We continue our look at refractory materials in the kilns that fire our work. Learn about the properties and differences in kiln bricks and insulating fibers so you can choose what will work best in your firings. 

Defining the Terms 

Refractory: In ceramics, a material that is resistant to high temperatures, typically over 1000°F (538°C). 

Tensile Strength: The maximum stress that a material can bear before breaking when the forces applied to it attempt to stretch it or pull it apart. All brittle materials are weaker in tension than
in compression.

Thermal Shock: A change in the temperature of a brittle ceramic that is so rapid that it causes the material to crack or even break.

Kiln Bricks

Kiln bricks, also referred to as firebricks or refractory bricks, are made from ceramic refractory material that can withstand temperatures achieved in kiln firings. The bricks typically line the inside of a kiln (including floors, walls, fireboxes, chimneys, and lids). Kiln bricks are available in different shapes and sizes, and their material makeup varies depending on the firing type, i.e. electric, gas, salt, soda, or wood. 

Kiln bricks come in two forms: hard brick and soft brick (1). Hard firebricks are typically very strong and dense, and are used most commonly as structural support in kilns. Soft bricks are less dense than hard bricks and they are excellent insulators, making for minimal heat loss and efficient energy use. Soft firebricks are light and porous, and are also much easier to break and cut to size. These are usually found in electric kilns or any kiln or furnace that requires insulation. They are typically not durable in the kiln atmospheres present in salt-, soda-, or wood-fired kilns. Soft bricks are manufactured using a slurry-casting process, which creates a network of microporosity that produces low thermal conductivity, good thermal shock characteristics, and very efficient insulators.

Hard bricks, also known as firebricks, super duty, or high alumina, are very hard, dense, and durable. These bricks are very heavy, at about 8 pounds each. The color varies some, but most are all yellow or tan to light orange in color. Hard firebricks are resistant to abrasion and chemical atmosphere, so they are typically used in the construction of soda, salt, and wood kilns.

Insulating firebricks and hard firebricks share the same chemistry. To make them resistant to higher temperatures, their manufacturers reduce the quantities of fluxes and silica and increase the proportion of alumina in the recipes of bricks intended to be used at higher temperatures.

For art kilns, most hard bricks are made from fireclays, which are mainly hydrated aluminum silicates. Their temperature rating is adjusted with varying amounts of alumina or high-aluminum clays. 

Kiln bricks are available in a variety of shapes, though a standard brick is rectangular and measures 9×4½×2½ inches. Rectangular bricks measuring 9×4½×3 inches are becoming increasingly popular, since the 3-inch firebrick provides 20% more insulation. That’s a significant improvement in a kiln’s energy efficiency.

When choosing hard bricks for outdoor kiln applications, consider the following:

High heat duty bricks: Economical, high-duty firebricks that will withstand firings to cone 9. These bricks are suitable for backup linings, and other areas that encounter moderate operating temperatures. They are not suggested for abrasive conditions.  

Super duty bricks: Exhibit high strengths and low shrinkage, as well as good resistance to thermal shock. Can be used as the inside lining of a kiln and will withstand very high firings. 

70% alumina bricks: A chemical- and ceramic-bonded 70% alumina brick that can be used for soda-kiln applications and will withstand a firing to cone 38. 

1 Chart comparison of insulating soft firebricks and hard firebricks. Photos and information courtesy of Lynn Manufacturing, Inc.

Ceramic Insulation Fiber

Refractory ceramic fibers are synthetic fibers produced by the melting and blowing or the spinning of calcined kaolin clay or a combination of alumina, silicon dioxide, or other oxides. The fibers are needled into lightweight, flexible blankets and contain no inorganic binders. 

Ceramic fiber blankets (2) look like soft, puffy cotton but are durable, flexible, and have superior tensile strength. This lightweight, blanket-like material will keep heat in and is great for filling in small draft holes and helping to slow down your kiln’s cooling. Ceramic fiber blankets are used for applications from 1500°F to 2300°F (816°C to 1260°C). Kaowool ceramic insulation blanket withstands direct flame, continuous use at 2200°F (1204°C), and intermittent use at 2400°F (1316°C). Above 2400°F (1316°C), it will begin to shrink, become brittle, and eventually start to melt at 3200°F (1760°C). Ceramic fiber blanket is available in a variety of thicknesses, as well as in varying densities. In general, the denser a blanket is, the lower its insulating value will be.

2 Ceramic fiber blanket. 3 Ceramic fiber board.

Similar to ceramic insulation fiber in material are ceramic insulation board and ceramic fiber paper. Ceramic fiber board (3) is a high density, vacuum-pressed insulation produced through a wet-forming process using alumina-silica fibers and binders. It has high temperature stability, low thermal conductivity, and resistance against thermal shock and chemical attack. Beyond industrial kiln applications, ceramic insulation fiber board can be used for insulating support to brick and castable on kiln interiors.

Compressed ceramic fiber paper (4) is processed from washed, spun, high-purity fibers formed into a highly flexible sheet. Fiber paper has high tensile strength and offers low thermal conductivity.

4 Compressed ceramic fiber paper. Photos courtesy of Lynn Manufacturing, Inc.

Fiber paper contains an organic binder to provide increased handling strength at room temperature. Because of its high purity chemistry, this refractory paper resists both oxidation and reduction. This paper can be used as a backup lining, hot-top lining, or parting plane in a refractory lining. If it becomes wet due to water, steam, or oil, its thermal and physical properties will return upon drying. It is recommended for continuous use at temperatures up to 2300°F (1260°C). 

Caution: Do not touch, tear, or cut ceramic fiber insulation unless you are wearing gloves and a properly fitting respirator. Only use fiber blanket or fiber insulation in a well-ventilated area, even if you are outdoors.

the author Dave Finkelnburg is a studio potter, practicing engineer, and a regular contributor to Ceramics Monthly. He earned his master’s degree in ceramic engineering from Alfred University.

Topics: Ceramic Kilns