Firing the TDI Downdraft Kiln Conversion

The following text is from the first revision of the TDI Downdraft Kiln Conversion book published April 2022. The revised edition details several new ideas with more detail on firing. Many people contributed ideas and feedback making the conversion work better and more easily. The Firing the TDI Kiln text is oriented towards a more novice potter transitioning to gas reduction firing. The Evening-Out-the-Temperature process at the end of the firing is for anyone that doesn't already know this method. It is standard for updraft kiln firing but not usually performed on larger downdraft kilns. 

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.

Presented following 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, no problem, it’s your kiln so do your own thing.

First, a quick note on cone packs. Because the TDI kiln can heat up so quickly there is a real possibility that cone packs may explode. Recently, I was going to fire and brought cone packs and ware outside to the kiln shed. Because of weather, I decided to wait and brought the pottery back inside but left the very dry cone packs in the shed. A week later I fired and a pack exploded. Considering it was January with low humidity, I was rather surprised. Making sure that glazed ware and cone packs have dried long enough is always good. One method is to candle the kiln keeping it below 100C/212F for long enough that moisture is evaporated out, a common procedure in school environments. My solution is to use cone holders/plaques and also self-supporting cones (especially 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.

Initial Firing Guide

This section has 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.

The way I see it, the purpose of the initial test firings is to get some 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
  • Body reduction
  • Climb to around Cone 7 ~ 1235C/2255F
  • 60C/140F 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 to 1.5 cones.

Missing in the initial test firing process is evening-out the top to bottom temperatures at the end of the firing, which is also a phase of moderate 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.

Initial Ware & Glazes

For the initial firings in a new TDI conversion, I make a bunch of quick boring pots mostly around 8” tall. 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, 3/4ths full is a good mid-point.

The photo below is an initial test load on my last TDI conversion. Mass-wise it is almost 3/4 full. I included some pots fired in a previous Cone 10 initial test firing just to provide some additional mass. I’ll use pots from the lower shelves in a prior test firing since the glaze is often not fully developed and place them near the top.

I use only a couple of glazes in the test firings on Laguna 900, a dark iron-rich clay. They are old 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.

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 Stuff

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 sample log included at the end of this book (example 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.

One primary item to document is the width of the flue dampening blocks. I leave a heavy metal ruler next to the soft-brick blocks. I find this easier than holding a ruler to measure it.

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

The time and temperature are the other 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 7 and then 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 my mobile phone’s 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 times 10 and you get degrees per hour. Easy. For 3 minutes, multiply times 20. For 10 minutes, multiply times 6.


It is important to understand that the propane pressures and dampening can be very 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.

Following, I will describe a recent initial test firing #3 and what my goals/expectations were in each firing sequence. The kiln was a 23”x27” and the log of major PSI/dampening changes looks like this – 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. Light the burner is described on page 12.


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 burner propane 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. Place a soft brick wedge under the kiln lid for about 15 minutes and then closing 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 temperate climbed to 210C/410F. I use cone holders and self-supporting cones rather than cone packs, so the fast rise wasn’t an issue.

The same process can be done for the 28” kiln with one burner left off during the candling.

Main firing start

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 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/600F 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 the 45 minutes. I don’t mind a slight creep and find that I like to keep the temps around 910C-915C or 1670F to 1679F.

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.

For the 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 too heavy. The clay over-reduced and did not look as good.

For the third firing, I settled on 2.0 PSI and a 2 5/16” flue dampening stalled 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 2.0 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.

Have some fun with 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.

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 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 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 I kept the pressure at 2.0 PSI and opened 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 the results.

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/190F 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/284F to 320F and then it will slow down to around 50C to 60C/122F to 140F 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/226F 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/275F 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/158F and I opened the dampener another 1/4” to 3 3/4”. This increased the rate to about 100C/212F per hour. It slowed again to about 40C/104F 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/140F 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 third firing, I made 1 pressure change and 3 dampener changes in this sequence. By comparison, in the first firing I made 3 pressure changes and 7 dampener adjustment, both wider and smaller. In the second firing, I made 1 pressure adjustment and 6 dampener adjustments.

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.

60C/140F 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/140F 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.

For the third test firing, I continued with 2.1 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/104F per hour climb again, I opened up the dampener 1/8”. The same thing again at about 7:06 PM, I opened dampener up 1/8” to 4 1/4”. At 7:22 PM Cone 10 was down and I shut off the kiln.

A goal in this test fire sequence is to have light reduction. During the top to bottom evening out procedure, detailed later, the reduction will be moderate, so getting the numbers for light 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. 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.

The first photo following is from the third test firing being discussed and shows light 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 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.

Beside 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 wait for cone 10 to fall. Second, it might take longer than one hour and you find yourself doing a lot of larger and small tweaking with the dampener block. 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/140F per hour climb rate.

When Cone 10 goes down, the propane gets shut off. The dampening blocks are moved to completely cover the flue exit. A soft brick 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 and kiln stand 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 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 5 to 6 hours 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 energy and slightly higher PSI and dampener settings in order to get to Cone 7 in 5 to 6 hours. Use your clay and glaze results to determine whether sufficient reduction is occurring. 

Once you have the firing pressure and dampener changes down to something that looks like the log on page 57, one can move on to the next section and begin with evening-out-the-temperatures process.

Evening-Out-the-Temperature Process

Evening-out-the-temperatures is basically just a process whereby the gas pressure is reduced significantly and then the dampener opening is reduced to the point that the kiln stalls. After stalling, the dampener block is moved slightly, increasing the opening to create a small climb in temperature. The goal is to have moderate reduction and a very slow climb rate. Too much gas PSI (and heat energy) requires more dampening and this can create too much reduction that may have a negative effect on the ware.

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 very successful pottery.

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/90F to 100F per hour.

In order to keep the temperature slowly rising, the dampening blocks require constant attentions 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 a couple of hours, especially when the kiln is heavily loaded.

For the 23”x27” kiln, I drop the pressure from 2.1 PSI to about 1.5 PSI and reduce dampening to about 3 3/4”, 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/185F per hour until about an hour to the end at which time it goes down to 50C/120F 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.

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


Gas Kiln Conversion - Downdraft

April 2022 - A second edition of the TDI Downdraft Kiln Conversion book has been published and is available at the same Amazon link below. The book details ideas from many people, including from the Facebook group, and a major addition is text discussing a method for conducting the initial test firings and also the Evening-Out-the-Temperatures process at the end of the firing. This text is was added to a new blogpost: http://www.sebastianmarkblog.com/2022/04/firing-tdi-downdraft-kiln-conversion.html

January 2021- The Facebook Group TDI Downdraft Kiln Conversion has been created to provide a place to share information including building the conversion, firing schedules, and pottery results. www.facebook.com/groups/4124895130900261/

February 2020 – I appreciate the many people have given me feedback relative to the post or questions with their conversion. From this communication, I’ve realized that I assumed a level of understanding and construction ability above many people and have not adequately explained to all how to get it done using tools typically owned. To remedy this, I have put together a conversion manual that explains the process in more detail, shows why specific items are important and how to measure them, and current sources for tools and components.

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

Thanks, and if you have any questions, I can still be contacted at borissquare@gmail.com

The following details gas downdraft conversions of two kiln sizes – a 23” by 27” and a large 28” by 32”. The goal was to fire cone 10 reduction. The design is relatively inexpensive and easy to construct from commercially available parts. Simplicity of both design and use was a major objective. Both kilns fired very well and excellent reduction and repeatability were obtained. Photos, firing schedules, and pressure/flue size settings are provided following. Specific components/methodology were employed and using different burners or dimensions may not produce the same results requiring recalculation of the inlet and flue sizes. I would also suggest reading and understanding the entire process before doing a conversion.

The photo below of the 28”x32” conversion essentially provides the design. The 28”x32” was an Olympic gas updraft and thus had smooth walls. The 23”x27” was a Skutt electric and had the element grooves.

There is a tremendous amount of information available concerning kiln designs. There are also numerous examples of kiln electric to gas conversions in both text and video. Through research, I noted that there were some very insightful comments in website documents written by Marc Ward with Ward Burner Systems. Marc also provides equipment including drilled orifices in case you don’t want to buy a #50 bit and drill yourself.

Ward wrote “Draft is the life's breath of a gas fired kiln”. Most of the kiln construction and conversion problems, even from documents written in the 60’s and 70’s, related to issues with draft. Kilns are constructed and then flue sizes and external flue piping are rebuilt and tweaked until the kiln maybe works. Numerous books and articles detail flue/chimney heights and size calculations, and many seem to be providing a flue of around three times the draw length of the kiln, including the exit length. The draft is created by the ducted rising hot air, so the higher the chimney the more suction/pull. From a simplicity standpoint, the attached exterior chimney adds both expense and difficulty to the construction process.

Many electric to gas kiln conversions have been built with an interior flue made with firebrick. Three issues with this design are the amount of space the bricks and flue take up, the added difficulty in building and cementing the bricks, and the oft-requirement of additional flues added to the top of the kiln to increase the draft/suction.

Discovering a German design that uses cordierite shelf material for the flue provided the success for our small kiln conversions. The beauty of the German design is that the thin shelves become super-heated, providing energy to the exiting gases creating a better draft. The shelves also speed up the conversion process. Once all of the parts were obtained and readied, the 28”x32” kiln conversion took about two hours.

Another important consideration is the ratio of primary air to secondary air for the burner. The burner hole drilled in the bottom shelf is 3.5”, which provides for an adequate ratio of primary to secondary air relative to this specific design and the draft it produces. With the flue undampened, the kiln will fire in complete oxidation.

In order to have even temperatures between the bottom and top of the kiln, there is a consistency of past designs/descriptions of having close to a 1 to 1 ratio between the width (or depth) and height of the kiln. The width/depth is related to the path of the flowing gas/heat from the burners up, around, and down to the flue entrance. For the 23”x27” electric to gas conversion, the flue entrance is 2” high and 2” kiln posts are used to support the ¾” bottom shelf, yielding an interior of approximately 23” by 24” tall. Also using 2” posts and flue opening for the 28”x32” downdraft conversion, the interior measures approximately 28” by 29” tall.

The flue wall is made from square kiln shelves and the conversion utilizes the inexpensive MR-750 Venturi Burners. The brass orifice that comes with the MR-750 is drilled to #38 (0.1015”) and is too large. Order new ones from Ward Burners and have them drill them with a #50 drill bit (0.07”). Or, if you have a #50 bit, order “Brass Spud Orifice #70 Drill Blank Starter Hole” from www.thebbqdepot.com - $3.14 each.

In order to ensure controlled and consistent firings, an adjustable pressure regulator was used along with a decent quality pressure gauge. The 0-30 psi pressure regulator was purchased from a local grill/propane company and the brand was Marshall Excelsior, made in the US. The pressure gauge was average quality 0-15 psi. A standard 12 foot (¼” Inside Diameter) propane hose was used and can be purchased with 3/8” flare fittings attached. The hose does not have to be high pressure so any ¼” ID rubber hose can be used and grill companies can make them with the flare fittings attached.

A propane tank fitting is attached to the pressure regulator input and the gauge is T’d in directly to the output, rather than placing the pressure gauge at the other end near the burner. Due to the slight flow restriction of the 12’ hose, the pressure readings will be higher and thus allow for a more sensitive adjustment during firing. It’s also easier to set pressures at the regulator.

Following are dimensions of the two conversions. Since shelves and kilns vary, the dimensions may change slightly.

23x27 Specs

A – Flue to inside burner wall
B – Flue width
C – Flue depth
D – Flue flat dimension
Flue Area
22.3 Sq. In.
E – Center hole for single burner

F – Burner hole diameter
Burner hole total area
9.6 Sq. In.
Flue shelf size
2@ 16” square x 3/4”
G – space between burner and burner wall/kiln wall
Approx. 1/4”

28x32 Specs

A – Flue to inside burner wall
B – Flue width
C – Flue depth
D – Flue flat dimension
Flue Area
35.6 Sq. In.
E – Burner/hole center distance
F – Burner hole diameter
Burner hole total area
19.2 Sq. In.
Flue shelf size
2@ 20” Square x 1/2"
G – space between burner and burner wall/kiln wall
Approx. 1/4"

In both kilns, the flue wall was placed on a small 2 inch post. Thus, the flue opening for the 28x32 was B-17.25” times 2” or 34.5 square inches, less 1 square inch for the small post, yielding 33.5 square inches.

Photos 15 and 16 show the 28x32 burner assembly, which must be made first. The MR-750’s are screwed into the iron pipe fittings. The MR-750 has ½” NPT threads and ½” NPT pipe and fittings were used. Required are: 4-elbows; 2- gas cock valves; 2-‘T’s; 2-6” length threaded pipe; 2-1” pipe; 3-2” pipe; ½” NPT to 3/8” male flare (for 12’ hose connection); needle valve and adapter fittings to get it into the steel pipe ‘T’; ¼” copper tubing; and, the copper tube T. Thread seal was also used on the pipe threads. Note that the burner photos show a cap over the hose flare fitting, used to keep dirt out.

After the burner assembly is made, an accurate measurement of the distance between the burners can be done, then the holes in the kiln bottom can be drilled with the 3.5” hole saw. A pilot hole was drilled first as the hole saw was not deep enough for a single cut on one side. For the 23x27 kiln, one burner is used and an ‘L’ shaped burn assembly can be made to make it stable. Later, to position and secure the burners, several daubs of Liquid Nails Construction Adhesive were used. Due to the height of the kiln stand, a ½” shelf was placed under the 28x32 kiln burner assembly to get a ½” space from the burner to the kiln bottom. The shelf was glued to the floor. Liquid Nails is very hardy (and easy to use) yet can be cut apart with a razor knife if it needed for repositioning, etc.

Drill burner holes in kiln bottom and position on stand.

Photo 1 shows the placement of the first shelf and the groove cut in the soft bricks on the 28x32. Start by marking and sawing on the bottom kiln section first. To establish and mark the groove position, a ½” wide 20” wood piece was placed on the top of the bottom kiln section (or use a ruler) and squared up by getting a constant C dimension. The actual flue shelf/wall can also be placed on the kiln wall to mark the groove. Note that the shelf may be recessed slightly into the groove.

Two lines are needed, inside the flue and outside. Measure from inside and outside of the wood piece, or the actual shelf, to the center kiln brick corners – ends of dimension D (inside kiln). Mark the lines on the bottom kiln section bricks and using a stiff saw, eyeball a square cut. The shelf width was 20 inches and the width of the grove was lightly wider.

Carefully stack and align/secure another kiln section. Measure, mark and cut, and repeat until all sections are cut. Afterwards, use a piece of coarse sandpaper attached to the edge of a board to smooth out the cut (Photo 20).

Photos 2, 3, and 4 shows the placement of the flue wall shelf pieces. Using twisted or straight pieces of Nichrome kiln wire about 1.5” long and a pair of needle-nose pliers, push the wire against the flue wall and into the brick, with approximately 3/8” sticking out and holding the flue in place.

I then jammed some ceramic fiber into the space on the side of the wall, first wetting it in a soupy refractory cement. The ceramic fiber helps holds the wall in place and seals it. After the bottom flue wall is in place, the top wall can be held in place and marked for cutting. Photo 5 shows the top wall piece, which was about 10” tall. The 10” cut off piece was then used as the burner wall (Photos 11 & 15). A standard cheap tile saw was used to easily cut the shelf.

Install the top flue wall the same as the bottom. A very small amount of cement was placed between the wall pieces and a small amount of fiber was used as the cut was not perfect (Photos 6 & 7).

Photos 9 and 10 show the placement of the bottom 2” shelf posts and also soft brick pieces cut to 2” tall that are used as baffles. The baffles make the exiting gases return from the burner side of the kiln and helps produce more even temperatures over the bottom shelf. The un-baffles areas were approximately 8” to 9” wide so as not to constrict the exit gas flow, providing around 16 square inches per side.

Photo 11 shows the 26” diameter by 3/4” shelf cut to fit the kiln. The cuts were based on the shelf being slight off center towards the flue as shown so as to have the gases exit through the unbaffled areas under the shelf. As shown, the space along the edge of the shelf provided an area of approximately 16 square inches per side for exiting gases. Cuts to the shelf were made with the tile saw.

Photo 11 also shows a soft brick baffle between the burners measuring 1” wide and around 5 inches tall. This keeps the 10” burner wall shelf piece from inadvertently leaning into the burners.

Photos 12 and 13 show the exit hole though the top. Note that it is to the side – this is the strongest place to put it but care must still be taken when opening the kiln lid to not overstress the bricks around the opening. The cut is carefully made with a flat saw and marked by measuring the flue on the top kiln section. Since the 28x32 was originally an updraft kiln, the round center exit hole in the lid was filled with cemented and pinned soft brick.

Photos 26 to 32 show the 23x27 electric to gas conversion. Missing from Photo 31 were soft brick baffles placed similar to the 28x32 to control exiting gases.

Firing Schedule
The attempted/ideal firing schedules were based on information from Val Cushing, John Britt, and Walford Campbell, and were tweaked for the two kilns. Initial issues encountered were too fast a warm-up and very fast firings. The too fast warm-up once caused a top cone pack to explode. There did not appear to be any adverse effects from the fast firings, however, Δ10 reduction firing in 4 hours just seemed too short a time. For both kilns, a firing time of approximately 7 hours appeared reasonable and included a 30 to 45 minute initial warm-up.

Noted about the firings is that the adjustable burner plate on the MR-750 burners was kept at around 5/8 to 3/4 of an inch throughout the firing. Reduction was made solely by restricting the exit flue, as shown in Photo 25. Soft bricks were used for dampening and in the following schedules the flue dampener opening is the distance between the bricks. It was found with both kilns that the flue area was larger than it needed to be. Both kilns will fire cleanly in oxidation dampened to around 9 inches. For the 28x32, this means that the flue has an excess area of approximately 9 square inches, equivalent to moving the flue wall around ½ inch, adding ½ inch to the kiln area. However, changing the flue dimensions may affect the firing – it would require experimentation.

As mentioned, the regulator in our conversions connects to a 12’ hose with a ¼ inside diameter. Varying the length or diameter will change the pressure verses flow characteristics and so the pressures will have to be found by experimenting. The valves on the burners are turn full on during the firing and gas flow is controlled by pressure only. When pressures are adjusted, tapping a fingernail lightly on the gauge is required to get an accurate reading.

The pilot light system is simple and is made from ¼” copper tubing, crimped at the top and drilled with a 1/16” hole. It works to light the burners. The 28x32 is warmed up with one burner and the pilot light was blown out and had to be relit to start the second burner. The pilot gas is shut off once the burners are running. We do not leave the kilns unattended during firing and no thus Baso safety system was employed.

Reduction commences at 900C and the kiln is stalled for 45 to 60 minutes with a constant temperature target of around 925C and not to exceed 950C. The gas pressures and flue dampener openings may have to be tweaked for your kiln and very small dampener changes can stall or increase kiln temps. A balance between gas and dampening will be found. No black smoke or soot is made during reduction. The appearance of soot shows over-reduction settings. Soot is carbon – reduction happens with unburnt fuel and carbon monoxide gas.

After the heavy reduction, the flue dampener is opened up and the temperature is allowed to increase quickly up to about 100C lower than the Δ10 temperature. The dampener settings listed following provide for a continued light to moderate reduction during this climb. Nearing the end of the firing, cones and color are used to affirm that the temperatures are relatively even from the top to bottom of the kiln. The top has always been slightly hotter than the bottom and ware is placed accordingly.

Photos 35 and 36 show some kiln packing. In Photo 35, the shelves were staggered vertically and placed toward the sidewalls causing the flame to zig-zag down though the shelves making for even temperatures. Photo 36 had taller bottles and pieces that made for a more abstract placing, but also had good firing results. One must try to visualize the flame going vertically to the top and flowing over the ware and through the shelves. For example, a shelf placed against the side wall and over a shelf also placed against the side wall will create a ‘dead’ spot near the wall that could experience lower temperatures. For the 28x32 kiln, 24” half round shelves were used with approximately 1” cut off the sides for fit – as shown in Photo 38.

Photos 22 and 33 show the propane bottles used. Bottles are placed in a tub and a small trickle of water is sufficient to keep the gas from freezing up causing pressure/flow loss.

Photos 23, 24 and 34 show the stove pipes used to exit gases from the building. The pyramid-shaped hood directly above the kiln was made from 24” wide sheet metal roofing panels and pop riveted together. 6-inch steel stove pipe was used to connect to outside the building.

Photo 37 was included as it shows a small shelf placed to block wind gusts during a windy firing day.

The following schedules are based on firings and can set up guidelines as a starting point. The primary objectives are an initial slow to moderate warm up, then fast climb to 900C, 45 to 60 minutes in heavy reduction, moderate reduction to 1182C, 60C per hour climb to 1282C with possible requirement to even out temps by dampening. The hotter the kiln gets above 950C, the slower the increase per hour if nothing is adjusted. Small changes in gas pressure and dampener opening size can have large effects in temperature rise or stall. Keep a log to learn the kiln.

At the end of the firing, the flue is completely closed off and a thin soft brick piece is placed over the burner providing somewhat of a block for the burner inlet. 

For questions or your comments/improvements, I can be contacted at borissquare@gmail.com


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