The inside of the firebox? Firebrick, or brick made from refractory clay has almost zero moisture content and is recommended by most masons for firebox construction. Two sided fireplaces are very difficult to build as the back of a conventional fireplace is part of the smoke path. Heat and smoke run up the back of the fireplace and hit a curving or sloping wall, the smoke starts to "curl "from the movement and hits a "smoke shelf" that sends the curl up the chimney. Double sided fireplaces do not have a back, so smoke must rise straight up and even a minor draft can interrupt the flow of heat and smoke up the chimney...making for a smokey fireplace. A master mason familiar with and experienced in building two sided fireplaces is a must.