By way of introduction, I am loath to cleaning coins. Still, there are many beauties in my collection that are simply crud-inflicted. Would subjecting them to a jewelry "sonic bath" possibly degrade them? I cannot see how such a spa treatment might degrade the integrity of the coin's surface - or am I being numismatically naive?
the problem is that the "crud" you speak of has probably corrupted the surface underneath and your cleaning it will reveal the imperfections. That said, I have cleaned a few coins that were, as you say, simply crud-inflicted, and been happy with the the results SOME of the time. If it's a big glob of yuck that completely covers the coin, like old bubble gun, I use an old Popsicle stick to get the major yuck off, since that will minimize the scratch damage. For glue, there are solvents. Sometimes I resort to a metal tool and go at it very carefully, which almost always winds up scratching it. Sometimes its better to have a scratch than all that crud. But bottom line, do NOT expect perfection will sparkle when you get done. You will have a flawed surface. Clean, but flawed.
I've been meaning to try ultrasonic cleaning, but haven't. I've seen hints here that the big grading companies sometimes use it as part of their conservation service. If you do try it, whatever you do, don't put the coins into the usual metal basket, and don't pile coins together. Any place where metal touches the coin's surface is going to get rubbed tens of thousands of times a second. The result won't be pretty.
I've thought a lot about ultrasonic cleaners. I use one in my shop to clean my eyeglasses, machine parts, etc. In thinking about its use for cleaning valuable and important coins, I agree with every caution already mentioned but I have a few other thoughts to share that go beyond the "don't do it" advice. My advice is more on the lines of "If you decide to do it, maybe think about these things first...". I believe it is critically important to prevent the coin from interacting with anything other than the cleaning solution and whatever is used to suspend the coin in the cleaning solution. I have "thought-experimented" with such things as a machined coin edge clamp made of nylon, HDPE or other materials that are significantly softer than the coin, that can be machined, clamped on the edge securely, and that are inert (or mostly inert) to the cleaning solution utilized. The clamp needs to be somewhat secure on the coin since the ultrasonics will tend to vibrate it loose (it sure does with the screws on my eyeglasses!). The clamp needs to have a thickness that is slightly thinner than the coin to prevent a "shelf" that will capture crud falling off the coin. The clamp and coin need to be suspended from overhead into the solution such that the coin is vertical and the crud that is shaken loose drops down and away from the coin. If the coin and crud interact it will have negative effects on the coin. It might be beneficial to use one of the little propeller-type mixers to circulate the fluid over and above the natural circulation that the ultrasonics induce. Duration of cleaning should be short. I use about 10 seconds with my eyeglasses in a solution of water and two drops of Dawn dishwashing soap. The glasses come out with all the oils, greases, dust, etc. gone from even the deepest crevices. Of course, there is always the possibility that what lies below the crud is worse than the crud. And, there have been claims that the implosive bubbles that form on the surface during ultrasonic cleaning can damage the surface of the coin. I have never seen any evidence of this but then I have not tried it on high-grade coins. I would say experiment on inexpensive proof and MS Lincolns and look very closely at the surfaces before and after. If anyone pursues these ideas, please share your findings with us all.
