Kilns: Fiber or Firebrick?

Kilns: Fiber or Firebrick?
By John S. Hohenshelt

There has been much discussion regarding the differences between brick and fiber kilns in light of the introduction of bronze and copper clays into the marketplace. This article explains the differences in these two insulating materials for kilns in relation to the firing requirements of the different metal clays.

The ceramic fiber kilns were designed originally to fire silver clays. Low-fire silver clay formulas can be fired successfully in less than 30 minutes, and kilns were developed to take advantage of this feature. These kilns use high temperature ceramic fiber with embedded elements. The fiber has very low thermal storage, which means it does not take much heat energy to heat up, and stores very little heat energy when fired. This characteristic of ceramic fiber kilns (also known as muffle kilns) allows more of the heat energy generated to heat the firing chamber and the metal clay instead of wasting it on heating the walls of the kiln. It also allows the kiln to cool faster, with the result that pieces can be both fired and cooled quickly. Most of the silver clay formulas currently available can be fired and cooled in less than 30 minutes.

Most firebrick kilns cannot perform these firings as quickly as the fiber kilns, so artists cannot fire as many projects in the same amount of time in a firebrick vs. a ceramic kiln. Also, firebrick kilns take longer to cool to a temperature at which an artist can remove the metal clay comfortably. Fiber kilns have shown their ability to fire hundreds of loads reliably year after year.

During the past two years, bronze and copper metal clays have been introduced to the marketplace. These clays require much longer firing than the low-fire silver clays. Firing cycles can be as long as ten hours, including long hold times, and require controlled cooling rates. Ceramic fiber kilns with embedded elements are not the best choice for firing these base metal clays for two reasons. First, the fiber does not store thermal energy (heat) as well as insulated firebrick for these long hold times. Firebrick kilns’ superior ability to store thermal energy allows firing temperatures to be maintained for prolonged periods while using less electricity and requiring less time with the elements running. It also allows the kilns to cool down gradually, at a controlled rate. Second, heating elements have a limited lifespan that depends on the amount of time electricity flows through them. The longer firing cycles for base metal clays shorten the lifespan of the elements more quickly, so the elements will need to be replaced more often than fiber kilns that are used for firing silver clay in shorter cycles.

As an example, a silver clay artist who does 200 30-minute firings in a year runs the kiln for a total of 6,000 minutes or 100 hours. A bronze clay who fires in 10-hour cycles may fire only 100 times, but puts 60,000 minutes or 1000 hours of wear on the elements. The bronze clay artist ends the year with only half the number of firings as the silver clay artist, but ten times the amount of wear on the elements and the kiln!

It’s similar to what happens when you drive a car. A 30-mile trip puts much less wear on the car than a 600-mile trip. Just as it is the total number of miles, not the number of trips, that takes a toll on your car, it is the total number of hours, not the number of firings, that puts wear on your kiln.

When the elements wear out on a firebrick kiln, they’re relatively inexpensive to replace. Replacing the elements in a fiber kiln can be costly, because the elements are embedded in the fiber muffle and the entire muffle must be replaced.



To summarize, a fiber kiln with embedded elements is the best choice for firing silver clay. They can be used to fire bronze or copper clays, but doing so will significantly reduce the number of firings before the elements (and, therefore, the muffle in which they are embedded) need to be replaced. A firebrick kiln is the best choice for firing metal clays that require long firing cycles. Firebrick kilns can be used to fire silver clay, but the total firing time will be significantly longer vs. fiber kilns because of the slower heating and cooling rates. The best choice for you will depend on the type of metal clay you use most often.

Hohenshelt_John_July09_W1John Hohenshelt began working summers at Paragon Industries at 13 years old.  He performed almost every job in the production facility from operating machinery, welding, crating, to stacking kilns.  After attending university (B.A. in Mathematical Economics and M.A. in Political Science, Juris Doctor.) He spent three years as a Captain in the U.S. Army. In 1998, he returned to Paragon Industries to work as a plant manager and learned electrical engineering and product design from his father.  In 2001, he was named President of Paragon Industries.



Article reprinted from Metal Clay Artist Magazine Vol. 1 Issue #4.

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