Get to know your anodes and cathodes and electrochemistry beckons. For as much depth as you wish on this subject, visit the National Association of Corrosion Engineers (NACE) site.
Anodes are that bit of the metal surface from where metal is dissolved, releasing charged metal ions into the surrounding wet soil whilst retaining electrons in the uncorroded metal. Cathodes are the nearby surfaces on the metal to where the electrons flow and get used up in a 'cathodic reaction'. Quite often the cathodic reaction produces ions that form scale or react with the metal ions and form 'corrosion product' .
Both scaling and corrosion on coins and artifacts are electrochemical processes. Given the right set-up these undesirable encrustations can be made worse or removed. The figure below shows the basic arrangement:
In this arrangement, the anodic and cathodic sites on the coin have been forced to separate by the use of a d.c. power supply. It is usual to connect the coin to the -ve terminal of the power supply to make the coin the cathode: the anodic process is transferred to the 'counter electrode' and so the coin cannot corrode. With this arrangement, and by controlling the d.c. current and electrolyte composition, it is possible to chemically attack the scale and corrosion product without damaging the underlying metal.
A good coin holder is a cable tie affixed around the perimeter of the coin. With a fine silver or copper wire (to match the coin's base composition) wrapped around the cable tie, this arrangement can be used to trap the wire against the coin edge to make electrical contact. The free end of the wire now goes to the -ve terminal of the power supply. A suitable counter electrode is a stainless steel bolt. Titanium or graphite rod is much better, even a bit of old gold would do the job nicely. But whatever material is used, make sure the electrical connection to the counter electrode is made outside the electrolyte bath. Copper wire connections do not last long when immersed, they corrode away to powder!
The electrolyte can be any one of the corrosives given in the chemical cleaning section: dilute sulphuric acid, formic acid. Finally when wiring-up the battery, variable resister and ammeter, make sure you don't blow the ammeter apart by pushing too much current through it. Polymathematica uses a d.c. power supply to provide the current (cheaper to run than burning up batteries overnight). Typically 1 mA per square centimetre of coin surface (don't forget it has two sides) is a good starting point.
From here on it's practice. The reading list in Background Knowledge gives a collection of excellent books which address the subject (amongst others) of electrochemical cleaning.
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Last Update, 1-July-96