Quite an array of approaches to cleaning were suggested in the first three days after this was posted to the Numism-L list, but none of them got it right. One was, however, on the right track. Can you tell which?

I'll make the wild guess that the coin was cleaned using techniques I have used with some success to remove crusty build up. Neither will help corrosion but removes deposits which seems to be the problem with the coin 'before':

Drizzel hot melt glue on the coin surface and allow to cool. Peel away the glue and anything that sticks to it. Repeat as necessary and do the other side. Time: 5-10 minutes. Second choice: Soak in a solution of Calgon water softener and brush occasionally. Depending on the degree of crust: 2-24 hours.

I have not tried it personally but I suspect this coin could also have been done in a rock tumbler with plastic pellets. Have you tried this one? I have no idea how long this takes.

I believe you have dipped the coin in a very light acid solution, kept an eye on it and brushed it with a nylon brush every two hours. When the result was nice, you cleaned the coin in a pure water solution and then soaked the coin with olive oil to make the small irregularities disappear. I think it took about four days for the process and twenty minutes of actual human involvment.

Okay, so I'll take a guess: You cleaned it with a soft brass brush. Time: 3-5 minutes.

My first try at doing this would be to use an ultrasonic machine with the bath perhaps containing some liquid soap and perhaps some ammonia.

My guess is that you soaked the coin in olive oil and used a bamboo 'pick' to clean off the loosened debris.

Did you just use olive oil and a tooth brush?

Perhaps you used an eraser.

My guess would be for your technique would be a blunt-ended dental tool used under a decent microscope.

Okay, so how was it done?

You may be surprised to learn that the last method in the list came the closest.