Fire bricks are special heat-resistant bricks used in outdoor ovens, kilns, fireplaces and other high-temperature devices. They keep heat inside the firebox, reducing waste and preventing a fire or a heating element from damaging the wall on the other side. These bricks withstand cracking more effectively than ordinary brick but can crack or chip over the course of their lives.
Incorrect Brick Type
Different kinds of fireboxes use different types of fire brick. Most fireplaces and wood-burning ovens contain medium-duty fire brick, made up of about half silica and capable of standing up to about 2,000 degrees Fahrenheit. Pottery and glass kilns often require higher temperatures. These devices use heavy-duty fire brick, which contains more alumina and can reach temperatures of up to 2,640 degrees without damage. Using brick with a rating too low for your firebox can cause it to crack or break.
Exposure to extreme temperature inside an oven, fireplace or kiln will crack ordinary brick in short order. Fire brick can change from cool to hot without suffering serious damage, but they may nevertheless break down over time. Tiny hairline cracks won't affect the way your firebox works. Larger cracks can allow heat to escape to the outside. They could keep ovens and kilns from reaching the correct temperature, and cause fireplaces to work less efficiently.
You can repair cracked fire bricks using fireplace mortar or "fire cement." This material is specially formulated to withstand high temperatures that would crack or damage ordinary mortar. Use it on cracks 1/8 inch deep or shallower. For deeper cracks, fill the bottom 1/8 inch with fire cement, allow it to dry, then add another layer. Always clean the surface carefully before applying this material. After the cement or mortar has air-dried for 24 to 30 hours, heat the firebox to 500 degrees Fahrenheit or greater for one to two hours. This cures the repair completely and keeps it from crumbling.
Poor maintenance can increase the risk of cracks in fire brick, especially in high-temperature applications such as kilns. Paragon, a kiln manufacturer, recommends that kiln owners vacuum their fireboxes regularly to keep particles of clay and sand away from the bricks and heating elements. Coat the floor of the kiln with kiln wash, a protective coating, before firing. Avoid firing wet materials, since they release steam that can crack bricks.
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