Modern coins are generally not cleaned because that lessens their value. Ancient coins frequently have dirt and corrosion (patina) products as blemishes. I generally ask two questions when dealing with each coin. Will cleaning be likely to make the coin more readable? Sometimes the dirt and patina actually enhance the readability, as for the Herod I coin in this exhibit. Will cleaning limit the growth of continuing corrosion? There is a "bronze disease" whereby a coin will continue to deteriorate, especially in a humid environment.
I generally avoid chemical cleansers and acids. Hydrochloric acid may rapidly remove some corrosion products but will generally leave a rough, pitted surface that is unattractive. I may use a detergent to loosen the dirt. I may use a paint solvent to remove someone's varnish coating (put on to limit further corrosion). I may use a silver polish after mechanical cleaning is done.
I mostly do mechanical cleaning under a stereo dissecting microscope. It is able to zoom from 8 to 40 power, but I use the 8X setting. A 10-watt light bulb hanging on the microscope is sufficient illumination and cool enough to touch. I place the coin on a block of soft wood or firm foam plastic. My tools are mostly dental tools of various types, especially the tiny points. I also have a small hand-held drill chuck that holds a needle or pin point. With these tools I carefully chip and scrape off the corrosion and dirt, avoiding serious scratching of the metal. A tool with a broad flat surface (worn out dental tool) heals most tiny scratches. A hard toothbrush provides a good brushing. Hand rubbing (thumbs) provides a final polishing. Practicing on cheap, bulk, uncleaned and unidentified coins helps with skill. Such mechanical cleaning needs much time and patience, and I do so to relax as I listen to good Christian radio music and programs.
or general topics:
Home, B.C., Birth of Jesus, Bible coins, Other Gospel topics, Book of Acts, Nero, Jewish Revolt, Revelation, Official Christianity, Byzantine, Contrast, Details