Cleaning and Preservation of Coins - A Complete Guide, page 13
CHEMICAL COIN CLEANING: Ammonia Solution, Vinegar & Salt Immersion Baths
(CONTINUED)
The following methods can be also used to remove the coin's crust, coin metal oxides, coin tarnish and some types of coating.
• Ammonia Solution Immersion Bath
The coin is immersed into a 10% Ammonia (NH4OH) solution. The container must be tightly covered with a lid, and the coin must be immersed completely (unlike in the picture below). The process begins immediately and is evidenced by changing color, from clear to dark-blue or violet, of the solution, starting around the coin. This is an effect of ammonia not only on the dirt, but also on the coin's patina, basic copper sulfate, that is dissolving in the solution.
Solution In 2 Hours
The coin is immersed for a few minutes or continually taken out, soaked with several changes of water and brushed. Great care must be exercised here. If the coin is left for longer time, the entire bath turns dark-blue, and the coin completely loses its patina and turns red.
A brief immersion in 5% hydrochloric acid then follows for neutralization. This is how the coin looks like after the treatment; the remaining green spot on the coin, the only area that was not immersed into the solution, is left for comparison.
If dark spots remain under the previous verdigris, they are simply left there, because copper retarnishes very quickly. The copper coin then can be either brightened (see "Brightening Copper Coins" on page 21) or tarnished (see "Tarnishing Copper Coins" on page 22) or artificially patinated (see "Artificial Patination of Copper Coins" on page 23) over the entire surface, or on parts of the surface by using a Q-Tip.
IMPORTANT NOTE: If the coin is placed flat on the container's bottom instead of being suspended into solution (as shown on the picture), it must be turned over a few times (at equal time intervals) while staying COMPLETELY immersed. When the coin with ammonia solution on its surface is taken out of the bath, THE COIN MIGHT BE SERIOUSLY DAMAGED IF EXPOSED TO THE AIR for a long moment!
• Vinegar and Salt Bath
This effective and inexpensive method is used to quickly clean coins destined to be exchanged at the bank, for example, clad types, nickels, and pennies. A large glass jar is half filled with distilled white vinegar. Then the coins of the same composition are added.
NOTE: Coins of unlike compositions mixed together will definitely stain each other using this method.
Next, add a tablespoon of table salt while stirring the coins continuously with a wooden spatula. Within minutes you will see the results. Stop stirring when you think the coins are clean enough. Coins left too long in this mixture can actually have their surface features etched away. A fresh mixture of vinegar and salt for each batch of coins is recommended.
After immersion, the action of the chemicals must be stopped by rinsing or washing the coins with distilled water. In many cases, a neutralization must take place to stop the further attack on the coins. This neutralization is accomplished by immersing the pieces in a solution that reacts oppositely. Acid is thus neutralized by alkali, and alkali by acid.
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