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I have a ultrasonic cleaner for cleaning dry pen nibs. I was going to test it with some darken up coins, maybe with mild dish detergent and water and see what kind of result that bring. Or would you use some other agent.
AAHHHHHHHHH. To start with please try to remember that dish soaps are all different. Some contain materials that will really mess up your coins. Dish soaps are manufactured differently so that people can purchse the ones they THINK works best. If they were all the same, then there would only be one manufacturer.
When you say water, remember, that too is different under different situations. Tap water can contain Flourides, Chlorides, etc. If well water may contain almost anything on Earth. If run through a water softener, now contains NaCl.
DO NOT use those types of cleaners on coins.
Most types of electrolictic methods do more damage than good.
As to VINEGAR. Only use this if your planning on adding your coins to a salad. Makes for a nice crispty, crunchy touch.