TDI Downdraft Kiln Conversion

TDI Downdraft Kiln Conversion 2023 Revision

The November 2023 book revision presents new and revised text and delves into refinements, tips, and information largely gleaned from comments and questions within the Facebook TDI Downdraft Kiln Conversion group posts and emails. The kiln build has evolved over the past 6 years with great input and experimentation of several serious potters. The evolution has resulted in more even firing temperatures and knowledge of the firing parameters and conditions. The overall inlet/flue design and dimensions have not changed from the original 2018 blogpost. 

The Firing the Kiln section was largely revised and was included and updated to the Firing the TDI Conversion blogpost, dated April 2022. The October Further Study 2023 blogpost includes the Deflection Block and other items and was also updated for the revised book text. 

The Amazon link to the book is the same: https://www.amazon.com/dp/B084DH88GH

The original blogpost: 2018/07/gas-kiln-conversion-downdraft

Firing the TDI Conversion blogpost: 2022/04/firing-tdi-downdraft-kiln-conversion

Further Study 2023 blogpost: 2023/10/tdi-downdraft-kiln-conversion-further

The purpose of the book is to explain the conversion of a typical 23” and 28”- wide electric kiln to a TDI propane fueled downdraft kiln. The TDI kiln came from me wanting to continue high-fire pottery after the local school had equipment issues and is the result of research and various prototypes made back in 2016. The original TDI kiln firings were good and I was asked to share the design dimensions and build. The idea for writing this book is to provide much more detail of the conversion and firing processes and help promote at-home or small studio reduction firings that can produce very reliable and repeatable results.

My advice:

  • Plan a stock build by following the book. The TDI kiln works very well and the design has been proven by many happy potters.
  • This will require some effort. The more you put into understanding gas reduction firing and the TDI kiln conversion process, the easier the conversion and the more successful your firings will be.
  • Before beginning the conversion, read all of the book and try to visualize and understand the building process. Before firing, read the firing sections to completely understand the firing process. Every detail has a reason behind it.
  • Plan the initial firings (and every firing) with a list of gas pressures, flue dampening sizes, and temperature target points. Try and visualize the gas flow path through the ware, packing accordingly. Use a firing log to document glaze, kiln, and firing conditions. Be patient. After the firing, analyze the pressure/dampening/temp-change-per-hour numbers and choose numbers for the next firing. Eventually, you’ll have confidence and a set of pressure/dampening numbers that makes firing outcomes relatively repeatable.

What is not in this book and has to be learned elsewhere is a basic knowledge of the reduction high-fire process. Britt says in his The Complete Guide to High-Fire Glazes, “Without a proper understanding of kilns and firing principles, successful firings are merely random occurrences”. I recommend the “Kilns, Firing and Safety” chapter in this book. Search the internet for “reduction firing”. A good firing procedure resource was from Val Cushing. Search for: Val Cushing High Fire Process pdf. Another one that explains terminology, energy parameters, and the building process is the Kiln Book by Frederick Olsen, which can be a reference for converting different sized kilns or for making design changes.

TDI Kiln Overview

The following provides a quick summation of important items in the build and firing process and sometimes why certain things are done versus other possibilities.

Kiln Size for Conversion

The manual details two kiln interior sizes: a 23” width by 27” tall, and a 28” width by 27” tall. These are approximate dimensions since different kiln manufacturers have slightly different interior sizes. The two recommended burners will handle larger and smaller sized kilns and there is a discussion of possible larger sizes depending on using one or two burners. The inlet and flue cross-section sizes are set constants based on either using one or two of the burners discussed.


Both the MR-750 and the LSMIITTH cast iron propane forge burner have been used and work. These are not expensive relative to the overall cost of having one’s own gas kiln. For alternative burners, provided are specific references to the amount of heat (BTU’s) verses propane pressures during the typical firing so as to help in assessing other burner alternatives and the potential amount of heat required.


The hole in the burner that the propane gas flows through is termed the orifice size. The TDI build is based on a #50 orifice, also called a #50 drill size or 0.07 inches. The MR-750 comes with a #38 orifice (larger hole size) and is removable and MUST be changed to a #50. The LSMIITTH burner has a fixed smaller #70 orifice hole that can’t be changed. It must be drilled to a #50 sized hole – a #50 drill bit can be purchased online. Yes, smaller drill number – larger hole size.

Pressure Gauge/Regulator

The TDI firing process references pressure in Pounds per Square Inch, or PSI. A 0 to 3 PSI pressure gauge is used and is mounted to the pressure regulator attached to propane tank. There is a hose of about 8 feet attaching the pressure regulator to the burner. This gauge location and the long hose makes it easier to adjust the gas pressure during firing and provides a finer adjustability at low pressures. The 0 to 30 PSI regulator shown in this manual provides very acceptable fine adjustments and is not too expensive.

3.5” Burner Inlet Hole in Kiln Base

For an efficient and complete combustion of the propane, there is a specific amount of air required. The air going through the burner itself is called primary air, and the air going around the top end of the burner and through the inlet hole is called secondary air. This must be in balance and is based on the orifice size and the pressures used for the TDI firings.

The burner is placed below the kiln and has a vertical flame path up the wall. This may mean that the kiln stand needs to be set on small bricks for the 9” long MR-750 to fit underneath. A vertical flame path is very efficient and helps maximize the pottery ware area. Side entry with deflector blocks was considered but the loss of kiln setting area combined with the loss of heat radiated through the deflector blocks made this a less desirable option for such small kilns. Noted is that the LSMIITTH burner is only about 7 inches tall after assembled with a 90 degree elbow.

Flue Cross Section Sizes

The TDI flue dimensions are detailed and minimum flue cross section areas are explained. These dimensions were calculated and based on the BTU’s generated by the burners so as to obtain both reduction and also Cone 10 temperatures. Making them larger might just waste a bit of ware setting space and making them significantly smaller might cause the kiln to stall and not reach your desired temperature.

Propane Tank Sizes

I would recommend using two 30lb propane tanks for most firings. A single-burner TDI kiln will use about 20 pounds of propane and having a second tank is just a precaution. Use one for firing, and then the second one for the next firing knowing you have a 10 pound reserve should something go amiss. The two-burner TDI kiln may consume around 30 to 35 pounds of propane so both tanks will be used in the firing. I find that 30lb tanks aren’t too heavy to move around and put in the back seat of the car, securely seat-belted, to take for refills. An empty 30lb tank weighs about 25 pounds, and about 55 pounds full.

Pilot Lights

The basic burner set up uses a very simple pilot light and, after several years of firing the TDI kilns, I’m not sure it is really needed. The original 1/4” tube with a hole in it is a very inefficient pilot setup and a better alternative is presented in the manual. For the first 2 to 3 hours, the kiln should not be left unattended in case the wind blows the flame out or there is a regulator failure, although this is not very likely and I haven’t experienced it. Once the interior is red hot, or around 900C/1652F, the propane will ignite from the kiln interior and a pilot is not needed. This is then the time after that I might feel comfortable leaving the kiln for a short bit of respite. However, without a BASO type safety system, the kiln should not be fired unattended.

Firing Guidelines

There is a long section on firing the kiln and doing the initial test firings to learn the attributes of your TDI kiln. Having a plan and approaching the firing analytically will speed up the learning process for getting repeatable results. Even if you are an experienced potter and gas firer, one still has to learn the PSI/dampening setting numbers to achieve anticipated results.

The firing temperature numbers and schedule that I use and refer to are largely from the writings of Val Cushing and what was taught to me by Walford Campbell. The clay and glazes I use are for Cone 10 firings. Others may use or teach different schedules and, like any gas kiln, other firing temperatures can be used for different clays and glazes.

Even Top to Bottom Kiln Temperatures

At best, kiln temperatures within gas kilns can vary around the kiln 1 cone or more and the expectation of having completely even temperatures is not realistic. A temperature evening-out process is described in the firing section that may be recognized by potters experienced with firing updraft kilns. Kiln packing also has a significant effect on hot and cold spots and just has to be learned and is also described in a section following. Know that many people are getting relatively even temps in their TDI’s with hot and cold areas anticipated. Rather than blaming the kiln, record and analyze the ware placement and the evening-out process during firing. Take photos of the packing to help you remember. If you’re still getting poor results, maybe pose a question on the TDI Facebook group.

Shade for Seeing the Cones

Available on Amazon for looking into the peep hole to see the cones - Forney 57005 Lens Replacement Hardened Glass, 4-1/4-Inch-by-2-Inch, Shade-5 Green. I like Shade 5 as it isn’t too dark.

Alternative Sized/Shaped Kilns

The only advice I can give regarding different sized kilns is based on my experiences with the two stock TDI conversion designs and information from potters who have done oval TDI conversions. Two important parameters are burner power relative to the cubic foot size and heat loss through the walls.

A single MR-750 burner will provide enough power for a 7 cubic foot kiln with about 30% additional BTU power available without changing the inlet hole size or PSI gauge, equating to a maximum single-burner kiln size of around 9 cubic feet. The two MR-750 burners used in the 10 cubic foot 28”x27” kiln conversion have a considerable excess of power and can likely work for kilns up to around 16 to 18 cubic feet.

This is a good spot to introduce the term BTU, which is British Thermal Unit and how heat energy is measured per hour. One gallon of propane produces approximately 91,500 BTU’s per hour. A 30 pound tank holds 7 gallons of propane.

The MR-750 or LSMIITTH burner with a #50 orifice puts out the approximate BTU’s listed below. For the 23” kiln TDI conversion, the typical firing pressures range from approximately 1 to 2.5 PSI (pounds per square inch), indicating 55,700 to 88,100 BTU’s. The 28” kiln has similar pressures but usually not much over 2 PSI.

The guiding dimensional factors in the conversions are the burner-inlet size and the relative flue cross section area. The burner inlets are 3.5” in diameter. The flue cross section areas were made larger than my calculations showed as a safety factor and to account for parasitic drag in the flue. The approximate flue cross section area for the TDI single-burner is 22 square inches and for the two-burner conversion it is 35 square inches. These areas work for the stock TDI’s but may not work if the kiln size and/or BTU’s requirements are significantly increased. That’s part of the experiment.

The other consideration is heat loss through the walls for larger kilns, including oval shaped kilns. L&L Kilns did a BTU analysis for their Easy-Fire and Jupiter kilns and it is available on their website. For their 23”x27” kiln with 2.5” thick brick at 1285C/2350F, the heat loss was listed at 18,593 BTU’s per hour, or about 907 BTU’s per square foot of kiln surface area. For the 23”x27” kiln with 3” thick brick at the same cone 10 temperature, the heat loss was 15,439 BTU’s, and for the 28”x27” it was 20,204 BTU’s, or about 763 BTU’s per square foot.

So, this means that the 23”x27” TDI kiln with 2.5” brick, at near cone 10, and with a propane pressure of around 2.5 PSI, 18,593 BTU’s of the total 88,100 BTU’s being produced by the burner are going through the wall, and only about 69,500 is heating the pottery. Larger oval kilns have more surface area and thus even greater heat loss through the bricks.

Alternative Pilot Lights and Ignitors

The pilot shown in the original build is pretty simple and basic. There are many after-market or replacement pilot light assemblies that can be used. One that is easy to install is the MCAMPAS Pilot Burner and 750 Millivolt Thermopile Assembly available on amazon. Using a hacksaw it can be detached from the mounting plate and connected directly to a 1/4” copper tube. This one is less susceptible to blowing out in the wind. Another inexpensive item is a piezo-electric grill ignitor and MCAMPAS makes one with two leads called 2 Set Piezo Spark Ignition Kit. The ground lead is attached under a burner clamp. The photos following show how I attached them. I attached a screw to the piezo tab and then used a hose clamp to secure the screw to the pilot light. 

Alternative Burner

The MR-750 burners are usually easily available in the US but this isn’t always true of other countries. There are many burners available with Forge Burners being the most prolific and lowest cost option. Forge burners are typically designed to have a tight blue flame to be used in small foundry kilns/containers. For the small ceramic gas kiln, what is best is a larger more turbulent flame. 

A forge burner that appears available in most countries through Amazon or AliExpress is the cast iron LSMIITTH propane forge burner. The nice thing about this burner is that the burner pipe is threaded with 3/4” pipe threads allowing a 90 degree elbow to be used, which means it can easily be placed under the kiln. The venturi burner is rated at 100,000 BTU’s and has an adjustment for primary air. The orifice hole that comes with the burner is smaller, around #70, so it can drilled to a #50 (0.07”). It will require a pipe thread 3/4” 90 degree and around a 2 1/2” threaded 3/4” pipe nipple. Below is a product picture. 

After attaching the 90 elbow and the nipple, and drilling the orifice to #50, below is the burner I tested. The burner is 7” tall and so should fit under most kiln stands. I placed it about 1/2” under the kiln bottom. 

For the burner tests, I just used a large clamp to hold it in place. In the photo below, shown is a mount made of a U-bolt and an inexpensive strip of galvanized metal called a Strap Tie, drilled for the U-bolt. 

A note about drilling the LSMIITTH orifice. I suggest to unscrew the handle and remove the needle valve stem before drilling and then after make sure there are no metal bits inside. The needle valve has two lubricated O-rings and metal bits may stick requiring removal with a pin or the like. 

The MR-750 with a #50 orifice will provide around the same BTU’s at the listed pressures. Assuming a proper and complete combustion, it is the pressure and the orifice size that determines the BTU’s, so the LSMIITTH burner with a #50 orifice will do the about same. However, I think there may be some flow restriction in the needle valve part of the LSMIITTH burner and the pressures were a couple of tenths higher at higher temperatures. It may be prudent to get a 0-5 PSI gauge, just in case. Since the LSMIITTH burner is throwing the same amount of propane in the kiln, the inlet hole size and flue area needn’t change. 

For the 23” kiln TDI conversion, the typical firing pressures range from approximately 1 to 2.5 PSI, indicating 55,700 to 88,100 BTU’s. The 28” kiln has similar pressures but usually not much over 2 PSI. So an alternative burner selected should have at least the capabilities listed. Burners of significantly more power should be double checked to ensure that they burn properly at lower than intended BTU’s and/or gas pressures.

Noted is that the set up and adjustment to the burner plate (primary air) should be done in place under the kiln and with the lid closed. A burner tested to open air will have unlimited secondary air and the quality of the burn may not be indicative of the combustion confinement within the kiln. The plate should be adjusted so as to provide a more bluer flame with limited yellow. A mostly yellow flame usually indicates incomplete combustion needing more air. Incomplete combustion may not provide for sufficient heat and the kiln may stall at higher temperature, and the atmosphere may be in continuous strong reduction. I adjusted the LSMIITTH burner plate to about 1/2” and it seemed to work fine. 

Note: If you are going to use the LSMIITTH cast iron forge burner, do not use the 2.5” deflection block presented in a prior blogpost. The LSMIITTH forge burner flame is very tight, just over 1”, and not a nice blustery mess like the MR750. During a test firing and using a mirror, I could see that the whole flame was hitting the deflector block and shooting towards the first shelf, well below the middle of the kiln. I did not continue to work with the LSMIITTH burner past doing two test firings, but I think that a wide deflector block up near the top just below the lid would be better with maybe a slight bump block down low to give some turbulence to the flame. I would cement and pin both blocks in the same way previously discussed.


TDI Downdraft Kiln Conversion Further Study

This Further Study post delves into some helpful tips and information gleaned from comments and questions within the Facebook TDI Group posts, conversations, and emails. The kiln build has evolved over the past 6 years with great input and experimentation of several serious potters. The evolution has resulted in more even firing temperatures and knowledge of the firing parameters and conditions. The overall inlet/flue design and dimensions have not changed. 

Build Features and Improvements

Flame Deflection Block: 

This is a big and important addition researched by Lyle Nicholson and Matias Azocar and has resulted in smaller top to bottom temperatures differences. The current deflector block I am using is cut from the end of a softbrick and the part that sticks out into the kiln from the wall and measures 2.5” high. It now has a bit cut off of each corner, as shown, as the middle area in the kiln was a solid cone higher and now I usually get around 1/2 cone difference, sometimes with spots at 1 cone different. It is cemented in place and I use a wire pin through into the kiln wall just in case. 

The original block and location. 

Now with a bit trimmed off.

Note: For some reason a few people think that having 1 cone difference top to bottom is unacceptable. From conversations with others and my limited large-kiln experience, it appears that most gas kilns have hot and cold spots and getting around a 1 cone difference is common. Potters pack the kilns remembering the various temperature spots with glazes that match. Reduction also varies around the kiln, again with glazes located accordingly. 

Peep Hole: 

The peep holes on most electric kilns are small and it is sometimes difficult to see the cones in the TDI kilns. A fix is to enlarge the hole and make a large peep plug out of softbrick. I only rounded out the middle peep hole and then made the plug using a hole saw, although it can be made by sanding it round. L&L Kilns uses a 1” diameter round peep plug that costs about $14 to $16 and is a good alternative. 

Burner Wall Height: 

The current burner wall height is around 5 inches, which provides enough separation of the down-flowing gases yet works with the deflector block. 

Rounded Flue Exit Holes in the Lid: 

To relieve stress in the lid, a rounded edge flue exit hole is recommended. The edges follow the same relative dimensions as detailed for the original cut-outs. It can be done with either a hole saw or shaped with sandpaper around a dowel rod. 

Propane Pressure Gauge: 

Initially the gauge was a 0-15 PSI (Pounds per square inch) and now we use a 0-3 PSI gauge that makes it so much easier to tweak small and repeatable adjustments. 

Note that the reason the gauge is at the tank and not the burner is that there is flow drag inside the 10 or 12 foot long supply pipe that creates a higher pressure reading at the tank verses at the burners. The higher reading makes for an easier and more accurate pressure reading given the quality (low cost) of the 0-3 PSI pressure gauges we are using. 


The orifice in the MR-750 MUST be changed to a #50. The MR-750 usually comes with a #38. A #50 is smaller than the #38, so if a #38 is used way too much propane will be dumped into the kiln causing it to stall out before reaching the higher temperatures. There is a balance between the amount of propane and air that results in a proper or complete burning of the propane gas. 

How Fast Can I Fire the TDI Kiln: 

Because of the relatively thin walls of the electric kilns as compared to larger store-bought gas kilns, there is a proportionally higher heat loss through the walls at higher temperatures and it thus requires a powerful burner. Combine this factor with the smaller size and smaller amount of ware in the kiln and one is able to have a very fast initial heating and ramp up.

After candling, the initial ramp up to 900C/1652F usually takes about 2 hours. Body reduction then takes about 45 minutes. The climb to around cone 7, 1235C/2255F, takes around 3 hours. Then there is the last hour of 60C/140F per hour climb to cone 10. 

So, the total firing time to cone 10 is about 7 hours including about a 15 to 30 minute candling/warming while setting up the temp meter, water bucket for the propane tank, chair, snacks, and a book. Lower cone reduction firings will obviously be somewhat less, but the same basic procedure. 

Keeping a Firing Log: 

I suppose I’ve already stated this several times, but without an accurate log of the firing numbers and conditions (PSI, dampening, & degree/hour change), it is difficult to establish baseline numbers that will make the firing outcomes consistent and repeatable. Firing is both science and art. The science is the knowledge and understanding of the baseline numbers. The art is the packing, visualization of the flame path, knowing the hot/cool/reduction areas, and the subtle tweaks during firing that produce the desired results. 


Because the kiln can be heated to 900C/1652F in about 2 hours, many have switched to using self-supporting cones rather than cone packs. I personally had several cone packs explode even after long drying times and low humidity conditions. I also use cone holders while using up the regular cones I already have. Self-supporting cones are easy to place around the kiln as witness cones. 

Temperature, a Pyrometer, and Cones:

The placement of the pyrometer probe is usually in the middle section of the kiln and closer to the flue wall than where the burner gases are rising. It should not be near the flame for obvious reasons. Note that when packing ware it is very important to not have items to close to or blocking the probe otherwise the reading may not be accurate. The pyrometer reading can be used for most of the firing and is accurate enough for the body reduction phase and for calculating degrees per hour. For the end of the firing, I think the cones should be relied upon. After a few firings, you may notice the temperature is within a few degrees of the same reading when the cone shows you’re finished. For example, the self-supporting cone 10 chart shows 1285C and I consistently get 1273C to 1278C on the pyrometer, so I know I’m close but still use the cone as the primary indication. 

Changing the Design - Experimenting

This depends on one’s priority. If your priority is to be able to fire your gas-reduction pottery and that is most important, follow the TDI design as detailed and learn how to fire the kiln. It is not complicated and often there are people you may know who can help with pipe fittings, measuring, cutting shelves, and setting it all up. The stock TDI conversion per the manual works very well and produces great repeatable results once the nuances of the kiln are determined and recorded. 

If you are a person who enjoys building and experimenting, and I am one of those people, then experiment away. If you come up with an evolved better firing kiln then build and fire it and you can decide if you want to share the details and explain how to build and fire your design. There have been a few FB posts with nice looking side entry burners designs but it doesn’t help people to suggest something may be better than the stock TDI design and not provide specific details, dimensions, and firing procedures. 

By placing the kiln stand legs on bricks, one can use the MR-750 burner from below with a nice vertical flame so I am still unclear of the advantage of side entry except in countries where the MR-750 is unavailable. 

Over the last couple of years I have been amused by FB posts that simply and authoritatively state that the TDI kiln won’t work, sometimes with a reason why, without actually building one. Yet I dislike and find it misleading for people to post text that says the TDI kiln doesn’t work when significant changes have been made to the design. These have included smaller flue areas that have led to stalling, use of weed burners, side entry burners, not using a #50 orifice, and too small burner inlet opening. This doesn’t help the purpose of the FB group, which is to support those building a stock TDI conversion and those who have experimented, discovered, and shared improvements with design details. For example, a weed burner produces a wonderful turbulent flame and numerous people have suggested them as a cheap alternative but what about sharing the size of the inlet hole verses a specific make/model, primary verses secondary air, changes to the flue area, firing details and temps, etc. This might be helpful.

Climbing off the soapbox now. 

Oval Kiln Design:

Oval kilns provide a unique conversion and the potential for more even top to bottom temperatures with larger ware areas. Shown following are photos from Michael Buckley for a 16 cubic foot oval kiln conversion using two MR-750 burners with the flue area of about 30 square inches. Oval kilns typically range from about 10 to 16 cubic feet in interior size. This compares to around 7 cubic feet for a 23” x 27” kiln and 10 cubic feet for a 28” x 27” kiln. 

A single MR-750 burner will provide enough power for a 7 cubic foot kiln with about 25% additional BTU power available without changing the inlet hole size or PSI gauge, equating to a maximum single-burner kiln size of around 8.5 to 9 cubic feet. The two MR-750 burners used in the 10 cubic foot 28”x27” kiln conversion have a considerable excess of power and can likely work for kilns up to around 16 to 18 cubic feet. 

The guiding dimensional factors are the burner-inlet size and the relative flue area cross section size. As mentioned in the conversion manual, the the flue sizes were made larger than the calculations showed as a safety factor and to account for parasitic drag in the flue. The flue area for the original two-burner conversion was 35 square inches and I think 30 square inches would be the smallest I’d make. The difference in 30 verses 35 square inches accounts for less than 1/2” of flue wall displacement and loss of ware area.  

So in conclusion, the burners and inlet construction should follow the dimensions as stated for the 28” kiln conversion and placed on one end of the kiln, the flue is placed at the opposite end. The flue wall size will have to be calculated so as to have around 35 square inches of area. Search the internet for “area of a triangle” for how to calculate the area off of your measurements – a photo following shows how they divided the triangle areas to make the calculation.  

As of the December 2023 revision, I have not personally converted an oval kiln. Some friends had an oval planned for 2021 but had to put it on hold. The kiln we were going to convert was an old Blue Diamond with an interior width of 25” by about 41” long and 29” tall. With about 14.5 cubic feet, I decided that two MR-750 burners would be sufficient given that they have more than enough power for a 10 cubic foot 28” conversion. We went with using 18” by 18” square kiln shelves for the flue wall, recessed into the kiln wall. The flue cross-section area was about 37 square inches. The layout of the burners and flue was to be similar to the Buckley oval discussed, with the burners on one side and the flue at the opposite side.

The larger ovals have sizes that range from 10 to about 19 cubic feet. If I were tasked to convert one of these larger kilns, I would still plan on using two burners as I think they most likely have enough power to reach Cone 10. It might take a bit longer during the initial climb phase and sequence from body reduction to Cone 7, but I think it’d work. Firing propane pressures for the two-burner 28” conversion generally range from 1.5 to at most 2.5 PSI. The MR-750 burner will still work well at 2.5 to 4 PSI and should provide sufficient power.

12/29/2023 – Perry Brown provided the FB group some good information from his 16 cubic foot oval conversion. He is using natural gas at 7” WC pressure and MR100 burners. The burners put out 90,000 BTU’s at 7” WC, so 180,000 BTU’s total. He was able to get to 1200C before it stopped climbing, only 85C left to get to cone 10. He added a third burner and was easily able to get to cone 10. This showed that the issue was not stalling due to draft, but not enough heat energy. Perry’s conversion had a flue cross section area of about 37 square inches.

The takeaway is that for 16 cubic foot oval conversion, more than 190k BTU’s will be required. For a propane conversion using MR-750’s, it will probably require at least 4 PSI, putting out around 120k BTU’s total, or maybe slightly more for safe measure. This is in the good burning range of the MR-750, which is rated up to about 10 PSI before things go awry. It may also be necessary to use a 0-5 PSI gauge rather than the 0-3 PSI.

Cheers and good luck. 


Firing the TDI Downdraft Kiln Conversion

The following text has been updated from the second revision of the TDI Downdraft Kiln Conversion book published November 2023. The revised edition presents more detail on firings as included following. The Firing the TDI Kiln text is oriented towards a more novice potter transitioning to gas reduction firing with just a quick overview for experienced potters. The Evening-Out-the-Temperature process at the end of the firing is for anyone that doesn't already know this method and it is the same as is done in updraft kiln firings. 

The TDI Downdraft Kiln Conversion manual is available on amazon at: https://www.amazon.com/dp/B084DH88GH

The Facebook Group - TDI Downdraft Kiln Conversion: http://www.facebook.com/groups/4124895130900261/

Firing the TDI Kiln

By most standards, a 23”x27” (7 cubic feet) converted gas kiln is pretty small with approximately 5.5 cubic feet of pottery setting area. The 28”x27” (10 cubic feet) isn’t much bigger with about 8 cubic feet of setting area. The larger gas downdraft kilns found at ceramics schools and potteries can be three to four times larger. These larger kilns typically have firing times of 10 to 14 hours and they can be relatively reliable as far as reduction and temperature hot and cool spots. A lot of what can be read or studied about gas reduction firing is often from the perspective of these larger kiln types.

Several experienced potters who work in the large kiln environment expressed reservations about being able to fire a TDI kiln in 6 to 7 hours and achieve good results. TDI firings over the past 5 years have proved those opinions incorrect, but it is important to learn and understand why and how the firing process works so as to understand important sequences and elements. The temperatures of large kilns fired in school settings are usually ramped up much more slowly and then often have 6 to 8 hours after body reduction to the end of firing. What is important is to have a sufficient amount of time for glazes to develop their characters.

The small TDI kilns can get to 900°C/1,652°F in 2 hours after the initial candling. Cone 10 can be reached in another 2 to 3 hours after body reduction, especially in the two-burner 28” kilns. However, many glazes will simply not produce the results they are capable of in such a short time. A good plan is to allow at least 4 hours from the end of body reduction to Cone 10 going down. So, after the initial candling it is possible to have a 6 to 7 hour firing and have great glaze result. But there are also glazes that might be a lot more interesting after giving them 6 or more hours after body reduction to Cone 10 down.

Things to know or learn for the firing include tracking temperature rise rates, body reduction, glaze reduction, the concept of heat-work verses temperature, and the final process of evening out the kiln towards the end of the firing.

John Britt's book, The Complete Guide to High-Fire Glazes, has a good section on Kilns, Firing, and Safety and describes well the firing process. Search the internet for “Val Cushing High Fire Process”. Cushing has a lot of good resources and a simple firing method that produces great results. Britt shows an R1 (basic reduction) firing time of 8.5 hours with about 4 hours from the end of body reduction to the Cone 10 going down. Both are a little longer than the TDI firings but they also don’t increase in temperatures as fast initially and don’t increase as fast between body reduction and around Cone 7 when things are slowed down.

The TDI Kiln can be fired in both oxidation and reduction. For an oxidation firing, simply leave the flue exit dampened to about 8 inches. One downfall of straight oxidation firing is that, without a dampening process at the end (dampening causes reduction), the kiln temperatures top to bottom are going to be a cone or two different. Glazes will simply have to be placed accordingly. Though not always, a 28”x27” kiln will tend to fire more evenly simply as a result of the lower height to width ratio as compared to the 23”x27” kiln.

For reduction firing, the flue is dampened to control the draft and create the reduction atmosphere. The MR-750 burner has an adjustable plate that I set to an opening of 1/2 inch and do not change. In some kilns, the burner plates are closed down to restrict the primary air causing incomplete fuel combustion and the reduction atmosphere. Closing the burner plate is not necessary with the TDI Kiln as flue dampening provides more precise and repeatable control.

Basics Process

First, I’d like to present the basic process for more experienced gas-firing potters. Following is a more extensive discussion for those not as familiar.

On a following page are basic Cone 10 firing schedules for the 23” and 28” conversions. Your kiln may end up with slightly different numbers, but these should provide a good starting point. The following numbers were gleaned after 3 or 4 initial firings to get my numbers figured out on my newest 23” single-burner conversion and have proved pretty consistent in subsequent firings.

From the main firing start I plan on two hours to 900C/1652F, the beginning of body reduction. After candling, which may be from 15 to 30 minutes while I set up everything, I simply set the pressure to 1.3 PSI with the bricks 8” to 9” apart. My last firing of a 3/4 loaded kiln took two hours two minutes at this pressure setting. At first, the climb was around 800C/1440F per hour, then 430C/774F after 30 minutes, slowing to 252C/454F at 1 hour, and 96C173F at 1.5 hours. At 900C/1652F, I dampened down to 2 3/8” and set the pressure at 1.8 PSI. This gives me a miniscule climb and if it gets to 930C/1706F, I dampen another 1/16” or so to fully stall the kiln. After 45 minutes of body reduction, I open the dampener block up to 3.25” and set the PSI to 2.0 for the climb to Cone 7 or about 1230C/2245F.  

I plan for a 6.5 hour firing. With the two hours and 45 minutes through body reduction and a one hour time period from Cone 7 to Cone 10 trying to maintain around 60C/108F per hour climb, that leaves around 2.75 hours for the climb from around 930C/1706F to 1230C/2245F. This equates to an average climb of about 109C/196F per hour.

My target points (or when I change something) have become 1050C/1922F, 1150C/2102F, and 1230C/2245F. After 930C/1706F, the initial climb will be around 180C/324F per hour then degrade to around 80C/144F per hour close to 1050C/1922F, where I bump the PSI up to 2.1 and open the dampener to 3.5”. At 1150C/2102F I go to 2.2 PSI and 3.75” on the dampener. After about the 2.75 hours from 930C/1706F, the climb has slowed to about 60C/108F per hour. I open the dampener to 4” and log the temperature every 10 minutes or so and calculate the climb per hour. Usually, every 20 minutes I open the dampener 1/8” and it ends up at 4.5” after about an hour.

I then do an end-of-firing-evening-out process, which takes around 5 to 10 minutes. I start when Cone 10 is bending near the 2 o’clock position and finish when the Cone 10 is down. Before I added the deflector block, the evening out process was much longer and has significantly been reduced with minimal top to bottom temperature differences. I still like to do the evening out process just in case kiln packing has caused a void or stagnant spot, or some other cooler area.

That’s basically the firing. It is pretty simple once the PSI and dampening numbers have been figured out and firings have become very consistent and repeatable. It is different from firing the larger production gas kilns and I think this is due in large part to the thinner walls of the conversions, as compared to the larger kilns, and the resultant heat losses as the temperature increases.

Firing Elements


Because the kiln can be heated to 900C/1652F in about 2 hours, many have switched to using self-supporting cones rather than cone packs. I personally had several cone packs explode even after long drying times and low humidity conditions. I also use cone holders while using up the regular cones I already have. Self-supporting cones are also easy to place around the kiln as witness cones. I haven’t seen a difference in using regular cones verses self-supporting cones – in the photo below are self-supporting Cone 9, 10, and 11. 

Temperature, a Pyrometer, and Cone Placement

The placement of the pyrometer probe is usually in the middle section of the kiln and closer to the flue wall to keep away from where the burner gases are rising – it should not be near the flame for obvious reasons. Note that when packing ware it is very important to not have items to close to or blocking the probe otherwise the reading may not be accurate. The pyrometer reading can be used for most of the firing and is accurate enough for the body reduction phase and for calculating degrees per hour. For the end of the firing, I think the cones should be relied upon.

Initial Ware & Glazes

For the initial firings in a new TDI conversion, I suggest making a bunch of quick boring pots mostly around 6” to 8” tall, which will provide about 3 levels of ware. I try to have the kiln around 3/4ths filled. The kiln heats up faster for lighter loads and as a result, less propane pressure is required to get the same temperature per hour increases than in a heavily loaded kiln. So, 3/4 full is a good mid-point in establishing base propane pressure/flue-dampening numbers.  

The photo following is an initial test load on my 2021 TDI conversion. Mass-wise it is almost 3/4 full. I included some poor pots already fired just to provide some additional mass. 

I use only a couple of glazes in the test firings on Laguna 900, a dark iron-rich clay that shows well the effects of reduction. They are known glazes that provide good feedback on both reduction and temperature. Two come from Digitalfire.com, the G2571A - Cone 10 Silky Dolomite Matte Base Glaze and the GR10-E Ravenscrag/Alberta Slips Celadon Green. The Dolomite Matt is a good indicator of temperature and will get nice and glossy at the high end of Cone 10. The Celadon Green is thicker than a traditional celadon with a deep forest green color and is a good indicator of the extent of reduction. Neither will run off the pot if it gets a bit too hot.

Stoney White or Mamo Whites show reduction by browning, especially when the coating is thin. Same for Shino glazes. For the most insightful first firings, glaze predominantly with known reduction reactive glazes with varying coating thicknesses. Making and bisquing enough bowls for around 3 consecutive firings will also help to remember firing details and gain confidence and knowledge.

Basically, what one should use are well-known simple glazes that provide a high degree of reliability. Dark clays are good as one can ascertain the level of body reduction being done.

Measuring Degree Change Per Hour, PSI, and Dampening

An important part of the initial firings is documenting what is happening. Many experienced potters have traditionally used the graph method of logging the firing. They create an X/Y axis line trace and the slope of the line tells them what they need to know about the firing. Britt uses it in his High Fire book and it provides a great representation of the firing and differences between firing types.

If you’re not a potter experienced in using graph logs, I suggest using the log similar to those presented following. It provides an easier way to interpret what is going on in each firing sequence and establish and record your base firing pressures and flue dampening sizes.

The time and temperature are the first items in the log. Obviously, temperature provides specific points in the firing. I consider the pyrometer fairly reliable for most of the firing including body reduction. For the final part, I use the pyrometer for an indication of degree per hour changes, but I 100% rely on the cones to let me know the firing has reached Cone 10. Pyrometers indicate temperature. Cones indicate “heat-work”, which is what you want to know. Research and understand “heat-work” if you’re not familiar with the concept.

Later, I will discuss some degree-per-hour guidelines for parts of the firing and also relate it to the ideal functioning of the cones. I use a calculator to figure degrees per hour. Record the difference in temperature and the number of minutes between readings. The calculation is:

Temperature Difference times 60, then that number divided by the number of minutes between readings.

So, if the kiln went from 1,234° to 1,249°, a difference of 15° in 17 minutes, then degree per hour change is:

15 times 60 = 900, then 900 divided by 17 = 53 degrees per hour

To keep track of what is going on without a lot of fuss, I typically set my mobile phone timer for 6 minutes and mark down the temperature. After 6 minutes, I mark down the new temperature and multiply the temperature difference over the 6 minutes times 10 and you get degrees per hour. Easy. For 3 minutes, multiply times 20. For 10 minutes, multiply times 6.

Without an accurate log of the firing numbers and conditions (PSI, dampening, & degree/hour change), it is difficult to establish baseline numbers that will make the firing outcomes consistent and repeatable. Firing is both science and art. The science is the knowledge and understanding of the baseline firing numbers. The art is the packing, visualization of the flame path, knowing the hot/cool/reduction areas, and the subtle tweaks during firing that produce the desired results.

Measuring propane gas pressure is easy – just get it right off of the 0-3 PSI gauge.

To measure the width of the flue dampening blocks, I leave a heavy metal ruler next to the soft-brick blocks as shown in the photo on the following page. I find this easier than holding a ruler to measure it. Recently, I forgot my ‘readers’ glasses (and was too lazy to go into the house and get them) and had the dampening blocks and extra inch and a half open from body reduction up to the end evening-out process. I simply made 1/4” or 1/2” adjustments per my schedule without actually noting the total dampening block distance. The kiln did fire faster than usual and I didn’t think about why this was happening. The result was the loss of a whole load of Shino glazed pots that showed no reduction effects. It is really important to properly measure the flue dampening. 

Judging Reduction

First, reduction is performed by dampening the flue opening restricting the draft. The round plate on the burner can be adjusted to lessen the airflow through the burner (primary air) but this method is not required and is also difficult to adjust with accurate repeatability – just leave it at 1/2” to 5/8” open. Adjustment of the flue dampener bricks is easy, measurable, and repeatable.

As previously mentioned, the firing numbers and schedule that I use and refer to herein are largely from the writings of Val Cushing and what Walford Campbell taught me and the clay and glazes I use are for Cone 10 firings. Others may use or teach different schedules and, like any gas kiln, other firing temperatures can be used for different clays and glazes.

The first reduction you will encounter is the body reduction (heavy reduction) at around 900C/1652F (~ Cone 010) with a maximum of 950C/1742F (~ Cone 08). By dampening the flue to a small opening, the kiln will stall out, meaning the temperature stops climbing, and might even begin dropping.

To test the amount of heavy reduction, make small 1/16” to 1/8” adjustments narrowing the dampener opening until you see smokey soot or see black soot accumulating on the inside of the exhaust hood. You might have to increase the gas pressure slightly to keep the kiln from falling below 900C/1652F. Soot is not good and means you dampened too much and are just wasting fuel, so open up the dampener a little and the kiln will be in heavy reduction. If then the temperature begins to climb too much and get near the maximum, lower the gas pressure very slightly to stabilize the temperature. Follow the tweaking procedures discussed later to maintain body reduction.

Since it may be too light out to see the reduction flame during body reduction, one method to verify reduction is to quickly open a dampener block 2 to 3 inches and the increase in draft flow will ignite reduction atmosphere in the flue and you’ll see a quick pop of reduction flame above the lid. Reduction verified, dampen back down to continue body reduction.

One other way of noticing reduction is that there will be an acrid smell. It is not a good indicator of how much reduction, just that reduction is present.

After the body reduction phase, there is the long climb to around cone 7, 1235C/2255F, which should then allow about a 1 hour slow 60C/108F per hour climb to cone 10. Reduction numbers during the long climb phase is usually medium to light depending on your glaze requirements. I typically use medium reduction, which works for the glazes I previously mentioned.

After the firing, judge the reduction and write the plan for the next firing. For dark clay bodies like the Laguna 900 I use, too little reduction results in the body having a light tan color. The heavier the reduction the more red/brown the clay color. Having some pots with tan colors and others with dark red/brown means that the reduction atmosphere was not evenly distributed and this can be caused by two factors. First, if dampening was not aggressive enough, the good reduction may only have been near where the flame was, with the bottom shelf typically having the least. Second, if there are spots in the kiln where the gas was not flowing due to the kiln packing or maybe a shelf being too close to the side wall creating a dead gas-flow pocket, there may be less reduction effects.

One glaze I use and recommend that is a good indicator of reduction is the Rhodes Stoney White matt. Thin areas of the glaze will turn the most brown with reduction and with thicker areas a yellowish white. From Rhode’s pdf’s available online, the Cone 10 recipe I use is: Dolomite 22.5, Custer Feldspar 48.9, Grolleg Kaolin 25.1, Whiting 3.5, and Zircopax 7.5. Specific gravity of around 140 and a 3 to 6 second dip makes for thin enough coats.

Cones can also show the effects of very strong reduction by having an uneven or blistered surface.

After about 3 or 4 firings, you should have some numbers that give repeatable results, or at least narrowing the range of possible PSI/Dampening numbers. For a recent TDI conversion, the firings went from way too little reduction, then way too much reduction, then just ok (a tad weak) reduction on third firing, which only required small tweaks with subsequent firings.

Tip: It is hard to see the reduction flame in broad daylight. Try and time the firing so the last 2 to 3 hours will be in darkness (if you don’t have a shed you can make dark). 3 hours in darkness means you will have 2 hours in ramp up to cone 7 and you will be able to see the reduction flame more clearly. It will grow in size and intensity as the kiln gets hotter. For the last hour, you will be able to maintain a nice reduction flame and have more confidence in the pressure/dampening tweaks.

Initial Firing Guide

The following sections have been written primarily for the more inexperienced gas-firing potter and it contains a lot of minutiae. Since I don’t know what people don’t know, I’ve tried to make it as comprehensive as possible without delving too far into areas that should probably be learned from books like Britt’s or Cushing’s PDF’s.

Presented is a method for less experienced potters to learn the kiln and set up a firing schedule to produce some good and repeatable results. After, there is a section on evening out the kiln temperatures at the end of the firing. If you don’t like my schedules, it’s your kiln so do your own thing. And if you’re firing to a lower cone temperature, just modify the climb to the last hour for your pottery, the procedure is the same.

The purpose of the initial test firings is to get some base numbers. I want a set of base pressures and flue dampening sizes that correspond to specific sequences in the firing process. Once one gets the base numbers, the firings can be reliably repeated and the numbers more accurately altered when necessary. The specific sequences discussed for an initial Cone 10 reduction firing are:

  • Candling
  • Main firing start
  • Climb to body reduction
  • Clay Body reduction
  • Climb to around Cone 7 ~ 1235C/2255F
  • 60C/108F degree per hour climb to Cone 10 down

The sequences will produce a very basic firing and I am not worried about top to bottom temperature differences yet – most likely the differences will be around 1 cone.

At the end of the initial test firing process is evening-out the top to bottom temperatures, which is also a phase of moderate to strong reduction. After you get your base firing numbers and feel comfortable with controlling the kiln, you can delve into the art part and play with the evening-out process at the end, which adds time to the firing and also contributes to more interesting glaze results.

It is important to understand that the propane pressures and dampening can be different from one firing to another and yet Cone 10 going down can be achieved. A lower pressure and more open flue dampening can produce an expected temperature per hour climb rate yet create a more oxidizing type firing. This may produce little reduction and not be the desired outcome. Conversely, a higher propane pressure can be used with a smaller flue dampening opening, also leading to an expected temperature climb rate yet the firing might have too much reduction with bad glaze outcomes. We’re looking to determine the happy medium.

If you read the firing text following, it might become evident that in the firing process the temperature change rate per hour varies considerably, but what is important is that there is a certain allowable time period between certain target events. What I shoot for is a 2-hour timeframe from burner full on to body reduction. From the time the burners are turned on full after candling until body reduction, the initial temperature climb rate per hour is very high and then it slows down considerably by the time it gets to clay body reduction, or about 900C/1652F.

After the end of the 45 minute clay body reduction, the propane PSI is increased and the damper is opened up. There is initially a rise of around 180C/324F per hour then it degrades to around 80C/144F per hour, at which point I increase the PSI a tenth and open up the dampener 1/4”. Again, there is an initial steep rise in degrees per hour change that then degrades to a slower rise after a while. This process gets repeated several times and one can see that, looking at temperature climb per hour, there is a sawtooth shape to the climb rates verses time. In the beginning, because it is new and both the technique and numbers have to be learned, there will be a lot of these sawtooth adjustments. I think the process can be reduced to one PSI/dampening number from the main start to clay body reduction, and three adjustments after body reduction to Cone 7, which is the beginning of the last hour of a Cone 10 firing. Even though I have my numbers and am just sitting there for most of the firing, I still monitor the temperature change rate every 10 to 15 minutes just in case the ware load is causing a too fast climb or maybe a too slow climb and I have to make an adjustment.

The Story

Following, I will describe the 3rd firing of one of my initial test firings and what my goals/expectations were in each firing sequence. The kiln was a 23”x27” and the log of major PSI/dampening changes is shown following – I left off the many readings in between. The results were good and I think this can be used as a starting point for other 23”x27” TDI kilns. I hope that understanding this “story” might help you visualize your firings.

I do note that this recent conversion was of an Olympic Updraft that has 3” thick walls and no element grooves. As a result, it is more efficient and for the higher temperatures I added 0.2 PSI to my actual numbers. For a 2.5” thick kiln wall, it may require another tenth or so. This will be more obvious after judging the firing times/temperature climb rates and the amount of reduction, as described later.

Another tip would be to make a significant change to the numbers after the first firing, as I had done, either for more reduction or in the direction of less reduction. When the firing progresses from too much to too little, or vice versa, you can then pick numbers in between to hone in on the best ones. Ultimately, making significant changes will usually speed up the learning process since result differences will be greater.

The 23”x27” firing schedule below was adapted from the third test firing and is used in the text following. 

For the 28” kiln, I have included a rough schedule as shown below. The process and procedure is essentially the same as the 27” kiln text, just with different pressure and dampening numbers. The warmup was done with one burner only, then the second burner turned on.

Lighting Pilots and Burner

For safety, the lid should always be open when the pilot light is lit. First, check that the gas valve on the burner assembly is closed as well as the pilot light (pilot needle valve clockwise for closed).

Important: Ensure that the gas regulator knob is unscrewed to loose (counter-clockwise) so that there is no chance of applying over-pressure that may damage the 3 PSI pressure gauge when the gas is turned on.

Next, I open the valve on the tank in the water bucket, and only one propane tank at a time. Next, screw the pressure regulator knob in until the pressure gauge reads a slight movement, not more than 1/2 PSI.

With a grill lighter lit and near the pilot light, open the pilot light needle valve and adjust to get a flame about 1-1/2 to 2 1/2 inches long. The needle valve is very sensitive and takes a minute movement to get the right flame size. Since the 10’ hose has to fill with propane, it may take a long while for the pilot to fully light. As the propane fills the pipe it will begin to sputter a bit and then finally stay lit.

Gently open the burner value and the burner should light immediately – notice the pilot flame gets smaller. The pressure can be adjusted up or down slightly to have a small flame for candling warm-up. The flame may sputter a bit initially as air is purged from the hose line. Don’t leave it unattended for the first few minutes until a good steady flame is being produced.

If you have trouble lighting the burner and you feel that some propane has filled into the bottom of the kiln, shut things off and just wait a few minutes. Propane is heavier than air and will seep out the inlet hole and then the lighting procedure can be repeated.

After the PSI has been set for a small candling flame, an option is to turn the burner off and note the reading on the PSI gauge and the size of the pilot light. Record the PSI and and note how many turns the pilot value needs to off and back on and the flame size. This will be the burner lighting numbers for the firing.

If using a Baso safety valve and pilot light assembly, follow the directions for the valve and remember that it will also take a while to purge the hose of air and have the pilot light stay lit.

Another option mentioned later is to use a grill style piezo-electric lighter, which makes it easier to light both the pilot and the main burner.

Here is a setup and initial procedure checklist that can be copied and referred to.

Candling and Main Firing Start

The TDI kiln is capable of very fast initial climb rates that can have bad effects on wet glazes and cone packs due to moisture. Candling helps dry things out. Listed is a pressure of under 0.1 PSI. With the pilot light lit and making a large flame, I turn on the valve for the MR-750 burner and then reduce the pressure as much as possible and still have a bit of flame in the burner. It is generally not good to have a flame down inside the burner body, but if it is small, it won’t hurt anything over a short period. After the burner is lit, I close the lid, however, if there is a moisture concern, place a soft brick wedge under the kiln lid for about 15 minutes and then close the lid for another 15 to 30 minutes to dry things out, or longer if necessary. In the log above, I used about 0.1 PSI and after 35 minutes the temperature climbed to 210C/410F. I use cone holders and self-supporting cones rather than cone packs, so the fast rise wasn’t an issue.

To start the main firing, I turned the gas pressure up to 1.3 PSI and turned the pilot light off. Below red-hot, I don’t leave the kiln unattended just in case the wind blows the flame out or something else happens. For the 28” kiln, an initial pressure of around 1 PSI should be good. The flue dampening was set to 8” and not adjusted until body reduction.

Climb to Body Reduction

Generally, I plan for about 2 hours from main firing start to 900C/1652F and body reduction. I was at 210C/410F at the start, which is 690C/1242F from 900C/1652F. So, for a 2-hour timeframe, an average climb rate target of 345C/621F per hour would be work. After initially going to 1.3 PSI, the actual initial climb rate will seem very high but settles down after a while and doesn’t seem to hurt anything.

As shown in the log, my 1.3 PSI equated to 2.2 hours and an actual average climb rate of 320C/576F per hour. This was the third test firing of this kiln. For the first firing, I used 1 PSI and the rate slowed after a bit, so I kept increasing to 1.2 PSI and it took 2.5 hours to get to 900C/1652F. For the second firing I used 1.4 and got to body reduction in just under 2 hours.

So, it appears that 1.3 to 1.4 PSI will be a good baseline pressure going forward. I can set this pressure and probably not have to make any adjustments until 900C/1652F body reduction. After the first hour, I will still monitor the climb rate every 20 minutes or so just to make sure.

For the 28” kiln, an initial pressure of around 1.0 to 1.3 PSI should be good.

Body Reduction

Body reduction is a period of heavy reduction affecting the clay body, which is done before the glaze melts and seals off the clay. Recommended times are around 30 to 60 minutes and I generally maintain it for 45 minutes. The range of acceptable temperatures is about 900C1652F to not above 950C/1742F. Potters also use Cone 011 and 010 as indicators to begin body reduction.

The objective is to stall out the kiln and then maintain a temperature for around 45 minutes. I don’t mind a slight creep and find that I like to keep the temps around 910C to 920C or 1670F to 1690F.

There are two concurrent ways a kiln will stall. First, if the fuel is increased too much, there will not be enough oxygen for a complete burn relative to the draft/airflow and the temperature can stop increasing and even decrease. The second way is to close down the dampeners restricting the draft/airflow with the result that there will not be enough airflow relative to the amount of fuel and the incomplete burn causes the kiln to stall. Both of these methods are used at the same time – dampening and increased propane pressure.

For the initial firing, dampen first to around 2” to 2 1/4” and wait a few seconds for things to begin to happen. Then increase the gas pressure slightly to stop the temperature from dropping. Increase again if needed.

For my first test firing, I kept the gas pressure at 1.2 PSI and closed the flue opening to 1 3/4”, stalling the kiln. There wasn’t enough fuel going in and the body reduction was poor for most of the ware.

For the second firing, I chose 2.2 PSI and a dampening of 1 7/8” stalled the kiln. Unfortunately, there was too much fuel and the reduction was way too heavy. The clay over-reduced and did not look as good.

For the third firing, I settled on 1.8 PSI and a 2 5/16” flue dampening stalling the kiln. The body reduction was really good and towards the end of the 45 minutes I had let it creep up a bit to about 939C/1722F.

Even when you find the pressure/dampening size that stalls the kiln and produces the results you want, there will still be a bit of tweaking during the 45 minutes. I leave the pressure at 1.8 PSI and make very small changes to the dampening opening. A dampening brick adjustment of 1/16” can make the temps rise or fall and so it requires constant attention. Sometimes I get it pretty perfect and just sit as the temperature stays almost constant or very slowly rises over 45 minutes staying within the range.

Have some fun with learning this, like letting it climb slowly to 930C/1706F and then close the dampener slightly and bring it back down to 915C/1679F, and then hold with slight dampening block tweaks. This is good practice of the process and patience that will be used in later firings when you are doing the same thing in order to even out the top to bottom kiln temperatures at the end of the firing. It is a lot less stressful to learn it here than at the end of the firing when you are also trying to watch the cones go down.

For the 28” kiln, a pressure of around 2.0 PSI should be good. Just dampen down to stall the kiln out and tweak to hold the temperature. Observe the results and make changes accordingly.

Climb to Around Cone 7 ~ 1235C/2255F

The objective here is to find the pressure and dampening settings that will bring up the temperature from body reduction to around Cone 7, or ~1235C/2255F. During this sequence, there should generally be light to medium reduction and the timeframe should be about 2 1/2 to 3 hours. The timeframe and desired reduction may change later commensurate with your glaze requirements but for initial firings, one is just trying to learn baseline numbers.

The log shows the pressure going from 2.0 to 2.2 PSI and the dampening from 2 5/16” to 3 1/4”. I usually just open up the dampening, leaving the pressure where it is, and observe and write down the results. Then adjust the pressure to keep the reduction/temperature rise going. Several adjustments will likely be required.

Since I want to go from around 950C to 1235C/1742F to 2255F over 2 1/2 to 3 hours, an average climb rate of about 100C/180F per hour is indicated (285C divided by 2.75 hours = 104C per hour rate). What actually happens is that the initial climb rates will be much higher, around 140C to 160C/252F to 288F and then it will slow down to around 50C to 60C/90F to 108F when the temps are close to Cone 7 - 1235C/2255F.

During this sequence, I keep an eye on the per hour climb rate with more frequent observations the closer it gets to Cone 7. I adjusted the dampener opening, making it larger by small increments and waited for something to happen. After about 45 minutes, a 6-minute check showed that the temperature climb rate had dropped to about 108C/194F per hour. So, I increased the gas pressure to 2.1 and opening the dampener 1/4” to 3 1/2” and the climb rate popped up to about 135C/243F per hour. It had been slowly dropping in climb rate per hour for a while (I was taking temperature measurements every 6 minutes), but I was patient and waited until it was pretty low and then made the adjustment.

About 40 minutes later, it had slowed down to 70C/126F and I opened the dampener another 1/4” to 3 3/4” and also increased the pressure to 2.2 PSI. This increased the rate to about 100C/180F per hour. About an hour later, it slowed again to about 40C/72F per hour so I opened the dampener to 4”. Being at approximately the Cone 7 temperature, the rate after a 6-minute test was about 60C/108F per hour, which was good.

Being the third firing, I had some baseline number ideas and so there was much less tweaking than in the first two firings. In the third firing, I made 3 pressure changes and 4 dampener changes in the after body-reduction sequence. By comparison, in the first firing I made 3 pressure changes and 7 dampener adjustments, both wider and smaller. In the second firing, I made 1 pressure adjustment and 6 dampener adjustments (and had too little reduction).

It is best to adjust only one thing at a time and then wait for something to happen, verified by a 3 or 6 minute temperature rate per hour check. The initial firings will have more adjustments. After a firing, look at the log and pick some incremental pressure settings and their related dampening sizes that you think might be good and mark them for use as a baseline in the next firing. If they are off slightly, then adjust next time as you hone in on the best baseline settings.

Judging these results and for the next firing in the after body-reduction to Cone 7 sequence, I would plan on using 2.1 PSI then probably 2.3 PSI as my two changes, with 3 or 4 dampener size changes – 3 1/4”, 3 3/4”, and then 4 1/8”.

60C/108F Degree Per Hour Climb to Cone 10 Down

As mentioned, for the initial test firings, I think the best thing to do is to try and keep a 60C/108F rate to the end of the firing and to not worry yet about the full-on process for evening out the top to bottom kiln temps. From Cone 7 - 1235C/2255F it should take about an hour for Cone 10 to go down. Note the witness cones on each shelf and there might be a large difference in top to bottom, or the middle might be hotter as a result of the deflector block size. For now, record all of this information on the kiln log.

For the third test firing, I continued with 2.2 PSI. The dampener was at 4” at the Cone 7 temperature point. I did 6-minute temperature rate calculations and when, after about 17 minutes it had slowed to around a 40C/72F per hour climb again, I opened up the dampener 1/8” and increased the PSI to 2.3. The climb slowed again at about 7:06 PM, and I opened dampener up 1/8” to 4 1/4”. At 7:22 PM Cone 10 was down and I shut off the kiln. So there were lots of 6 and 12-minute calculations written down but not shown in the previous sample log.

A goal in the initial test fire sequences is to have light to moderate reduction. During the top to bottom evening out procedure, detailed later, the reduction will be moderate to strong, so getting the numbers for light to moderate reduction here works. In order to see reduction (assuming one is not using an oxygen probe), it is better to finish the firing at night, as previously mentioned. My small shed’s doors are open during the firing so I time the last 1 to 2 hours of firing to be at dusk to dark outside.

As should be noted by this point, reduction is adjusted by dampening the flue and there is a relationship between the temperature rise and dampening. So, if one wants more reduction, the block may be dampened a small increment. But then the kiln temperature does not rise as fast as you want, so the gas pressure must be increased. Initially, one will tend to make larger adjustments but once you gain confidence, you’ll notice that you make fewer adjustments. And, that the adjustments are more related – the dampening increment is not quite as much knowing that you will probably be increasing the gas pressure. 

The first photo following is from the third test firing being discussed and shows light to medium reduction near the end of the firing – the reduction flame is there but not too large or bright. Something like this is what you want for the initial test firings. The second photo following is moderate to heavy reduction, also near the end of firing, with a larger brighter and more ragged flame.

As with the prior Climb to Cone 7 sequence, in the first few firings there might be lots of tweaking to try and keep a steady rate increase and the kiln may stall a few times. This is fine, you are both learning how to tweak and control the kiln and also recording and figuring out the baseline numbers to use later.

Besides it taking about an hour for Cone 10 to go down in this sequence, two other things can happen. One, the temperature can increase too fast and even exceed the Cone 10 temperatures listed in the Orton cone charts. Just dampen down a bit and the rise can be stalled or even lowered, and then wait for Cone 10 to fall. Second, it might take longer than one hour and you find yourself doing a lot of large and small tweaking with the dampener block and even stalling it out at slightly near or above the Orton cone temperature as indicated by the pyrometer. This is ok too and will actually serve to even out the top to bottom temperatures. Record and note the PSI and dampener opening numbers that you think made the increase more stable and closer to a 60C/108F per hour climb rate.

After a few firings, you may notice the temperature is within a few degrees of the same reading when the cone bend shows you’re finished. For example, the self-supporting cone 10 chart shows 1285C and I consistently get 1273C to 1278C on the pyrometer, so I know I’m close but still use the cone going down as the primary indication.

When Cone 10 goes down, the propane gets shut off. The dampening blocks are moved to completely cover the flue exit. A soft brick piece about 1/2 inch thick is placed on top of the burner to somewhat block off the burner port, and it also protects the copper pilot from radiating heat. Now, it’s just a matter of waiting a day or so for a kiln opening.

Again, the purpose of these initial firings is to find the pressure and flue dampener setting that provide acceptable sequence times and results for the candling, climb to body reduction, body reduction, climb to around Cone 7, and then climb to Cone 10 down. Eventually you will have baseline numbers that will give you a relaxed and repeatable firing for the 6 hours or so to get to a Cone 7 ~ 1235C/2255F temperature. A lightly loaded kiln may require less PSI and slightly smaller dampener opening than the baseline otherwise the firing may be too fast. Conversely, a heavily loaded kiln may require more heat energy and slightly higher PSI and larger dampener settings in order to get to Cone 7 in 6 hours. Use your clay and glaze results to determine whether sufficient reduction is occurring. 

Once you have more confidence in you firing abilities or have the firing pressure and dampener changes down to something that looks like the log previously shown, you can move on to the next section and begin with evening-out-the-temperatures process.

To help with recording and comparing firings, there is a sheet on the last page that can be copied. It has items most important to the firing outcomes and should provide a visual path to setting up pressure/dampening numbers to get the firing process under your control. I assume that reduction will be done, so I note temperature climbs to body reduction, which just helps plan times for light or heavy kiln loads. Kiln loads will greatly affect the time of the firing, so if one is looking for that two-hour climb to body reduction with a heavy loaded kiln, the gas pressures may be significantly higher. Yet, the body reduction PSI and dampening will likely be about the same.

The Climb to Last Hour numbers entry is just to set the initial PSI/dampening to get the climb going, also affected by ware load. During the last hour to whatever cone temperature that may be, I think following the Orten Cone temp rise per hour listing seems to work well. The Last Hour Sequence numbers entry is also just to set the initial PSI/dampening.

Evening-Out-the-Temperature Process

Evening-out-the-temperatures is basically just a process whereby the gas pressure is reduced slightly and then the dampener opening is reduced to the point that the kiln stalls. The goal is to have a very slow or no climb rate. Even though the temperature is held almost constant, the heat work continues, which is what makes the cone go down and ware mature. The back pressure caused by the over-dampening helps even out the temperatures in the kiln.

Walford Campbell has an end-of-firing method he learned firing gas updraft kilns in his pottery in Jamaica. Walford studied ceramics at Derby Lonsdale University, Derbyshire, England. On returning to Jamaica, he joined the faculty of the Edna Manley College of the Visual and Performing Arts in Kingston and also started his own very successful pottery. His procedure is used as follows.

Firing cone 10 reduction and after the 900C/1652F body reduction period, he brings the temperature up at 2.0 to 2.5 PSI (for his 28”x32” kiln) until Cone 7 falls. The gas is then reduced to 1.5 PSI and the flue dampening is reduced to the point that the kiln stalls. From there to Cone 10 going down, he opens the dampening slightly and maintains a very slow rise in temperature. The rise might be only 30C to 40C/54F to 72F per hour.

In order to keep the temperature slowly rising, the dampening blocks require constant attention and tweaking. Small adjustments can make for surprising large changes in the temperature rise of the kiln, or it might start going down in temperature. What does happen is that the top to bottom temperatures in the kiln are evening out and when Cone 10 is falling he shuts off the kiln and closes the flue dampening blocks. The temperature on the pyrometer is not relevant and only the cones are used to judge the firing (look at the Orton cone chart for Self-Supporting cones and slow, medium, and fast rates and the difference in temperature indications). This process can take an hour but sometimes it goes on for longer, especially when the kiln is heavily loaded.

I do the evening out process slightly different in that I wait until Cone 10 begins to bend. For the 23”x27” kiln, I drop the pressure from 2.1 PSI to about 1.8 PSI and reduce dampening to about 2.5”, and the kiln stalls. The dampening block is then opened up about 1/8” and I wait to see what the climb rate is. I’ll do 3 and 6-minute rise rate checks and tweak the block accordingly. Adjustments can often be as little as 1/16”. The indicated temperature is not relevant to the actual Cone 10 going down, yet it still shows a target climb rate since, in this process, the final temperature might be in the 1260C to 1280C/2300F to 2340F range. If it does climb to around 1280C/2400F in my kiln, I will stall it out there and then wait for Cone 10 to drop.

Your kiln might have different numbers and it is important to remember that the pressure and dampening numbers will range from a low pressure/dampening that will stall the kiln with almost no reduction (in oxidation) to a higher pressure/dampening that will produce high reduction. What you’re looking for is a pressure/dampening that creates a nice light to moderate reduction, as evidenced by the flame and, of course, the final glaze results.

There are other resources for the evening out process. Florian Gadsby makes interesting YouTube videos and in this one, “Packing a Kiln Load of Pottery and Gas Firing in Reduction” at 10 minutes, he is firing his Rohde KG-340 kiln, which has an interior flue made of thin refractory material like the TDI flue wall and 4 bottom vertical burners. Take aways from this are that after heavy reduction, the dampener is only slightly opened to get a climb rate of around 85C/153F per hour until about an hour to the end at which time it goes down to 50C/90F per hour. It takes 4.5 hours from after heavy reduction to finish and the constant reduction and strong dampening is what evens out the kiln so he gets about a 1/2 cone difference (seen from cones in his other video). He also does a crash cool down to 1000C/1800F, which he says makes the colors brighter, then he closes it up for a slow cool. The strong dampening and slow per hour climb rate at the end even out the top to bottom temps.

Perry Brown, from the TDI Downdraft Kiln Conversion group on Facebook posted: I am working on a TDI conversion right now, but the kiln I have been using for several years is a small Olympic updraft. I have found that the keys to even firing are patience (i.e., not bringing the temperature too quickly) and actually letting the temperature rise stall a couple of times during the firing (i.e., holding at, for example, 1300 deg and 2000 deg for about 30 min each) to let the temperature even out bottom to top.

If this whole evening-out explanation seems overly simplistic and somewhat anticlimactic given the previous hype, well it is. Once the baseline numbers of the firing are established and the tweaking process is learned, the evening-out is not difficult, but it does require patience.

To finish, below is a photo of Walford’s 23”x27” and the 28”x32” kilns towards the end of the double firing and showing nice reduction flames. Good luck and have fun!