BBR Ceramic Kiln – First prototype test – Advice Needed

[amo]

Hello D32 nation!

I am here today to kindly ask for your advices to improve the use and conception of my first prototype: A Batch Box Rocket ceramic kiln.

The goals are:

- Creating a small ceramic kiln for a small quantity of potteries

- Clean burn

- Fast firing: between 2 and 6 hours

- Good efficiency with low wood consumption

- Later : the ability to manage the excess or lack of oxygen in the kiln chamber to choose oxidation or reduction of the ceramics pigments.

Sketch up plans of this first can be found Here.

It is a small kiln, built with reclaimed fire bricks (unknown grade) and refractory concrete slabs (1300°c). The whole thing is insulated with 100mm aerated concrete (Ytong / Siporex). All is linked with clay and sand mortar. (Kaolin clay and mica / quartz sand for high temperature parts).

The burner is based on a 180 mm BBR core. But it has been modified in order to optimize the overall compacity and the layout due to available materials dimensions.

The ceramic firing chamber is placed above the riser exhaust. It is meant to act as a bell. The hot gases exit from the firing chamber by a small siphon and are evacuated by a 150mm pipe, 1.6 m long.


The ceramics are loaded from the top, by removing the two layers of slabs (aerated concrete and refractory concrete).

For the first tests, the temporary door is a concrete slab. The main air inlet is either on the two sides of the door (DSR2 style) or on the bottom of the door (first BBR style), depending on how this slab is placed.

No difference noticed on the smoke production regarding the door style, or full open.


The two big modifications from the standard BBR 180mm made were:

- Shortening the riser: to help lowering the overall height.
Hoping the kiln chamber would act as an afterburner / second box due to high temperatures and flow turbulences caused by ceramic obstacles.

- Increasing depth of the firebox: to act as masonry support for the siphon and chimney. Although the firebox is longer, I load it with one pile of 30 / 50 cm logs, so it would not contain much more fuel as the regular firebox should.

- No secondary air feed:
o As my welding / metal work is not so brilliant,
o As secondary feed is not included in DSR1/2/3, Uzume’s masonry stoves, and observations about secondary air not being so important in clean burn (https://www.uzume.fr/post/nous-retirons-l-air-secondaire-du-batchblock-v1)

I already made two burning tests. The temperature was monitored in the middle of the firing chamber with a K thermocouple from china.

First tests went up to 700 °C in 2,5 hours.
Second test went up to 910 °C in 4 hours. Not able to go higher and hard to maintain this temperature.

Both times:

From 20 to 500 °C with one batch, clean burn, the temperature goes up really fast. Much faster than recommended to avoid cracking and breaking of the ceramics. But both times de ceramics were well fired with no damage.

At first lighting, the combustion is a little smoky but becomes very clean after a few minutes. (no visible smoke).

After 500 °C / 600 °C the temperature increase slows down.

Regular refueling is needed to avoid it’s decrease.

Draft always seems very good, with no backdraft / smoke or flame coming out of the door. The kiln makes a nice and strong rocket sound.

The big problems are:

- As regular refueling is needed, the firebox gets filled with a pile of embers. As the pile grows, embers from the bottom don’t have access to fresh air. Their combustion slows down, and the pile grows. When filled with embers, the firebox is not providing as mush heat as it could to help the increase of temperature.

    => Idea : Placing a grate on the floor of the firebox, under de logs to help air going through the embers. As we sometime see Peter adds two metal bars for this function ?

- When the firebox is refueled by adding a log, the combustion starts to be highly smoky and polluting, with a thick black smoke coming out of the exhaust pipe.
After long minutes the combustion gets clean again and after a few more minutes the temperature start to decrease in the kiln chamber.

I can imagine that the firebox gets too hot: being constantly fired and being so close to the high temperature kiln chamber. Fresh logs introduced in the too hot firebox would get gasified faster than what the core is able to burn.

Ideas:

o Making a secondary air feed. Which would not need a complete rebuilt.

o Insulating the firebox from the kiln.

o Expanding the height of the riser to its standard size.

I know that the modifications I made to the standard BBR core change the behavior of the burn and is hard to predict.
Although I am asking for you advices on wich improvements are prior and for ideas I might have missed.  As some changes are more time/energy/materials consuming than others.

And I can also ad a reference to a ceramic kiln, which inspired me, built by the stove builder “ Warmte op maat “. It's a kiln base on Peter’s DSR1, made from IFB and metal structure. The performances are outstanding: up to 1064, in 2.5 hours, with 16 kg of wood.

See :

http://instagram.com/p/B_MjMMGjTEG

http://instagram.com/p/B_9dN-zjOgC

Some might ask why I did not just built the same: Mostly because I tried to use reclaimed materials available.

Security details : I am aware construction and firing test took place not in a completely safe environment. Water buckets, water hose and extinguishers were around.

Please excuse my technical English as a French native speaker.

Thanks a lot ! 

Inside Plans :





Second Firing temperature curve:





Kiln ceramic chamber loaded with random ceramics









Lighting





Strong Burn





Thick black smoke after refuels





Building Pictures :

[hanee]

Hi Amo,

Wow, what an exciting project! It looks to me like you're not far off from reaching your goals.

Unfortunately, I'm not very qualified or experienced enough to advise you on what is causing your black smoke (too much air is what I think I've gleaned from reading Peter/Vortex/etc posts in the past... but as I said, I'm not qualified here!). Hopefully someone experienced will provide some advice on the technical combustion issues.

Some question for you that may help the discussion:

1. What temperature/cone are you trying to reach? I'm guessing you're aiming at a Cone 06/05 (in and around 1000C), based on the design that inspired you (the instagram post).
2. Have you evaluated what the heat-loss side of things looks like? Did you measure the temperature on the outside of the heater while firing to see what your loss looks like? How about temperatures after the firing chamber just before exhausting out the chimney?

A good way to look at the problem, in my opinion, is to break it in two. You have a firing chamber (which is planned to function as a bell), and you have a combustion unit. They are two separate problems. One challenge you've created is that you've integrated the two a little strongly by expecting your firing chamber to also function as an afterburner/riser. That may make it hard to modify things as you go.

Many wood fired kilns (and gas fired kilns, for that matter), rely on more than one combustion unit. In the case of wood fired kilns, it often has to do with managing the loading process. If you had two fully functioning batchboxes feeding exhaust into one bell, for example, you may be able to avoid many refueling problems. At the practical level, it means that you needn't nurse the firing quite so much and are able to add large batches at a time: as one firebox completes its cycle the other one can be starting up. Others will chime in as to whether two cores, both alternately loaded in full batches, would avoid the reload combustion problems altogether. (Perhaps they would create draft issues depending on how the chimney(s) or bell is set up).

Whether you have one or two cores contributing heat, I do think that separating a design so that it can be conceived as a firing chamber (which can be expanded/rebuilt/modified as needed) and as a well-tested and standard core would make it more robust and flexible. As long as the core functions well on its own and the firing chamber is heavily insulated and has enough space around the wares to function correctly as a stratifying chamber, with the ware at the top functioning as the mass, and the exhaust at the bottom, the exact design of each should be fully independent (decoupled/interchangeable).

As for coming up to temperature too quickly, and then struggling to keep rising, here is my possibly-naive interpretation:

The rapid rise is probably coming predominantly from increases in exhaust temperature during the beginning of your initial full-load burn cycle. You've placed your firing chamber in the middle of your exhaust line so it's going to have very little averaging-out or buffering of the heat. It's also probably getting a lot of flame licking the wares (if the afterburner is functioning at all) which will be hard on your ceramics. Some kilns even have something called a "bag wall" to ensure that concentrated flames and blasts of uneven heat do not get to the ceramic ware. Unless you're looking for special effects like Raku, the goal is clean, even heat -- combining an afterburner/riser and firing chamber into one unit is going to make controlling the heat ramp very difficult.

For the slow rise after the initial jump, I'm guessing that the average air temperature within the fire chamber reached 500C and then started to plateau because it takes quite some time for the firebricks to get up to that temperature all the way to their outer edge and for the insulation to start making a meaningful effect. Only once the bricks are up to the air temp, and only if they are prevented from losing heat via adequate insulation on the outside, will their temperature and the temperature of the other ceramic ware rise much further. Until then they will be storing and conducting heat out of the chamber. The instagram kiln you linked to appears to have perhaps 6" of what looks like high performance ceramic wool insulation and almost no interior mass, which could explain it's rapid rise and limited fuel use.

A firing chamber is a dynamic of mass and insulation. The mass you add in the kiln helps to make temperatures rise and fall slower and more evenly. Raku kilns are often built with no mass at all to get up to heat quickly (and Raku clay bodies are formulated to be able to handle the thermal shock). Adding mass to a kiln is only for smoothing and slowing temperature gradients. That is it's only purpose other than being a durable surface. The insulation, on the other hand, helps you to reduce heat loss and thereby increase temperatures given the same incoming heat-energy. Every time you fire a kiln with mass, you've got to nearly fire all those bricks, not just your wares.

Think of it as three components when it comes to the kiln design.

1. Heat source (I will leave it to the core experts to help you on that).

2. Insulation (to allow heat to accrue)

3. Mass (to smooth temperature gradients and control "ramping up" and "ramping down")

I'm guessing you're heat source has a few problems, both in managing refueling and in efficiency/particulate, but my bet is that you would have eventually reached your firing temperature once your mass was warmed up, unless you were sending too much heat out the flu or out the sides. Perhaps you would have reached your 6hr goal. A few more temperature readings in different places could be very instructive as to how things are functioning.

Hopefully those with more experience show up soon, but I thought it was worth throwing in whatever limited, hopefully-correct knowledge I could! Sorry, I am notoriously wordy! Bon chance!

[fuegos]

"But it has been modified in order to optimize the overall compacity and the layout due to available materials dimensions" & that I think is your problem.BBRs J tubes & riserless cores all need to be built to the EXACT dimensions as stated or they won't work.The firebox can be lengthened but all the other dimensions are critical as has been discovered with 1000s of hours of work.It seems that you have achieved a pretty good result with regards to temperature so maybe a rebuild would solve the smoke problem.materials need to be cut to fit the design not the reverse.but thanks for posting you're in the right place for advice & support, don't give up !

[josephcrawley]

Stalling is a very normal thing in wood firing. Look it up and you'll find a ton of discussion about this phenomenon.

Nils Lou has an excellent and helpful book on wood firing that might help.

About the black smoke it's a common batch box problem as far as I know. You have a very hot fire a huge pile of coals and when you throw in a new batch of fuel you end up with a lot of pyrolysis all at once. The system isn't designed for this. Same with the coal build up. If I load my stove three loads in a row I end up with a firebox half full of coals. Once the Ware chamber is hot enough given adequate air you should get combustion in that area as well with flames leaping out of the chimney. It's what the old timers refered to as basting off. It's a rather frightening experience.

I'm not saying it can't be done but people have been wood firing for a very very long time and to my knowledge it has not been done cleanly yet.


Fred Olson's kiln book may also be a valuable resource. Good luck and keep posting this is a very interesting project.

[hanee]

Unfortunately that Nils Lou book is out of print and hard to come by. Goes for around $100 in the US and I'm betting even more difficult to get a copy of in France my bet. You'd probably be better off checking out the Ceramic Arts Network (ceramicartsnetwork.org) forums before trying to track down such books. Or find local resources. But they are all likely to not have ever considered trying to wood fire more efficiently.

As I stated in the other kiln thread you were participating in, my opinion is that there is little/no motivation among wood fired potters to use less wood or burn more cleanly. I don't think many will understand your motives.s They also generally are interested in firing large volumes of work due to the labor involved in wood firing. Even their 'small' kilns are usually quite massive.

I have digital copies of several kiln books, among them the Fred Olson one. Unfortunately that one has only two designs covered in the "small wood kilns" section (both of which are quite large). One is the Olson FastFire, the other is Steve Mills "Double Cross-Draught Kiln". The FastFire is a large beast as far as my understanding. The Steve Mills kiln is a little more manageable but still definitely not particularly efficient. It's made to be taken down and put back up as needed on location, but it's still a monster compared to what you've built.

You're existing design, my bet, may already perform better for a small load than either of those kilns (which are often shown in photos with black smoke and flames belching, and rarely have adequate insulation).

I think your original inspiration of the kiln shown on instagram shows that, in fact, efficient firing is possible. If his consumption numbers are correct and considering the installation within a building and the pristine firing chamber in his photos, he's clearly far far ahead on efficiency of anything you'll find in a wood fired kiln book or ceramics forum.

If he was willing to opensource his design and share it that might be your best bet as a starting point. I've got a hunch that much of his success has to do with the amount of insulation he's using. I wish my kiln project was not so far off in the future as it'd be great to get some back-and-forth on experiments with a few people building variations out there.

[hanee]

Regarding the balance of mass and insulation in a firing chamber: take a look at Ian Gregory's kilns (note of course that it's a bad idea to be handling ceramic wool in this way):

www.ian-gregory.co.uk/kilns.html

You can see that the only real function of the mass is to control ramping if the heat source is not easily adjusted. Other that, mass is only going to increase wood consumption and firing time.

I find it telling that the firing chamber in your instagram link appears to be almost completely made of ceramic wool insulation, with perhaps Cordierite kiln shelf for surfacing (just a guess based off one photo)... that could have far more to do with his efficiency than the particular nuances of his core design. Scaling down the need for heat is the first step in efficiency. What pollutes more or wastes more wood, a 90% efficient source that must provide 60kbtu/hr for 6 hours or a 70% efficient heat sources that must provide 60kbtu/hr for 2 hours? That instagram kiln has vastly lower energy needs, most likely, way before the core design even comes into the picture.

[coastalrocketeer]

I would be inclined to recommend low mass, highly insulative firebrick, and or ceramic fiber blanket as the INSIDE face of your bell. I believe the mass of your current side wall and ceiling, are sucking up most of your heat production, and causing most of your losses. I often see at least 5-6” thickness of these low mass insulating materials on the interior of small ceramic kilns. Often 3” of lightweight insulating fire brick, then an outer wrap of ceramic fiber blanket.

[amo]

Thanks to all of you, it is very heartwarming to receive all your remarks, advices and encouragement!

I would like to reach at least 1050°C and someday probably with further designs 1300 °C to be able to fire stoneware.

I started thinking about this prototype by trying to estimate the heat loss with heat transfers formulas. I was not able to estimate the heat loss by the chimney, so I just decided to build a first prototype to have a first experience, and improve it empirically.

After 4 hours of the second burn (910 °C max), I could still place my hand on the aerated concrete insulation, top and sides, and feel it lukewarm. Sadly, I did not think of taking the temperature of the exhaust. I will think about it for next tests!

I also like the Idea of two cores functioning alternatively very much. Experimenting would show if drafts and synchronization problems are hard to manage.

Using refractory insulative materials will help solve all the issues, as you all say and as the Instagram reference, I posted. With such insulation the overall efficiency would be much higher, firing would be faster and the firebox wouldn’t be clogged with embers, and much less refueling would be needed.

I am indeed looking for flame effects on wares. I was inspired by Phoenix kilns, which gives beautiful effects on ware. An example of Phoenix style kilns: youtu.be/diUWORNA-B4?t=1281

On this example, the inside of the ware chamber is also lined with ceramic wool.

I received the dense firebricks free from a friend who dismantled a masonry barbecue. And, with luck, this barbecue contained the exact number needed for this prototype. It allowed me to experiment with fire for a very low budget.

Big changes on this prototype and reconstruction with expensive insulative materials might take some months to be implemented but I’ll keep posting next tests and improvements.

Books:
I read Daniel Rhodes “Kilns” book, but I did not learn much more than all you can read on Peter’s and Uzume’s websites.
I did not know about Nils Lou and Fred Olson’s book and I would love to read them! Indeed, they are hard and/or expensive to find in France.

Regarding the pyrolysis "shot" that happens when I refuel at high temperature. Maybe a (big?) secondary air would help feeding oxygen in the ware chamber and burring this black smoke. This secondary air could be preheated around the chimney and forced by a fan? With this concept: youtu.be/nJIt7Fc2-VY?t=364

Thank you all for helping me getting on the right rails !

The constructor of the DSR1 well insulated kiln kindly sent me some pictures of the build :

[hanee]

amo,

My notifications weren't working right and I guess I never saw your post. As you work towards a design I'd suggest that you may want to have a different "hot face" material than the backup insulation. A good design is a dense refractory backed up by insulation. Also, my understanding is that you could use a refractory insulation for the first inch or two, and then switch to something like mineral wool (which starts to melt at 1000C, but which, if insulated from the firebox by a layer of ceramic fiber, will never reach those temperatures).

The best economical example I've seen of a good approach so far (and the one I'll be using for my kiln if I get to build it before the end of summer), is this one by Joe Finsch (hopefully the link the PDF works, otherwise I can post a copy I downloaded):

gedpotteryblog.files.wordpress.com/2013/10/make-your-own-kiln.pdf

It's a catenary-arch design. I intend to use a regular BBR core (with the taller riser), so the exhaust enters near to the top of the arch, and then an exit chimney at the bottom or a little below the floor level. The riser could be within the insulated catenary firing-chamber, with the firebox being on the outside.

What I also like about this Joe Finsch 'clay kiln', is he seems to have had success with a fairly simple fireclay based refractory lining that uses normal clay-forming practices, whereas a lot of 'casting' refractories seem to require a lot of finesse in de-bubbling for good casts (plus a mold). The 'clay kiln' is also designed for soda/salt or wood firing, so the inside will take a lot of abuse from fluxing agents. That's why it's important to line it with a 'hot face' that's more of a dense refractory. If he's having success with soda/salt firings, then it should definitely work fine with wood firing.

Also, in his design, you'll notice what I was saying about using Rockwool/Mineral Wool for a second layer of insulation after an initial more refractory ceramic fiber blanket. Finally, the whole thing is covered in a vermiculite concrete over wire mesh. It makes a nice sandwhich with a hard/durable interior and exterior and an economical yet substantial amount of insulation throughout. The interior clay mix also relies on some paper probably both for early green-strength and resistant to thermal shock and perhaps to make it marginally more insulative.

Anyhow I think basically putting one of these firing chambers over a typical BBR design with a tall riser would be geometrically efficient and give a good even interior temperature with minimal surface area -- it also solves the age-old problem of how to do the roof by employing the catenary arch rather than having to have a stronger/thicker or reenforced material to span a great distance over a cubic design.

Anyhow that's where I'm headed when I get the chance...

One other note, in some traditional wood firing kiln designs I've seen there is a way of dealign with excess coal/ash buildup (mainly referenced in Pioneer Pottery by Michael Cardew -- though I'm guessing it's common elsewhere I just hadn't seen it described well before). This is basically to have a chamber below the stoking box. I know the Vortex Stove has an ash collector integrated into the design, and it's sort of like that. I'm not sure how integrating a larger ash/coal falling chamber underneath the batchbox would potentially modify the performance, but it seems to me that it would be a potentially elegant and simple solution to needing to keep stoking fuel. The ashes and excess coal would be knocked or fall down to the lower chamber as needed to allow more continuous stoking. I'm not sure how you'd either integrate it into the combustion system or otherwise make use of that excess coal (perhaps towards the end of the firing for slower down-firing or introduced into the firing chamber perhaps for certain effects?).

Another thing to note is that many of the desired effects of wood firing come from inefficient burns: my understanding is that any 'reduction' vs 'oxidation' firing is inherently heavy in Carbon Monoxide. Clean burning is oxidation. But most wood firing specifically tries to achieve a period of 'reduction' once it reaches it's desired temperature.

[hanee]

Oh, and thank you very much for the additional photos of the instagram DSR kiln!

[Forsythe]

hanee has provided a lot of excellent info here. I just want to expand on some of what he's said with some additional thoughts / ideas that may be useful.

Apr 30, 2022 12:31:12 GMT -8 hanee said:

As you work towards a design I'd suggest that you may want to have a different "hot face" material than the backup insulation. A good design is a dense refractory backed up by insulation.

THIS. The hot-face can be accomplished either by (1) using a firebrick lining with the ceramic fiber behind it — or by (2) creating a hot-face lining directly upon the rigidized ceramic fiber, using the ceramic fiber as a substrate to form a hard, dense, highly-refractory interior "skin" (very similar to a DIY home metalworking forge or furnace.)

I'm currently working on a portable kiln build using option 2, with an accordion-folded ceramic fiber kiln body, rigidized with fumed silica, and then lined internally with a highly-refractory, InfraRed-reflectant interior hard coat for the hot-face. The interior hard coat uses a mix of zirconia ["Superpax-Plus", which is very finely-powdered zircon flour often used as a glaze opacifier], kaolin, and small addition of "Veegum-T" [a highly purified, superfine magnesium-bentonite used to deflocculate glaze slurries into colloidal suspensions and/or make highly-refractory, non-plastic clays both shapable and easily-vitrifiable]

(Side note: I'm also experimenting with a version of that recipe using a powdered metallic-aluminum addition to high-fire and sinter that blend into a super-tough, high-alumina, zirconia-impregnated hot-face, making use of the aluminothermic reaction within the clay body itself, between the SiO2 [present in the kaolin] and the powdered metallic-aluminum — inspired by some old research papers & expired patents on the subject of producing high-fired, high-alumina firebricks and refractory bodies with low fuel input... but that's a topic for another, later thread.)

The zirconia and veegum-T idea was inspired by DigitalFire's website. (Which is a wealth of useful info, in general.)

digitalfire.com/material/1724




There's also this PDF from Joppa Glassworks who sell a 50/50 mix of zirconia/colloidal silica as "z-wash" for kiln lining and furnace-making with a very similar technique:

www.joppaglass.com/burner/z-kit/z-kit_Lo.pdf





—> Worth noting: the difficulty in getting the zirconia / silica slurry to bond to ceramic fiber board is almost certainly because of the organic binder used to form those ceramic fiberboards into rigid shapes. I'd be willing to bet that if that fiberboard were first fired to burn off the organic binder — with the zirconia slurry applied only after the ceramic fiberboard was burned clean — that he would no longer have any problems with adhesion between the zirconia coating and fiberboard substrate.
—> Also worth noting: the cracking and separating of the zircon coating he mentioned is almost certainly due to the mix's lack of clay content. The firing temps he's using aren't anywhere near high enough to sinter-bond the zircon... and zircon is normally only held in a suspended matrix between vitrified silica and alumina (clays) in most zirconia coatings, very often with a magnesium-bearing bentonite. Because his mix utilizes only silica, that means the zirconia particles [after firing] are held in place within nothing more than a simple silica glass, which is going to be much more brittle and vulnerable to thermal shock than a clay's aluminosilicate matrix would be.

The best economical example I've seen of a good approach so far (and the one I'll be using for my kiln if I get to build it before the end of summer), is this one by Joe Finsch (hopefully the link the PDF works, otherwise I can post a copy I downloaded):

gedpotteryblog.files.wordpress.com/2013/10/make-your-own-kiln.pdf

It's a catenary-arch design. I intend to use a regular BBR core (with the taller riser), so the exhaust enters near to the top of the arch, and then an exit chimney at the bottom or a little below the floor level. The riser could be within the insulated catenary firing-chamber, with the firebox being on the outside.

What I also like about this Joe Finsch 'clay kiln', is he seems to have had success with a fairly simple fireclay based refractory lining that uses normal clay-forming practices, whereas a lot of 'casting' refractories seem to require a lot of finesse in de-bubbling for good casts (plus a mold). The 'clay kiln' is also designed for soda/salt or wood firing, so the inside will take a lot of abuse from fluxing agents. That's why it's important to line it with a 'hot face' that's more of a dense refractory. If he's having success with soda/salt firings, then it should definitely work fine with wood firing.

Also, in his design, you'll notice what I was saying about using Rockwool/Mineral Wool for a second layer of insulation after an initial more refractory ceramic fiber blanket. Finally, the whole thing is covered in a vermiculite concrete over wire mesh. It makes a nice sandwhich with a hard/durable interior and exterior and an economical yet substantial amount of insulation throughout. The interior clay mix also relies on some paper probably both for early green-strength and resistant to thermal shock and perhaps to make it marginally more insulative.

Anyhow I think basically putting one of these firing chambers over a typical BBR design with a tall riser would be geometrically efficient and give a good even interior temperature with minimal surface area -- it also solves the age-old problem of how to do the roof by employing the catenary arch rather than having to have a stronger/thicker or reenforced material to span a great distance over a cubic design.

agreed. I love how accessible that design is, and the caternary arch is the most structurally-stable form used for kiln-building... especially if you're pushing into stoneware and porcelain firing temps. ...I do worry a little bit about the longevity of his fireclay interior hot-face with soda/salt-firing, though. 70% alumina is normally required for the interior lining of soda kilns... and typical, non-kaolin fireclay is usually only in the ballpark of 18-34% alumina (very often they're on the lower end of that spectrum) — with kaolin clays normally in the range of 35-43% Al2O3.

One other note, in some traditional wood firing kiln designs I've seen there is a way of dealing with excess coal/ash buildup (mainly referenced in Pioneer Pottery by Michael Cardew -- though I'm guessing it's common elsewhere I just hadn't seen it described well before). This is basically to have a chamber below the stoking box. I know the Vortex Stove has an ash collector integrated into the design, and it's sort of like that. I'm not sure how integrating a larger ash/coal falling chamber underneath the batchbox would potentially modify the performance, but it seems to me that it would be a potentially elegant and simple solution to needing to keep stoking fuel. The ashes and excess coal would be knocked or fall down to the lower chamber as needed to allow more continuous stoking. I'm not sure how you'd either integrate it into the combustion system or otherwise make use of that excess coal (perhaps towards the end of the firing for slower down-firing or introduced into the firing chamber perhaps for certain effects?).

Definitely. With wood-fired kilns, ash removal below the grate is necessary to keep stoking, and that slant-floored slot design aids not only in funneling the ash out of the firebox — it also channels the burning coals toward each other at the center, making room for more fuel additions without having to rake the coals themselves.

There's also a design for making firegrate bars out of fireclay (instead of using steel, which doesn't last long) available in The Self Reliant Potter by Henrik Norsker, which is linked-to and discussed here: donkey32.proboards.com/post/37550/thread

included in that brief discussion is the "test kiln" build from that book which very closely resembles the earliest version of Peter's DoubleShoebox rocket:

Another thing to note is that many of the desired effects of wood firing come from inefficient burns: my understanding is that any 'reduction' vs 'oxidation' firing is inherently heavy in Carbon Monoxide. Clean burning is oxidation. But most wood firing specifically tries to achieve a period of 'reduction' once it reaches it's desired temperature.

Yes, that — but also — one goal of clean woodburning combustion is to reach extremely hot burns very quickly — a rocketstove needs to reach 600°C as fast as possible, because it's only at or above 600°C that the pyrolizing woodsmoke's carbon particulates can be burnt cleanly into CO2, rather than carbon monoxide and/or soot. ...However... In pottery, it is necessary to very slowly ramp-up and ramp-back-down the firing temperature in the range between 210-600°C — because that temp range is where quartz-cristobalite inversions occur within the clay bodies [which are changes in the silica's volume due to crystaline phase-shifts at those temps]... and progressing too quickly up or down through that 210°-600°C range will cause thermal shock cracking and shattering of the clay wares being fired.

This normally isn't a problem for a small kiln only doing low-fire pieces — because the clay wares can be loaded into a "bell" above the rocketstove's riser, allowing them to be "fired" only indirectly with the flue gas exhaust, downstream, after complete combustion... but when you're doing stoneware or porcelain, the clay-firing temps have to be significantly hotter... and that means the pyrolized woodgas needs to be actively burning in direct contact with the clay bodies. In practice, this means building the clay-firing chamber inside of what would otherwise be a rocketstove's riser (or secondary combustion chamber.) ...and that then means having to burn cooler and thus dirtier on the ramp-up and ramp-down kiln stages. (That's even aside from the oxygen-deprived "reduction-firing" method mentioned above)

...so, unfortunately, you can fire cleanly with wood — or you can avoid shattering clay pottery while woodfiring — but you can't do both at the same time. (And that's why some modern wood-burning kilns are now built to be multi-fuel or even electric/woodfuel combinations, allowing for slow ramping of temps with electricity or gas, and only switching to woodfuel once the kiln has reached the above-600°C range, then switching back for cool-down — or bricking-up the firebox doors and draft-holes, and closing the chimney flue; relying on a highly-insulated kiln structure to hold the heat for a slowed cooling.)


Personally, I really, really like the design of Steve Harrison's portable wood-fired stoneware kilns... If not for the fact that he's in Australia, I might be inclined to buy one from him (he and his partner Janine King have a lovely little blog— and they're still recovering from the devastating Australian brush fires that burned them out not long ago) ...but this post is already getting kinda long, so I'll follow-up in a sec with links, pics, and notes for his designs.

[Forsythe]

Here's that Steve Harrison portable stoneware kiln design, from his blog, TonightMyFingersSmellOfGarlic.com, with Janine King, which follows the progression of design iterations, and some of them including a gas/wood-firing multifuel option:



tonightmyfingerssmellofgarlic.com/2016/02/07/new-small-wood-fired-kiln/

I have spent a little time over the summer break building another version of my portable stoneware wood fired kiln. These kilns are a direct response to seeing and working with Stefan Jakob’s ‘Ikea’ garbage bin raku kilns. Such a fun idea! And they work really well too, but only at the lower temperatures used for raku. It made me think about if it would be possible to make a reliable stoneware version of this kiln. Not in an Ikea bin, but in a custom made stainless steel monocoque box frame. The answer that I have been developing over the last half dozen years in my spare time is Yes!

This one solves all the problems identified in the last version, that although it could get to stoneware, some of its components weren’t likely to have a long life. I abandoned the ceramic fibre lining, as it doesn’t last for extended periods of time at very high temperatures where there is a lot of wood ash. The fibre turns glassy and peels off, like glaze shelling off, exposing new fibre, which then dissolves, the ash glaze slowly eats its way through the lining in this way.
We first experimented with a ceramic fibre lined stoneware wood fired kiln back in the late 70’s and early 80’s (see Handbook for Australian Potters P289-291.) In that kiln I used the new material at that time called ‘saffil’ board, that was mostly composed of alumina fibre. A 10 mm. hot face lining of this material lasted 30 stoneware firings before it was eaten away in the hottest part. These new little kilns use light weight refractory insulating bricks as the lining.

...As an aside: With a refractory hard-coat lining as mentioned in my previous post above, that durability and longevity issue with ceramic fiber is more than solved — and the kiln would last even longer than when made with lightweight insulating brick, due to the woodash slag resistance and IR-reflectance of the zirconia refractory lining.

I couldn’t allow myself to recommend or to sell anything that wasn’t up to scratch and capable of delivering a long working life, so the development has continued, designing out the apparent flaws as they made them selves known. So now the design is a little closer to completion. I have designed version 5, so I hope that after that is built and fired a few times, everything will be settled down and we will have a very long lived and reliable small portable kiln. I think that we could say that we are now moving from prototype to beta testing stage. Perhap there will be something that we can sell to other potters with like minds. Just like we do with the more substantial gas and electric kilns that we build here – only much cheaper.

The improvements in version 4 meant that we could fire it up to 1,000oC in one hour. This part of the firing could easily go very much faster, but we have cracked kiln shelves in the past by going too fast below red heat. We then took the firing from 1000 to 1280 in another hour, finally soaking at 1280oC to 1300oC for the last hour until cone 10 was over. We got very good reduction colour in the glazes in that time frame. I was amazed what a couple of extra hours could achieve, in terms of quality. After all it’s not all about intense speed. We can already do that. This is more about getting very good quality results with a minimum of expenditure of effort and fuel.

I spent a few days working out how to create this little wonder of a kiln, to enlarge it to use a 12” x 18”  (300mm. x 460mm.) kiln shelf in the setting, and still be able to cut the frame out of one sheet of Stainless steel with no or minimal wastage.
I’m sure that there are a number of potters who are with me and like minded in this regard, potters who are thinking just the same as me. How can I achieve lovely wood fired results without firing for days and creating loads of smokey pollution. I think that this sort of little fun kiln will be very good for potters with an interest in wood firing, but without the large work flow required to fill a larger anagama kiln, or a suitabe place where so much smoke can be created day after day. This little kiln is definately not smoke free, but the smoke is minimal.

As it turned out, this was a very relaxed and easy firing using dead brushwood and small, dead, fallen branches as fuel. There are always loads of eucalypt paddock falls all around our property from season to season. We collected 3 wheel barrow loads, one of kindling twigs and another two barrow loads of small thin branches, up to 50 mm in dia. We ended up using only 2 of them.

tonightmyfingerssmellofgarlic.com/2016/04/06/small-portable-stoneware-wood-fired-kiln-cont/

We have just fired the 5th incarnation of our little, portable, stoneware capable, wood fired kiln. It fired very well, again easily in 3 hours. It just cruises along at its own pace. 1 hr to 1000 oC and then 2 hrs to 2 1/2 hrs in reduction to stoneware cone 10 over.
This variation was to test out the new chimney arrangement and that worked perfectly, some much better than the previous one. I am very happy with that. Another problem solved!

I have it adjusted now so that as the temperature reaches 1000 oC. The kiln automatically goes into reduction. I don’t have to use a damper to make this happen. The kiln continues to rise steadily over the next two hours without having to alter any settings in the firebox. It’s lovely.

tonightmyfingerssmellofgarlic.com/2019/07/18/winter-wood-firing-workshops/

We are very busy all this month with our winter wood firing workshops. We run these every winter, and it has become a reliable part of our income. We will be doing 4 firing workshops this month. We are currently half way through the series. 2 down and 2 to go.
So far we have had some of the best workshops that we have ever held. Excellent results keeping the students very happy. AND a zero rate of losses so far. Pulling red hot pots from a kin at 1,000oC can be challenging. The clay has to be of the right kind and the firing just right. Fortunately, we have a stock the very fast firing and fuel efficient small portable wood kilns that we build in our kiln factory, on-site. We have a number of these at our disposal these days. They work remarkably well, they are a joy to work with. We take about an hour to fire them up to top temperature from cold for the first firing. The work is unloaded using metal tongs...

tonightmyfingerssmellofgarlic.com/2017/02/21/more-portable-woodfired-kilns-the-dual-fuel/

I have been working on my portable wood fired kiln designs and tweaked it a little bit more. I am up to design variation No.9 now, and working on number 10.  I have designed a way to have these kilns fired by both wood or LP gas, but not at the same time! It’s a simple method of changing the setting so that it can work with either fuel, with just some minor adjustments.

3 to 4 of these little amazingly fuel efficient kilns are sufficient to fire the work of about 10 students, as the kilns cycle through the day. I have built a few different shapes and sizes , so that we have a taller chambered kiln and a wider setting kiln as well as a cubic  chambered kiln, we manage to get through all the different shaped work in the day.

Apart from the firebrick-vs-zirconia-lined-ceramic-fiber design, one other thing I'm aiming for is a downdraft design — which would be even more fuel efficient, capable of reaching much higher firing temps, and would make for more-evenly distributed heat throughout the kiln's interior. Having a direct-updraft chimney exit on top loses a lot of usable heat to convection, whereas a downdraft flue exit at the bottom-rear of the kiln would only exhaust the coolest gases, after they had travelled through the whole kiln setting and all the ware being fired... all due to the same hot-gas free-fluid dynamics as the bell theory in rocketstove and masonry heaters. ...But, those are fairly minor adjustments to Steve's original design, which is pretty freaking sweet to begin with.



*Edited to add some close-up photos of the Steve Harrison kiln's firebox, which appears to be wide, low, and deep — spanning the whole internal area of the kiln setting.

[hanee]

Forsythe , thank you for posting that information on the portable stoneware kiln. That seems like some very fast firing he's doing: I'm guessing he's not bisque/single firing but glaze firing?

One problem in all of this type of work is that it's easy to lose a lot of time on R&D and add complexity into a process. If you're doing it for its own sake, then that's all good and fine. If designing kilns is a hobby, then there's really no reason not to explore and innovate every last detail. If someone was a potter, on the other hand, they might be losing a lot of energy that could develop them further as a potter.

For me, my hobby/career is figure sculpture, not combustion technology or ceramic science. I just happen to live off-grid and have a good supply of wood and am enough of a purist that I'd like to know I'm not polluting or wantonly wasting resources. And I've already got my time quite divided as I've got much work on our self-built house and homestead to do in our limited season here in Maine.

I am guessing many others out there are in similar situations. This is why people buy EPA-certified box stoves and buy or build a gas or wood kiln from a set of pre-established plans in some book. The pursuit of simplicity and efficiency can be a complicated thing: c.f. Zircopax . Though many times things that seemed complicated become simple over time, a lot of time there's a good reason why people weren't already using our brilliant solutions. Sometimes the reason ends up technical, sometimes it ends up practical. And yes, sometimes there is no good reason and it truly is just a problem of conventionality, irrational market forces, and other things, but I usually question whether feeling like there's got to be some definitely superior solution out there is a kind of hubris: after all, nature has its balances and there are rarely absolute advantages. In my experience, progress and innovation is as much sideways as it is forward, and it's easy to see the distance instead of the direction.

These notes aside, I'm wondering how the net efficiency of one core vs another even matters above certain temperatures, or whether the 'burn profiles' of BBR/RMH technology necessarily even matches what's ideal in ceramics (particularly with a need for controlling ramp -- though I'm a bit of a skeptic on that, since as a sculptor, I regularly fire extreme uneven thickness work often with no grog in it at all fairly quickly and have rarely experienced any losses; but my work usually sits around for months drying).

My understanding of combustion is severely limited, but as far as I can tell, there are two issues at play in the design downstream of the firebox: air and heat. The idea being that they're trying to mix the air and fuel together at sufficient temperatures. The insulated riser is part of that, but once you're operating within an insulated chamber to begin with -- i.e., once your entire bell is insulative and possibly reflectively coated too, not absorptive -- then any way you slice it, you've got a secondary burn chamber. It just may lack the ideal geometry for mixing (though a well packed kiln has plenty of 'stumbling blocks' already) or it may have insufficient oxygen (and need one or more air ports added or managed better).

I find it interesting that, whereas in Peter's, or Vortex's or Matt Walker's core designs every little proportion and detail seems to matter, in Steve Harrison's design there appears to be no special geometric considerations of firebox or chambers with regard to venturis, etc, other than the basic need for a grate, fitting the fuel length and volume he expects to use, and fitting within the kiln's overall dimensions nicely. Is that because Steve Harrison has no Testo, or is that because a large, super insulated chamber is going to burn pretty clean unless it's air starved (or not yet heated up)?

I'm wondering how much these things even matter. I've seen videos of people low-firing with a pile of sticks in a poorly insulated adobe kiln in a matter of an hour -- how much inefficiency can there really be in that case? What's the embodied energy of the refractory insulation they'd have used?

Similarly, traditional pit firing and open firing often use less total fuel than an equivalent kiln firing to equivalent cone. Yes, in order to reach higher temperatures you need a kiln, but the efficiency up to bisque-temperature is not bad from the wood-use perspective despite the obvious fact that there's no secondary burn, etc. This appears to have to do with heat-transfer mechanism and good prep, per Michael Cardew's "Pioneer Pottery" speaking of African open-firing traditions:

"One of the most surprising features of this tradition is that the firing never takes more than two hours, and often as little as one-and-a-quarter hours; and yet the 'clamps' contain very large pots which in any ordinary kiln would need a firing schedule of at least forty-eight hours..."

He then goes on to attribute the difference to two things: Preheating the ware to bring it down to below atmospheric moisture content and closer to 0% before firing, and firing with coals, not fire.

"The blazing flames given by the fuel when first lighted go up into the air and so, though nearly all their heat is wasted, they do not crack the pots. These are fired rather by the glowing of the embers [emphasis added] of the grass and the brushwood floor; and the abundant ash from the fuel on top helps to blanket and protect the ware."

I'm curious as to how this plays into things. Remember we have a certain minimum draft-speed of air traveling through any wood or propane kiln and all the heat has to get transferred to the ware from the BTU embodied in those hot gasses before they escape. I assume that means through a mix of convection and radiation (whereas an electric kiln is more fully radiation). Perhaps the direct-coal firing can have a surprising efficiency because it is primarily radiation (and even a bit of conduction) with lower convective losses from the requirement for draft. More like an electric kiln once you've got the coal bed established. Probably gives off a lot of CO though.

Comparing the Steve Harrison kiln to RMH/BBR designs, and again with the idea that the ware-chamber must-needs be functioning as a secondary burn chamber: it's interesting that Steve Harrison's kiln shoots fire out the chimney. That seems to be a pattern in wood-fired kilns that reach higher temperatures. My guess is that they therefore are burning cleanly, at least at that point in the cycle, but that the heat is not being used efficiently. My understanding is that the flame is from the last of the woodgas components being burned. Basically it's the equivalent of what's happening out of an insulated riser in an RMH/BBR. Except the heat's just going out into the environment. My suspicion is that more complete combustion would have to happen earlier in the process to be utmost *efficiency*, but that in terms of pollution, both might perform the same. And, it's worth noting, if the minimum size fire you need to maintain clean combustion is already producing more BTU than your ware can handle per hour, that heat is a loss either way.

Finally, in terms of efficiency, we must also always remember that scale is everything: if you live in a house in Florida and make 3 fires a year, the efficiency or cleanliness of a BBR is completely irrelevant. How often are people firing pots (or sculptures in my case) and in how big loads? Those dynamics change what's acceptable or ideal.

While I believe there is much that could be simplified by recognizing the firing chamber as being a kind of secondary-burn chamber, I've tended to try to achieve some kind of intellectual simplicity for myself by trying to separate the two.

The way I figure, if I can see the firing chamber as a simple entity, and the heat source as an independent more complex variable, then I'm more likely to have success, since I've got no interest in mastering combustion technology or acquiring a Testo. This much-simplified approach doesn't involve designing or understanding combustion and secondary combustion systems:

Firing/ware chamber:
- hot air enters, transfers to the ware, exits. Use stratification (must be bottom exit) or force flow through ware before exiting (up/cross/down all can work with right design but traditionally down/bottom-exit disperses heat more evenly). Reduce heat loss through insulation and possibly radiant barrier.

Heat source:
- pre-established/tested wood core (functioning identically to a propane burner where the combustion is as complete as possible before entering the firing chamber).

The only issue I see is that, if frequent stoking is required for higher temperatures, then there is probably no pre-established/tested BBR/RMH core that provides for that already. You'd be modifying the core with a grate of some sort (like Steve Harrison's firebox), at which point you're no longer in a tested design and could probably simplify things substantially by seeing the firing/ware chamber as a secondary-burn.

But, what I don't get is, why should there be frequent stoking? If you've got sufficient insulation you've most likely got an excess BTU problem, not an inadequate BTU problem (unless you get the core scaled down very tiny). The problem (on paper) seems to definitely not be one of insufficient BTU and constant stoking. Instead, the problem would seem to be getting up to heat across a more clay-friendly 6 hours instead of 1 hour while maintaining complete combustion. But I suppose I will find that one out empirically!

So in that spirit, just remember, amo , that apparently sound intellectual advice probably is of little use compared to tried-and-tested experiments.

On that note, I still think the best option is to separate the ware chamber from the combustion core, because if these forums are a lesson in anything, then the attempt to improve efficiency of combustion isn't going to be a one-time thing. You'll either settle with something that 'works', or tinker with it over and over. Nice to know in the mean time the bulk of your kiln will continue to work whether it's running on one wood-core or another (or even gas).

My hunch is that, after initial startup, given the same well-insulated bottom-exit firing/ware chamber, the differences between Steve Harrison's firebox, Peter's firebox, Vortex's firebox, and some old kiln firebox design from 1950, is going to probably account for the difference of a few hand fulls of wood and a couple puffs of black smoke... the manual impact you have on the fuel/air management will probably amount to the same amount of efficiency differences as the differences in combustion geometries... my guess is that in this particular application, the best performing and cleanest burning firebox will be the one that's easiest to manage the burn cycles with, not the one that has the best lab efficiency and emissions.

[Forsythe]

May 4, 2022 9:39:48 GMT -8 hanee said:

Forsythe , thank you for posting that information on the portable stoneware kiln. That seems like some very fast firing he's doing: I'm guessing he's not bisque/single firing but glaze firing?

He does stoneware glaze firing and raku with them. Bisque firing could certainly be done in one, but he uses an electric kiln powered by solar panels for his bisque firings.

One problem in all of this type of work is that it's easy to lose a lot of time on R&D and add complexity into a process. If you're doing it for its own sake, then that's all good and fine. If designing kilns is a hobby, then there's really no reason not to explore and innovate every last detail. If someone was a potter, on the other hand, they might be losing a lot of energy that could develop them further as a potter.

well, to be fair, he is a potter and widely published author on ceramics. He also lives off-grid and champions sustainable living. He had a problem that needed solving, [his student workshops] and so he created a solution for it which met the needs he had — and it's worth noting that it took several design iterations to get there. It looks like a simple design — and it is— but there's a good deal of R&D behind the ability to fire those wares in 3-4 hours with 2 wheelbarrows of deadfall branches... rather than 12-16 hours with 8 wheelbarrows full.

For me, my hobby/career is figure sculpture, not combustion technology or ceramic science. I just happen to live off-grid and have a good supply of wood and am enough of a purist that I'd like to know I'm not polluting or wantonly wasting resources. And I've already got my time quite divided as I've got much work on our self-built house and homestead to do in our limited season here in Maine.

I am guessing many others out there are in similar situations. This is why people buy EPA-certified box stoves and buy or build a gas or wood kiln from a set of pre-established plans in some book. The pursuit of simplicity and efficiency can be a complicated thing: c.f. Zircopax . Though many times things that seemed complicated become simple over time, a lot of time there's a good reason why people weren't already using our brilliant solutions. Sometimes the reason ends up technical, sometimes it ends up practical. And yes, sometimes there is no good reason and it truly is just a problem of conventionality, irrational market forces, and other things, but I usually question whether feeling like there's got to be some definitely superior solution out there is a kind of hubris: after all, nature has its balances and there are rarely absolute advantages. In my experience, progress and innovation is as much sideways as it is forward, and it's easy to see the distance instead of the direction.

That's fair. ...but also: I don't think anyone is twisting your arm [or anyone else's] to build something you don't want to DIY, for a need you don't have, and subject yourself to a cycle of experimentation and iteration that you don't have the time for, nor the interest in. Kinda begs the question of why you'd be here in the first place if you have no interest in these things.

I personally never thought I would get into stove design, but I had two largely-unrelated needs for which I'm still working to create a satisfactory, durable, sustainable solution, and that led me to it. Need is the mother of invention, and I don't think it's quite accurate to characterize Steve's (or Peter's or Trev's or Matt's) work as wasted time and energy that could have been spent more productively elsewhere, making them a better potter or whatever-else you think they should have spent their time on.

These notes aside, I'm wondering how the net efficiency of one core vs another even matters above certain temperatures, or whether the 'burn profiles' of BBR/RMH technology necessarily even matches what's ideal in ceramics (particularly with a need for controlling ramp -- though I'm a bit of a skeptic on that, since as a sculptor, I regularly fire extreme uneven thickness work often with no grog in it at all fairly quickly and have rarely experienced any losses; but my work usually sits around for months drying).

Well... yeah. Of course. When you're firing something thick like sculpture, it can absorb the micro-cracking from quartz-cristobalite inversions in some areas, while the thickness of the piece ensures that there's still enough structural integrity to hold together until those weak spots can "heal" back together during vitrification, once the whole piece is reaching maturation temp. Firing thin clay wares is a whole other matter, and care must be taken not only to avoid shattering the wares — but also to avoid cracking the kiln shelves and collapsing them into all the work you've set in the kiln. It's not an uncommon phenomena to be skeptical of... like...at all. It happens even to the most experienced and well-seasoned ceramic artists with PH.Ds on the matter — like Steve. How anyone with any ceramic firing experience —even if only sculpture— could be skeptical of that ubiquitous potters' struggle against losses caused by too-rapid firing...is... well, it's truly baffling, tbh.

My understanding of combustion is severely limited, but as far as I can tell, there are two issues at play in the design downstream of the firebox: air and heat. The idea being that they're trying to mix the air and fuel together at sufficient temperatures. The insulated riser is part of that, but once you're operating within an insulated chamber to begin with -- i.e., once your entire bell is insulative and possibly reflectively coated too, not absorptive -- then any way you slice it, you've got a secondary burn chamber. It just may lack the ideal geometry for mixing (though a well packed kiln has plenty of 'stumbling blocks' already) or it may have insufficient oxygen (and need one or more air ports added or managed better).

I find it interesting that, whereas in Peter's, or Vortex's or Matt Walker's core designs every little proportion and detail seems to matter, in Steve Harrison's design there appears to be no special geometric considerations of firebox or chambers with regard to venturis, etc, other than the basic need for a grate, fitting the fuel length and volume he expects to use, and fitting within the kiln's overall dimensions nicely. Is that because Steve Harrison has no Testo, or is that because a large, super insulated chamber is going to burn pretty clean unless it's air starved (or not yet heated up)?

I'm wondering how much these things even matter. I've seen videos of people low-firing with a pile of sticks in a poorly insulated adobe kiln in a matter of an hour -- how much inefficiency can there really be in that case? What's the embodied energy of the refractory insulation they'd have used?

So... comparing a rocketstove for heating and/or cooking to a kiln for firing clay bodies is a bit like comparing a bathtub to an olympic swimming pool. I mean they're both containers of clean freshwater into which human beings submerge themselves, right? So why not just have the olympic swimmer do laps in the ol' claw-footed tub? Makes the pool seem like an awfully inefficient use of water just to exercise-in when you could sit there and flail your arms & legs in a perfectly-good bathtub. Ammirite?

There's a massive difference in the amount of wood used — and thus the amount of heat produced, and therefore the stability of the burn. Small fires are far more tricky to burn cleanly...even within J-tube rocketstove design, it's a lot easier to make a 5 or 6-inch system chug along happily, while a 4-inch system (or smaller) is a lot more prone to choking. When you're burning a small amount of wood (as in 4-5kg for a rocketstove to cook or heat your house for 12 hours) those geometric design specifics matter a whole hell of a lot more than when you're loading that same 4-5kg of wood every 15-45 minutes over the course of 4-6 hours in a burn chamber many, MANY orders of magnitude larger. In the latter case, the fire is massive and the heated chamber is hot enough — everywhere, all throughout it's interior — to ensure secondary combustion of the smoke (and yes, you're right) if it has enough oxygen. The small fire's margin of sufficiently-heated combustion area —the difference between smoking and complete secondary burn— is very small and very fickle. It really is a matter of port shape and cross-sectional area of flue path and air intakes. The combustion in a batch box -vs- that in a kiln is a lot like the difference between the combustion in an oil lamp -vs- that of a grease fire. Big fires just inherently burn easier than small ones, and huge fires are a lot more capable of sucking-in the oxygen they need to keep going without putting themselves out or over-cooling.

Comparing the Steve Harrison kiln to RMH/BBR designs, and again with the idea that the ware-chamber must-needs be functioning as a secondary burn chamber: it's interesting that Steve Harrison's kiln shoots fire out the chimney. That seems to be a pattern in wood-fired kilns that reach higher temperatures. My guess is that they therefore are burning cleanly, at least at that point in the cycle, but that the heat is not being used efficiently. My understanding is that the flame is from the last of the woodgas components being burned. Basically it's the equivalent of what's happening out of an insulated riser in an RMH/BBR. Except the heat's just going out into the environment. My suspicion is that more complete combustion would have to happen earlier in the process to be utmost *efficiency*, but that in terms of pollution, both might perform the same. And, it's worth noting, if the minimum size fire you need to maintain clean combustion is already producing more BTU than your ware can handle per hour, that heat is a loss either way.

Well...no. Not at all. Fire shoots out of the chimney (and spy-holes) when the kiln is in the reduction firing mode you mentioned earlier. The pyrolized woodgas doesn't have enough oxygen in the kiln to burn — which is the point. That's what changes the clay and glaze chemistry from oxidation to reduction ...When O2 is not only depleted from the air in the kiln's internal volume, it is being ripped off the surface molecules of clays and glazes —which are all oxides— by the oxygen-starved fire. An oxide having some of its oxygen pulled away by chemical reaction is said to be chemically "reduced" in a "reduction" reaction. The O2-starved woodgas then ignites as soon as it comes into contact with oxygen again at the chimney exit. It has absolutely nothing to do with an inefficient use of heat or producing more BTUs than one needs, but rather about the conditions required for that type of "reduction" ceramic chemistry to take place.

Finally, in terms of efficiency, we must also always remember that scale is everything: if you live in a house in Florida and make 3 fires a year, the efficiency or cleanliness of a BBR is completely irrelevant. How often are people firing pots (or sculptures in my case) and in how big loads? Those dynamics change what's acceptable or ideal.

For sure. Even though a kiln burns a lot more wood all-at-once, it's the indoor woodburning heater's cumulative woodburning everyday, twice-a-day, which will outpace the kiln's particulates & pollutants when run all heating season.

While I believe there is much that could be simplified by recognizing the firing chamber as being a kind of secondary-burn chamber, I've tended to try to achieve some kind of intellectual simplicity for myself by trying to separate the two.

The way I figure, if I can see the firing chamber as a simple entity, and the heat source as an independent more complex variable, then I'm more likely to have success, since I've got no interest in mastering combustion technology or acquiring a Testo. This much-simplified approach doesn't involve designing or understanding combustion and secondary combustion systems:

well okay. More power to you, I guess. But what benefit to you is this forum where those things are literally the topic of the whole place?

The only issue I see is that, if frequent stoking is required for higher temperatures, then there is probably no pre-established/tested BBR/RMH core that provides for that already. You'd be modifying the core with a grate of some sort (like Steve Harrison's firebox), at which point you're no longer in a tested design and could probably simplify things substantially by seeing the firing/ware chamber as a secondary-burn.

On that point, I agree with you. Trying to make a rocketstove into a fully-capable stoneware or porcelain kiln is kinda like trying to make a bathtub into a swimming pool. They're just not designed for the same use-cases.

But, what I don't get is, why should there be frequent stoking? If you've got sufficient insulation you've most likely got an excess BTU problem, not an inadequate BTU problem (unless you get the core scaled down very tiny). The problem (on paper) seems to definitely not be one of insufficient BTU and constant stoking. Instead, the problem would seem to be getting up to heat across a more clay-friendly 6 hours instead of 1 hour while maintaining complete combustion.

...because you need a steady heat. That's why you keep stoking. If you didn't, your internal kiln temps would be a mess of a roller coaster, and you'd never know when the clay had matured sufficiently before it began melting-down... either that, or you'd pull underfired, crumbly wares from a sloppily-tended firing that didn't stay hot enough for long enough... It takes a while for the clay ware to finally reach the same temperature as the flames around it, and then you've got to "soak" at a specified holding temp for a period. Clay's maturation into ceramic is a matter of temperature AND time...that's the whole premise behind using pyrometric cones to measure the amount of thermal work done to the clay body. It's not just hitting a target temp with a sudden, single blast of heat and then letting the temp drop because stoking would be inefficient. That would be like thinking you could get stewed vegetables by charring them with a creme brûlée torch all-at-once, then throwing them into a bowl of water to serve. The chemistry just doesn't work like that.

But I suppose I will find that one out empirically!

...or you could talk about it with people who may have already tried something very much like you're proposing, in a forum like this, where we discuss combustion technology and ceramic science... thus saving yourself a lot of precious wasted time and energy you could have used to make yourself a better sculptor... (as your opening premise suggested of everyone else involved in literally doing that very same experimental thing.) But that would require a little bit of interest in combustion technology and ceramic science, which you said you have none of. so...I'm really not sure how to be of help with your pickle of a catch-22 there.

[hanee]

Hi Forsythe , unfortunately, I think you've rather dramatically misrepresented or misinterpreted a few things I said. Most are technical things, but these two are egregious enough that they need to be addressed as they are about the community:

May 4, 2022 12:47:12 GMT -8 Forsythe said:

Kinda begs the question of why you'd be here in the first place if you have no interest in these things.

May 4, 2022 12:47:12 GMT -8 Forsythe said:

I don't think it's quite accurate to characterize Steve's (or Peter's or Trev's or Matt's) work as wasted time and energy that could have been spent more productively elsewhere, making them a better potter or whatever-else you think they should have spent their time on.

I'm happy to take responsibility if I did not express myself clearly, but the angle you've come at this from, in terms of misinterpretation or assumption, seems rather extreme, and I believe there is a lack of good faith in your re-interpretation.

As is, I can't see how your reaction relates to my actual text. I'd suggest you or anyone else concerned simply re-read my first couple paragraphs of my previous post.

I'm quite appreciative of those who put work into DIY combustion technology, refractories, and so forth, but not everyone on the forum is here in order to mix their own custom refractory or create a completely new core, let alone apply some advanced chemistry to solve problems that this or that research paper is still struggling with (thus the Zircopax joke, which I had expected you would think was cute/funny and interpret in a friendly way). Fortunately there are creative, knowledgeable people on these forums, such as yourself, who have put much time and attention into such things and who freely share their work throughout their process.

Many of us are here mainly because we are trying to use these technologies to satisfy a basic need (such as keeping warm with limited resources) or to further enable other pursuits. These uses are, in other words, a "means to an ends". It sounds like you may have taken some offense at the idea that for people who are here primarily just trying to be intelligent end-users, it might be a distraction to get deep into R&D and complex details, or to attempt to do everything from scratch.

I think I said that part clearly before, but if there is indeed offense, I'll re-quote myself and say "if designing kilns is a hobby, then there's really no reason not to explore and innovate every last detail". That statement doesn't in anyway conflict with the second statement, that if trying to "innovate every last detail" might hinder your primary life interests, you should be cautious about not wading into novel R&D.

The example I gave was someone wanting to develop themselves as a potter but losing a lot of time trying to innovate "every last detail" on a kiln. If the original poster was primarily interested in firing pots and ended up spending half his summer testing different combinations of refractory coatings on ceramic fiber, or trying lots of different home-made refractory linings, that would be energy lost from pottery (which may or may not ever be regained from a slightly more advanced kiln).

Certainly many people are on these sorts of forums because they're looking for best practices and established designs and don't intend to attempt innovating unless their use-case seems to not be covered. That doesn't mean those people don't belong (or "beg the question" of why they'd be here), nor do their priorities and interests being different imply that others who prioritize R&D on refractories are wasting their time. One thing does not follow the other.

A painter may be losing energy trying to grow and process his own flax into linen, and to do so in some entirely new way, while a gardener may be having a wonderful time playing around with growing flax in new ways. Some people are both gardeners and painters. Many people aren't. Where's the conflict?

On this forum there are builders, there are designers, there are those who are just looking for answers about whether a technology will suit their application, and there are many others. All of this creates a community. I'm glad you're a part of that community and I've greatly appreciated your contributions.