Detected this one at the weekend. Would like to know how to get it back to silver without damaging
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Start with a strong solution of dish wash liquid and a toothbrush. Then maybe lemon juice soak or vinegar soak and more toothbrush. Just dont use anything that will scratch the surfaces. Electrolysis can be useful sometimes…
Cloudy ammonia and or Apple cider vinegar 
try
How to Clean Coins Safely and Easily or take it to a coin dealer to ask.
I wouldnt use a mix of eg baking soda and vinegar tried on a brass looking style hinge and it rusted it somehow