Comment Re:study confirms expectations (Score 1) 159
That's actually a good question. Inks have changed somewhat over the past 5,000 years, and there's no particular reason to think that tattoo inks have been equally mobile across this timeframe.
But now we come to a deeper point. Basically, tattoos (as I've always understand it) are surgically-engineered scars, with the scar tissue supposedly locking the ink in place. It's quite probable that my understanding is wrong - this isn't exactly an area I've really looked into in any depth, so the probability of me being right is rather slim. Nonetheless, if I had been correct, then you might well expect the stuff to stay there. Skin is highly permeable, but scar tissue less so. As long as the molecules exceed the size that can migrate, then you'd think it would be fine.
That it isn't fine shows that one or more of these ideas must be wrong.