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.
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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.
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:
- 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.
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 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-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!
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