Heat riser material

I have been trying to find a suitable material to form my own burn chamber and heat riser. I came across a product called castable refractory clay. Basically you can form your own firebrick in any shape. My idea was to use a rigid cardboard tube to construct the j-tube then use the refractory clay to make a mold then burn out the cardboard. I would love to hear any and all comments on this idea. Thanks

Erik; Fireclay/perlite and a little castable refractory makes a core and riser , and is probably cheaper than all refractory clay. You want a form with square corners for your core and you use the round concrete form for the riser. watch (broaudio) youtube ( cast core part 1-2.) A round core will not burn as well as a square one. a copy of ianto evans book rocket mass heaters is also a good place to learn about rmh.

Hi Erik, always nice to see someone who spells their name correctly! heheh

There are a number of recipes for mixing castable refractory. If you search that term you'll get a lot of threads. I've not tried this, so I do not have a recipe to share, but others may.

From what I've seen folks do not really make individual bricks, they cast much larger portions of the fire box (allowing for stress cracks - casting in two pieces, instead of one, for example). It can be informative to look up info on casting on-site lintels and capping stones for masonry heaters too, paying special attention to such things as properly (and easily) mixing and then aggitating the mix to get the air bubbles out.

You might also look over at Donkey's forum: http://donkey32.proboards.com/#category-1

Fire Riser: perlite and fire clay.

You can also look up the various YouTube videos of Ernie and Erica Wisner. A number of them show the mixing of this basic perlite-clay mix. It is the same mix that is used to insulate around a number of their installations. This is the same thing shown in the video mentioned above. There are only two ingredients (three, if you count water), perlite (most folks buy perlite in 4 cu. ft. bags) and fire clay (often bought in 50 pound bags). However, if one can find good clay on-site that too is an option.

Shortly, I'm going to make my first in-door install with kind of fire riser. Mix the clay all watery in a wheel barrow or something similar, and add some perlite and mix it in, and keep doing that. It is said to be initially surprising how much water is absorbed by the perlite, so start very wet. The idea is to get as much perlite into the mix as possible, with the clay there just to form the matrix that holds the perlite together and allows it to stand/hold together. It ends up being pretty dry, almost but not quite crumbly. You can make the mix into little balls, and they 'pop' them with some finger pressure. You'll see this on the various videos that discuss it.

I do not recall typical ratios off the top of my head. These are useful for an idea of what to expect, but from what I've seem, most folks end up doing it by feel, on-site, for each batch that is mixed.

Then you need two forms, and inner sleeve which may end up burning out and an outer form which may remain intact. Concrete pier forms are popular choices, as is ducting. Whatever you use, most folks seem to want a minimum of 2-inches of perlite-clay for the walls of the riser, and 3-inches is even better. For a 6-inch riser, a 6-inch diameter tube sitting inside a 12-inch tube provides a 3-inch wall (assuming the tube are centered properly).

It looks pretty straight forward. Dusty too. Both the clay (if a dry powder) and the perlite can sent a lot of small particles into the air, so a respirator with a particle filter may be a wise investment (I bought one a year or two ago for something like $30, so they are certainly affordable personal protection for one's lungs).

Thanks for all the replays you are all more helpful than I expected especially since this is the first forum I've belonged too. I hope I can be as helpful in the future once i get the hang of this. You are correct regarding the clay its $$$. I'm sure with all the good advice and resources I will come up with a good mix and will share what I've found works

Interesting about the shape of the riser, goes to show how much I still need to learn. And yes my names spelled correctly, or at least the old way lol.

I'd add a couple other thoughts to the cast riser... My own experience messing around with this stuff was that you need to balance the amount of clay (binder) to the perlite (insulator). Too much clay and it's not as insulative...not enough, and the whole thing will be crumbly and unstable. You can add furnace cement to the mix, which will stabilize it a lot once you fire it. You can also thin the furnace cement with sodium silicate to make it easier to mix in (and the silicate will also make it stronger when fired). I'm also working on a whole new type of insulated riser that will hopefully be easy to make, light weight, super insulated, and (relatively) cheap... Should have details in the next month or so...

Hi Erik,

Check out my videos on making the clay/perlite/furnace cement/sodium silicate mix. I think it is an excellent option for making your own core and riser to whatever dimensions you want.

Instead of first mixing the clay and water, then adding in the perlite. I prefer to mist water onto the perlite, then dust on some fireclay. Mix and repeat many times. (I think it's much easier to keep the clay content down to maintain good insulation properties. This method also uses much less water, which should help to avoid cracking and extended drying times). Then have someone slowly drizzle a thin stream of the thinned furnace cement while you mix it into the perlite. I don't think the sodium silicate is even necessary, you would probably be fine to just thin the furnace cement with fireclay slip.

For my RMH, I decided to use firebrick as the inner form for the core instead of some sacrificial form like cardboard or hardibacker. This way the feed tube isn't at risk of the wear and tear from loading and adjusting the wood.


Making a little stove to test the mix-


Making heat riser sections-


You can also check out my thread over on Donkey's forum, it goes through the process of coming up with the mix, which was originally inspired by Matt's videos (broaudio on YouTube). I think that with a combination of using some variation of my perlite mix with the construction techniques in Matt's cast core and riser videos, you could have a high performance core and riser relatively quick, easy and low cost.
http://donkey32.proboards.com/thread/1298/fireclay-perlite-barrier-furnace-cement

Here are the cast core and riser videos from Matt, broaudio.





edit: embedded linked videos and added Matt's videos

Sorry to try to reopen a pretty old thread, but I'm totally curious about the use of cinder blocks as the channel for the exhaust as well as the heat storage medium. It's been a good chunk of time since that video was posted, is the cinder block system still working fine or did the blocks pop/crack? Anyone know the maker of that system and how it's doing?

Ok, just realized, I clicked on a related video, not one of the original ones embedded in this thread. Sorry for the confusion

I wish I could give better advice but I'm still trying to figure it out myself. I did however have a question about what fivers could be added to my perilous mixture instead of shredded fiberglass? Also what could I coat the finished burn chamber casting to help protect the surface. I guess it would be the same as protecting a Cobb bench. Thanks

Erik; Protecting a cob bench is quite different from coating the burn chamber. In a burn tunnel you can paint on sodium silicate (water glass) apply high heat and this will harden the casting, helping to protect it. A cob bench is finished to resist water / abrasion / dust ,and to look good. Shredded fiberglass is desired in the core & riser for its resistance to heat and to bind the mix together . Straw / shredded cedar bark / hay or any fibrous material is used in lower heat areas near the top of your bench to bind and insulate the outer layers of cob. No binder is needed in the bulk of the mass at all.

Thanks for the good advice. Fiberglass it is then! I was referring to coating the outside surface, I would assume it would remain cool enough to handle a coat of something. What do you think? Thank you

Erik; glad you asked... a fireclay / perlite cast core should have a thick layer of insulative cob over it. If you cast a refractory core then you need insulation (perlite )around it . After that its personal preference. My core is surrounded by brick others go with all cob but you want to insulate around the burn tunnel to keep your heat on the inside. After my greenhouse rocket has been burning all day I get readings of 150 F on the outside of the brick surround.

Ok please forgive me but is a refractory core the same as a cast clay peri lite mixture. I thought they were one in the same. I was planning on casting my core with a mixture of refractory clay, perilite, and Portland cement or rutlands refractory furnace cement. Thanks again

Erik; Sorry to confuse you. Some people are buying 50 pound sacks of high temp refractory cement (expensive )and casting cores with that and skipping the fireclay perlite mix. When I used matt walkers mix I had 2 50# sacks of fireclay one 4 ' bag of perlite some shredded fiberglass and a 2 gal tub of "refractory cement" , This style core should be covered by several inches of insulated cob, although with a brick surround you could pour perlite between the walls.

Hi. i don't know if this is usefull to you too, but I've been experimenting with Aerated Autoclaved Concrete blocks, aka Ytong. I don't know if they're available where you live. They seem to be highly temperature resistant, are feather light, and can be shaped to form with a simple handsaw.

I built a core already and did some succesful burn tests. I created a small you tube channel where I'm posting my progress in tinkering with a rocket stove design to create a terrace heater.

Here's the link.

https://www.youtube.com/watch?v=8Svl0rSOQJo

I'm also very curious on the remarks of the experts on usage of this material.

Regards

Ivo

AAC could be produced in a cottage industry, the temps are well within a household ovens range and the pressure required shouldn't be hard to achieve. This means to me that it would be a candidate for casting at the home level as well as for shipping due to its relatively light weight. If it can stand the heat over time.

Check out my videos on making the clay/perlite/furnace cement/sodium silicate mix. I think it is an excellent option for making your own core and riser to whatever dimensions you want.

Maybe I am not completely clear on the purpose of the clay. It is somewhat insulative but mostly used as a binder yes; many just use the perlite and clay and water. So why not just increase the perlite content and forget the clay, use enough cement/water glass to stick it together and that's all, perlite (pre dampened w/ water? why?)/furnace cement/sodium silicate.

For that matter, why not just perlite and sodium silicate only? Maybe w/ shredded fiber glass to act as "rebar" for a bit more structural integrity.

Geoffrey Levens wrote:

... perlite (pre dampened w/ water? why?)

Perlite is light, and floats around easily, getting in your nose and lungs. Not really good for you. So some folks like to spray or mist it down when working with it.

The perlite I've gotten locally is super tiny particle size as compared to what I've been seeing in people's pics and videos. I'll have to see if I can find a different supplier locally. It just seems like the larger bits would be easier to work with. (That said, the tiny particles do insulate just fine.)

And I found it a little difficult to guess the proper water to perlite ratio in my fire riser. Each batch I ended up mixing in more perlite. I have discovered that the final batch has too much perlite in it to hold shape on it's own after drying. The top couple inches of my riser are crumbling, as the card board pier form burns away. This is just a test build, and it still gets plenty hot, so I don't see it as a disaster or anything super negative, but at the same time, it is not what I was trying to do.

I also do not know if this is greatly effected by what certainly appears to be a much smaller particle size in my perlite as compared to that which I see others (apparently) using.

"Feeling" the texture of the perlite-clay mixture is not something I have been able to do by watching videos, looking at pictures, or reading descriptions. These are all helpful, and offers useful tips, and suggestions, and I think have gotten me in the right ball park, but still are not the same thing as actually getting your hand buried in perlite-clay and feeling the differences.

There is clearly a too wet stage. But there is also what appears to me as a pretty wide range through which the "snow ball" of perlite clay will hold together, and still "pop" apart when pressed with opposing finger pressure. I'm now thinking that when one is in the early stages of that phase, that's good enough. And don't be overly concerned with the water-clay stickiness factors. I think, you can expect to be able to squeeze water out and have a resulting film of water-clay on your hands/gloves whilst the perlite-clay mix is in a good, usable "popping" ratio.

So I'm presently thinking, that the "popping" of the snow ball is the more relevant point to watch. But even that has a range of ratio in the useable end of the mixture, and I think it is fair to observe, that ratio is also effected by the intended use. What I ended up with at the top was too crumbly (after drying) to hold together unsupported, but it would be perfectly fine as an insulative layer, if support was provided (between brick walls, or something like that, as may be built around the feed tube and burn chamber, and the exterior face of the build, for example); in fact, as insulation, the least clay used the better it insulates. (More clay = more structural support; more perlite = higher insulating value.)

I would also observe, that so far the higher clay content is working well as the supporting medium for the fire box. I believe that was on the order of 1:6 ratio of clay:perlite. It is not the same material, so it may not be at all comparable, but I did find a roofing spec sheet that gave insulation values of mixing the perlite with various ratios of cement, and 1:3 and 1:6 were common enough ratios to be included. That was what prompted my thinking of that range of ratio (1:3 to 1:6, clay:perlite) for using under the fire box, which of course take the weight of the brick and barrels above it.

Interesting observations. Nothing like actually getting your hands in it to tell you what's going on!

I am very curious about the use of clay though.

Also a couple other ideas/questions have popped up, so far nothing via searching:
1) How would mix of only perlite and sodium silicate work?
2) How about castable refractory cement only, no perlite?

I did find one thread, lost the link, where someone used either only cement and perlite or cement only but I was not clear from what they posted which one. They did say their tests showed it to insulate very well and be super hard/strong against impace

Here it is

https://permies.com/t/30654/rocket-stoves/Cast-Refractory-Cement-Combustion-Chamber

Turns out he did add 5 gal of perlite to bring up the volume needed due to cost of the cement, but mostly cement

Sadly I went the cheater route and bought castable refractory cement so I have no recipe. Used two 55lb bags of KS4V+ (rated to 2,600 degrees) and mixed in about a 5 gal. bucket of perlite to reach the volume needed as each bag of cement was roughly $60. I built a 6" system, but if you went for a 4" the two bags alone would be plenty, and an 8" you'd probably want 3 bags plus a little perlite as filler. Mixed just like regular cement, poured just like cement, and after 3-4 prolonged burns there have been no stress cracks or signs of problems in the burn chamber.

The beauty of casting your chamber the way I did is you have no pinholes or leaks. You get an extremely efficient chamber for a fraction of the cost if you ordered one, and you don't have to mess around with bricks and clay slip.

Geoffrey Levens wrote:Interesting observations. Nothing like actually getting your hands in it to tell you what's going on!

I am very curious about the use of clay though.

Also a couple other ideas/questions have popped up, so far nothing via searching:
1) How would mix of only perlite and sodium silicate work?
2) How about castable refractory cement only, no perlite?

I did find one thread, lost the link, where someone used either only cement and perlite or cement only but I was not clear from what they posted which one. They did say their tests showed it to insulate very well and be super hard/strong against impace

1) I've not worked with sodium silicate. I have no thoughts on that question.

2) I have not done this either, but nor would I. There are two primary characteristics desired in the fire box design used in the rocket stove concept, whatever the configuration used (J-style, or batch box):

a) Withstand very high temperatures (well over 2,000 F, although 2,200 F rated fire brick is said to stand up to years of use; 3,400 F is about the theoretical hottest burn one may get from wood), and

b) Keep the temperature very high during the burn; most typically by insulating the fire box.

Therefore, if you only use refractory cement, you are only meeting one of these two primary characteristics. That would be my concern.

Maybe it is possible to make a purely refractory cast, and then insulate around that with perlite? It would depend upon cracking and other stress issues of the core, and I do not know those details. If the core survived, in theory, it would be fine to make it out of a purely refractory material, but it would tend to shed heat more rapidly than desired (I would expect), so it would need to be enclosed within a suitable insulating body. So ultimately you are not going to save in net materials, and you are adding potentially another step (insulating the core), and possibly introducing failure points of which I am unaware (in the core).

In other words, I don't know if people add the perlite into the core because it also helps reduce stress and cracking (the refractory material would come to temperature more slowly, and that should induce less stress on the core structure itself, and offer a slower rate of expansion/lower stress), or if they only add the perlite to the core material so as to combine steps. It would be important to investigate the stress and mode of failure experienced by a purely refractive core.

That said, pretty much everyone still "insulates" around the fire box, even when building an insulating perlite-refractory poured core. The core material is insulating on the order of keeping the internal core temps at that 2000+ F range. That is a different function of the "insulation" around the cob mass. That insulation (straw and cob, for example) is to keep users and guests from getting burned by touching the surface of the masonry thermal mass, which is obviously hotter near the fire box, than in the far end of the thermal mass more distant from the fire.

So there are layers of insulation being employed, and with different goals in mind.

Core insulation is to keep the fire as hot as possible - like 2,000 F hot. Ideally, we want to burn all the smoke and gasses inside the fire box and bottom of the fire stack (typically temperatures are too low to accomplish this at the top of the fire riser).

Fire box insulation is to keep the temperatures in the vicinity of the fire box from damaging structures (floor, wall, etc) or people or materials (like pillows and blankets).

Thermal mass insulation is only used to moderate temperature in the mass, and really to call it insulation is misleading. The preferred technique is to simply have enough thermal mass that the surface heat is comfortable and does not present a fire hazard. This is really the same way a fire brick works. We sometimes speak of fire brick as insulating the fire, but it does not; it transmits the heat slowly, largely because it has metal added to it to increase its mass and tolerance to heat (maximum temps as well as rate of expansion), moderating the rate of heat transfer; but that is not the same thing as behaving as an insulating body. (Kiln brick is different than fire brick in this regard.)

When using fire brick to build the core, the fire brick transmits the heat much more slowly than the life time of the fire itself. Therefore, we loosely (and inaccurately) call it an insulator. But we enclose that fire brick in something like perlite-clay to both help retain that heat in the fire box as long as practical, and to slow the rate of heat transfer toward the surface of the heater. This too will moderate, average out, that final heat absorbed and transmitted throughout the mass.

But the heat has to dissipate somewhere. If the mass does not return to room temperature before being heated again, it starts from a higher "resting" temperature and will reach a higher final temperature too (assuming the same net energy in both burns). This is why one or two burns a day, every day or every other day, all effect the temperature of the mass, of course. I think we all know that.

The point that is easy to overlook, is that if the heat does not get carried away, it builds up, even inside insulation.

So yes, the perlite clay is insulative. But if we are using it to protect our structures (floors, walls, etc) without allowing a flow of air to carry away the heat, it keeps building up, unless given sufficient time to return to room temperature. So we have to be aware of that in air-tight areas of our build.

As to your last point (un-numbered) that reminds me of "caps" used in masonry heater construction. I have seen them use vermiculite too, in place of perlite (both will work; I suspect perlite is a better choice in high temperature areas). The biggest point that comes to mind here, is temperature. Concrete comes apart at high temperature, so if you are using it in your build you need to find out what the temperature rating of the concrete you are using is rated at, and stay below that in areas where it is being used.

I would expect however, that the lost link was talking about insulated concrete, be that vermiculite or perlite. And depending upon the temps (and building code) there may be a refractory added as well. But I do not recall any of the popular cap recipes off the top of my head.

If it had just been concrete, it would not have been an insulator. Of course, if the temps are low enough, I see no reason why one couldn't use additives other than vermiculite or perlite. (But that seems to be entering a different area of use than in RMH design; perhaps swimming pools or something like that; but then, I'd be tempted to just pour the concrete and insulate around it.)

And remember to vibrate out the air bubbles in a castable core. It can be thick stuff, so the vibration must be sufficient to the task. I've seen hammer drills used for this in making caps for masonry heaters, and Peterberg built his own vibrating table when making his cores.

Erik, thank you! Likely we cross posted so you missed my follow up; he did use refractory cement + some perlite as his material. Only been short time but his tests show good insulation plus structural integrity vs impact.

I have been reading and reading about casting and design tweaks for the core. Must admit that the more I read, the better those Dragon Stove cores are looking! Well worth the price.

Any idea of how long they are likely to last under heavy use? I would be burning daily for 6 months/year in a bell mass heater.

Geoffrey Levens wrote:Erik, thank you! Likely we cross posted so you missed my follow up; he did use refractory cement + some perlite as his material. Only been short time but his tests show good insulation plus structural integrity vs impact.

I have been reading and reading about casting and design tweaks for the core. Must admit that the more I read, the better those Dragon Stove cores are looking! Well worth the price.

Any idea of how long they are likely to last under heavy use? I would be burning daily for 6 months/year in a bell mass heater.

No, I have no idea. All I know about them is they are based on Peterberg's design, so I expect they work well. Peter really has put a lot of time into this line of research.

But I have no interest in paying a bunch of money for something I can build myself. Of course, I tend to be short of money and I enjoy projects! So maybe that's just me, heheh. Given the time, even if I had the money to buy a Dragon outright, I'd much prefer building my own from scratch. I enjoy the process.

At the same time, I do want the process to stop at some point, and render an operating masonry heater

I started by building a few basic J-styles outside, then building on of them inside. But I think I'm better suited to the batch box (closer to a traditional masonry heater, operationally), so next step will be to build a Peterberg style batch box outside, get used to it, and take temperature readings, and then design a safe version for use indoors (they are said to get hotter than the same sized J-style).

In the meantime, I'll use the temporary J-style this winter, for the experience of living with it. I think there is a lot of value in that.

My top priority will be a good working heater. I like projects a lot so likely would follow that w/ backyard experiments, casted J tubes, batch boxes, see what I could do. Fun for sure and I was a bit of a pyro even as a little kid. My mother used to graciously let me play in the fireplace making matchstick/alum foil rockets and other fun stuff. I think she figured better where she could monitor what I did.

Geoffrey Levens wrote:My top priority will be a good working heater. I like projects a lot so likely would follow that w/ backyard experiments, casted J tubes, batch boxes, see what I could do. Fun for sure and I was a bit of a pyro even as a little kid. My mother used to graciously let me play in the fireplace making matchstick/alum foil rockets and other fun stuff. I think she figured better where she could monitor what I did.

I don't see any reason you couldn't build your own RMH by next winter. And have it working well. You have 10 months between now and then.

Just get the fundamentals down. Read the "book" to start with, and then maybe get a set of the Wisner plans. That'll pretty much cover the J-style fundamentals. It's not that difficult, really. Geometry and fire safety is the lion's share of it. And a good chimney system. Well, that and practical building techniques. There is that, heheh.

Maybe I shouldn't say the same thing about the Peterberg batch box, given I haven't yet built even one of them, but it doesn't really look any more difficult than the basic J-style RMH. Different, yes, but it looks like it ought to be easy enough to build.

But to each their own.

With regard to the Dragon, do investigate the materials and the temperatures. Metal in high temperatures is a temporary measure in a relatively short span of time. Try to find people how have been using them for a few years. How long have they been making them?

Masonry heaters have been built which last generations. And I see no reason why a well built RMH, J or box, ought not be the same. That would be the standard against which I'd personally measure it. But that's just me. If the RMH doesn't out live me, I've done something wrong

Erik Weaver wrote:I don't see any reason you couldn't build your own RMH by next winter. And have it working well. You have 10 months between now and then.

Must admit I don't even have the land yet, then need to build the house. I am currently on a looooong meditation retreat in South America, coming back to US end of April, then starting serious land search. I am designing, sourcing materials, and well obsessing a bit, during my breaks from meditation; I hope to be ready to just start when I find the land.

You make a lot of good points. I have read that batch boxes are a bit more forgiving than J tubes in their geometry requirements. But I go back and forth between which I will end up wanting to live with. I guess can always switch out one for the other, esp if building w/ cob or adobe brick and clay mortar.

Geoffrey Levens wrote:

Erik Weaver wrote:I don't see any reason you couldn't build your own RMH by next winter. And have it working well. You have 10 months between now and then.

Must admit I don't even have the land yet, then need to build the house. I am currently on a looooong meditation retreat in South America, coming back to US end of April, then starting serious land search. I am designing, sourcing materials, and well obsessing a bit, during my breaks from meditation; I hope to be ready to just start when I find the land.

You make a lot of good points. I have read that batch boxes are a bit more forgiving than J tubes in their geometry requirements. But I go back and forth between which I will end up wanting to live with. I guess can always switch out one for the other, esp if building w/ cob or adobe brick and clay mortar.

It seems clear both styles work. And given your time frame, you have time to get them built, so there's no need to feel you have to buy a ready-built design (if you *want* to that's a different matter).

Really, it boils down to psychology. What's your mode of behavior? Would you rather read whilst the mass heater runs on it's own? Or would you rather side beside it and fiddle with the sticks every few minutes? I'd rather read than fiddle with the sticks, therefore I'll end up with a batch box, of that I have no doubt.

On the other hand, if I liked fiddling with the sticks I'd build a J-style. And if I were building it in a cabin where I did not expect to have books or to be watching DVDs I might opt for a J-style to give me something to do heheh.

Watching the fire is another consideration. Either style can have a piece of ceramic glass installed in the side facing the room. I think this is best in the feed tube of a J, not the burn chamber, due to heat considerations. And from what I've seen on YouTube, the batch box gives a better fire display. The J-style never really burns more than the bottom few inches of the wood. The batch box does at some point become engulfed in flames. And if you like watching a fire, than might be worth the relaxation/meditation value too

But you're quite right, cob is forgiving. Certainly the small amount I've worked with so far has been, and everyone says that is its nature. I see no reason to think otherwise. Changing from one style to the other is not going to be a big deal, provided there is adequate space around the build, and there is enough elbow room to work comfortably. There is nothing in the materials to make it difficult to change; quite the opposite, using these materials makes it easier, if not inviting, to modify.

You might also read the thread here on Pernies about the tipi, however that is spelt. The person offered the opinion they found it quite comfortable. For someone with similar tastes it would certainly comfortable enough to live in whilst building a permanent house. And if planned a head of time, perhaps the RHM built into the tipi could then become an outdoor sitting area for enjoying cool spring and fall evenings? If you like that sort of thing.

Happily there are lots of good options! heheh

Well you nailed it! I am both a reader and a fire watcher. Sounds like the batch box wins hands down for me. Funny in a way, this whole investigation started w/ having dinner at friend's house in Sept who have an antique wood cook stove they got on eBay. Really nice looking, huge hunk of iron, pretty nifty design but not nearly so efficient nor user friendly I think. Just it had a certain "romantic appeal". One thing led to another and now I am pretty much down to bell type mass with cook top over the heat riser and now, likely a batch box.

I do have plenty of time to play w/ the core in the meantime. And I have in my mind a simple design for an oven that would just sit over the cooktop when needed or when immediate heat release is not needed.