POTTERY PRODUCTION PRACTICES: Controlled Firings: Kiln Design Matters

February 2, 2006
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Part 1: Refractories

Every aspect of a kiln's design is a tool that can be used in its operation and control. Whether you decide to build or buy your next gas-fired pottery kiln, keep in mind that features such as the insulation and kiln furniture, burner orientation, exhaust system, thermocouple location and door design will significantly affect the kiln's firing performance and efficiency.

Every aspect of a kiln's design is a tool that can be used in its operation and control. Whether you decide to build or buy your next gas-fired pottery kiln, keep in mind that features such as the insulation and kiln furniture, burner orientation, exhaust system, thermocouple location and door design will significantly affect the kiln's firing performance and efficiency.

Insulating refractories play an important role in a kiln's operation. Lighter weight refractories will absorb less heat than heavier refractories, which can save fuel and shorten the amount of time required to fire the kiln. They can also reduce the amount of time required for cooling.

The ceramic industry standard for a fiber-lined kiln is to use an 8- to 10-lb density fiber for the side walls and a 10- to 12-lb. density fiber for the arch. Photo courtesy of Jerry Wagner.

Refractory Types

Commonly used insulation includes hard refractories, insulating fire brick and ceramic fiber. Hard refractories typically consist of standard and super-duty hard brick, as well as castables. These refractories are very dense and have a significant heat storage capacity. They are often used in salt and soda kilns, as well as wood-, coal- or oil-fired kilns in which the discharge velocity of the burner can cause refractory erosion.

Insulating fire brick (IFB) are available in various temperature ratings, but 2300 or 2600°F IFB is sufficient for most pottery kilns. By definition, IFB have a better insulating capacity than standard or super-duty fire brick and can therefore reduce fuel consumption.

Ceramic fiber is an excellent insulator and is the least expensive refractory lining to heat. It can be specified by temperature and/or composition. For example, Kaowool(r) blanket is an alumina/silica fiber with a

continuous use limit of 2000°F and a classification temperature rating of 2400°F.

Cerablanket(r) is an alumina/silica/zirconia fiber with a continuous use limit of 2400°F and a classification temperature rating of 2600°F.*

Ceramic fiber can be purchased by the roll or in a fabricated modular form with internal anchoring systems, and is specified by the density of the fiber (6, 8 or 10 lbs). Keep in mind that the weight of a fiber lining is based on a 12 x 12 x 12-in. module. For example, a 12 x 12 x 12-in. fiber module of an 8-lb density fiber will typically weigh 8 lbs, while a 12 x 12 x 8-in. module of equal density will weigh proportionally less. Most fiber manufacturers can custom make a module of any dimension, but the standard module is 12 in. wide x 12 in. long at a variable thickness.

The industry standard for a fiber-lined kiln is to use an 8- to 10-lb density fiber for the side walls and a 10- to 12-lb. density fiber for the arch. Industrial kiln building practices typically allow for 8 in. of fiber lining on the walls and ceiling of a kiln firing to cone 10.

Some ceramic fiber insulation has been linked to the inhalation of free silica, which is believed to pose a health risk, so it's important to request material safety data sheets (MSDSs) and follow the supplier's recommended guidelines when working with this type of insulation.

Temperature Limitations

It should be noted that industrial construction practices provide for all refractories to be de-rated by 150°F from their maximum temperature rating. For instance, a K-26 IFB (2600°F material) should be used in a kiln that operates at a peak temperature of less than 2450¡F. Similarly, a K-23 IFB (2300¡F material) should be used in a kiln with a peak operating temperature of less than 2150¡F.

When refractories are used at or near their rated limits, their effectiveness as an insulator begins to diminish, and the material can experience premature failure. Some common symptoms of refractory wear include:
  • Excessive shrinkage
  • Slumping or softening of the refractory material
  • Bloating or expansion of the refractory material
  • The development of a glassy hot face surface
  • Excessive grain growth, which leads to hot face surface compression and can cause spalling


Don't Forget the Furniture

When designing your kiln, be sure to consider the weight of all kiln refractories (brick/fiber/castables) and all setting refractories (posts/shelves), along with the maximum potential weight of unfired pottery and the volume of air required to be heated. (In industry, air is measured in pounds; 1 lb of air is equal to 1 lb of ware.) Once you have defined the total mass to be heated, you can then determine the total amount of energy (in BTUs) required to achieve the peak temperature, as well as the volume of air required to cool the kiln.

Editor's note: Part 2 of this article, which focuses on burners, exhaust systems, thermocouple location and door design, will be published in the Fall 2006 (September) edition of Pottery Production Practices. To find or purchase books on kiln design and construction, click on "Books" in the blue menu bar.



For Further Reading

Olson, Frederick L., The Kiln Book, 3rd ed., Krause Publications, 2001.

Remmey, Bickley G., Firing Ceramics, Vol. 2, World Scientific Pub Co Inc., 1997.

Rhodes, Daniel, Kilns: Design, Construction and Operation, 2nd ed., Chilton Book Co., 1981.


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