Due to the fire-retardant properties of earthen materials, hearths are generally crafted of bricks, blocks or stone. When you build a hearth of these materials, you'll use grout to hold them together and fill the spaces between pieces. When choosing grout for the hearth, it's important to keep the main function of the hearth in mind.
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Sealing the Hearth Materials
Depending upon the type of material you build the hearth with, you may need to seal the hearth's surface prior to grouting. When you grout porous materials, such as slate, the uneven surfaces can cause issues. According to Tim Carter of Ask the Builder, sealing the hearth materials with a grout-release sealer prior to grouting creates a level, nonporous surface that allows you to evenly apply grout and then wipe the slate clean of the grout that gets on the surface.
Grouting After Installation
As with any stone or bricklaying job, you install grout after the mortar or adhesive holding the material in place dries. This takes approximately 24 hours with most adhesives; consult your manufacturer's instructions. Once the adhesive is dry, spread grout liberally over the surface of the bricks, blocks or stone, and push the grout down into the spaces to fill them. Wipe any grout from the surface of the materials before it dries with a wet cloth. Once grout dries on the surface of the hearth made of porous materials, it can be difficult to remove.
Grouts to Use
Since the grout of a hearth faces constant heat and even the occasional touch of a flame, you must use a grout that can hold up under the temperatures. Grouts labeled "heat-resistant" are made specifically for this purpose. Fireclay is a heat-resistant substance that you can use in the same way as grout and holds up well under the heat in a hearth, according to Carter.
Over time, the grout in a hearth may develop cracks regardless of how well you care for it. When the grout in the hearth cracks, you can repair the cracks only by replacing the grout. To replace fireplace grout, you must first remove the grout that's already in place. Remove grout from a hearth with a chisel or screwdriver and hammer, then brush the surface clean. Afterward, reapply grout using the same method as above.