The substance that cements fireplace bricks together is called mortar. Loose or crumbling fireplace mortar should be repaired as soon as possible. Deteriorating mortar leaves the bricks and the intact mortar more susceptible to heat damage from blazing fires in the fireplace. It is, however, a fairly normal occurrence and you can expect it to happen every few years or so if you routinely use your fireplace. With a couple hours and a few tools, you can patch those crumbly areas on your own.
Dig out the old, loose mortar with the scoring tool until the joint is ½- to ¾-inch deep. The brick on either side of the joint should be mortar-free.
Clean the joint. Bring the vacuum hose right up close to the joint and brush it with a paintbrush while you vacuum to collect the dust.
Apply water to the joint and to the surrounding brick faces using the same paintbrush. It's necessary to wet the brick to ensure that the new mortar adheres well.
Follow the manufacturer's instructions for mixing mortar. Use room temperature water, and mix a small amount. If you mix too much it will harden and go to waste. Stir the mortar with a utensil until it takes on a pasty texture.
Load mortar onto the edge of the brick trowel and line up the edge of the trowel with the joint. Holding the tuck-pointing trowel in your other hand, use it to pack the joint with mortar. Fill it halfway then, pressing firmly with the pointing trowel, pull it along the joint two or three times. Continue pulling the trowel along the joint, pushing more mortar into the joint, until it's filled.
Scrape off excess mortar with the brick trowel and wipe the brick immediately with a damp sponge, avoiding the joint.
Run the brick jointer over the soft mortar to give it a smooth finish.
Brush the entire surface with a stiff-bristled brush when the mortar has sufficiently hardened, after anywhere from 30 to 90 minutes.