My experience has been that these chips are pretty sensitive to cleaning. At least the inlayed ones. My 50 cent chips are the dirtiest and the most worn. I tried a warm Dawn soak with some light toothbrush scrubbing and felt that I did some damage to the inlays, without really making much progress on the gunk stuck down in the mold. So I stopped and am just using them as is.
The chips on the left are after cleaning and the chips on the right are pretty similar to what they looked like before. If you look closely, you can see some print missing and some discoloration to the inlays on the left. And they are really not any cleaner.
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I believe
@chipinla put some pretty heavily used hot stamps in an ultrasonic. It took off most of the hot stamp, but he was pleased with how well they cleaned up. He posted some pics and commentary somewhere, but I don't recall the thread. Maybe he will chime in.