We usually encounter refractory materials in the kilns that fire our work. Learn about the properties and differences in kiln furniture so you can choose what will work best in your studio. 

Defining the Terms 

Cordierite: A low-expansion magnesium-aluminum-silicate refractory formed by heating a mixture of talc, clay, and mullite to about 2462°F (1350°C).

CTE: Coefficient of thermal expansion, the length per unit length that a ceramic material will expand on heating or shrink on cooling per degree of temperature change, typically given as units times
10-6/ °F.

Mullite: An aluminum-silicate refractory formed by heating certain clays to temperatures above 2192°F (1200°C).

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

Silicon Carbide: An extremely hard compound of carbon and silicon formed by melting a mixture of silica and carbon at temperatures above 2732°F (1500°C). 

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 Furniture Science

The first thing you need to know about refractory kiln furniture is that not all refractories are created equal. Composition, flexural strength, and maximum working temperature are the most important things to look for.

Refractory fiber and soft brick are used to insulate kilns. Kiln shelves and kiln furniture are made from much denser refractories. Let’s start with shelves, the posts that hold them up, and the setters used to support ware on the shelves.

Mullite, cordierite, or silicon carbide (usually abbreviated SiC) are commonly used refractory materials for shelves and posts in ceramic-art kilns. Mullite is made from a mix of alumina and clay. Its fired composition is nominally 68% alumina and 32% silica. Mullite shelves and furniture are less available than those of cordierite. The latter is made from a mix of talc, clay, alumina, and silica. Pure cordierite is almost 14.8% magnesium oxide (MgO). However, because mullite is almost always added to the shelf body, the composition data for kiln shelves will typically show around 9% MgO. High-alumina shelves are cordierite-mullite-alumina and will have even less MgO. High-alumina cordierite shelves are stronger and have a higher maximum working temperature.

Cordierite-mullite shelves are, on a first-cost basis, the least expensive shelves available. The CTE of cordierite is also the lowest of common kiln-shelf materials. Cordierite shelves, however, can be weaker and have a lower maximum working temperature than SiC. Virginia ceramic artist and educator William Schran has written that cordierite shelves are not recommended for use in wood and salt/soda firings. The vapors of sodium in salt, soda, and wood firing as well as potassium in wood firings shorten their useful life. Because of its lower strength, a shelf of cordierite-mullite needs to be thicker than many mullite or SiC shelves, and is therefore heavier. The weight of shelves and posts in a firing may exceed that of the ware. That means the energy cost to heat the load may be mostly to heat shelves and supports. Given rising energy costs, lighter, if more expensive, shelves may pay for themselves over time. Still, cordierite is the best choice for many kiln applications. Electric kilns used at lower firing temperatures permit use of thinner, thus lighter cordierite shelves—though all shelves warp if their working temperature is exceeded. 

Corelite shelves from RESCO Products, Inc. are cordierite-mullite shelves engineered to reduce weight and increase strength. The company rates them for use up to 2336°F (1280°C).

SiC is used to make stronger kiln shelves and supports that also withstand higher temperatures. Unfortunately, SiC is also an electrical conductor and thus it can be dangerous if used in electric kilns. SiC also has a higher CTE than cordierite. That makes SiC less resistant to thermal shock. Because of their greater strength, SiC shelves can be made thinner and lighter yet still have the same load-bearing capacity. The weight reduction means better energy efficiency. Thin SiC shelves also take up less space in the kiln. 

Not all SiC is created equal, however. The lightest, densest SiC shelves I’m aware of are the nitride bonded ADVANCER line made by Saint-Gobain. They’re very well made, which is reflected in the cost. Other forms of SiC shelves are available and offer higher strength and similar thermal properties. However, as the percent of SiC in the product decreases, so does its maximum working temperature. 

With the possible exception of ADVANCERs, all kiln shelves need kiln wash applied to the top surface. If you regularly flip shelves over to help them stay flat, you’ll need to clean off the top first and then apply wash to the other side to keep wash from falling on to your ware. They also need to be stored in a dry location away from snow, rain, pooling water, and humidity. Refractories can absorb moisture and then explode when the water turns to steam in a kiln. If in doubt, heat the kiln slowly up to the boiling point of water. I like four hours of heating at less than 190°F (88°C) to dry out ware and refractories.

Kiln Shelf Comparison Chart

Above: Kiln shelf comparison chart courtesy of Bailey Pottery Equipment Corporation. For more information please visit www.baileypottery.com.

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.