We used a combination of fire bricks, that won’t split in high heat, and some less-expensive regular red bricks that we had lying around, to create an arch inside the firebox and provide a thermal mass. The fire will burn at one end of our evaporator, heating the full length of the bar as it climbs our brick “ramp” toward the chimney at the other end. The large thermal mass created by the brick will help to both retain and regulate the heat and provide thermal stability over the long amount of time required to evaporate sap.