A citric acid bath might be the go with a light rub with bicarb. I have found some of my 1800s very hard to clean and needed multiple treatments. The results have been satisfying as a clean silver looks awesome.
With a few coins I have had no success completely removing a caked on black layer, suspect that is due to bushfires and a reaction to the 7.5% alloy in the coin. I have not treated them with stronger acids for fear of damaging them but I understand all silversmiths pickle their Stirling silver in acid to remove surface impurities prior to polishing, so I guess there would be a method that removes the black layer without too much damage.
With a few coins I have had no success completely removing a caked on black layer, suspect that is due to bushfires and a reaction to the 7.5% alloy in the coin. I have not treated them with stronger acids for fear of damaging them but I understand all silversmiths pickle their Stirling silver in acid to remove surface impurities prior to polishing, so I guess there would be a method that removes the black layer without too much damage.