Tips and Tools: Flat-Top Kiln Bill Merrill

Appears in the April 2023 issue of Ceramics Monthly.

Compared to other kiln-roof designs, the flat top offers ease of construction and relatively lower weight of materials.

The advantage to a flat-top kiln roof is that it’s easy to construct and is lightweight, as each part of the roof can be handled by two people. Another advantage is the entire kiln only needs straight K insulating brick.

When starting construction on a flat-top design, the first thing to consider is the size of your kiln. For a new kiln, it is best to visualize it at full scale. Lay out the perimeter of your kiln design in firebrick. Lay in your kiln shelves and place a row of bricks to represent the kiln walls. This layout will give you the width and depth of your kiln.

With the kiln width decided, begin building the roof rows. Line the bricks up, butted together, standing vertically. The rows will be held together by a length of threaded rod fed through a hole in each brick. Measure the center of the hole to be drilled in each brick (2 1/2 inches from the top edge and 2 1/4 inches from the sides). Use a masonry drill bit that is ¹⁄₈ inch larger in diameter than the cold-rolled ³⁄₈-inch-diameter threaded steel rod (1). This allows the bricks to be perfectly aligned when putting the rows of brick together. Tip: A simple way to ensure all the bricks are drilled exactly in the same spot is to construct a jig out of scrap wood and clamp it in place on the drill press (2). 

High-temperature wire will secure the threaded rod to an angle iron placed across the top of each row. To avoid gaps between the bricks, carve a small channel on one side of some of the bricks to house the 16-gauge nichrome wire. Expedite this process by making a wooden template that connects the bottom curve of the drilled hole to top of the brick in a V shape, with the widest part of the triangle as wide as the angle iron (3).

Mark the channel lines onto one side of every third brick. Using a straight edge and a ³⁄₁₆-inch drill bit or utility knife, carve a channel from the drilled hole following your drawn line. Make the groove deep enough to allow for the nichrome wire to lay flush. 

Lay out your row of bricks, placing a brick with wire every three to five bricks and including one in the center. To stagger the roof seams, every other row should have a brick cut in half lengthwise and one half of the brick used on each end of the row. 

Insert the ³⁄₈-inch threaded rod (use coarse #16 thread) through the brick. Place the wire under the rod and let it travel through the groove. Compress the bricks together with the nichrome wire extended under the rod, up through the grooves. The wire needs to be long enough to go above the angle iron and be twisted together (at least 12 inches long). On each end of the threaded rod, place 2-inch fender washers with a 1/2-inch center hole and a compression washer to help hold the brick, then use a wing nut to secure. 

After the wire has been put in place above the angle iron, twist the wire by hand about two revolutions, enough to allow it to start to be pulled tight. Use a pair of vise grips to pull the wire tighter. Lift the row off the ground slightly and twist the wire slightly tighter. Make sure you don’t twist too many revolutions or the wire can break as it is fairly brittle. The wing nuts can be tightened with your pliers. 

After the rows are constructed, place them together on the floor (4). Before you install the brick rows on your kiln, lay a thin, ¹/₈-inch layer of Fiberfrax on the top of the kiln walls. This will seal the roof and walls, and will also cushion the brick resting on the walls.

Start placement of each row from the front of the kiln. If the bricks are not quite perfectly straight, loosen the wing nuts slightly and tap the brick into place. Then put on the second row. Complete the same step on each row as you go. There will be a perfect alignment of each row of bricks from front to back. To keep each row together, use mechanics wire around the overhanging threaded-rod ends (between the wing nuts and washers) to tighten the first row to the second, etc. (5). I have also used a large wood clamp to hold all the rows together. 

Now you have the roof completed (6, 7), and the last thing to do is cut strips of Fiberfrax and put them between the angle-iron rows of brick. While not necessary, it would make the kiln even tighter. 

My kiln can take five days to cool. If you have things planned out with your firing schedules and let the kiln cool at its own rate, the kiln will last longer and the majority of the glazes will be richer in color. 

Here are the measurements and supplies used for my stoneware kiln:

Measurements:

• 8 bricks wide (72 inches) and 6 bricks deep (54 inches)
• Interior space is 54 inches wide and 36 inches deep 
• Height from the burner-port floor to roof is 70 inches, with a raised floor to make a flue channel
• Height from the top of the flue channel to the roof is
  65 inches 
• The flue opening is 8 inches wide × 5 inches tall 

Supplies needed for the roof: 

• 12 rows of brick to construct the roof
• The angle iron for the kiln roof is 2 inches wide × 2 inches tall × three ³⁄₁₆ inch thick × 72 inches long
• Twelve 58-inch lengths of ³⁄₈-inch threaded rod, with wing nuts for each side of the rods (the threaded rod should be 4 inches longer than the width of the kiln) 
• Nichrome wire: lengths of multiple wires can be measured after determine the length of nichrome wire needed to attach the steel rod running through the brick to the angle iron

the author Bill Merrill is professor emeritus of art at Peninsula College in Port Angeles, Washington. 

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