Yes, I know, read the instructions first! That said, if you accidentally fill the forge with charcoal first, it seems you lose it all. I did this and had to break the forge to empty it... and lost the charcoal. Re-read the instructions. Placed the ingots first (this isn't very intuitive since the ingots end up on TOP of the charcoal anyway, and this is not how the earth kiln works... you pile items logically from the bottom up). Then I attempted to light it with a torch by crouching... and removed my ingots. Can't place them back on, or empty the forge... break the forge, lose a second set of charcoal.
Why not use the same mechanic as the campfire with the forge? Build one, right click, add charcoal to the fuel slot, add your ingots to the top slot, and POOF! It lights up exactly the same. Seems like two totally different mechanics for exactly the same result.
Still loving the game, but things like that take a touch of the fun out of it when you are just starting off and learning new mechanics.