Coins recovered from being buried in the ground, or found on the sea bed, may present special problems due to chemical reactions between the metals and the salts present in earth or sea water. Different metals need different conservation methods. Brass coins are best cleaned with hydrochloric acid. A 100 per cent application of hydrochloric acid does not damage the brass provided that the coin is not left in the acid too long. Gold coins are mostly found in a perfect state of preservation. A dilute solution of hydrochloric acid will be sufficient. A bubbling reaction will be seen when the matrix and acid come into contact with each other. After a coin has been cleaned in acid, it is advisable to drop it into an alkaline solution afterwards to neutralize the acid and to prevent any further chemical reaction. Coins should be treated in an electrolytic solution to remove salts and other corrosive chemicals. The correct and effective cleaning of salvaged coins is a specialized science and no fixed method can be endorsed.
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| Obverse of 1652 Brabant ducatoon after being cleaned |
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| Reverse |
Salvaged silver coins will acquire a dark patina of silver oxide and are the most difficult to clean. The coins of the outer layer of a lump of silver will, naturally, not be so well preserved as those on the inside, most of the latter being in perfect condition.
Even if it is found encased in matrix, it will still be coated with a thin layer of silver oxide. Care should be taken not damaging the surface of the coin. A silver coin removed from a layer of matrix must be dipped into a 100 per cent hydrochloric solution. A violent reaction will take place as the hydrochloric acid attacks the lime and iron oxide encrustations that make up a typical matrix deposit. The coin must be left to soak in the acid until the reaction has stopped. Wash thoroughly in a soap solution. The silver coin should now be clear of most oxides or matrix encrustations and details such as dates and mint marks will be visible. However, the silver oxide layer will still be present as a black coating barricading details on the article.
To remove this oxide the coin must be placed in an alkaline solution and an electric current of 12 volts passed through it. The coin must be attached to the negative terminal of a car battery and the positive terminal attached to a stainless steel plate. The coin and plate should be about 5 cm apart. When the current is switched on, a stream of bubbles will be visible. This dissolves the oxide and repels it. Large pieces of black oxide will be removed from the coin. The solution will soon be discolored. About 10 to 12 coins can be cleaned in a liter of solution, depending on their size. After a minute the coin must be removed from the electrolytic solution and washed in a strong detergent. It will now have a dull silver color. To restore the original luster of the piece, carefully clean it with a toothbrush and detergent. Coins cleaned in this way are still acceptable to coin collectors.
