Yesterday Steve finished laying the slate in the entryway and started working on the hearth! Here's the hardybacker that the slate tiles will attach to, attached to the walls.
Here's a close up of the air space between the sheetrock and the hardybacker, showing the 1/2" copper pipe spacers. It's a pretty clever system. He uses long screws, slides the pipe over them, and tightens it all down to the wall. It's very sturdy. There's an air gap at the bottom and the top will be left open, which will create a convective air current. It'll simultaneously keep the sheetrock from getting hot and act as a heat exchanger to warm the air in the room.
The brazilian black slate tiles will be attached to the hardybacker, and also will wrap around the sides so the air gap doesn't show.
I better think about getting some firewood!