Cleaning and Preservation of Coins - A Complete Guide
CHEMICAL CLEANING OF COINS BY REDUCTION AND ULTRASONICS
2) Reduction
Reduction is the decomposition of oxidation by separation of the oxygen, chlorine, sulfur, etc., from the compounds of oxygen, chlorine, sulfur, etc. making up the oxidation on the coin surfaces. This method is based upon the galvanic current formed by the contact of two different metals.
The direction of the current in a solution is from the minus to the plus metal, corresponding to the so-called electromotive series in which metals are arranged:
| Gold (Au) | +1.40 Volt |
| Silver (Ag) | +0.80 Volt |
| Copper (Cu) | +0.34
Volt |
| Hydrogen (H) | 0 Volt |
| Lead (Pb) | -0.12 Volt |
| Tin (Sn) | -0.14 Volt |
| Iron (Fe) | -0.44 Volt |
| Zinc (Zn) | -0.76 Volt |
| Aluminum (Al) | -1.66 Volt |
Each of the metals exemplified above can be electrically positive or negative, depending upon whether it contacts a metal above or below it in the series. For example, if iron and zinc are placed in a caustic soda solution, iron is the negative and zinc is the positive pole of galvanic cell.
As the current decomposes the water into the constituents of hydrogen and oxygen, reduction begins immediately, manifesting itself by evolution of hydrogen bubbles. This hydrogen works mechanically on the rust, by breaking it off, and also chemically, by decomposing it into the iron and oxygen of which it consists.
In case of reduction with zinc and sodium hydroxide, the copper coins are laid between zinc chips, over which 5% sodium hydroxide (caution: caustic soda) solution is added; it must completely cover the zinc. The coins should not touch one another.
According to the electromotive series, a current flows between the metals, reducing the metallic part of the deposit on the copper to metallic part copper, and loosening it. The zinc oxide that occurs as part of the deposit on the zinc is dissolved in the sodium hydroxide.
This method can be done with other metals and liquids too, keeping in mind the electromotive series.
3) Ultrasonics
One of the safest ways to clean ordinary collectible coins at home is to use a commercially manufactured ultrasonic wave cleaner which is used by many jewelry and watch firms. This method allows to accomplish a simultaneous chemical and mechanical treatment of maximum efficacy: cleaning occurs even deep in the metal and, therefore, practically in the smallest pores and indentations.
The coins to be cleaned are placed in special containers filled with the liquid - plain water with household
dish detergent or commercial cleaners designed for specific metals. A generator
within the cleaner produces high-frequency sound vibrations, not perceptible to
the human ear, between about 20,000 and 10,000,000 cycles per second, which are
transferred to the containers via a transducer.
These vibrations exert a very strong mechanical action: hundreds of imploding vacuum bubbles dislodge dirt without abrasion by this action; the cleaning fluid is forced into all the irregularities and the impurities are drawn out of them. At the same time, the metal surface is set into intense vibration so that corrosion and other foreign deposits are broken loose and separated.
Using a mild detergent solution, ultrasonic cleaners work very well on silver coins. Copper and nickel coins may require stronger detergents or oxide removers to dislodge grime and corrosion.
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