Work mortar into any small cracks in the chimney thimble using a trowel or putty knife, if it is only slightly cracked. If it is actually broken, it will need to be replaced, and you should skip this step and go to Step 2. To repair a crack, mix the mortar to a fairly watery consistency so that it's easier to work into the cracks. Fill the cracks from both the inside and the outside of the thimble. Use refractory mortar, a special kind of mortar that is resistant to the high temperatures that will be present in a chimney thimble.
A chimney thimble is a circular ceramic fixture that protrudes from the side of a chimney, usually about 6 feet above floor level. Chimney thimbles are designed to accept single walled black stove pipe from a wood stove. Because chimney thimbles are made of ceramic and are essentially pottery, they are vulnerable to damage if they are hit with something. You may be able to repair a slightly damaged chimney thimble, but a smashed one will need to be replaced.
Remove a chimney thimble that is badly broken by knocking out its pieces with a hammer and chisel. Clean away any remaining chips of thimble and mortar by tapping the edges of the hole in the chimney with the hammer and chisel.
Spread fresh mortar around the circumference of the cleaned out hole.
Set a new thimble into the hole and move it back and forth to maximize contact with the wet mortar.
Hold some mortar in your fingers, reach in through the thimble into the inside of the chimney and work the mortar into the joint between the inside edge of the thimble and the inside face of the chimney.
Fill in any holes in the mortar around the edge of the thimble on the outside face of the chimney, then smooth out the surface of the joint with a trowel. Allow it to dry for 24 hours before attaching a stove pipe.
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