Refractory Materials
Refractory materials (materials that can withstand high temperatures) are used in constructing and maintaining ceramic studio kilns. Fire brick, ceramic fiber, and castable refractories are the three forms of refractories used in kilns, but fire brick is the most significant.
A Brick with Many Faces
Fire brick is a generic term encompassing any brick that withstands repeated heating and cooling at various temperatures. Additionally, fire bricks must be able to withstand different atmospheres, provide various structural or insulating qualities, and, due to the difficulty of cutting them, be available in a variety of shapes to add flexibility to kiln design and construction.
Hard and Soft
There are two types of firebrick: hard brick and soft brick. Hard bricks are very dense and durable and are used for their structural qualities. They are often found as the main building component of large kilns, chimneys, fireboxes, and burner ports—anywhere around the direct flame. Soft bricks are lightweight and made from a refractory clay body containing combustible materials. Also known as insulating firebricks (IFBs) When fired, the materials burn out, leaving a sponge-like matrix of air pockets, which provide insulating qualities to the brick. These bricks absorb about half the energy as hard bricks during a firing. Soft brick ranges from 2000°F to 3300°F and is used as the brick of choice for constructing electric kilns or as insulating liners in reduction kilns.
Grades Are Important
The main ingredient in fire bricks is fireclay, which contains mostly alumina and silica, elements capable of withstanding high temperatures. Hard bricks are available in several grades, depending on their composition and properties, which determine their most efficient use in construction. High alumina compositions start at 50% alumina and increase alumina content to 98% for the highest purity and most expensive. A potter would rarely require an alumina content exceeding 70%.
Low duty: Typically rated to 1750°F maximum service temperature. It is primarily used for fireplace chimneys and contains 24–26% alumina.
Medium duty: Temperature rating to 2700°F maximum service temperature. Uses include backup linings, lower-temperature ceramic kilns, and chimneys containing 34–38% alumina.
High Duty (first-quality firebrick): Temperature rating to 2850°F maximum in purely heat service. Certain atmospheres can reduce this temperature rating by several hundred degrees and contain 36–40% alumina. Uses include boilers, ceramic kilns, chimneys, and backup linings.
Super Duty: Its temperature rating is 2900°F in pure heat service. Certain atmospheres can reduce this rating by several hundred degrees. It has the same uses as high duty, where higher temperatures are involved, and it contains 40–44% alumina.
High-Fired Super Duty: Temperature rating to 3000°F. A higher-burned version of super-duty firebrick is designed to lower porosity, increase physical strength, and improve resistance to alkali attack and carbon monoxide disintegration. It contains 40–44% alumina.
Most bricks are pressed or extruded. Straights, arches, wedges, keys, rotary kiln blocks (RKBs), and square-edge tiles are common shapes. Larger pieces are typically produced by air hammering the brick mix into wooden or steel molds sized for the desired shape dimensions.
The standard refractory brick size is 9×4½×2½ inches, also known as one brick equivalent (beq). This size is the most commonly used in pottery kiln construction. However, an equally popular standard size used in industrial furnace construction is the 9×4½×3-inch series. The 3-inch series brick reduces the number of joints in the kiln.
Straights: The standard straight is 9×4½×2½ inches or 9×4½×3 inches and is available in larger sizes up to 18 inches long, 9 inches wide, and 4½ inches thick.
Square edge tile: This term refers to “big” straights, which comprise 12×12 inches up to 24×12 inches with thickness ranging from 1½ to 3 inches.
Soaps: A term used to describe a half brick in width such as 9×2¼×2½ inches or 9×2¼×3 inches.
Splits: A term used to describe the thickness of a brick thinner than 2½ inches, splits come in 1, 1¼, 1½, and 2 inches.
Arch: This shape tapers in thickness along the width over its entire length and is used to make sprung or circular barrel arched roofs 4½ inches or 6 inches thick.
For 2½” thick brick:
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- #1 Arch: 2½” to 2⅛”
- #2 Arch: 2½” to 1¾”
- #3 Arch: 2½” to 1″
For 3″ thick brick:
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- #1 Arch: 3″ to 2¾”
- #2 Arch: 3″ to 2½”
- #3 Arch: 3″ to 2″
- #4 Arch: 3″ to 1″
Wedge: This shape tapers in thickness along the entire brick length and is used to make sprung or circular barrel arches 9, 12, or 13 inches thick.
For 2½” thick brick:
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- #1 X Wedge: 2½” to 2¼”
- #1 Wedge: 2½” to 1⅞”
- #2 Wedge: 2½” to 1½”
For 3″ thick brick:
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- #1 Wedge: 3″ to 2¾”
- #2 Wedge: 3″ to 2½”
- #3 Wedge: 3″ to 2″
Skew: A shape with a specific taper on one side or end to enable a sprung arch to be built. Four major types are available in both side and end skews that relate to the arch’s rise.
Recommendations
Here are some recommendations that will significantly assist you with your firebrick purchase.
The vast majority of pottery kilns are constructed of high-duty or super-duty brick.
Use extreme care when considering used refractories for building kilns. In most cases, you won’t know the conditions under which they were exposed. If they’ve been removed from a furnace, it’s usually because the lining failed. Properties of refractories deteriorate with exposure to extreme heat, chemical vapors, mechanical stress, and thermal cycling.
While insulating firebricks can be easily cut with a saw, hard bricks require special equipment, like a tile-cutting saw, to cut. Design your kiln to take full advantage of available shapes and, when possible, to minimize the number of hard bricks you must cut.