The flames in your chimney's fireplace can reach temperatures that are high enough to crack most normal masonry bricks used to build your chimney. You can build a firebox inside the base of the chimney that can withstand the heat, using specially formulated firebricks and refractory mortar to create a shell within the chimney. The bricks are set into place much as you would standard bricks, so building the firebox is no more difficult that creating any ordinary run of bricks.
Things You'll Need
Solid masonry blocks
Check the base of the chimney for the presence of a hearth. You'll need a hearth that's 20 inches deep and 4 inches thick within the chimney, constructed of either concrete or solid masonry blocks. If none is present, then attach a wooden 2-by-4-inch plank across the front base of the chimney with masonry screws.
Create a batch of concrete in a wheelbarrow by adding water to concrete mix until it's the same consistency as a thick batter. Fold the water into the concrete using a garden hoe to drag the dry mix from the bottom of the wheelbarrow above the added water. Pour the concrete into the chimney, filling up the space to the 4-inch level of the plank across the front. Run a wooden screed over the surface of the concrete along the top of the plank to level the surface and to settle the concrete into any voids created by the pour. Run a steel trowel over the surface to smooth the concrete further, then allow it to cure for a full day before continuing.
Create the backup masonry for your firebox using 6-inch-thick solid masonry blocks to build a short wall. This backup masonry will surround the firebrick lining the chimney. Lay the solid masonry blocks in runs, creating a three-sided wall with an opening along the front of the chimney. Butt the bricks against the chimney's wall, using the opening of the chimney as the stopping point for the firebox sides. Use a trowel to add 1/4-inch layers of refractory mortar between the blocks to hold them in place. Stagger the bricks on each run to add strength to the masonry wall. Cut the blocks when needed using a circular saw with a masonry blade. Construct the backup wall to the same height as the width of the front of the proposed firebox opening.
Check each block layer for level with a carpenter's level. Adjust the positioning of the bricks to ensure that they're level across the length of the run.
Cover the front of the solid masonry block wall with a 1/4-inch layer of refractory mortar.
Lay a liner of firebricks against the front of the backup masonry wall so the bricks are nested within the three sides of the masonry blocks, and are butted against the 1/4-inch layer of mortar along the front of the blocks. As with the backup masonry, place the firebricks as though you were building a brick wall, with each run's joints staggered in placement over the joints beneath. Build the wall of firebricks as high as the backup masonry wall, using the refractory mortar to resist the high temperatures of the flames. Use firebricks at least 2 inches thick so your combined firebox wall's thickness is 8 inches or more.
Mix a batch of regular mortar in a large bucket to the same consistency as oatmeal. Pour this mortar mix into any spaces between the backup masonry block wall and the chimney wall.