I found the issue! When you go to hell, the path of the tree in the normal world remains, so you can walk over the river of lava! I thought it was intended behaviour, but now that I managed to find how to use the sand to "trap" the tree and turn it into charcoal it makes sense that it would NOT be intended lol.
Is the glass just a red herring (or a sign you screwed up) or is there a hidden use for it?