I mentioned in a different thread that I might be able to help another member with removing some black residue or encrustation from an Athenian owl, and other members suggested I post my technique, so here goes.....but first, I have no idea if this is a commonly-used procedure or not; it has just worked for me on several coins over the years. I should make it clear that this whole discussion applies to silver coins only, not bronze, etc. Required materials are one juicy lemon, a roll of heavy-duty aluminum foil, several wooden toothpicks (definitely not plastic), some distilled water, a clean cloth, most likely a loupe, and about one hour per coin, accompanied by a healthy dose of patience. I roll out a foot or so of the foil, and simply cut the lemon in half and squeeze several drops onto the foil, making a small puddle of juice. If the entire coin is to be cleaned, I immerse it in the juice, or, if just a small area is to be cleaned, I use a toothpick tip to transfer some of the juice directly to that spot. It helps to use a loupe the see exactly where the juice is going in the latter case, and also for all remaining steps. To start, I wait just a couple of minutes, then take a toothpick and try to see if any of the residue to be removed will come loose or not by GENTLY picking (not scratching) through the juice at the affected area. Typically most material to be removed requires longer time than a few minutes, but it's best to start with a short elapsed time originally, and then continue on as needed. I usually completely rinse the coin with the distilled water after each attempt to remove the material, to see how much really came off, and also to make sure I'm not overexposing the surface to the juice (see cautions below). If the residue is still stuck to the coin, I add more juice and wait another 5 minutes or so, and leave the toothpick tip in the juice. This seems to soften the tip (which will eventually flatten out to mush), so that more pressure can be applied, but still without scratching the surface of the coin. I've found that most foreign substances on a coin's surface will be loosened up after about a combined 10 minutes of exposure to the lemon juice. This is especially true if that side of the coin is placed in the juice puddle next to the foil ("face down"). I'm clueless about chemistry, but it seems like there is some kind of reaction between the juice and the foil that really goes to work on tough residue. From here on out, it's simply perseverance and adding juice, picking with the toothpick, until the residue is removed. I've only had one or two coins that had residue or encrustation so firmly affixed to the surface that this technique was ineffective. A few cautions: if your coin is toned, exposure to the juice is likely to remove it from the coin, or at least lighten it. Also--leaving the coin exposed to the juice and foil TOO long might leave it with a bright, shiny appearance. This is where the patience comes in--do not give in to the temptation to just leave the coin in the juice for half an hour, thinking, "THAT should really do it!" You may end up with an unnatural-looking bright coin. Instead, keep applying the juice for short intervals, rinsing the coin in between, and you will see small improvements in the amount of material loosened each time. I'm attaching a few photos of a "success story"--this Alex tet from "Side" (happily a die match to Price 2949e!) was substantially encrusted by a black "tar" when I purchased it from a local shop. After several cleaning sessions, there are just hints of it left in the deepest recesses, not affecting the coin really at all (the dark areas on the reverse are not visible without the camera flash). I am interested in others' experiences with cleaning ancient coins; thanks.
Thanks for writing out your technique! I may have a coin or two that would benefit from decrustation. If I try it I'll take before and after shots and post here.
Your technique is a combination of acid cleaning and "electrolytic cleaning". Electrolysis is quite effective for removing toning (tarnish) from silver. Most people recommend using baking soda (a mild base) and water instead of lemon juice (a somewhat strong acid); either one will give you the electrolytic effect (in contact with the aluminum), but the baking soda is less likely to damage the underlying surface of the coin. Depending on what your "crust" is made of, it's possible that the baking-soda technique might not be as effective as lemon juice. I wouldn't think of using lemon juice on modern coins, but I know the rules for ancients and dug coins are different.
