Masonry fireplaces are constructed with firebrick and high temp mortar, to build the interior of the firebox structure. With age and use it is normal wear and tear for the joints in the fireplace and the firebrick themselves, to start to degrade. When you discover loose or deteriorated mortar joints/firebrick, cracks greater then 1/16th of an inch in width, it should be assessed for tuckpointing, brick replacement, or rebuild.
If you have a prefabricated fireplace, your firebox is constructed of 4 panels, (floor, back, left and right side.) Each panel is able to be removed, and replaced with either a manufacturer listed panel, or a custom cut-to fit panel. These factory built panels do crack under certain conditions.
You may notice hairline cracks in the panels, which is often caused by the heating/cooling process. The panels expand as they heat and contract as they cool. You may notice gaping around the panel vs. the panels touching which allows for this process to occur without causing damage to the panels themselves.
Larger cracks in your panels can be caused by impact from logs being thrown into the firebox. Too large of a fire being built, causing excessive heat to be applied to the panels. Even quick cooling or “Thermal Shock” from applying water to put out a fire/embers. Once the cracks reach a width greater than 1/16th of an inch (approximately the width of a dime) it is time to have the panels inspected and recommended repair completed.
Located above the firebox, your smoke chamber is what funnels the smoke and gasses from your fireplace into the lining system of your chimney. Often these smoke chambers are constructed of brick, in a step formation (corbeled) to create the transitional space. Unfortunately when the smoke chamber is not parged smooth, it allows the smoke and gasses to cool within the smoke chamber area before it makes its way up the chimney. This causes the bi-products of combustion (creosote) to form on the walls of the smoke chamber and flue system at a greater rate. Creosote, being highly combustible, is the cause of the majority of Chimney Fires or High Heat Events we diagnose.
The NFPA (National Fire Protection Association) recommends that all inner surfaces of the smoke chamber be parged smooth with an insulating refractory mortar.