Create the Firebox Using Firebrick

For a kiva firebox, you MUST use firebrick that can withstand extreme heat (regular brick can't). To get the traditional round shape, I cut each brick into thirds. Score the bricks 1/4" on each side with a skill saw (mortar blade), and whack them with a flat chisel to break into thirds.

The smaller pieces will taper easily. I don’t use full-sized brick because it's too hard to create the roundness that I need. Layer at the bottom using sairset mortar that is fire retardant (not regular cement), and work your way up.

I make a plywood template to mold my firebrick thirds and create the arch opening. To make the template easier to remove, I put tiny wood wedges at the bottom. When everything is set, you can remove those wedges and wiggle out your template.

As you go back to achieve depth in your firebox, taper the firebrick thirds tightly to get that lovely round shape. At this stage, you also start laying your adobe bricks on either side (using regular cement for the adobe) to create the base.