Make Sure the Joints are Strong
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.