Building a Small Flat-top Kiln By Mel Jacobson and Kurt Wild
Make your dreams of having a fuel-fired kiln come true with these flexible plans
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ne of the most difficult and important factors to consider when planning to build a kiln is the kiln’s "scale." The potential size of a kiln is thoroughly discussed, debated and pondered whenever a potter decides to build a kiln. Kurt Wild and I have tried to address some of the questions about scale that arise when designing and building kilns. Without question, the most important question to ask when considering the scale of a kiln is how many pots can you make in a specific amount of time? Just how heavy is your production? Be honest with yourself. Make sure that the kiln that you build fits your production schedule and accommodates the size of the pots that you make. Building a large kiln, and then agonizing over how to fill it with enough work in order to fire it on a reasonable schedule, is a waste of your time and energy. Many potters are exposed to large kilns in school or the gas kiln at the local art center. These are high-production kilns, used by many people, making hundreds of pots per month. In many cases, potters use these huge kilns as a model of what they should build. Since the plans are flexible, the "Minnesota Flat-Top Kiln" design by Nils Lou can be sized to fit almost any space or studio. We have built them as small as 10 cubic feet and as large as 60 cubic feet with a car system. A kiln of about 25 cubic feet is ideal for a home potter that has limited space.
Keeping in mind that the flu size does not need to change and the burner system is constant in a large or small kiln, you can build a kiln to suit your situation. These kilns can be built to use natural or propane gas. A small gas kiln can use a limited amount of fuel and be as safe as electric firing—just a simple shed roof is all that is needed for cover in most situations. I have built several of these kilns in garages. Just keep in mind that a good amount of space is needed around any fuel kiln. As always, you need to observe some caution when building a kiln! Please check with your local building/zoning department and the fire department for area regulations before moving ahead with this project. It would also be prudent to contact your insurance carrier about your coverage. It is better to be prepared before you start, rather than making changes after you are finished. These plans are meant to be a basic guide to building the kiln. You can take these plans and modify them to fit the space and size of the area that you are planning to use. It is very simple math to change or modify these kilns. Since these plans are for the kiln we built for Kurt, we have provided a material’s list, and Kurt offers notes and suggestions gathered during the building process. Following the basic ideas of Nils Lou but being a bit creative will allow you to have a kiln that is "the perfect fit." We also suggest that you
obtain a copy of "The Art of Firing" by Nils Lou. This book contains a wealth of information on building and firing kilns, especially the Minnesota Flat-Top.
The Building Process The Foundation The site for building the kiln should be a flat, clear area with a compacted gravel base or a concrete slab. The base for the kiln and stack are constructed using a combination of 8-inch and 12-inch concrete block. This kiln has a layer of 8-inch block positioned so the holes in the block run horizontally to allow air to pass under the kiln. A layer of 12inch block with the holes in a vertical position is placed on the smaller block, overlapping the seams. Another layer of 8-inch block is placed on top of the 12-inch block in the same pattern as the bottom layer. (See Drawing 1) Once the concrete block is in place and level, place the expanded metal or cement board on top as the base for the kiln floor. The Floor The floor of the kiln and the stack consists of 3 layers of brick: First, a layer of hardbrick; second, a layer of softbrick (K 23); and third, another layer of hardbrick. The two layers of hardbrick can be laid in the same pattern, but the pattern of the
March/April 2005 • PotteryMaking Illustrated
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middle layer of softbrick should be changed so the joints are staggered between layers. You should pay close attention to maintaining a level, flat surface with each layer of brick. This attention to detail will help ease the construction of the rest of the kiln. (See Drawing 2) The Walls The first course of the wall is a soldier course. The bricks are placed on edge so the layer is 41/2 inches high. You should start with the back wall (See Drawing 3) to position the flue (41/2 x 7 inches) and the burner ports (41/2 x 41/2 inches). You will notice that the flue opening is just slightly off center on the drawing to allow for use of a full brick on one side. This will not affect the kiln’s operation. Continue with the soldier course and allow for the door opening. These plans show the door on the front wall of the kiln (See Drawing 4) but in Kurt’s kiln the door was placed in the sidewall. Either option will work. Some insulation bricks will need to be trimmed to size using a hacksaw or a regular handsaw. Once the soldier course is completed, continue building the walls. Rows 5 through 16 are laid in alternating courses (See Drawings 5 and 6). When starting to place the brick for row 5, you may need to adjust the brick on the back wall so that there aren’t any joints above the flue or burner port openings. Be sure to maintain the proper width for the door opening as you build the walls. You also need to build in peepholes for viewing the cone packs and the inside of the kiln during firing. Some people have used a brick built into the wall that projects into the chamber as a shelf for the cone packs. You do not need to do this if you place your cone packs on the shelves with your work.
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The Roof The roof consists of insulating firebrick stacked on end in multiple rows and clamped together. The first step is to cut out a 3/4-inch plywood deck that is just slightly smaller than the inside dimensions of the kiln chamber. Be sure to include the opening left in the wall for the door. This piece is mounted in the kiln using 2x4 braces placed at a slight diagonal to allow for easy removal when you are finished building the roof. Be sure to use plenty of braces around the perimeter and in the middle of the form. The top of the plywood should be slightly higher than the kiln walls to allow for the thickness of the ceramic fiber placed on top of the wall as a gasket. Tip: Place a chalk line on top of the plywood to mark the center of the kiln from front to back. This line will help keep the roof square while placing the brick on top of the plywood. When placing the brick in position, dip the top 2/3 of each brick in a very thin slip made up of equal parts fireclay, common sand and water. This slip should be thick enough to lightly cover the brick’s surface, not just stain it. Not coating the whole brick will prevent clay particles from falling into the kiln during the firing. The clay slip on the other part of the brick will help hold it in position when clamped together in the next step. Set each brick in position with the clean end against the plywood. Work from the centerline out to each edge starting at the front of the kiln. Butt the bricks as securely as possible, small gaps will disappear when the bricks are clamped together. You do not need to dip the bricks that rest on the wall into the clay slurry. Place the three hard firebricks on each corner as shown (See drawing 7).
PotteryMaking Illustrated • March/April 2005
Tip: Although you can perform most of the construction process as an individual, it is a wise decision to enlist help from others for several of these steps, especially when building the roof. You are now ready to place the corner braces and tie rods in place. Make the four corner braces by welding pieces of 21/2 x 21/2 x1/4inch steel angle together and drilling holes in the appropriate locations. (See inset on drawing 7) Position the braces, add the tie rods, nuts and washers, and snug them up evenly. Place hard firebrick splits between the brick and the tie rods on each side of the roof (See drawing 7) before tightening the nuts on the tie rods. This will keep the bricks from bowing out or the tie rods from bowing in during the tightening process. Once you have applied a slight tension to the roof structure, remove the middle inside support. Replace this support with a shorter one and a hydraulic jack. Raise the center of the roof about 3/4 inch to cause a slight domed effect. Continue tightening the tie rods in an equal manner working from front to back and side to side in several steps. A torque wrench is recommended and you should tighten to about 40 ft/lbs of torque. The roof is now a slightly domed, solid slab. Once you fire the kiln, you will want to re-tighten the nuts to 40 ft/lbs to allow for any stretching of the metal tie rods or shrinking of the brick. Tip: It is a recommended practice to check the tension of the tie rods after several firings throughout the kiln’s life. Corner Braces You can now add the bracing for the corners of the kiln structure. This consists of a 2x2x1/4
inch thick angle iron that is approximately 76 inches in length on each corner joined by tie rod or cable. A unique feature on Kurt’s kiln is the use of 1/8 inch steel cable and eyebolts to join the corners together. The eyebolts are placed in holes drilled through the angle iron. The holes are drilled so that the top holes are 2 inches above the roof and the bottom holes are 4 inches below the hard brick floor. One end of the cable is looped through the eyebolt and fastened with a cable clamp. The other end is passed through the opposite eyebolt, pulled tight and fastened with a cable clamp. When all 8 cables are in position, slowly and alternately tighten the nuts on the eyebolts to create an even tension on all the cables. The Flue Box and Chimney Stack The flue box is designed to create a double Venturi effect on the gases coming from the kiln chamber. This system of restricting, then expanding, then restricting and expanding again is used to create a strong draft of the flue gases and turbulence within the kiln chamber. The flue box uses a combination of hard brick laid flat and soldiered to make the 1-inch slot for the damper (see drawing 8). The damper is made from a 1-inch thick piece of Kaowool "M" board. The top of the flue box is the second area of restriction before entering the chimney stack. This opening is the same size as the opening from the back wall of the kiln into the flue box. The chimney stack is composed of 10 inch inside diameter galvanized pipe and Kaowool sleeves approximately 93/4 inch in diameter. These will slide into the metal pipe providing you with a ceramic fiber chimney. The chimney stack should March/April 2005 • PotteryMaking Illustrated
sit on a piece of the ceramic fiber board or blanket to provide an airtight seal against the brick of the flue box. The chimney on Kurt’s kiln is 10 feet in height. The height can vary to some degree without affecting the firing of the kiln. If your kiln is going to be located inside, the chimney needs to extend through and beyond the peak of the roof. It will be supported and secured by the roof structure. If the kiln is outside, you will need to support the stack either by having a shed over the kiln or using a guy wire system to hold the chimney in place. Burners The burners for this kiln can either be a high velocity propane or natural gas with forced air. Your burners should have the required safety devices such as a pilot/thermocouple safety shut off system and be installed to meet safety codes in your area. The burners are placed on either side of the chimney with the flame entering the chamber against the inside of the wall. Target bricks can be used to direct the flame where needed. These bricks can be moved to fine tune the firing of the kiln. Other Notes The internal measurements of Kurt’s kiln are 311/2 inches wide by 36 inches deep by 341/2 inches high for a total of 22.6 cubic feet. The actual setting space, using 2 12x24-inch shelves side by side set 2 inches off the floor, is 10.8 cubic feet. Facing into the kiln, the shelves are set so that they are 4 inches from the left wall and 2 1/2 inches from the right wall with the flue opening. The
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shelves are placed so there is a 6-inch space on both the back and front side of the shelves to act as the flameways for the burners. Materials list for Kurt Wild’s version of the Minnesota Flat-Top Kiln: 51 Standard 8-inch cement blocks (71/2 x 71/2 x 151/2 inches) 12 12-inch cement blocks (71/2 x 111/2 x 151/2 inches) 3 4-inch cement blocks (71/2 x 31/2 x 151/2 inches) Expanded metal or cement board as the first layer over the cement block and before laying the brick. For the kiln proper, 1 48x54 inch piece or 2 24x54 inch pieces are required. The stack base requires 1 18x18-inch piece. 800 K23 insulating firebrick 9x4 1/2x2 1/2 inches (this amount includes about 20 extra bricks to cover breakage and or cutting). 220 hard firebrick (straights) (9 x 41/2 x 21/2 inches) 8 #2 split hard firebrick (9 x 41/2 x 2 inches) 2 #1 split hard firebrick (91/2 x41/2 x 11/2 inches) 2 1 1/4-inch split hard firebrick (9 x 6 x 11/4 inches) 4 thin split hard firebrick (9 x 41/2 x 1 inches) 1 21/2 x 41/2x12-inch or 121/2-inch hard firebrick (for over damper slot) 12x24-inch shelves as desired Shelf supports as desired 1 piece of Kaowool 48x54 inches or 2 pieces 24x54 inches (optional/desirable added insulation over top of kiln) "M" board for damper and base of metal stack sleeve
10 12-inch Kaowool sleeves (81/2 inch ID x12 inch length; 91/2 inch OD), to line galvanized metal stack 2 5-foot-long sections of heavy galvanized pipe for stack (to be lined with the Kaowool sleeves). Using standard 2-foot lengths of galvanized pipe is not recommended, as the Kaowool sleeves do not readily slide down the pipe. Any heating and sheet metal shop can fabricate 5-foot lengths. Be sure to provide the shop with a Kaowool liner to ensure a decent fit. 1 72-inch piece of 2x2x1/4-inch angle iron is required for fabrication of the 4 roof corner braces. 1/2-inch cold rolled rod is required to connect the roof corner braces: 4 52-inch long pieces are required for the sides 4 45-inch-long pieces are required for the front and back (each rod must be threaded 3 inches on each end). 16 1/2-inch nuts are required and 16 1/2-inch washers. 4 Corner, upright angle iron braces (2x2x1/4 inch) 76 inches in length. 1/8-inch steel cable to fasten upright braces (you may want to have the cable cut to size at the hardware store as it is difficult to cut without proper tools). This kiln required 4 pieces, 56 inches long and 4 pieces, 48 inches long 16 5/16x4-inch eye bolts 16 5/16-inch nuts 16 5/16-inch washers 16 cable clamps
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PotteryMaking Illustrated • March/April 2005
Kaowool Layer
Stack
Corner Bracket
(See Materials List for rod length)