How to clean Roman, Greek or Byzantine coins
When you buy unclean coins whether being Roman, Greek or Byzantine (this are the ones I know by experience), most of the time you will have to put a lot of effort to remove the hard crusted dirt formed from being underground for several hundred years.
There are several ways to clean coins from excavations, but I will explain you the method I use in order to avoid to ruin the patina.
As soon as I receive the coins, I wash them with warm soap and water to remove the first layer of dirt. You should use a soap with a neutral pH that does not contain lemon or vinegar, to avoid damage. Allow the coins to soak in the soapy warm water for about 30 minutes. During this time, you have to brush the coins with a toothbrush with hard bristles and you will notice immediately that much more dirt will come off already with this system.
Dry the coins thoroughly with a soft cloth without scrubbing them and then divide those that are already clean enough. You can repeat this process several times until the dirt and deposits continue to come off the coins.
At this point pour olive oil in a plastic container and immerge the coins that need further treatment. Let the coins soak for 5-7 days until the oil softens the remaining dirt on the coins.
After 5-7 days of soaking, remove the oil, wash and then brush the coins again; you must repeat this passage for as long as the dirt attached continues to come off.
At this point, some coins will be well clean, while others will still have hard dirt plates attached and will need a more drastic approach. You need to remove these patches of hard dirt using first a toothpick and then, if it’s not enough, a sharp metal tool like a scalpel. Using this tool very gently, apply a lateral pressure on the hard dirt plates, being careful not to damage the patina.
I usually use this last process only as a last resort for coins that are really in bad shape because it’s really easy to ruin the patina of the coin and personally I scratched several coins with this method; don‘t use it unless you really have to.
The patina is the oxide layer that has formed over the centuries; a coin with a patina intact is a must for a collector, and therefore more valuable.
You can also use ultrasonic cleaners such as those for cleaning jewelry, for good results and to speed up the cleaning process, of course, we must consider the costs of such equipment.
Once the coins are finally clean, treat them with “Renaissance wax” to protect them from the degenerative processes of the metal.