The only thing that can really mellow tannins is time, as the previous poster said. I think the last wine I added tannin to, I added 1/8 tsp. It should be used sparingly, because it will give the wine quite a "bite". Next time, add a tiny bit at a time and wait a few hours and taste it. You can always add more, but you can't take it out once it's in there!
That plum wine sounds awesome!
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