When members of the Northwest Potters Guild (NPG) in Portland, Oregon, needed to repair the roof of their flat-top car kiln, they decided to do an upgrade at the same time. Guild member Bruce Beattie reports the result is saving them on fuel with every firing.
Defining the Terms
Refractory Fiber: Heat-resistant insulation fiber capable of continuous use at over 1832°F (1000°C) and primarily composed of alumina and silica.
Thermal Conductivity: Rate of heat transfer through a given thickness of a material at a given temperature, in units of BTU-inch/hour-square foot-°F (watts/meter-K).
Density: Mass per unit volume of a material, in units of pounds/cubic foot (kilograms/cubic meter).
Linear Thermal Expansion: Increase in length of a sample of a material with increase of temperature, in units of inches/inch°F (meter/meter°C).
Flat-Top Kiln Design
The flat-top kiln design has been around for decades. It typically uses two fuel-fired burners located near floor level, one on each side of the flue exit, firing toward the front of the kiln. Larger versions often make use of a kiln car as the kiln floor and part of the front wall of the kiln. The car rolls out, allowing it to be loaded conveniently from both sides.
A flat-top kiln roof constructed from insulating fire brick always faces the problem of thermal expansion of the brick. During every kiln cycle, the brick heats up and consequently expands. Then during cooling, the brick shrinks by the same amount.
The design of a brick flat-top roof requires that the bricks be held in compression, literally squeezed by the external threaded steel rods on the outside of the roof (see 1). Unlike ceramic fiber, insulating bricks are brittle and tend to be crushed slightly by compression. During each kiln firing, the bricks expand with the heat increase, but there is no room for the expanding insulating bricks to grow. Instead, they are compressed against each other until some small amount of crushing of the bricks occurs.
Regular tightening of the rods that hold the bricks in place maintains the bricks safely in compression so they don’t fall out of the roof. However, that also ensures further crushing of brick occurs in the subsequent kiln cycles. Eventually, brick crumbs begin falling from the roof, cracks form, and finally the roof itself begins to fail.
An arched roof can grow upward to relieve most of the effect of thermal expansion. However, the arch requires more material than a flat top and is more time-consuming to build. It also applies lateral force outward against both side walls of the kiln. To utilize a sprung arch roof requires lateral support for both side walls of the kiln, support that is unnecessary with a flat-top design.
Ceramic fiber offers important advantages over traditional insulating fire brick in a flat-top kiln roof. First, the fiber weighs much less per unit of volume. Also, it offers lower thermal conductivity per pound of insulation. Fiber can be installed faster than brick. Finally, and of great importance with a flat-top roof design, fiber is compressible, so it resists damage due to thermal expansion each kiln firing cycle.
Comparing materials rated to cone 10, hard fire bricks weigh about 150 pounds per cubic foot (PCF), soft insulating fire bricks about 38 PCF, and ceramic fiber weighs less than 10 PCF. That means that a fiber roof of the same thickness as insulating fire brick would still weigh only about one quarter as much.
Building a Flat-Top Kiln
Unlike a traditional arched roof, the design of a flat-top roof calls for insulating fire bricks placed on end on a flat form. When the form is removed, the bricks are held in place by a thin coat of refractory mortar and the compression of threaded rods on the outside of the roof.
While the flat-top kiln design is inexpensive, easy to build, and fires well, like all kilns, it does not last forever. The roof requires regular attention to the nuts at the ends of the threaded rods to assure there is sufficient compression to hold the roof bricks in place. NPG members suffered some lost and damaged work when the roof of their kiln began dropping bits of crumbling brick while firing. Finally, some whole bricks fell out of the roof onto the top kiln shelf.
While discussing how to best repair the damage to their kiln that had occurred over time, guild members decided on a complete replacement of the kiln’s roof. The metal roof of the kiln shed is low, so there was not enough head room to replace the flat roof with an arched roof. Guild members, who met through ceramic art classes at Mount Hood Community College, say they fire their kiln to cone 10 in reduction at least 25 times a year.
Alternatives considered were refractory fiber in the form of blankets, boards, and/or blocks. The guild chose fiber blocks, which are made of the same refractory material as insulating fiber blanket and board but are both denser and thicker. After examining materials to use in replacing their kiln roof, guild member Bruce Beattie said, “We decided that cost, ease of installation, and time tipped the scale in favor of Pyro-Blocs.” Pyro-Blocs made by Morgan Thermal Ceramics were purchased through a local refractory materials vendor, High Temp Inc.
Pyro-Blocs (see 5) are available in four temperature ratings and thicknesses between 3 and 12 inches in 1-inch increments. For their situation, the guild selected MZR blocks that are 8-inches thick. The blocks are rated for a maximum temperature of 2600°F (1427°C).
Installation did not require any fasteners showing on the hot face of the Pyro-Blocs. Instead, the blocks have stainless-steel inserts built in. The inserts are fastened with nuts screwed onto threaded studs welded to the kiln’s steel shell (see 7, 8). The shell needs to be at least 1⁄8-inch thick to permit welding the studs in place.
Heat Retention and Loss
In considering the rate of heat retention/loss at the end of a firing after the rebuild, since only the roof was changed and walls and floor remain as originally built, the total thermal mass isn’t so much less that heat retention will be reduced very much. If NPG had gone to 100% fiber then it certainly would have made a difference. Pure fiber kilns typically have to be fired down to get slow cooling.
All images courtesy of Bruce Beattie.
the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his masters degree in Ceramic Engineering from Alfred University.