Is It Better To Clean Coins Or Leave Them Alone?

Another problem with olive oil, besides the variation in ingredients already mentioned, is that it can slowly oxidize, forming lacquer-like substances that may never wash off (other hydrocarbons such as gasoline do this if you leave it in a gas tank for a few months).

As a chemist (and not a coin-cleaning expert), I like acetone a lot better than water, since water facilitates many other reactions (acid etching and oxidation both go faster with water). If you did use water, I would use acetone last (again subject to the high purity warnings already mentioned. I use high-purity stuff at work, which is far more pure than distilled water).

I use acetone to dry things. It washes off the water, evaporates quickly and cleanly, is extremely unreactive, and won't dissolve metals, metal oxides, or metal sulfides (which pretty much describes coin surfaces). Of course, that's not to say when you make a loosely attached particle fall off it won't take some metal with it and be uglier underneath.

Denatured alcohol is different things at different times, often with water, so I don't trust it.

I'd also avoid anything that was stored in a plastic bottle. Not only can the plastic add to the residue problem over time, but most plastics are permeable, so even if the plastic has no additives, that adhesive from the label could eventually soak through. I work with eye-drops, and after years of finding things in the eye-drops that passed through the outer box, through the label, and through the bottle into the solution, I'm surprised they keep the water in. Oh, actually they don't. Many of the bottles lose 5% of the water contents per year, straight through the plastic, concentrating the ingredients by 5% a year.

What if I've been using strong ammonia (on an extremely crusty coin which cleaned up good) from the 80's? Oh well, I was careful and it worked...
Consider yourself lucky. Again as already mentioned in several previous posts here, solutions of just about anything can accumulate contamiates by just standing in their container. As to your statement of STRONG AMMONIA. That too is ambiguous. Was it household Ammonia of the shelf? High concentrated from a chem lab? Paint store?Note that most Ammonia products are used in the home so manufacturers purposely add additives to make it a little more pleasant. Many household Ammonias are composed of an Ammonium Hydroxide solution. basically meaning watered down. Many also contain Anionic Surfactant, a Non-Ionic Surfactant, Miscellaneous Perfumes, color clarifying agents and possibly so called inert salts. Also, pay attention to the none detailed items such as a color clarifying agent. Also, note the lack of percentages for the general public. Ever notice all the warnings on an Ammonia container?
Glad you said you were carefull. Any one of the previous noted substances could be dangerous to you and coins.

Stephen420. Why take chances with substances you do not have or know nothing about? With anything on a coin you do not like, first try just distilled water, in a heatable glass container like a coffee pot. Heat a little and remove. If that doesn't work, distilled water can be used for many things around the house so not a waste. Usually less than a dollar a gallon. Next note you can buy Acetone at any hardware or paint type store also for not to much money. Other so called cleaning agents may or may not work and there is the investment in a substance possibly not useable around the house ever again.