Cleaning and Preservation of Coins - A Complete Guide
MECHANICAL CLEANING OF COINS
NOTE: Coins should never be worked on with metallic objects such as knives or needles, because this will leave irreparable scratch traces.
1) Soaking Before Cleaning
Soaking of the newly dug coins can be done either in warm soapy water or in oils such as Olive Oil, Mineral Oil and Motor WD-40 Oil.
A. Soaking in warm water with soap or detergent helps loose and lift the dirt without doing anything to the coin itself, provided the water does not have any dissolved impurities, for instance, atmospheric carbon, in it.
B. Soaking in Olive Oil is an effective method for loosening the dirt, but, if the coin is left in for a long time, a slight chemical attack on coin metal may occur due to the presence of fatty acids and traces of sulfur compounds in olive oil.
The coins should not be laid flat on the bottom. For example, they should be placed on the imrovized "trays" - plastic strips cut out of a food container's cover, which have high edge. Thus, both sides of each coin are exposed to the oil.
Soaking Coins in Olive Oil
Degreasing with Acetone, which is not ion-forming solvent, must follow.
C. Soaking in Mineral Oil is a more safe method because mineral oil does not have fatty acids and, therefore cannot attack the coin metal, no matter how long the coin is soaked. Degreasing with Acetone, or Ethylene Chloride or Naphtha must follow, long enough to get all oil molecules out of any crevices and pores present in the surface of dug coin.
D. Soaking in WD-40 is very effective but not for a long while. WD-40 should not be left on the coin's surface because the oil will gum up and harden over time, thus making it difficult to remove.
2) Cleaning with Brushes
CAUTION: Even brushing coins with a soft toothbrush can leave microscopic scratch traces on the coin if loose dirt particles are present on the coin!
Below is a picture of a freshly dug bronze coin, Indian Head Penny, that is covered with hard dirt - a usual case, to be removed by brushing.
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