Before you use the firebox after a relatively long period of time, make sure it is properly cleaned. You will need to remove ashes and old wood as well as clean the walls and floor of the firebox. Before you move on to the next step, allow enough time for the firebox to dry completely.
After you are done cleaning the fireplace firebox, closely inspect it for visible damage. Check for loose bricks and big cracks and repair if found. If the firebox has somehow suffered extensive damage, it will be a wise decision to contact a professional because of the personal safety issues.
To enable the heat produced in the firebox to have more effect, cover the floor and walls of the firebox with a uniform layer of fireclay. This fireclay is specially manufactured mortar aimed at withstanding the excess amount of heat produced in a fireplace.
Refer to the manufacturer’s instructions manual that came along with the fireclay in order to determine the duration of time you will have to allow for it to dry properly. Generally, this time is around 48 hours but it may vary depending on the type of repairs.