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Fireplace caulking is a little different than caulking around a bathtub, for example. The goal of caulking a fireplace crack is not only to cover the crack but also to force the mortar compound into the crack. Fire boxes have thick walls to protect the surrounding structures from heat.

Is It Fire Resistant? Yes. Silicone is both heat and flame resistant. Thanks to the unusual chemical properties of silicone as plastic – it doesn't catch fire very easily and it doesn't act as a conductor of heat, which means that it is fairly fire-resistant.

While polyurethane does have its strengths related to its fairly low price, silicone is by far the superior choice when choosing a chimney sealer. Silicone is an inorganic substance that has high toughness against temperature fluctuations and UV rays.

QUIKRETE® Fireplace Mortar (No. 8620-21) is a silicate based refractory cement caulk for tuck-pointing damaged fireplace mortar joints and repairing cracked or chipped firebricks in fireplaces and wood-burning stoves.

Adhesive Strips

Available at many hardware and home improvement stores, small rolls of fiberglass-based stripping can be cut to size and applied in noticeable gaps between an insert and the masonry on the outside face of a fireplace, underneath the insert.

• If you notice bits of mortar on the floor of a fireplace, caulk the cracks using a silicate-based refractory cement caulking that resists heat. Preparation is the key to effectively caulking the cracks in a fireplace. Put on work gloves, a dust mask and safety glasses.

• The non-water soluble type of fire brick mortar is a mixture of aggregates and binders, which are high temperature resistant. This product is considered as the best all around firebrick mortar used in the construction of fireplace and chimney.

• Caulk is silicone, acrylic, or latex based, which is flexible and can help absorb movement, whereas grout can crack. Caulk may shrink or dry out over time, which is why it shouldn't be used in large installations or as a replacement for grout. Beside above, will grout stick to silicone? You WILL NOT be able to re-grout over the silicone.

• How to caulk without a caulking gun. Wet the tip of a finger in clear water and use it to spread the caulk evenly to provide a good seal. Add more caulk to fill any voids that appear as you work the caulk in. Allow the latex caulk to dry at least an hour before painting.