Automatic Furnace

It's time to upgrade our Furnace. Pick up all of your Furnaces. Place down a double chest, then attach a Hopper to its side. Place a Furnace on top of the Hopper. Place 2 more Hoppers on the side and top of the Furnace. Place another double chest on the top-most Hopper.

And we're done. Any Items you want smelted go in the top chest. Place a supply of Charcoal into the side Hopper. And Smelted Items drop down into the bottom Chest.

Save all of your extra Furnaces because we're going to use them later.