Tophat Circular Ring Kilns                                                    Hit Counter

Their Construction and Firing

By Michael Wendt August 20, 2008

2729 Clearwater Ave

Lewiston, ID 83501

below are two links for full resolution photos you can download and use for full size photos (very big files).

see tophat closed and firing      see tophat open ready to unload 

 

 

Top hat circular ring kilns are among the very easiest kilns to construct and thanks to the counterweight and pulley system, they are by far the easiest kilns to load and unload. It is especially important to assess the kind of work you intend to make before you select this design since the largest diameter pieces you can load will be 24"*, so if your work is larger, other designs are better suited to your needs. With that single limitation in mind, be aware that even pots as large as the entire kiln displacement can be easily loaded thanks to the low ledge height afforded by splitting the kiln body halfway up plus the 360 degree access which allows helpers to assist in the lifting and placement of huge pots. Splitting the kiln halfway up also reduces the required ceiling height, the size and weight of the counterweight system and makes stacking easier since the lower half provides an easy visual reference for stacking the kiln shelves.

Kiln sizes up to 12 cubic feet are very simple to construct, requiring just a few simple tools and some ingenuity. When I first started doing pottery in 1973, the common wisdom among potters was to buy or build the largest kiln you could ever imagine the need for and I made preparations to install a large car kiln once I had been making pottery a while.

To my surprise, these small ring kilns offered some distinct advantages over larger kilns that soon changed my mind about getting a huge kiln.

First, they are very fast firing, running cold to cold in twenty four hours or less, allowing much more flexibility than a huge kiln that might be fired only once or twice a month and which takes days to heat and cool. This not only allows the user more frequent feedback on the firing process but also makes the order process much easier to accommodate. Glaze experiments and mistakes also offer more frequent feedback. Product flow though the studio is faster too. Imagine an entire 100 cubic foot kiln load ruined by a glaze mistake and then imagine the same scenario with a 12 cubic foot kiln and the smaller kiln’s advantage becomes clear. At our peak in the mid 1980s we fired 24 kiln loads a month (using three tophats) which yielded an average of 200 cubic feet per month. The nicest feature of this capacity is more apparent now when we fire only 8 loads per month of the 12 cubic foot versions which total about 90 cubic feet per month. We still can fire twice a week easily to keep orders current.

Second, they are much cheaper to build and require much less furniture than a very large kiln. Moreover, since we have two that are the same design, they can share the same furniture, further saving expense. Additionally, high utilization rates mean a better use of capital when compared to the 40-100 cubic foot kilns so heavily touted by the majority of potters. Consider a $3000 kiln fired 8-10 times a month with an average load value of $1000.00:

A $3000.00 initial outlay returns $8-10,000.00 per month while a $12-15,000.00 large kiln fired twice a month still only returns $8-10,000.00 per month.

Third, the smaller amount of floor space means a smaller kiln room, further saving cost

Layout and Construction:

First, a diameter is selected. Most people choose either 23.5" or 28.5" inside diameter. The smaller size requires 10 bricks per ring so the included angle is 360 degrees /10 bricks = 36 degrees per brick. The larger size requires 12 bricks per ring so the included angle is 360 degrees/ 12 bricks = 30 degrees per brick.

Once selected, a compass is required to draw the inside diameter on a sheet of 5/8" plywood which will be used as the course layup pattern board to make sure every ring is identical and it is also used as a template for both the lid and the base. I use a sheet rock nail, some black annealed rebar tie wire and a number 2 pencil finely sharpened.

Wrap the wire around the nail after it is driven into the center of a 4' X 4' section of the plywood. The wire is then carefully wrapped around the pencil until the exact radius is reached and then the inside diameter is carefully drawn. Remove the nail and use a long straightedge to draw a center line across the circle to both edges. From these intersection points, a tape measure is used to measure one radius in chordal fashion to the edge left and right of each edge-centerline intersection point. This gives you 6 of the 12 facets for a 12 sided kiln. Next, carefully measure left and right from those 6 intersection points on the circle an inch more than ˝ the radius which will result in 6 sets of crossing pairs of lines.

Use the straightedge to connect these and the six other marks with fine lines drawn through the center.

Next take a framing square and project a line left and right of the radial lines where they contact the circle. These are the brick faces of the inside of the kiln.

Now, depending on whether you want the walls to be 2 ˝ " thick or 4 ˝ " thick, lay your Insulating Fire Bricks ( often abbreviated IFBs ) on edge touching that line or on the flat touching that line. This shows you exactly how and where the bricks need to be cut. I use a 10" miter saw with a carbide tipped blade, goggles and a dust mask. Before you cut any brick, get some cheap wood like pine boards and practice the layout, check the exact angles and fit the pieces of wood up until you have the angles set correctly since the scale on the miter saw is only close, not exact. I also constructed a stop block system made of a strip of metal so that I could make every brick identical. Cut all the bricks you need for all the rings plus a few extra to cover damage or breakage so that you don’t need to go through the whole set up procedure again later.

Stack one course of the bricks onto the board accurately, then trace carefully around the outside. Remove the bricks and cut the outside shape of the kiln board.

see layout directions

The lid and base bricks are stacked onto the board carefully to find the position that requires the least number of bricks to be cut. I trace the shape onto the bottoms of the bricks with a marking pen and then use a hand saw to cut them. I place them back on the board with the rest of the bricks and use a coarse 36 grit sanding block to finish truing and fitting the trimmed bricks.

Banding the rings is simple too! Go to a heating and air conditioning company and check to see if they have any strips of light gauge stainless steel sheet metal 4- 4 ˝ " wide and 10 feet long. Carefully wrap the band around the brick layer you want to band and mark the overlap if any. Remove the band and pop rivet two or more stainless steel radiator clamps which have been cut in half to the ends of the band, making it possible to loosen and tighten the band as needed. The bricks are mitered, then laid up on the pattern board and the metal ring is installed and tightened to sufficient tension to allow the ring to be lifted into place on the kiln hearth.

Both the lid and the base require both a clamping band and the use of Sairset® or some other kind of high temperature mortar. Cover the pattern board with a plastic sheet before laying up the bricks. I dampen the bricks before assembly to allow easier movement for best alignment. Once assembled, tighten the clamping band and let dry before moving. I cut the holes for the burner ports with a large drill bit followed by a threaded rod used to rasp the hole to the preliminary size. I enlarge them gradually over several firings until the kiln fires evenly. The top hole is cut using a 6" hole saw I made from a coffee can. I cut slits in the open end and bent them inward and outward alternately. In the end, I fastened a ˝" bolt which allowed me to turn it with a ˝" drill and cut the flue hole in the lid. I also recommend the installation of at least two lifting handles per ring and it works best to keep the weight of the rings down to an amount you and another person can easily lift.

see grooving photo

 As you work on the lift section ring, first install a 3/4" carbide router bit in the drill press and clamp a piece of angle iron to the drill press table in a location that will allow you to route a 1/4 " deep groove along the entire perimeter of the outside edge of each brick in the lift ring where the lift tabs welded to the lift ring will fit. The two lift rings consist of 4" x 1/4" steel or stainless steel flatbar formed to exactly fit the outside of ˝ the layup pattern board. A bolting tab with provision for two 3/8" grade 5 bolts is welded to each end of the lifting ring so that when bolted, they make a single ring that is an accurate fit to the bricks and in as true a plane as possible so that the bricks do not break when lifted.

A pair of large "ears" are welded to the ring 180 degrees apart. These ears have "U" shaped flatbar guides bolted to them and a provision for attaching the steel cables. The U guides need to be sized correctly to allow them to slide up and down the 2" steel support pipes that carry the cables and pulleys needed to connect the kiln to the concrete counter weight system.

The three "T" shaped pipe frames consist of 2" ID schedule 40 steel pipe sized for the head space you select. One is used as a guide for the counterweight and the other two act as guides for the kiln itself. Short pipe sections large enough to slip the 2" pipe OD are welded to the ends of the counterweight "T" as well as the kiln guide "T"s and have 3/8" nuts welded to them in such a way that 3/8" bolts can be used to clamp the kiln guide "T"s and the cross pipe at the other end of the kiln guide "T"s firmly in alignment. Obviously, all the pipes must be dead vertical and parallel to each other in order for the lift system to work properly. Buy four 4" diameter cast iron pulleys... two bored to 5/8" ID and two bored to 1" ID. Purchase two bronze bushings 5/8" OD x ˝" ID x 1 ˝" long and press them into the 5/8" bore pulleys. Once the "T" shaped kiln guide pipes are welded, measure 2" from the centerline of the vertical guide towards the wall end of the pipe and carefully drill a ˝" hole through both sides of the pipe so that a ˝" diameter bolts can be used to act as axles for the pulleys. On the counter weight "T", weld flatbar pads wide enough to accommodate two 1" pillow blocks which will support the 1" cross shaft used to synchronize the cable lift system to both sides of the counterweight.

The counterweight consists of a slide portion using the same "U" shaped bolt on flatbar slide guides bolted to a frame shaped like a capital "H" on its side. The lower portion of the "H" is "U" shaped to capture the small counterweight segments cast from concrete. I included a piece of rebar with a steel tab which had a mount hole pre drilled to allow the upper part of the counterweight to be bolted securely to the upper portion of the "H" shaped counterweight guide frame. The reason I made the counterweights of concrete (approx 40 LBS each) was to allow the kiln counterweight to be easily and quickly moved if need be. Also, the best lifting action comes when the kiln and the counterweights are as close to the same weight as possible.

Carefully clamp the cables to the mounting holes and tension them as close to the same as possible. On my kiln, I used a heavy turnbuckle on one of the cables to allow me to equalize the tension and make sure the kiln slid up and down easily.

Burners and firing configurations for these kilns are critical. Most Olympic styles now employ two burners but when they were based in Seattle, WA, they used six small burners which gave very even heat around the entire perimeter. If possible, I recommend this configuration of six 40,000 BTU/HR burners as opposed to the two burner design.

Some people report that their ring kilns fire too hot at the top while others complain the bottom is too hot. To obtain even firings, I believe a dual probe pyrometer is essential with one thermocouple 6-10" from the bottom and the other 6-10" from the top. In this way, the rise rate at both locations can be compared. I found my six burner kiln ran much hotter at the top than the bottom so I made some flame spreaders which sat directly below the bottom kiln shelf set 2.5" above the kiln bottom. I could adjust them outward over the burner ports. Using cone packs throughout the kiln, and moving them outward slightly more each load, I finally reached a setting that made the top and the bottom operate within a few degrees of each other. Make only very small changes each time to avoid overshooting in the other direction and set the damper very accurately each time to eliminate it as a variable. An oxyprobe is also a great tool for learning to fire this type of kiln. I learned to recognize the setting that was most efficient allowing the fastest rise rate possible which saves fuel and shortens firing time. We used to say:

"In by 10:00, over by 5:00."

Now days though, I have deliberately slowed to kiln firing rate by two hours near top temperature and the finished quality of the glazes is much smoother with far fewer seconds due to glaze blisters and other defects in finish.

Summary

Tophat ring kilns are ideal candidates schools, art programs or small home or production studios because they offer a unique blend of features that encourage potters to explore the medium. Most notably, they are very easy to build, load and fire. I have used them exclusively for over 35 years and plan to continue.

Michael Wendt