Fire bricks are lightweight, clay bricks that are known for their heat-resistant properties. Fire bricks are most commonly used to line and insulate kilns, furnaces and fireplace interiors. Fire bricks can be cut with an electric masonry saw, a fire brick handsaw or with a chisel and hammer. Do no attempt to use wood or metal saws and blades on fire bricks, as fire bricks are gritty and abrasive and will quickly damage your equipment.
Cut your fire bricks with a masonry saw and a diamond blade to achieve a clean straight cut. Masonry saws are either hand held or tabletop saws. A tabletop masonry saw is especially helpful when cutting large quantities of brick. Rent a masonry saw from a home improvement store, such as Home Depot, if you do not own your own. Wear safety glasses and a dust mask.
Opt to cut your fire bricks with a fire brick handsaw and a mitre box for smaller jobs. Fire brick handsaws cut the brick very easily because they are made with high quality nickel and steel and are resistant to the abrasive grains of a fire brick. These saws can be ordered from tool manufacturing companies like Carbide Processors, Inc. Miter boxes can be purchased separately. Set the bricks inside of a mitre box and fit the saw into the grooves to achieve straight cuts. Mitre boxes have grooves for cutting 90-degree as well as 45-degree angles.
Cut the brick with a masonry chisel. Draw a line around the brick where you want to cut it. Hammer a chisel along the line to score the brick. Make your score at least a half-inch deep. Set the brick in a clamp on a worktable so that the score line is just past the edge of a worktable. Give the side of the brick hanging over the table a firm whack with a masonry hammer. Use the chisel end of the hammer to break off any jagged pieces.