Same thing happened with a friends 45-70 loads but he was using lever revolution jacketed bullets and a Hodgedon or IMR powder, don't know which. I agree with one of the posters from the above links that it looks like Copper Sulfate and is probably due to moisture and possibly the slightly acidic finger prints from handling the cases and bullets. This is just speculation and it only happened with one batch. But we have our share of hot humid weather here too.
His powder seemed fine.
"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli
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