galvanic cell where the aluminum, being the less noble, is the anode and the silver coin the cathode.

The solution of baking or washing soda provides the electrolyte allowing the two metals to interact.

During the reaction, the aluminum foil is corroded and a byproduct is hydrogen gas. The hydrogen gas reacts with the silver sulfide to chemically reduce it back to silver. It is crucial that the silver and the aluminum be in constant contact throughout the treatment. If not, the galvanic action could act to preferentially corrode the copper in the coin’s silver alloy.

Commercial silver cleaning products that come with an “electrolytic cleaning plate” exploit exactly the same principals. The metal plates provided are usually zinc or a zinc alloy and work exactly the same as aluminum foil and baking or washing soda in water.

The primary downside to this technique is aesthetic. The conversion of silver sulfide to silver produces a form of silver that is referred to as “mossy” silver.

Mossy silver is rough and spongy, tends to tarnish rapidly and looks dull and lackluster. Many who clean silver using this technique to polish the silver with an abrasive technique to improve the look.

Abrasive cleaning techniques

Abrasive cleaners include cleaning cloths, pastes and polishes. The abrasives used in commercial cleaning products include quartz, alumina, titanium dioxide, hematite, calcium carbonate, clay and diatomaceous earth.

Cleaning cloths are usually a cotton flannel impregnated with one or more of these materials.

Pastes and polishes are made up of abrasive materials, detergents and wetting agents.

Abrasive cleaners work by abrading the tarnish from the surface of a coin leaving tiny microscopic scratches on the surface of the metal.

These scratches can affect the luster and eye appeal of a coin.

Whether or not a polished surface is considered acceptable or not depends on the particle size of the abrasive and basic physics.

Larger particles leave larger scratches than finer abrasives. If the distance between the scratches is less than the wavelength of light and no deeper than half the wavelength of light, then the surface of the metal will look smooth rather than scratched.

Sterling silver (.925 fine) has the same hardness as Plexiglas. If an abrasive product scratches Plexiglas, it will scratch silver. I recommend testing any abrasive cleaner you are considering on a piece of Plexiglas first.

Conservators commonly clean silver with a homemade paste of distilled water and precipitated chalk. Precipitated chalk is very fine and will not scratch Plexiglas. Abrasive cleaners should not be used on coins with a porous surface.

If you are considering using either of these techniques, I recommend experimenting with low-grade coins.

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