Cleaning antique bottles

Betsey
by Betsey
After my mother-in-law passed away we found 4 antique bottle that are covered in what I believe to be coal dust. They were in the basement and when the house was first built it was heated by coal. One bottle is Ski-Hi, Vernor Ginger Ale and Nehi that was bottled in CA. I have tried denture cleaner and other ideas but cannot get it all off. I am afraid to scrub too hard, Any suggestions!!
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