Pretty advanced BD on a very nice coin. Also the
bi-metallic aspect complicates cleaning options. Might be worth seeking professional
help.
If you decide to treat alone the basic steps are:
1) Get rid of as much of the light green powder as possible.
For this you can use toothbrushes, including ones with cut-down short bristles to make them tougher. Toothpicks. Bamboo skewers. Even
sharp steel pins if you
pick carefully. I think it is best done dry as much as you can but you could use water if you need to. The goal is to get rid of all the soft green stuff. This may expose shiny copper.
2) Dry.
This is a case where I would bake the coin to drive out all moisture. You should
search through this cleaning
thread for tips on duration and heat as I can't remember what I have used in the past or what others have done. I think I have put them on baking sheets at fairly low temperatures (150 F) for 30 minutes or so but can't remember for sure. Haven't done it for years.
3) Store or seal.
Once the clean and dry stages are done you can either leave it
as is and monitor it to see if the BD reappears, or try to seal it. Sealing reduces the chance of the BD continuing as it needs contact with the air to really advance - though even sealed coins can have the BD advance due to some chemical reactions already under way, or improper/incomplete sealing. But on the other hand sealing makes it more difficult to treat if the BD does advance as you have to remove the sealant before re-treating it. Depends in
part on the humidity level where you live. Personally I would clean it and watch it. Might not need sealing.
If you seal you can use renaissance wax, though in cases like this it might be worth trying to have someone seal it with benzotriazole. Benzotriazole is definitely a "don't try it at
home" thing - not until you know what you are doing at least. It is a listed carcinogenic chemical. You can't even buy it in many places. In effect is a fancy clear lacquer that stops the shemical reaction and
seals it from the atmosphere.
SC