I would suggest taking a look at the following video....
https://www.youtube.com/watch?v=cbuiV3RJou0
I have tried the following (rated from the MOST forgiving to the LEAST forgiving);
Dry tumbling - Vibratory tumbler using walnut and corncob media. Works OK. Won't clean everything off with copper. Works pretty good with silver. Slow, but VERY forgiving and hard to really damage a coin.
Ultrasound - Works pretty good. Fairly forgiving, although pitting can be a problem.
The following two methods are the most likely to damage an item if done incorrectly;
Electrolysis - Good for HEAVY scale. Pitting is a DEFINITE PROBLEM using this method. Lower voltages 9v-10v are a bit better.
Chemicals - Can work OK. Have to be REAL careful, pitting, staining, and stripping are problems. This is one of those areas that you will either have GREAT results or HORRIBLE or BOTH results depending on the circumstances.
Now I figure I will give wet tumbling with stainless steel jewelry media using rock tumbler like shown in the video a try.
I would say what many others say... If it appears to be of any MAJOR VALUE, you would be better off to take it and have it PROFESSIONALLY cleaned by someone who specializes in cleaning valuable coins/jewelry.
It really is possible to turn an extremely valuable coin or jewelry into junk by cleaning it incorrectly.