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I too have found that small slugs are responsible for damage to hepaticas in troughs or planted out. Rarely in pots under cover. I've never tried copper coins & I believe there are commercially available copper bands which are said to be deterrents. I suspect that, just as in the case of grit, ashes & other supposed remedies, slugs will ignore them.
Anne, yes I have some H. japonica. They all survived that "black frost" and flowered.