http://tinyurl.com/n476mmb

its a drum can..  I've got great draw..  seals are pretty good.  I am not yet insulated anywhere.  it gets hot,  but not very.  took a long time to get warm inside.  

If you look at the pic you'll see a bunch of fire bricks,  like 16 of them.  They are sitting on a steel plate.  I've got approx 4 inch tubing making up a j channel.  my vertical is over the 4 of the 1-2-4 ratio..  but as I said my draw is very good.  It all works,  just not hot enough.  There is a vast amount of heat being generated ..  around the j channel especially at the upward turn toward the oven.  I'll give my diagnostic and I hope somebody can set me right..  I need this thing working well enough to make some pizza by FRIDAY...  not too tight!

Insulation will help.  but if I am feeding an adequate system would it still get hot ..  just not maintain???  or what?

the steel plate that supports the fire bricks is not a tight connection.. I figure what is occurring is the j tube is sending that heat up and roasting the  #$%^& out of that steel plate which is not transferring sufficiently to the bricks. Since the draft is good and there is no leakage apparent..  then the heat is being created.  the exhaust is not excessively hot or smoky so the burn is good.  

The heat I reckon is being lost in 2 places.  
1.  the metal plate is getting hella hot and reflecting the heat downward to the can which then radiates it out

2.  the j tube at the lower section joint is red hot. so that is clearly a major loss.

concerns:

- drum can is too big for a 4" feed tube?
- j tube riser is too long and is losing heat on the way up.
-steel plate is counterproductive in that it is reflecting heat away from intended area...
- lack of insulation is obvious concern,  but I am certain that the other issues are bigger than that as I am not registering a lot of heat coming from various regions of the system. ..  (I have vermiculite and clay mix ready, but I wanted to test the overall system before committing all the material)

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I have 8"  pipe available but it seems huge!!
I could shrink the can.  
I'm certain the plate under the fire bricks is reflecting heat away..  so I can solve that ...  

Conclusions:

I think the can is too big for a 4 inch feeder if I want high internal temps ..   It's obvious that the up tube on the J should be sorter and I should insulate,  but i don't have confidence this will solve things.
I figure I need to make my channel bigger or my can smaller.  Is 8 inch nuts?    k gotta eat  /...  thanks!

Brendan, your link doesn't work for me error 404

SMAX,  I think issues are solved

One word, insulation!

https://permies.com/t/52544/metal-burn-tunnel-heat-riser

HEY SMAX,  I read a lot of that link and the summation seems that I have more problems than insulation.  metals being the leader.  I'm in Japan.  clay has been hard to find or costly.  fire brick is costly.  I have drum cans and regular stone and concrete..  all of which I've already destroyed in my experiments/...   I've gotten my hands on some fire mortar...  a ton of fired clay roof tiles..  drumcans...  vermiculite...  hmmm...  I still think my can is too big for the 4" feed...  but something is starting to occur in my imagine..  tired..  gonna sleep on it.  thanks!

Fired clay roof tiles could do the core and heat riser. I would say, go that route, making a 4 inch batch box. You will have more power. Fired clay roof tiles and mortar, for the box and heat riser. If you can make it permanent, make a roof tile and cement enclosure to hold the insulation around the core. And support your barrel.

Put the heat riser at the back of the barrel, or underneath the stones, but do it so the gases go to the rear end of the barrel first. Then place the chimney at the front, just a tiny smidge above the mouth of oven. So you have a vague bell effect. Use concrete and crushed tiles to make the mass above the barrel. And insulate on top.

At least this could sound like a good plan to me. If you use "ciment fondu" and crushed tiles as a charge for your heat riser, you couls have a really nice one. Use your existing pipe as a mold.

K...  just got up and having coffee... read your addendum...  I think I get what you're saying.   I have immense amounts of construction debris.. literally mountains...  especially these roof tiles..  stone fragments and urbanite/blocks  imagine a roof 15 meters on a side being replaced..  KAWARA  tiles are 25 cm on a side ..  curved.  I've experimented with them in oven construction as heat reflectors and insulation..  they are difficult to work with due to shape,  but I have a thick stack idea that may work..  like they can stack to them selves evenly.  the concrete fondue thought besides funny is kinda what I had in mind..  

OH MAN!!  what about the steel frame I built?  LOL  ..  so I've come to the realization that my idea of a portable device ain't gonna work out without some serious purchases of high temp ceramic insulators and precise welds etc...  so yea.,  I think I need to make this thing a permanent install...  so:

- I'll put it closer to the ground.
- eliminate the steel J.  
-create a kawara stack burn/fire feed area and put the drum on that.  
-I am going to increase my J tunnel volume..
- I will eliminate the steel plate that supports the fire bricks...  
- I'll build an insulating case around the thing out of stone debris and then bury the thing in a mix of stuff..  
-I'll decrease internal airspace where I don't need it and replace it with some heat absorbing material (stone/clay)

I have some clay straw mix that made up wall tiles which I can excavate from the debris..  I have vermiculite...

How does this sound ?    

I may try to leave the exhaust where it is for now...  ,  but build a baffle around it so that the air has to come up from the J,  go around  the oven stone, cooking area and then back down...  and out..    but if I have to move it..  you say near the door?   why not the back?  

Thanks
B

Brendan, word of caution, straight away.

"ciment fondu" i used because of the lack of translation. It's a dark clayey concrete, that we have in France. But in other parts of the world too. And often called ciment fondu. It is refractory.

Well, if you want portable. Keep what you have.

What is that pile of crushed stuff behind the tiles? If anything made out of red clay, you have a wonderful "grog" or "chamotte" ( kind of sand made out of already fired clay elements, which have been crushed. Easy to do)  nearly ready on hand. If you can't find clay, use ciment fondu. I'm pretty sure there's a supplier near you. Mix the red clay debris, which have already been fired once, with either clay or ciment fondu.  To make a batch, with your vertical pipe from the J tube as a mold, sacrificial piece. Which will die. But in a long time, since it's stainless steel. I would make  a shelf, lower than your pieces of flat iron you used to brace the feet. To hold the batch.

Make parts for your batch firebox, out of that grog and binder (again, clay or ciment fondu)  Either flat pieces, that you will hold with wire. Or something more refined, like Peter's castings.

Myself, i build out of whatever flat pieces i can find, like pizza oven slabs.

Then insulate the "core" (heat riser and firebox) with either rockwool, glass fiber insulation, or superwool. Stay away from the real ceramic felts/wools, as they are dangerous for your lungs. Hold the insulation with chicken wire or something alike.

If you want portable. I would just move the chimney to the very front. Over the mouth even. And insulate the arch. I would do this with the compressed rockwool we have here, and a piece of sheet metal. Shaping the two like the insides of the barrel, shoving it in somehow (cut the barrel at the back, and reassemble after, either welding or riveting)  And hold the two in place with bolts through the two layers and the barrel. Thought, there is many other ways.

Brendan.

As for your roof tiles, i know they're a pain to work with. But i gonna show you few examples of ovens made out of roof tiles.



With southern french roof tiles



From this page http://www.patrimoinegruissan.fr/Fichiers/Archit/four_pech_des_moulins.htm

They went over the top. But the remains of the first oven showed at the beginning of the page, show nothing but tiles and binder.  

Here, it's flat roof tiles




Again, flat roof tiles



From a french diy magazine  

https://www.systemed.fr/realisations-lecteurs/reconstruction-d-four-a-pain-traditionnel,2523.html

Just a few examples.

With your tiles, i would cut them perpendicular to the curve, , something like strips of one or two inches wide. This way, the curve wouldn't bother you that much anymore. Start with a fanned vault. And finish with a slab. Ok i know it's cheating. Or start a corbel vault. Then switch to fanned.  There's plenty of options.

That's a lot of good information,  merci!  My cafe opens in a few days so I'm going to do a quick and dirty and temporary fix today.  get it working without mortar for now. get rid of the extra steel and use the drum can as the coooking area.   I'll take some pics and make some notes.  this will give me a feel for things,  get past the 1rst week. (in japan there is a big spring golden holiday week, after that business is very slow for a bit) ---  basically I'll re-situate it and use a lot of mass around the can to keep the heat up..  so it will work no matter what imperfections it has.  luckily I have more wood than I know what to do with because I run a carpentry shop.  


K,  time to get to it!  I have muffins to bake!