A common definition of refractory is, “stubborn, obstinate.” While that’s not the “resistant to heat” definition used in ceramics, it is close to what refractory means. Refractory materials really are obstinate about melting. They resist the heat applied to them and don’t melt at normal working temperatures. Just what “normal” is depends on each situation.
Defining the Terms
Refractory: Resistant to heat.
Mullite: A naturally occurring (but more commonly synthetic) aluminum silicate mineral containing no flux elements.
Kyanite: A naturally occurring mineral calcined to produce the mullite used in refractories.
Silica: The oxide of silicon and the most common silicate mineral, chemical formula SiO2.
Silicon Carbide: A dense, strong refractory composed of silicon and carbon with chemical formula SiC.
Compressive Strength: Ability of a material to support weight, expressed as weight per unit area.
Thermal Conductivity: Ability of a material to conduct heat.
Keeping it Simple
Fire brick is made almost, if not entirely, from clay (figure A). The clay contains some flux and that determines how refractory the fire brick is. The less flux, the higher the useful temperature of a fire brick will be.
Insulating fire brick (IFB) has the same chemical content as hard fire brick, but also contains a significant volume of air. The air makes the brick a good insulator, but comes with trade-offs. The more air in IFB the more easily it can be crushed, cracked or gouged. This limits how high the brick can be stacked and may limit its working life.
Ceramic fiber is entirely different but also the same. It’s composed of very refractory glass fibers with LOTS of air between. That makes it a great insulator that’s also very light. Ceramic fiber commonly takes the form of a flexible, and somewhat soft, blanket. It is also formed into rigid boards for areas where blanket fiber cannot be used.
Castables are refractory materials that come as a wet paste or can be mixed to a slurry from a dry powder. They are placed, allowed to dry until stiff, then heated by firing in place, as inside a kiln, to achieve final cure. Castables are denser and thus poorer insulators than either fiber or insulating brick rated for the same temperature. They are useful for special shapes such as kiln floors and burner ports.
Three common elements, oxygen, silicon, and aluminum, make up almost 83% of the earth’s crust. These same elements are used to produce most of the refractories used to make ceramic art. As described above, aluminum silicate refractories are available as dense and insulating fire brick, blanket and board fiber, and castable refractory.
Users of these materials must learn two important qualities of refractories—thermal conductivity and compressive strength. Thermal conductivity is a measure of how rapidly heat moves through a material. Smaller numbers indicate better insulating properties—heat passes through the refractory more slowly (B).
Compressive strength is relatively high for hard brick but can be quite low for insulating fire brick. Thermal conductivity rises with brick density, which is an indicator of brick strength. One must accept that as strength of an IFB or castable goes up, its insulating quality decreases.
Selecting Refractories for the Studio
Whether you are building a kiln or buying one, repairing a kiln or adding additional insulation, or simply purchasing kiln furniture, having an understanding of refractories before you buy will help you make a more informed and economical choice.
First, determine the maximum temperature your kiln will ever reach before you select refractory materials. Make sure any brick, fiber or castable is suitable for at least as high a temperature, or higher, than that maximum. Why do I need a 2400°F refractory if I’m only going to fire to 2250°F? Refractories are made to function in certain temperature ranges. If 2200°F brick is available, it just won’t work, or at least it will have a short useful life at 2250°F, so you’re stuck using the next higher temperature material available.
Hard and Soft Brick
IFB is typically made to a standard of the American Society of Testing and Materials (ASTM). The standard, ASTM C-155, rates IFB by the maximum temperature in degrees Fahrenheit at which the insulating brick can be used continuously in a fully oxidizing atmosphere. Thus IFB labeled NC-20 or K-2000 are both good to 2000°F. NC-23 or K-2300 are good to 2300°F.
Two important features of IFB should be noted: First, wet IFB will be severely damaged and eventually destroyed if allowed to freeze. Even in warm climates, wet IFB must be carefully dried out before firing or the brick may be damaged by the moisture in it expanding rapidly and forming steam that can burst the brick.
Because IFB is such a good insulator, high-temperature brick may be only required at the hot (interior) side of the kiln. Lower-temperature brick can be used on the cool outer side of the kiln. This can both save expense and provide better insulation.
Hard brick is usually described as Medium Duty, High Duty, or Super Duty. Medium Duty firebrick has a working temperature up to 2700°F. High Duty brick is good to 2850°F, while Super Duty is rated for 2900°F. Hard brick may be durable, but because it is so dense, it absorbs about three times as much heat as IFB while bringing the kiln up to temperature, and is also a relatively poor insulator.
Like IFB, ceramic fiber should be protected from the weather. When used for hot-face kiln insulation it must be installed using special fasteners. Buttons made from fireclay or other refractory material and held in place with a durable wire such as Kanthal wire are necessary to secure both ceramic fiber blanket and board insulation.
Castables are high-temperature cements. Like regular cement, there is only so much time between when castables are first exposed to air and when they set up hard. It is important to be aware of how long this “working time” is when deciding how much castable to mix.
Castable will air dry hard but will still contain some moisture. Thus it is absolutely critical to follow the manufacturer’s guidelines when heating and curing the castable the first time. Nothing will be more certain to destroy castable than heating it too rapidly the first time it is fired and causing a steam explosion in the cast refractory. It is much safer to heat cure castable more slowly than required than risk ruining it by a heating schedule that is too rapid to allow all moisture to escape from the castable. It is important to remember that brick mortar used in kiln construction and repair is also a castable and should be heated slowly and carefully during the first firing.
Choosing Kiln Furniture
Mullite kiln furniture and shelves are excellent choices for use from low temperatures up through cone 10. That’s because they are made from kaolinite clay mixed with mullite, a particular aluminum silicate crystal with a melting point above cone 38!
A word is in order about silicon carbide kiln furniture (posts, shelves and setters). Silicon carbide is typically manufactured by combining silica sand with a carbon source and heating the mixture in a kiln or furnace in total reduction. This is a high-temperature process requiring furnace temperatures from 2912°F (1600°C) to as much as 4532°F (2500°C). Thus it is energy intensive, which these days we could just as well call energy expensive. Well-made SiC kiln furniture is very strong for its weight and highly refractory, easily withstanding temperatures up to cone 16, far above ordinary ceramic art firing temperatures.
There are two liabilities with SiC. First, it is a porous material that can absorb water. Unless that water is totally dried out before a firing, the moisture can turn to steam inside the kiln furniture, thus fracturing it. Also, processing SiC into kiln furniture is difficult and requires careful, sophisticated controls. Purchasers should always investigate the quality of SiC kiln furniture carefully
Cordierite is a magnesium aluminum silicate mineral synthesized at high temperature from silica, kaolin, and a magnesium source. Some iron is also present. Refractories containing a high percentage of cordierite make excellent kiln furniture because they are strong and very resistant to thermal shock.
This article was excerpted from the December 2013 issue of Ceramics Monthly, which can be viewed here.
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