Published Apr 1, 2020
Boris Robinson’s introduction to making pottery was at a well-equipped museum studio with two large gas downdraft kilns. When the studio closed, he had to switch to electric. He missed the effects of reduction firing, but realized buying a gas kiln was out of his price range, so he went on a mission to convert an electric kiln to gas.
In today’s post, an excerpt from the April 2020 issue of Ceramics Monthly, Boris shares how he built his DIY downdraft kiln, and gives some safety advice for those interested in converting an electric kiln to gas. - Jennifer Poellot Harnetty, editor.
Ps. For more details on how Boris figured out how to convert his electric kiln to gas, see the April 2020 issue of Ceramics Monthly.
Design Specifics for Downdraft Kilns
There are a few minimum design considerations specific to this conversion method that if changed may impact the firing performance. First, affordable MR-750 Venturi burners were used rather than cheaper weed-burners or the more expensive forced-air burner systems. The brass orifice that comes with the MR-750 is too large, but new orifices (drilled with a #50 drill bit (0.07 inch)) can be ordered from Ward Burner Systems. The smaller orifice size increases the velocity of the propane gas in the burner and also allows for a higher pressure and finer flame adjustability. The burner is placed below the kiln pointing upward. The burner hole in the kiln floor is 3½ inches in diameter with the top of the MR-750 placed ½ inch to ¾ inch below the floor. This size and placement provide for the proper ratio of primary to secondary air during combustion—primary air passes through the MR-750 burner, secondary air passes around the burner.
Placing firebricks on the lid to dampen the flue exit controls the draft and reduction. This kiln can be fired in full oxidation with a flue exit dampened to an 8 to 10 inch opening, or an area of approximately 23–28 square inches. Due to the drag caused by the walls in the flue, decreasing the flue area may negatively impact the draft, especially if converting an electric kiln that has large element grooves.
A 0-30 PSI pressure regulator and 0-15 PSI gauge are used to control the gas flow to the kiln. The shut-off valves on the burner pipes are left fully open and the dampeners on the Venturi burners are left at ¾ inch open and not adjusted. A reduction atmosphere is achieved by dampening the flue exit with soft bricks. For the 28×32-inch kiln and beginning at 1652°F (900°C), the flue exit is dampened to 2¼ inches to stall the firing and create heavy reduction. Dampener adjustments as little as plus and minus 1/8 inch are made to keep the temperature between 1652°F (900°C) and 1742°F (950°C) for around 45 minutes. Since both the indicated pressure and the size of the dampened flue exit can be recorded in a log, there is direct feedback to help learn the best firing settings.
Make sure that the space between the bottom shelf and kiln wall leading to the flue contains at least 25 square inches for a 28-inch-diameter kiln and 18 square inches for a 27-inch-diameter kiln. This ensures that there won’t be a draft restriction leading down around the bottom shelf to the flue entrance. This also applies to any baffle or supporting blocks under the first shelf. And lastly, the flue exit in the lid should be on the side of the kiln, not at the back next to the hinge. I found that placing the flue exit near the hinge weakens the area and bricks will crack when the lid is lifted repeatedly. Also, the spy hole must be on the side and not in the area above the burners.
I am really pleased with how easily the kiln fires, the amazing reduction, and the repeatability of results. Utilizing thin kiln shelves as a thermal draft inducer proved to be the key in creating this inexpensive and simple-construction design, and I hope that it will provide more potters with access to cone-10 reduction firing. For a more detailed description of the conversions, including additional photos, dimensions, and firing schedules, visit www.sebastianmarkblog.com/2018/07/gas-kiln-conversion-downdraft.html.
Safety for Downdraft Kilns and Converted Kilns
The converted kilns were all placed in a steel shed and a hood connected to 6-inch steel ducting vents the kilns to the outside. As I do not leave the kilns unattended during the 6 hours or so of firing, I did not install a Baso safety valve. Simple pilot lights are used to ignite the burners and are not needed once the kiln gets red hot. However, some locations may require a Baso system per code, especially if the kiln is attached to a household propane tank. I do not candle overnight, but instead have an initial 10–15-minute period where the burner is set very low and the lid is open about ½ inch, and then a slow warmup continues for around 15 more minutes. This pre-heating has been sufficient to warm things up and get enough heat flowing so the draft works once the lid is closed.
the author Boris Robinson’s education was in both the arts and the sciences. The two have intermixed throughout his life, from inventing and patenting a dry-gas mass flowmeter in the 1980s, to working with black-and-white photography and printing with carbon gelatin, and more recently to making Tokoname teapots.