Of course there are many types of "dip", and this would include acetone, rubbing alcohol, soap and water, olive oil, turpentine, gasoline, Jewel Lustre, etc. Some are mildly acidic, some stronger, some not acidic at all; some oxidize the metal surface free of "toning oxidants", some merely clean adherent detritus, dirt, etc. I am always suspicious of even toned coins as having been "cleaned" but there is a spectrum of cleaning (ie see the "finer" of the 1905 halfcrowns in the LCA auctions current sale), as well as retoning of cleaned coins, etc. (see ebay's famous "Greattoning" seller - LOL). I have seen, and indeed have, some coins that have been lightly dipped with little apparent loss of lustre. Interesting about the sandy soil bit as usually there are choride compounds lurking in such, but evidently not at Rob's locale...