The mortar acts as a cushion for the stone, both supporting it and holding the wall together. Spread a large dollop of your mortar on your concrete footing using your trowel and begin setting stone. Start the wall by setting a face stone in the mortar. Establish the first ring of face stones with mortar underneath each stone. With stones, there are two major things you’re looking for: good face stones and ratio. Face stones are stones that will make up the face of your wall; for walls that are straight, they should have a smooth flat face, for curved walls they should typically have a little bit of curve. Ratio is the process of incorporating stones of different size and shape in your wall. By alternating larger and smaller stones, you show off the character of each individual stone.
When the first course of face stones is in place, move on to the first row of fire brick. Trowel a layer of mortar along the inside edge of the face stones and press the first fire brick into place. Apply mortar to one end of the next brick before butting it against the first and pressing it into the mortar.
After building the first ring of face stones, set the first row of fire brick. The fire brick will form the flame retardant interior wall of the fire pit. The outside edge of the fire brick should meet the inner circle marked after digging out the footing, but double check by measuring 12 inches back from the face of the first row of stone. Place a dollop of mortar on top of the cement footing, and set your first fire brick. To set the next fire brick, butter the edge of the fire brick. To butter the brick apply mortar to the edge of the fire brick that will butt up against the previously set fire brick. Smooth the mortar – using your trowel – on all four sides so the mortar makes a pyramid-like shape and then squeeze the brick next to the previously set fire brick. Tap the fire brick into place with the end of your trowel, place a level across the two stones to ensure that the fire bricks are level, and make any adjustments by lightly hammering the brick with the blunt end of your trowel. If the fire bricks do not fit perfectly, create a smaller brick or plug by cutting a fire brick with a stone hammer and chisel or with a diamond blade saw or grinder. Fire brick also absorbs moisture when you set it, so use moist mortar.
After building the first row of stone and fire brick, continue building the wall of the fire pit by setting stone first and then bringing the fire brick up to the height of the stone. Each new stone should be set on mortar, so place a bed of mortar on top of existing stone work for each stone you set. Use the trowel to set the mortar, and set the mortar slightly back from the face of your stone – mortar will stain the face of your wall.
When building up the wall and the fire brick, avoid weak joints or 'cross joints' by spanning joints with larger stones. For the brick, this means placing one brick over the joint created by two and creating a 'running bond' pattern. For the stone this means playing with ratio and spanning stones to create a stronger wall. Fill in any area between the back of the stone and the fire brick with mortar and junk stone. Try to keep joints small by picking stones that fit well together. If a stone is near the size that you want it, but needs to be cut down in size, notched, or have a slight curve added to its face, shape the stone using your chipping hammer. Score a light line with the sharp end of your chipping hammer where you want your cut, put on your safety glasses, and using the sharp end of the chipping hammer, chip away.
After the first row of brick and stone, you may decide to build up one side of the fire pit to the height of the wall so you have a visual point to work from. Another method of building up the wall is building up the stone all the way around until you reach your desired height. As you reach your desired height, start looking for capstones. Pick a capstone from your capstone pile that has a smooth top and face. The fire pit, with the capstone, should reach the desired height of the fire pit. At the end of each day, take your jointer and dig out any excess mortar sticking out between stones. Then use your paintbrush and brush the mortar joint to smooth the joint line.