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Black oven for pizza baking - first build  RSS feed

 
Posts: 14
Location: Western Australia
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Greetings fellow pyros,

I am new here, my journey started with traditional dome shaped pizza ovens and ended here.
My plan is to use a repurposed electric oven with a RS I will cast using refractory materials.
I must say I am very excited to get this project up and running!!

My main concern is the cooking temperature. The goal is to bake pizzas at an average temp of 440-450c (842F), yet I can't find any info re the temp these converted ovens can operate at - taking into consideration the heat lose from opening/closing the oven door frequently. IF needed, I'll insulate 60% of the glass door, allowing some viewing area to watch foods cook.

Additionally I would like to enjoy some open flame going up from the riser into the oven itself, rather than just heat.
For an open flame a shorter riser or burn chamber will be required (?) Will it still draw if the oven door is open plus a small chimney sticking out the back of the oven?
        
Will it be a good idea to use fire bricks on all four oven walls to create a mass for a more even cooking? or will it only slow down heating the oven?

Also I should mention I am down under here in Perth Australia and some of the resources recommended here (i.e. refractory materials such as Fire Clay) are not readily available, or just very expensive.

I got a tonne more Q and also thinking about using a RS to heat up a Japanese style hot tub with the RS semi protruding directly into the water.. but that's for another thread...
looking forward and thanks for reading my post.
 
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You could look at batch rocket stoves, they Can be made to work with a short heat riser.
I don’t see how you can make a really good pizza oven without a brick cooking base, it is all about getting the moister away from the pizza base to brown it up without burning, a steel base won’t work like a porous brick, the moister will boil under the pizza  and make it soft.
 
Ethan Pitt
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Fox James, yes I agree regarding the fire bricks, I planned on cooking two pizzas at the same time, one on the bottom and the other on a pizza stone placed in the middle
I can't seem to understand how does the batch oven works, I can google lot's of photos and videos but no instructions on how to build one up.
 
Fox James
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I have never built a batch rocket but I do understand how they work, my own rocket stove was a learning curve to build and I possibly built it before I had done enough revision.
I assume this forum like many other forums, was a busy place a few years back but it Is differcult to find all the info you need now.
There is another forum but that is even less active but it has a lot of back dated info on batch rockets.
I build pizza ovens so I had the materials and equipment to cast my fire box and heat riser, if I were to build a batch rocket I would go the same way and cast a refactory cement version.
Anyway I encourage you to continue your revision  via YouTube and google until you can find what you need.
If you are lucky Satamax will see your post, he is very helpfull.
I think the other forum is called Donkey 32 or something like that.
 
Ethan Pitt
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Yeah I've been on that donkey 32 forum. Plenty of batch ovens and info there, but no specific explanation.

All I gather is, that there is a fire box between the riser and the batch.. I'm sure I'll get there in the end thanks



 
Fox James
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I am not the right person to try and explain but a batch rocket has a much larger fire box and burns a lot more wood in the same time as a J tube rocket would.
There is a lot more emphasis on secondary air, this enables an ever more complete combustion of all the wood gasses.
The result is more heat in a shorter time. 

As I have never operated a batch rocket I don’t know how much fly ash they produce but I have a feeling there will be quite a bit.
On that basis I am not sure if a direct ‘black oven’ would work as your pizza might be covered in ash.

I have a feeling a rocket rocket stoves would make a good oven but perhaps not the best for pizza.
That does not mean you can’t cook a pizza, just that you won’t have a large mass of 500c bricks to work with.

I made a crude video after a pizza party (cooked in a traditional pizza oven) two many drinks and a few pizza bases left over, we tried out a pizza base on my rocket fire hot plate. The steel plate was at a similar temperature as my pizza ovens brick base but all it did was burn rather than crisp up!
Post 61 here ... https://permies.com/t/80/86886/Rocket-stove-hot-plate
 
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Ethan wrote

My plan is to use a repurposed electric oven with a RS I will cast using refractory materials.



My advice is to go for it. Attached is a photo of my oven.

Pizza cooking times are now down to 5 minutes.

If you are interested, I can add more information. I had a problem submitting a longer reply to the forum so this is a bit of a test.

Cheers

Druce

Pizza-Oven-3.JPG
[Thumbnail for Pizza-Oven-3.JPG]
 
Ethan Pitt
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Wow Fox James you the one who built that hot plate, I've seen your post a while ago, great job mate!! 

If batch ovens create more ash like you say, it's no good. It's nice to be able to use the batch size piling up wood, with large logs of wood.
I've seen lots of videos where people use wood from stuff like wood pallets for their RS. I guess it easier to break down to small kindling but I wouldn't want to cook food with that stuff.
Then again, its always worth giving it a shot. Hack.. I might try to build both batch and RS oven should time permit.

Hi Druce, Thank you for the reply

That's exactly whats on my mind, since we are still renting here I don't want to build something permanent or expensive. T
My idea was to use as much insulation as possible and also create some mass inside the oven using fire bricks on all sides of the oven. 
How warm does it feel if you put your hand on the sides of your oven? Do you think it will improve a great deal if you insulate it better on the outside and some parts of the door. Using a ceramic blanket for that will be easy.
How bad is the drop of temp when you open the door?


Would love more info, don't hold back.Did you have to do much welding?

 
Druce Batstone
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Ethan
Here's more detail on the repurposed oven on a J tube rocket stove. You seemed concerned about heat loss. I don't think you need to worry. I have not insulated the top or sides of the oven with anymore than what was already in place. The sides are warm to hot of course.

The cooking speed s mainly a function of the quality of the fire not heat loss including door opening. I believe this is a feature of the rocket stove and a black oven. I guess it's possible to do a calculation but intitutively the hot gas flow is very high. The better the fire the more heat in to overcome any losses. Any loss on door opening is soon replaced by hot gas.

You will note a vertical length of 150mm SHS in the fire box. I found this greatly improved the quality of the fire. It creates a "P" channel and allows sticks to stand vertical so more sticks are firing in the base of the fire tube. The time to reduce a pizza was reduced from 1 to 1 minutes to 5 minutes with this modification. I'm sorry that I have yet to measure temperatures. I've just purchased a suitable thermometer and will use it on the next firing. Firing is with spotted gum kindling of about the same length. I doubt if I could get the same results with random diameter fallen sticks and twigs.

The rig can be moved but is still very heavy. I was able to buy an old oxy-acetylene carrier on Gumtree. The frame came from old bed frames. Yes there is a bit of welding but I used the corners of the bed frame wherever possible. The frame is riveted which gives a nice retro look. The oven was free (also on Gumtree). The cost and weight is in the castable refractory. If I was to make another, I would only use insulating castable. This has dense castable in the fire zone but I think insulating castable will withstand the conditions almost as well. I used the recommended proportions for a 6" J stove. Ash accumulates in the fire zone but is easily cleaned by hand when cool. The SHS extension is easily removed. Some fly ash comes up through the riser but very little accumulates in the oven.

You will note that two pizzas can be cooked at the same time. The lower one is on a tile. The top one is on a terracotta plant-pot base.

Hope this helps.

Druce

 
Fox James
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Your fire box looks well made, did you cast it in one?

The problem with low density insulating refactory is durability as it is quite soft and very prone to abrasion damage.

I totally agree about the fuel and how that can effect the fires performance.
Mine takes about one hour to get really hot, at that stage I can burn virtually anything but to get it there requires carful choice of wood.

I am very interested with your extension box, I tried that too but in my case i just got the fire rising up the fuel.

I have been messing around with a electric 2” tube fan lately, I strapped a length of 2”copper pipe to the outside of the barrel and fed the end into the firebox.
With the fan on the difference is immediate and in fact quite scary as the hot plate quickly becomes noticeably concave and the barrel starts pinging.

I am hanging up to build mark two but I need to study and experiment with mine a bit more first...
 
Druce Batstone
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Thanks apple givers. I learnt most of what I did from Permies. Hope I can give something back.

Fox, I cast the stove in two halves. Your casting is much better than my efforts. I've been following your thread since the beginning and greatly admire your workmanship and ability to communicate.

I used dense castable for the bottom part of the J for temperature and abrasion resistance. My work experience included specifying castables for wood burners used industrially. The duty requirements turn out to be an order of magnitude greater than short-duration, lower-temperature pizza oven duty. The temperature in the burning zone of the wood burner was high enough to form clinker and the burners could operate continuously. I've seen nothing like this in the fire zone of the Pizza stove. That's why I think insulating castable would last a long, long time.

Sorry about the lack of edits in my previous post. The extension piece reduced pizza cooking times from 12 to 15 minutes to 5 minutes. Your experiments with the fan tells me that the resistance to gas flow through the oven is much less compared to the resistance under your plate. I worried about burn back but was very agreeably surprised how easy it was to get the fire going up the riser. A gentle puff form my old lungs did the job!

The idea of making a stove top appealed to me too but the complications defeated me. You are aware of Matt Walker's work. Have you thought about using the glass cook top?

Cheers
Druce
 
Fox James
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Cool, I think my early experiment with an extended feed tube might of been a bit premature as there was still moisture in the refactory and vermiculite... I will try again.
Re the fan, I don’t fully understand why it is so effective but I think pushing pre heated air and possibility activating a secondary burn might be what is happening.

My favourite rocket stove entertainer is ‘Potty’ on you tube... love him or hate him his work is enterprising and effective.

Yes I am on the look out for suitable glass, I have a front glass from a metal halide light unit that I intent to experiment with.
I want a new thiner  steel top for my fire with a 4” hole directly above the heat riser so the kids can cook there marshmallows and I can place a wok over.
I hope the glass will cover the hole but it is hard to imagine the glass withstanding 500c
 
Ethan Pitt
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Druce:

"If I was to make another, I would only use insulating castable. This has dense castable in the fire zone but I think insulating castable will withstand the conditions almost as well. I used the recommended proportions for a 6" J stove."
And
"The temperature in the burning zone of the wood burner was high enough to form clinker and the burners could operate continuously"

- Do you mean cast the whole Rocket Stove from an insulating mix rather than refractory? Isn't the temp that builds up in a refractory RS creates the draw? Or you are just looking at getting the RS lighter for moving it around?

I am asking all this since I have seen threads of people reporting they can reach cooking degrees within 15 minutes. And my priority is a minimum of 430C inside the oven to cook pizzas as fast as a Napoli oven will do. Or am I just dreaming?


 
Fox James
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Hi Ethan, I was hoping someone with more knowledge would help you out but I can give you my input anyway.

That is very hot it such a short time, I see most rocket stove pizza ovens use an oil drum with some rock wool or similar thrown over the top.

I don’t know if you have a tight budget or time constraints but if I were to build a rocket fired pizza oven, I would be taking a different approach.

I think to achieve very high temperatures very quickly, you need thin steel and lots of insulation for the actual oven.
To get really fast heat up times I think you might have to avoid mass in the fire box and heat riser, perhaps look at ceramic fibre board and a ceramic fibre heat riser.
I would look at building a similar design to a brick pizza oven with the dome shape, perhaps you could find a giant wok or some other metal dome shape and cover that with ceramic fibre blanket.
You could place a pizza stone directly under the heat riser and let the heat ascape around the edge and into the dome.
I would have to do a few sketches to get my head around the exhaust but I don’t know how far you want to go with your concept?
 
Druce Batstone
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Here are some answers from a test firing yesterday.

I used a 5kg log split into kindling. The oven reached 250 C in 15 minutes and 300 C in 30 minutes from ignition. The maximum temperature was about 320 C (just off the scale in the cheap oven thermometer). There was no cooking involved. The temperature dropped by about 20 C when the door was opened in normal cooking mode. The temperature of 320 C was very stable for more than an hour when finally, the 5kg of log was just embers. The kindling was not very straight (knotty log) so I might have got a higher temperature with higher density packing in the fire zone but I think 350 C might be the maximum. I could also improve the transition out of the oven to the stack (two pieces of square down pipe) to get better draft which could increase the maximum temperature.

I purchased dense and insulating castable (Brisbane Australia). Yes I would cast the complete J stove in insulating castable to make it easier to move and to reduce cost. The temperature of the inner surface of the riser should be higher if there is less heat loss through the walls if they are insulated. Not that should have any great effect as you are looking for a chimney effect to achieve draft.

The oven I used is as Fox says - thin steel with insulation. I think there is a Japanese guy with information on the dome setup that Fox has suggested. From memory he used two stoves on either side. If you really wanted to get close to your objective I suggest you consider a larger stove (say 200 mm 8") or two stoves for more flexibility.
 
pollinator
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You have to consider 3 types of heat in your build; conductive - the heat in the stone or tile you put the pizza on, radiant heat that is coming from the wall of the oven, air heat from the the riser of the rocket stove.  So you have to weigh the positives and negatives of each at each level of your application  The more insulated the burn chamber and riser the sooner it gets to complete burn  Honey Do Carpenter was able to use aircreat with sodium silicate impregnation.
He also used it to insulate an oven. The aircreat cement has a high heat absorption and radiant factor while the air content has a high insulation factor.
For the oven, if I understand correctly you want the air/radiant heat to cook the top of the pizza before the conductive heat burns the bottom. So will you spend the time to build up radiant heat in the walls of the oven or depend on continuous flow of hot gass from the riser to cook the top of the pizza or combine the two by having enough mass well insulated in the oven that can build to radiant heat quickly?
 
Ethan Pitt
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Fox James wrote:  I don’t know if you have a tight budget or time constraints but if I were to build a rocket fired pizza oven, I would be taking a different approach.



My only concern is that we are renting, 5 years ago, I built a dome shape traditional oven, and had to demolish it when we moved out. Therefore, I wish to come up with something practical. Perhaps one that I can move around if needed.

I picked up a free oven already, I got a 44 gal drum too but not sure I'll use it.
Additionally I intend to cook other foods, not only pizzas. So it won't be the end of the world for my first build not to reach 450C, as long as I get a nice bbq flavour.



Fox James wrote: To get really fast heat up times I think you might have to avoid mass in the fire box and heat riser, perhaps look at ceramic fibre board and a ceramic fibre heat riser.




If you look at Druce's comment above, he reports temps of 250C in 15 min and 300C in 30 min. (thanks for that Druce much appreciated) There isn't much mass to retain the heat but a terracotta plate and a pizza stone, with no insulation.

With the information in this thread, theoretically it seems like I can reach and retain a good temperature with a properly insulated riser and oven, and some (not great) mass inside the oven.

i.e. the downside for a huge wok covered with insulation, or no mass (I have seen those on YouTube by the way) is that it will reflect a great deal of heat, but only while constantly being fed by the rocket stove, while a mass will reflect it evenly, even if the fuel gets low.

This is all assumptions of course and that is why i am here to learn. So please correct me if I am wrong.

Also I should mention that my idea of adding mass into the oven is removable, I won't use mortar to stick them. They flush very nice if you have seen the floor of a pizza oven.



Druce Batstone wrote: I purchased dense and insulating castable (Brisbane Australia). Yes I would cast the complete J stove in insulating castable to make it easier to move and to reduce cost.



What are we talking about here? Is the first concrete cement? the ones I saw at the local shop here are almost same price but I have no idea of the weight.

I have come across a rigid fibre FraxFibre Rigid Board 12mm, said to withstand temperatures of up to 900c. Very light! Issue is that it will need some sort of coating from the inside as it may dislodge or release some fine particles that may end in the food and I will need to get creative as how to get it into a round chimney shape.

Blanket: $95 610mm x 3.6mtrs
https://www.unifrax.com/products/blankets/fiberfrax-ceramic-fiber-blanket/

Board:
https://www.unifrax.com/products/boards/fibermax-duraboard-1700-and-1800/

Not the exact one but also a Frax product. $44 for 1000x610mm

Hans Quistorffif I understand correctly you want the air/radiant heat to cook the top of the pizza before the conductive heat burns the bottom. So will you spend the time to build up radiant heat in the walls of the oven or depend on continuous flow of hot gass from the riser to cook the top of the pizza or combine the two by having enough mass well insulated in the oven that can build to radiant heat quickly? [/quote wrote:

Yeah like I mentioned above, I don’t think it’s any good relying on just heat flow.

I have seen Honey Do videos but I don’t understand what is this aircreat? Is it something he came up with? Issue is, we get a very limited range of products here. I can’t even find fire clay.

I almost bought a bag of refractory cement on Monday but lucky you guys say it’s better to build the RS from insulated materials completely, so I am happy.  

 
Fox James
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Well as regarding temperatures I only have my own limited experience, I get around 220c on top of the 8mm plate very quickly but it’s takes an hour to get the fire box and heat riser glowing red hot. At that stage I have around 450c on the top centre, one more hour (two hours after lite up) I can see just over 500c and a slight cherry glow on the centre plate. I don’t know what the inside barrel temps will be but it will be hot!
This guy seems to get similar temps to me but had a faster warm up time, he Has lots of design features that you don’t normally see.
Check out his vids..... 
 
Ethan Pitt
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Fox James wrote: Check out his vids..... https://m.youtube.com/watch?v=5afOmOaFyKM



Yeah I've seen his videos he's a top bloke, I like the side walls he welded in the feed box and the secondary intake. I plan to use his vortex design for the riser.

 
Fox James
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If you look t his latest vid (just two days ago) he uses vermiculite  and ceramic board to make a riser.
 
Druce Batstone
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Re Dense and Insulating Castable Refractory


These are supplied in bags but do not have Portland cement in the mix. Have a look at a Perth based supplier www.woodfiredpizzaoven.com.au/refractory-materials/refractory-cements/insulating-render

As mentioned, the insulating refractory is normally used as a backing layer. I think it would be quite durable in all parts of the rocket stove. The cost per bag is much the same for dense and insulating but the volume is about twice for insulating.


 
Fox James
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I think it must depend on the manufacturer  because the insulating refactory I have used is soft and weak compared to dense refactory.
I build around 3 or 4 pizza ovens a year, I buy refractory in bulk and I have a good relationship with my supplier, I will give him a ring today and see if they sell a more robust insulating product than the one I have at the moment.
It would seem to make sense  to use a insulating product for certain areas at least.
I have tried mixing my own insulating refactory useing perlite and vermiculite but the results offer  very little structural security, I have often thought about useing a certain type of cat litter in the mix. There is a backed clay product that is very porous and very hard that might work.

Certainly the areas than receive the most abrasion like the bottom of the feed box will need to be very robust!

Interestingly... Potty, made a comment on his you tube channel, stating he does not think that a dense refactory heat riser needs any insulation to work!

From my perspective I can see the benefits of having a dense fire box and heart riser,  as once the dense material  does eventually get red hot, the heat is there to stay for several hours, you can then feed in large logs or large pieces  of hardwood.
Obviously, if weight is a factor then a lightweight produce has other benefits apart from insulating.
 
Ethan Pitt
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Druce Batstone wrote:Have a look at a Perth based supplier www.woodfiredpizzaoven.com.au/refractory-materials/refractory-cements/insulating-render



That's funny, this is the shop I was at on Monday. VERY limited supply there, only firebricks, refractory or insulating cement and ceramic blankets. I asked if he has any special insulating tiles or similar - nothing.

I remember when I build my first pizza oven I used pumice like bricks (very light) for the underlay. These bricks are so easy to work with and can be cut with a hand saw BUT I have no access to those as they were from a kiln factory that closed down!

This are the bricks: https://www.traditionaloven.com/articles/81/insulating-fire-bricks (the white color not beige)
 
Ethan Pitt
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Fox James wrote: It would seem to make sense  to use a insulating product for certain areas at least.



I wonder how will it perform if we say stagger the RS from insulating and fire bricks, like a mix up. And still insulate it from the outside.
 
Druce Batstone
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After the comments in this thread, I would rethink the all insulating castable rocket stove if I was ever to make another. There would be two castings - the lower section from dense castable (reinforced with stainless steel needles) and the riser from insulating castable. Both would have wall thickness of about 25mm. I would try to incorporate Peter Berg's trip wire and back corner curve in the dense castable (how I wish I could go to a workshop run by Fox).The two castings could be held together by 25mm of insulating board on all surfaces. I would still use a steel frame to hold everything together and for mounting the oven.
 
Ethan Pitt
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Druce Batstone I would try to incorporate Peter Berg's trip wire and back corner curve in the dense castable .[/quote wrote:

Check out the PPotty's latest video Fox mentioned for an another way of creating turbulence (vortex)

https://www.youtube.com/watch?v=kaBMBrDWsMM&t=849s    ; (around the 2:40 mark)

So simple!

 
Fox James
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Ha Ha, I live on the other side of the world mate.
I have been making moulds and castings for 40 years but the method I used to cast mine was not differcult!
I just got hold of some ridged house insulation, it is very common in Europe and so easy to shape.
Once shaped I painted it with fibreglass resin and just made a suitably sized wooden box to cast the unit.
The key is having a good way to vibrate the refactory mix so you don’t get trapped air or external voids, I have a proper vibrating table but it is possible to use other diy methods.
Once the mix has set, you just dig out the foam with a screw diver or such like.

Anyway I am enjoying the conversation, great to pass ideas back a fourth....
Vortex would be my choice but a trip wire would be no more differcult to cast.
 
Fox James
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I have just spoken to my supplier, he said the most resilient insulating material they sell fit for our purpose, would be pre formed low density fire brick but even they are quite easy to abrade.
However he sells 1400c ceramic board that could be cut and used for lining fire boxes or making heat risers.
 
Ethan Pitt
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Thank you for the conversation too Fox, It's great to share ideas.

Today I learnt whats Aircrate is finally.
I can't find the meaning of a P channel ?

Now I need to decide on:

1. design
I was thinking on following this
https://permies.com/t/54074/Rocket-Mag-heater-Rocket-Mag
I just hope it doesn't produce too much ash but I need to build one to know I guess. Or just go safe and build a J rocket. The idea of using large logs appeal to me. I really hope the food I will cook will taste as if it was cooked on the barbie.

2. Materials:
I found some interesting products on some factories who deal with furnaces and kiln. Looks more promising than pizza oven shops.

Here is an example https://www.furnace-technologies.com.au/refractory-materials

3. Insulate VS heat resistance.
The question still remains and further reading this afternoon has got me even more confused. I shell read more before I post assumptions here. There just too much to go through and some of the information is not backed by numbers and facts.

4. 4 is fake, I just needed one more number to say I really need to get my hands dirty and start casting!!

 
Fox James
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It is certainty not easy to find a clear line of information due to so many different designs.
I built mine to soon without enough info, so don’t rush into building one....
P channel is a method of suppling secondly air to the bottom area of the riser, it is named so due to the guy who developed it being named Peter!

If I were to build another J tube, I would want a supply of hot, cold and warm air, this means some form of P channel, a lid for the fire box, a step in the bottom of the feed chamber to stop fuel sliding into the burn tunnel. I would want a hight mass fire box and possibly  a low mass heatriser?
However I am steering towards a batch box design in which case I  don’t know how I would build one as I, like yourself, have not quite got my head around all the options.....

P channel for J tube .... http://donkey32.proboards.com/thread/355/small-scale-development
 
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