Norman, when I have an item with similar blackened spots, and especially when I can see some traces of coftgari, I apply a heavy coat of regular oil (like engine oil or WD40) all over it, and keep it soaked in oil for a while. Then I use soft cotton to gently wipe off the oil (no heavy scrubbing though!). The goal is to remove oxidation and dirt deposits to reveal any coftgari which may be "hidden" under. In your case I'd do the same: just put some oil and gently wipe it off using clean parts of soft cotton until you stop seeing dirty marks on it. and this is as far as I'd go, i.e. any further scrubbing and/or cleaning attempts will only make it worse and potentially create more "damage".
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