Firebrick is used to line fireplaces and kilns and is made mostly of silica and aluminum oxide. The bricks can withstand temperatures over 1371 degrees Celsius, but conduct heat poorly and tend to be brittle. When the bricks are installed or break, they are glued and patched with several formulations of a substance called kiln glue, kiln cement or kiln grout. The glue is sometimes mixed with ground firebrick and water to make a patching compound called kiln or refractory mortar.
Put on rubber gloves. All firebrick glues and mortars are stubbornly adhesive and some preparations include phosphoric acid.
Pour water into a paper cup. Pour a small amount of kiln cement into a second paper cup.
Dip a 1-inch paintbrush into the cup containing water. Wet the edges of the firebricks you want to glue with the brush. Wet any crack you want to seal with the brush.
Dip the wet brush in the kiln cement and stir. Continue to add water to the kiln cement until the cement achieves the consistency of ketchup.
Paint the edges of the firebricks you want to glue with the diluted kiln glue. Paint any cracks you want to seal with the diluted glue, as well.
Add firebrick dust to the diluted glue a little at a time and stir with a plastic knife to make refractory mortar. Add firebrick dust to the glue until the mortar has the consistency of thick mud.
Patch nicks in firebricks with the firebrick mortar, using the plastic knife.
Fire all firebrick glues and mortars to temperatures between 1,400 and 0-7.222 degrees Celsius depending on the directions that came with your kiln glue to set the glues and mortars.