That must have been painful as fuck.

hard to say how earth bending works, since it obeys fantasy story telling, more than science; And "earth" in the series is not well defined; But it would make sense if earth is a set of elements commonly found on soil, take for example metalloids, not quite metal this set contains carbon;
that can be manipulated with earthbending; Add carbon to iron and you get steel, you could for example metal bend steel by manipulating the carbon in it; while pure metals would be unable to be bended. In conclusion alloys and compunds that contain enough of certain bendable elements could be manipulated with earth bending; then in would be possible to create a compund of mercury that could be absorbed through the skin, and be metalbended, while pure platinum could not.