First Wood Fired Kiln Build, Problem Stalling

[twsincich]

Hello! 

During the pandemic I got interested in wood fired pottery and decided to try and build a small kiln. I had no experience in ceramics since high school and definitely not in kiln building, firing etc. so I just tried to read what I could and study photos of other kilns I could find online. It seems to work well but has problems stalling after 2000 deg f. which seems to be common. 

John Thies' "Manabigama" kiln was the closest thing I could find that was close to what I hoped to make so I tried to model it loosely after that since he says it easily fires to cone 10 in 8 hrs and I was hoping to use wood ash glazes.  https://ceramicartsnetwork.org/daily/article/An-Experienced-Wood-Firer-Shares-Kiln-Plans-for-a-Small-Instructional-Wood-Kiln 

I call my kiln the "Agnigama". Agni is the Hindu Vedic God of fire and gama for Anagama. 

I had not read Fred Olsen's "The Kiln Book" before I started and theres a lot of things I've learned since firing it and reading more about kilns and wood firing in general that would lead me to do things differently next time but I'm hoping I'll be able to tweak this design to be able to work at least better. Was I trying to save money by using as few bricks as possible? Yes. Did I think there had to be an incline in the kiln like an anagama kiln? Yes. Hence the thin walls, small chimney size, etc. I did think a thinner, taller chimney was better than a wider, shorter one but that was just a guess. Expansion joints? I know what they are now... The list goes on. 

I designed the kiln to use 2" x 3" sticker wood I get for free cut to 14" lengths. I usually only feed 1 to 2 pieces at a time for most of the firing. The last firing I tried splitting those pieces smaller and using strips of thin cedar shakes at the end of the firing to burn faster and hotter but didn't have much luck. Both firings have mostly been in reduction which has been a double sword. Turns out I wanted that for the glazes I used but also seemed to be due to poor draft design in the kiln rather than intentionally creating it. 

The first real firing I was able to get the temp up to 2250 deg f with the help of a fan in front of the air intakes and vigorously raking out coals. Fired normally at 300 deg/ hr then stalled around 2100. Total firing time was 17 hrs. All my cone packs seemed to indicate the cone 8 bending down completely and 9 starting to bend. Ash glazes and clay matured so I was pretty happy with that. 

My second and most recent firing did not go as well. First problem was rushing things and not everything was bone dry so I had a couple explosions when I hit 400 deg f. (That's an obvious problem to fix) Again, it fired great to 2000 deg f then again I had problems with stalling. I noticed that the air intakes were clogging with coals and  clearing the coals helped but it still would not get any hotter that 2180 after hours of trying different things before I decided to call it at 12 hrs. There were more cold spots in the kiln and many of the glazes did not mature this firing. Cones 6 and 7 seemed to be bent and 8 started but the pack in the back of the kiln fell over during the explosion but more glazes did not mature in the back so I think it was a bit colder there. 

I have a damper in the chimney 5 bricks up from the floor that can cover the entire chimney from 0-100%. I have tried adjusting it along with the air intakes in different ways to see how it affects the kiln but I seem to have to leave it wide open for 98% of the firings I've done. 

I think the main issues lie with the firebox, flues and chimney design since the fan did help increase temp but also blew dirt and coals all over the ware. This seems to point to a draft issue in the kiln design. After reading Fred Olsen's book, it seems there are some basic ratios concerning air intakes to firebox size to exit flues and chimney size I clearly missed. 

1) He recommends the firebox grate cross section area (L X W) be 10 times the cross section of the chimney. Mine is only 5 times. But the Manabigama I based it off of is only 2.5x.  Obviously, I know that thermodynamics doesn't just directly transfer up or down in a linear fashion and my kiln is much smaller than his so I wonder if its just not enough space to properly burn the wood in the first place or the chimney cross section area is too small. He does say wood chimneys are rarely less than 9" x 9" across. I tried to make it tall enough to clear my roof structure and just assumed bigger was better (more draw to pull air in) but then I read it can be too tall? Olsen's book seems to suggest it should only be 2-3 feet tall?

2) "Air intakes and exit flue to chimney areas should be equal to the cross section of the chimney"

My Chimney only has an interior cross section of 6.5" x 6.5" = 42.25" sq

My air intakes are 4.5" x 2.5" (x3) = 33.75" sq

Exit flue to chimney: 4.5" x 4.5" = 20.25" sq    <<<<<<   I BELIEVE THIS TO BE ONE OF THE MAIN CULPRITS 

Chimney: 86.5" tall Total interior area: 3654" cubed

3) Another culprit seems to be that my fire grate was only 2.5" off the floor and made from bulky fire bricks that choked up the air flow with coals easily. 

Original area for coals under grate: 291" cubed 

Ideas for increasing air flow and draft:

1) Planned new fire grate would open up the area under for coals to 1112" cubed, almost 4x more space! 

2) open up the air intakes to allow more air 

3) open up the exit flue into the chimney from 4.5" x 4.5" = 20.25" sq to 4.5" x 7.5"= 33.75" sq. 

I would like to open it up to 8.5" x 4.5" but then i'm worried it would cut through the main bricks holding the chimney up. 

 

Any comments, questions or suggestions would be greatly appreciated! I am out of my element here so any help would be amazing. 

 

Here is my kiln as it is now: 

 

 

Kiln chamber is a single course of hard firebrick laid vertically (probably not ideal but has held up so far) with two one inch layers of ceramic fiber over the top and then covered with 2" of sand/fireclay/vermiculite/plaster of paris. Chimney is single course of vertically stacked bricks after two initial rows of doubled up bricks. 

Kiln mid way through firing the second time. (I close the stoke door and peep holes during firing, this was just for photo)

wood for firing: soft pine 2"x 3" x 14"

Ash glazes and cone 8-10 clay all seemed to fire well the first time through. I was hoping for cone 10 but stopped at 2250 deg f and after the cone 8 had bent completely. 

First firing wares  

Ideas for increasing draft. Making the fire grate out of 3/4" rebar and raising it up to 6" off floor to open up the coal pit, opening up the air intakes to allow more air in and opening up the exit flue to be closer to the chimney cross section area. 

Comparison between the initial kiln design, planned modifications and the "Manabigama" kiln for reference. 

 

Edited January 9 by twsincich

[twsincich]

Cone Packs from first and Second Firings 

 

Temperature and Wood Used during First and Second Firings:

 

Start: 2:30 pm      End: 7am

Total: 17 hours

Wood Used: 271 pieces 

Start: 2 pm      End: 1 am

Total: 11 hours

Wood Used: 266 pieces

[twsincich]

Some pieces from the First Firing

[twsincich]

Pieces from the Second Firing

 

[neilestrick]

@twsincich Welcome to the forum. It's really difficult to fix this without seeing it, as there are at about a dozen variables at play here, but based on your numbers, your ash pit it probably too small, as is the flue opening, as is the air intake. But it may not be. It may be that you're just letting all the heat out the chimney and everything else is fine. You may just be stoking too heavily or too lightly. But based on the fact that it does better with a fan, and that you have to leave the flue wide open, the air seems to be the problem. Like any fuel burning kiln (gas, oil, wood, coal, or whatever), you have 3 factors to deal with during the firing- fuel, air intake, and damper. They all work in unison, and adjustments to any one of them will require adjustments to the other 2 to get the rate of climb and atmosphere you want. You're maxed out with the damper, so the other 2 are limited.

Is that base solid concrete? Is the floor of the kiln directly on the concrete? How thick is the floor of the kiln, and what material is it?

[twsincich]

Thank you so much for your reply Neil! 

I totally get its difficult to fix over the computer, appreciate you taking the time to look at it. 

I also wondered if the wide open flue was just pulling all of the heat out but i tried closing it to 30% and 70% near the end of firing while adjusting the air intakes and only lost temperature. But that could be me messing something up too.  

The base is a 4 inch slab of concrete. 

Two layers of regular red brick with air slots under the firebox and chimney  ( I was worried about the slab overheating)  

1 layer of soft firebrick at 2.5" thick 

floor, walls and chimney is a single layer of hard fire brick 2.5" thick 

2 inches of ceramic fiber over hard brick arches 

2 inches of fireclay/sand/vermilculite/plaster of paris cover

 

 

Edited January 9 by twsincich

[Biglou13]

How thick is your floor?   How tall is your chimney?  
 

I need to study post more but first idea is  taller chimney. 
 

sometimes you have to ride out the stall with more time.   

Edited January 9 by Biglou13
Addition

[C.Banks]

I followed the 10:1 rule from Olsen (firebox : flue size) on a similliar project and found I did not get enough pull with a 9 foot chimney of 81 inches in area. I came to the same conclusion that the exit flue is too small. I liked the idea of a fast fire kiln but in my case there were other issues with the depth of my firebox in relation to the heigth of the kiln.

I haven't got back to it but I'll rebuild with twice the flue size and look more closely to a ground hog deisgn with a low, wide chimney instead of the tall, narrow fast fire design.

Established kilns I've been a part of have run into heat saturation issues atfer too many hours of struggling with what amounted to overloading. I bring this up in repsonse to @Biglou13 as I'm sinscerely curious if your design has the mass to succeed as anything other than a fast fire. I don't mean this as a criticism I only mean to work out what expectations we should have from kilns with less mass.

[Russ]

18 hours ago, twsincich said:

Exit flue to chimney: 4.5" x 4.5" = 20.25" sq    <<<<<<   I BELIEVE THIS TO BE ONE OF THE MAIN CULPRITS

Yes. It should be close to intake. Its very hard to make it bigger while firing . The damper will control the size IF its made larger than needed.

18 hours ago, twsincich said:

Another culprit seems to be that my fire grate was only 2.5" off the floor and made from bulky fire bricks that choked up the air flow with coals easily. 

Original area for coals under grate: 291" cubed 

Yes also. You need lots of room under the grates for the coals and to also keep the grates from melting IF youre using metal grates. I try to not let the coals build up more than half of the space below the grates. It will choke off the incoming air and cause stall.  .....wood kilns need LOTS of air to fire correctly.

18 hours ago, twsincich said:

Any comments, questions or suggestions would be greatly appreciated! I am out of my element here so any help would be amazing. 

Adding air with a blower of some sort will cut firing time in half or more and will increase temp IF you make sure youre not under feeding the fire. Overfeeding will also affect but not nearly as much. Watch the chimney exhaust. .. a little smoke at stoke is ok and then see it clear within several seconds is good. Smoke all the time thru firing.. no good. Too much fuel.

Also your wood size is what Id use at the beginning of a firing till about bisque temp. After that its smaller stuff with way more surface area for burning. Take one of your 2x3 pieces and split into 4 pieces. Itll burn way faster and release more btu's quickly.

 

[twsincich]

Thank you all very much for your replies! Means a lot to me that you'd take the time to help out. 

@Biglou13 

The base is a 4 inch slab of concrete. 

Two layers of regular red brick with air slots under the firebox and chimney  ( I was worried about the slab overheating)  

1 layer of soft firebrick at 2.5" thick 

floor, walls and chimney is a single layer of hard fire brick 2.5" thick 

2 inches of ceramic fiber over hard brick arches 

2 inches of fireclay/sand/vermilculite/plaster of paris cover

Chimney: 86.5" tall , 6.5" x 6.5" interior cross section, Total interior area: 3654" cubed

Concerning your and @C.Banks comments about overall kiln mass and saturation issues, I totally agree with both of you. I'm also curious as to whether there is inherently enough overall mass to be able to fire up to cone 10. I'm not really interested in firing fast at all, more with being able to get up to cone 8,9,10 reliably to be able to use stoneware and ash glazes. The reference to the Manabigama being able to fire to cone 10 in 8 hrs was just in showing me what was even possible with a kiln of that size. ( I thought wood firing just always took multiple days)  I really enjoy the firing process, no need to rush through it. 

As I was building this kiln I started to realize how small and thin it was in relation to other kilns. I realized I hadn't seen any other kiln without the floor, walls and chimney at least 4.5" thick. I decided to put the soft firebrick under the floors half way through the build because I realized I may not be able to keep any of the heat inside. After I finished the chamber I started to worry I should have also added a layer of soft brick along the entire chamber walls before covering it with the fiber and fireclay/vermiculate shell. And now that I have read Fred Olsen's book I do wonder if the kiln itself is inherently too small to be able to burn wood hot enough and have the thermal mass to be able to hold in the heat long enough to even fire to cone 10. 

But the fact that I've been able to get it seemingly to cone 8 with undersized everything gives me hope I could get it a little hotter, fingers crossed. 

As for riding out the stall, I agree, that's what I was trying to do during the first firing. Around 12:30 am and 2150 is when I couldn't get any more rise in temp so I just road it out for another 4 hours trying to adjust fuel, air intake and damper and then tried using the fan and aggressive coal raking to get it up to the final 2250. Then I waited another hour to see if I could get any more and decided to call it. I had never fired a kiln before so I had no clue what to expect so I just figured I'd see what happened. Then I actually ran out of wood and was running around my barn trying to find things to cut up at 6:30 am. The second firing I tried for another 2 hours to get something to work after top temp and decided to call it early. 

@Russ 

I looked at the bricks again last night and feel confident I can open up the exit flue to 4.5" x 9" for a total of 40.5" sq (2x bigger) and my new air intakes would be 47" sq (which i can plug up or adjust during the firing) and the chimney cross section being 42.25" sq makes them all pretty close or adjustable. 

I am realizing that I definitely need more air! and the fire grate design I was using was too low and small. I think my new design will definitely help open that up. I've even thought if I try firing again after applying these changes and they don't help, worst case scenario, I could knock out the floor of the firebox and move it down one or two brick layers to make even more space for the coals and maybe knock out the chimney to make it wider, possibly taper it. (it is only dry stacked with fireclay mortar from the damper up)

When I started to split my wood pieces near the end of the firing it did help burn faster and hotter for a little bit before dropping again. I definitely noticed that those 2" x 3" pieces are a little too big for the end of firing when it needs to burn hotter and quicker with more surface area. They were definitely having a hard time burning up and even just one or two pieces was keeping the atmosphere  smokey and reduced for a long time. That was my thought with using .5" strips of cedar shake at the end too.  (I have an entire boat filled with old cedar shake shingles 1"-3" x 18" ) I will definitely make sure to split more before firing next time for that reason!

Thanks again for all the help!

Edited January 9 by twsincich

[Russ]

4 hours ago, twsincich said:

am realizing that I definitely need more air!

More air, more air and more air. If you can force air into the firebox and have that part adjustable  (a motorized fan of some sort with a speed controler) you can overcome alot of the"issues" youre having with undersized intake, exit and chimney size. My suggestion is to do as you proposed...enlarge your exit flue, enlarge the ashpit and forget about the chimney... with forced air chimney height becomes a non issue.  Adding more exterior insulation also helps alot.  When i first built my kiln i had some of the very same problems. It would take 12 to 16+ hours to fire. Ive now got it down to 5hrs +or- to c10 in the coolest parts which for me is the lower back portion .  When the blower was added to force air it cut the time down to 8 or 9 hrs.  Since then Ive changed stacking configuration leaving room for the flame to pass thru more readily along with a few more tweeks to get it to a consistent five hours. And its several times larger than yours at eighty cubic feet stackable space. Tons of wares, shelves and furniture along with the hardbrick mass to heat up.  So keep firing and tweeking things around. Dont worry about making mistakes... its how we learn!

[Russ]

4 hours ago, twsincich said:

And now that I have read Fred Olsen's book I do wonder if the kiln itself is inherently too small to be able to burn wood hot enough and have the thermal mass to be able to hold in the heat long enough to even fire to cone 10. 

Im going to say no, its not too small. With adjustments you should be able to get it to c14! The flames off the wood are hotter than that.  You just have to release the btu's stored up in that wood quickly with enough oxygen to burn it.

[neilestrick]

Another though- this kiln design is not really made to fire fast. The only reason it might fire fast is because it's small. It's a mini anagama, and anagama are not efficient, fast firing kilns. That said, even large anagama can get to cone 10 much faster than people typically fire them. A kiln that is fired for 3 days can get to temp in less than a day no problem, but they are fired that long in order to build up ash, giving the surface effects that are desired from this type of kiln. But still, 18 hours is a lot slower than 8 hours. So take your time and don't rush it. Let it soak, as we say. Anagama have a very small ash pit to firebox ratio, so the ash pit will fill up quickly if you try to fire too fast. Also, small pieces of wood burn quickly and produce heat quickly, but they also produce ash quickly so they're not great. You can quickly fill up the ash pit and then your air flow drops. Once you open up the flue and intake, I would definitely try firing with larger pieces of wood, like fireplace size logs, even if you can only fit a couple at a time. The little stuff is fine for early in the firing, but you need to get to bigger logs as soon as you can.

Are you using a pyrometer to time your stokes?

[twsincich]

thank you so much @Russ and @neilestrick!  I feel much more optimistic now than I did last week! 

"MORE AIR!" Will be my new mantra. 

I'm not really interested in firing fast at all, more being able to get up to cone 8,9,10 reliably enough to be able to use stoneware and ash glazes. The reference to the Manabigama being able to fire to cone 10 in 8 hrs was just showing me what was even possible with a kiln of that size. I figured wood firings just always took multiple days. I now get you COULD fire them faster, just depends on what type of effects your after. I'm happy to ride the firing as long as it takes. 

For now it seems like I'll plan to make the enlargements to the air intake, fire grate and exit flue, take my time loading and let it kindle for a day then start nice and early in the morning to be able to take my time and test out different firing rhythms and cycles however long it takes! Bigger and smaller pieces of wood for different parts of the firing too. 

[Kelly in AK]

You’ve gotten tons of excellent advice here and provided great documentation for people to help you. My first impressions were: 

Beautiful kiln, you’re going to get there, just a few tweaks away. The first two are general principles, the third applies directly to your kiln, forced air or not. 

The firebox always has to be larger than you think, wood needs a lot of room to combust. 

There has to be significant space under the grate, air is what’s making the wood burn hot enough (air’s got to go under the burn, not over it).

Double brick that chimney! Insulate it. At least to the height of the kiln. The higher the better. I’m not going to give you a long story, I’m just telling you in all sincerity it will change things dramatically. Air is key, and that column of air has to stay hot to create the draw needed to make wood burn at 2300.°

You’re advancing along this learning curve rapidly, Olsen and Rhodes’ guidelines are excellent, but, as you may have noticed by looking at many kilns, not set in stone. The groundhog kilns of the Carolinas were my inspiration. I built a mini, and double bricking the chimney took it from cone 6 with difficulty to cone 11 with ease. 12 hours. 

[twsincich]

Thank you so much for your kind words @Kelly in AK! Everyone here has been incredible so far. 
 

I agree, I wish I’d made my firebox bigger. Bigger cross section and space under the grate. Luckily It sounds like I have some options and I could still drop the floor down if it’s still not enough. That way the air is coming in low enough to heat up before reaching the wood like you suggested. 
 

My first thoughts before starting anything was, the smaller the better. Cheaper to build, Fire, easier, etc etc. The main thing I’ve learned during this whole process is bigger is always better! You can always brick in a flue, make a bag wall if the kiln is half empty cause there’s too much space, would be easier to load if I could fit in, and with a wood fired kiln the firebox is probably never too big. 
 

I love your idea about double bricking the chimney! It’s really only doubled up the first brick and a half, would be easy to go up to the damper that’s 8” above the top of the kiln. Does that sound like enough, at least to start? Would you suggest hard brick or soft brick? I was told soft brick insulates better and wouldn’t be an issue since it’s outside of the kiln atmosphere? I mortared the chimney bricks with high fire alumina mortar up to the damper so nothing would seep out. But I have some extra bricks of both hard and soft. 
 

I was reading about the groundhog kilns in the Carolinas and loved those! They sound great. I grew up in Florida, wish I’d known about them then!

 

[Russ]

16 hours ago, twsincich said:

I figured wood firings just always took multiple days. I now get you COULD fire them faster, just depends on what type of effects your after. I'm happy to ride the firing as long as it takes. 

It can take a long time  if thats what you want you want.  So in my experience burning wood with bark will produce the most fly ash. Im not saying that debarked wood doesnt produce fly ash but not to the extent as bark does. Keep this in mind if youre looking for that effect. It doesnt take lots of time for this to happen. And ash will only begin to melt and form a "glaze" at the upper end of your firing temp.

As far as firing time... if you really dont mind spending 12 plus hours firing (Im getting too old for that) by all means do it. If you want a naturally aspirated kiln then raising the chimney high high high is what you might need to do. It increases draft rate which in turn brings in MORE AIR....  enlarging the firebox and inlet will also bring in MORE AIR.

Let us know how the next firing goes!

[Kelly in AK]

Getting it double thick to the top of the kiln will certainly make a difference, I can only guess if it’ll be enough. Since you’re so close, I suspect it will be. Soft brick is better for that outer layer, yes, better insulator. 

[twsincich]

@Kelly in AK @Russ @neilestrick @Biglou13 @C.Banks

 

Hello to all who have posted help, just wanted to give an update since my most recent kiln firing with your suggested modifications. Thank you again SO MUCH for your kind words, advice and encouragement! 

I am happy to report that I believe the firing went very well! I was able to fire the kiln up to a top temperature of 2394 deg F and the cone 10 on my cone packs in the front and back were completely bent. I think I may have even gotten to cone 11 in the very front but do not have cones for 11 so more of a guess. Started the fire at 10am and bricked up the kiln and closed the chimney damper all the way at 12:30 am for a total fire time of 14.5 hrs, which I am very happy with.

What took so long? Well, I thought if I learned slip casting I could fill the kiln up quicker for another firing to test the modifications but of course that took a few months longer that planned. 

The modifications I made were:

1- Enlarged the exit flue to almost double the size it once was. Drilled small holes along the line I wanted to cut and then used progressively larger bits until the block fell right down. Not very pretty but perfectly functional and my major concern was cracking the pieces of brick left, so, success there. 

 

2- Rebuilt the fire grate with 1 1/4" rebar and T bar steel that I picked up from the local junkyard. Little slump in the front (I'll add a support brick in the middle next time) Worked great as a cheap, replaceable fire grate that raised the wood up about 5 inches from where it was last time and opened up the coal pit area under it significantly. I could tell when I was firing that the coals superheated the wood, which burned better and faster and I only cleared out the coal bed minimally this time whereas the previous firings I would have to constantly rake out the coals once I hit 1900 deg F and would end the firing with a large pile of coals on the ground after. 

How many firings will it last? I don't know but easily and cheaply replaceable. 

3- Opened up the air intake space in the kiln door to allow more air in. As the coals seamed to burn down better and faster and the kiln seemed to draw well the entire firing, this seemed to help a lot in connection with the raised fire grate. 

 

4- Insulated the chimney more up to the damper. I didn't have as many soft bricks as I thought so I just used a mix of hard and soft for this firing with some simple fireclay/sand mortar to see if it worked before I did anything more substantial. (I have to drive 3 hours to Portland for firebricks/clay/most things I need) Seemed to help! There was always a strong pull in the kiln and flames shooting up when I would stoke. 

[twsincich]

As for the firing itself, I started much earlier than previous firings to give myself plenty of time and options to be patient and learn the kiln. After blowing up so many pieces being in a rush last time I let everything dry for a few days after finishing glazing and let it all candle with a heat lamp in the kiln for another day. Started the fire at 10am and wanted to hold it under 200 deg F for at least 2 hrs to make sure everything was as dry as it was going to get. Then I slowly starting to increase the temp until I hit around 500 deg F and then followed my usual climb rate of about 150 deg every half hour. No particular reason for 150/half hour but my first fire followed that and it seems to work. This was all done by simply using little cedar shake strips burning in the air intakes. After i hit 500 I started to put a piece of my cut 2x3x14" pine pieces onto the fire grate and would used splits pieces of those to stoke the air intakes. This rhythm of putting the full piece on the grate and then using split pieces in the air intakes seemed to work well to regulate the climb and get the wood to burn all the way down efficiently. Once I got to around 1800 deg F I started putting the single pieces onto the grate and there was a sufficient coal bed to burn the wood without stoking the air intakes with split pieces. After about 2000 deg F the climb started to slow down and I started to test different damper, air intake stoke and wood addition ratios. I learned that keeping the air intakes clear and allowing the coal bed to burn down allowed the wood to burn quicker. (I'm assuming because it was hot enough and more air intake) I also realized that instead of using full 2x3x14" pieces, if i split them into 3 or 4 pieces and added them onto the fire grate more often they burned quicker and the temp climbed easily. I was easily able to get to 2394 deg F (but NOT 2400 haha and I used a lot of wood and time to try to get there) At top temp I closed the chimney damper all the way and bricked up the kiln. I am fairly confident I can get it up to temp quicker and more efficiently than this time, with less wood now that I have gotten familiar with the kilns habits. 

 

 

Edited April 7 by twsincich
Added Firing Stats

[twsincich]

The pieces came out really well, especially since this was only my 4th time firing a kiln and first time slip casting anything! I made a cone 10 stoneware body and a cone 10 porcelain body to cast with and made 16 new cone 10 glazes to test out as well. So I definitely risked all my marbles that I'd be able to get the kiln up to temp! Everything in the very front split apart which I am assuming because it was directly near the burning wood and sometimes the wood did hit the pieces. So that is an easy lesson to fix. I'm thinking maybe some kind of bag wall of bricks would help? Ideally I would probably want the firebox and grate to be much lower to prevent this but without a huge rebuild I'm hoping I can figure something simpler out. Everything behind those fired wonderfully! 

 

 

and some glaze testers 

 

[Kelly in AK]

That’s a beautiful piece (kiln) of compact design. I’m delighted you worked through some bugs. The rebar will sag and spall, you’re aware, but the thickness you chose will make it last longer and they’re a consumable. The next hundred firings will really teach you something.. Bravo.

[Russ]

Good job! Firing a wood kiln is a very grand dance.     Porcelain is quite delicate and wont hold up to that heavy level  of flame impingement like a good grogged stoneware. It seems they were overfired by a few cones!  You might try to use stoneware pieces to act as a bagwall.     The rebar will sag. Even more so if the coals build up and block the inrush of cool air from underneath .    I see you didnt weld them to the tbar so you could just turn them over each firing and get several more out of them.  I pull mine and lay them on a concrete slab and beat them straight with a sledge hammer.

Keep up the good work and let us know about your next firing.

[Mark C.]

More chimney (taller for sure to suck better).

That dampermaybe low as you said 5 bricks up from floor, How tall is that in inches? Low. if it was waist high the flame/heat expands out of the chamber and then gets reduced down and then expands again after the damper The damper that close has little expansion space which will slow draft down a s well. A taller stack may help this flaw.

I cannot tell how much flame expansion space you have?

The double expansion is spelled out well in a few books like Nils Lou  space age kilns.If I recall-its be awhile so I hope i have that right. I biult the double  expansion stack in my salt kiln and its works great with less taller chimneys .I have 7-8 feet brick and 8 feet of heavy stainless pipe

Read up on making cone pack making. Cones should face slightly away from each other so than do not fall on each other for better reading-good temp records count

[neilestrick]

Well done!

Don't be in a hurry. If you burn larger pieces of wood, you won't have to stoke as often and it won't clog up the ash pit. Ideally you should be able to fire without having to scoop out the ash pit much at all, if ever. This type of kiln it not at all efficient, so let it go slowly, take your time, enjoy the process. Remember that you're also heating up all those bricks and shelves, which is a lot of mass.