The growing popularity of metal clays such as BRONZclay™ and COPPRclay™—not to mention fine-silver and sterling metal clays such as PMC®—has sparked a great deal of discussion of the differences between firebrick and ceramic fiber kiln interiors.
As a metal clay artist, how do you choose the right kiln for your clay work? Let’s take a look at the differences between the two materials for kilns and what they mean to the firing requirements for the different metal clays. Much of the information in this post is excerpted, with permission of the author, from an article written by John S. Hohenshelt, president of Paragon Industries, a well-known manufacturer of kilns. Thanks, John!
Ceramic Fiber Kilns
The inside of a ceramic fiber kiln is made of spun brick. The same materials that make a firebrick are heated to melting, injected with water, and forced through nozzles at high pressure to form fibers. The fibers are collected, compressed and laid together to form a soft, pliable blanket. Heating elements are then embedded in the blanket and are not visible in the chamber. Because a lot of air is trapped among the fibers, the blanket is a terrific high-temperature insulator.
Ceramic fiber kilns were originally designed to fire fine-silver clays. Some fine-silver clay materials require less than 30 minutes to fire with excellent results. Paragon developed its “SC” line of kilns to take advantage of the quick firing properties. The ceramic fiber has very low thermal storage, so that it takes less energy to heat up and stores very little heat energy during firing. This allows more of the heat generated by the elements to go toward heating the firing chamber and its contents rather than be held in the walls of the kiln. This also allows the kiln to cool faster after firing is complete, so that the pieces inside cool more quickly. Most firing on fine-silver clays can be accomplished in less than half an hour.
The inside of a firebrick kiln is a wall of interlocked refractory ceramic blocks that are lightweight, somewhat porous and can withstand high temperatures while offering low thermal conductivity (great for energy efficiency). The heating elements are set into grooves in the brick walls and are easy to see.
Firebrick kilns can’t accomplish firing processes at the same speed as the fiber kilns and take longer to cool enough to let the artist safely remove the fired clay. As a result, artists can’t fire as many projects in a given period of time as they could with ceramic fiber. Over the years, the ceramic fiber kilns have shown their reliability, firing hundreds of loads year after year.
The Next Generation of Metal Clay
Enter the more recent development in metal clay: bronze, copper and sterling clays. Bronze and copper clays require much longer firing times than fine-silver clays do. Some of the firing sequences run for up to ten hours and include long hold times—quite a difference from the fine-silver firing programs.
Fiber kilns are not the best kilns for these clays. First, because the fiber doesn’t store thermal energy as well as insulated firebrick and requires more electricity, running longer, to keep the temperatures consistent for these long hold times. Second, heating elements have an expected lifespan based on the amount of time electricity flows through them. The long firing times decrease the number of firings a kiln can accomplish before the elements need to be replaced, so that elements need replacing more often than they would firing only silver clay. Because they’re embedded into the ceramic fiber, the elements cannot be replaced separately. Instead, the entire firing chamber (fiber and elements) must be replaced as a single unit.
A firebrick kiln, on the other hand, with the high heat retention of the firebricks, easily holds temperatures while using less electricity and with the elements running for shorter periods of time. When the heating elements do need replacing, because they’re set into grooves in the firebricks and exposed, the relatively inexpensive elements themselves are simple to remove and replace.
The Wear & Tear Factors
Think of the wear factors this way: A silver-clay artist can fire 200 times in a year and have only 100 hours of actual usage time (1/2 hr. per firing x 200 firings) on the kiln’s elements. A bronze-clay artist may fire only 100 times, due to the longer firing time per cycle, but will have about 1000 hours of usage time on the kiln’s elements (10 hrs. per firing x 100 firings). The difference between 30 minutes per use and 600 minutes per use (10 hrs. x 60 mins./hr.) is significant. The bronze-clay artist ends the year with half the number of firings but ten times the wear on the kiln and its elements.
Put another way, you could think of firing like driving a car. If the trip is only 30 miles, the wear on the car is much less than it is on a 600 mile trip. It is not the number of firings (trips), but the minutes of firing time (miles) per trip that ultimately matter.
So, What Kiln Do You Need?
How do you choose the right type of kiln for your needs? The answer comes down to: What do you work with most?
A ceramic fiber kiln, with fast-heating embedded elements, is the best choice for firing silver clay, and it can be used for longer firing programs. The artist should keep in mind, though, that the kiln will produce fewer firings over its life as a result of firing these materials.
A firebrick kiln will be serve better, and longer, for firing the metal clays with long firing times, and it can also be used to fire fine-silver clay. The kiln may not fire these pieces as quickly as the fiber kilns and will take longer to cool enough for the artist to handle the pieces, but the elements will last longer between replacements.
Which type of kiln do you use? What do you fire in it? Share your experiences; other metal clay artists would love to have your point of view. And, if you have questions, Rio’s Tech Team is always there, ready to answer your questions.