A castable burn chamber

Broaudio presents a method of casting a burn chamber on youtube. The vid is one month old. I don't think it has cooked long enough for him to speak from experience. But, the concept sure is intriguing!

His mix ratio: 1 part Lincoln 60 Fire Clay to 1 part Perlite. Then, 14 parts of this mixture to 1 part of Refractory Mortar. Fiberglas mesh is also added. He vigorously heats the casting up 24 hours after it has set. It develops small cracks and shrinks. He claims (to paraphrase) the mix loves the high heat for drying and structural integrity. Since it will buried in thermal cob he's not concerned about cracks.

The link:
http://www.youtube.com/watch?v=7ANMXGrxgnE

Two concerns: 1) Will his recipe hold up compared to stacking industrial manufactured fire brick? (heat approaching 2,500 degrees)
2) Is his method of drying most effective?

What I understand from one supplier: The industry markets soft fire brick and hard fire brick. Soft brick is for insulating and kilns with clean heat. Hard brick is for facing volatile environments like wood fire and salts-- salts for the purpose of glazing. Hard bricks are more robust and will tolerate hotter temps. This led to my concern. Any ideas? Thank you in advance. Tom

Hi Thomas. That's my video you linked to. I'm sure you will get a lot of different opinions on your question, but I will give you mine and a little more detail.

I have a stove in my home that uses the same process, cast with only fireclay and perlite. I've been using it as the sole heat source for a little over a year now, and it is holding up beautifully. I do mess with the feed opening once a month or so as it wears away, but it's as simple as a handful of mud slapped in place.

I added the furnace cement and fiberglass after experiments over the last year showed me that it makes a casting that is quite a bit harder than my pure clay/perlite core. I have more confidence sharing the harder mix, although I continually build outdoor versions with just dug clay and perlite. The furnace cement is expensive, but it does set up hard.

My opinion is that I much prefer the highly insulated core over a heavy firebrick core. In Paul's recent podcast with the Wisners they note that they felt like their cast core performed 80% better than their heavy fire brick builds. After building quite a few of both I feel that my insulated cast core shows the same improvements over my heavy brick builds. To me, the (potentially) lower durability of the casting is a worthwhile trade off for the higher performance.

The only durability issues are at abrasion points, there is no deterioration in the burn tunnel or riser at this point.

If I could give you one piece of advice, it's make some quick and dirty stoves in your yard with mud and bricks and whatever you might have around. Get a feel for how they work, what holds up and what doesn't, etc.

I hope that helps, it will be interesting to see what other opinions you get on the topic. Good luck on your build, whatever way you decide to go.

Hi Matt,

Thanks so much for your quick response. I'm dying to get going. Just waiting for the ground to thaw and gain code approval here in upstate New York. I presented Ernie and Erica's 8" stove plans to our local codes office. That was a terrific first meeting.

I'm so glad to hear you've had some time to see positive results develop, that the model is holding up well. I'm very content to observe what works well for others and replicate. I'm gobsmacked when developers talk of the expense of certain items, like furnace cement. In the big scheme of things it is such a bargain. The repair bill I got last week, for my 20 year old efficient gas furnace was $450! What's a gallon of furnace cement, clay, duct, tile, brick and a barrel in the end worth? Way more than that!

The next logical question-- have you built your heat riser with this mixture? I envision stuffing this material between 8" and 10" stove stacks. Would metal stack be necessary? Perhaps cardboard sonotubes as mold structures? Will it support itself after they burn off?

Thank you again!
Tom

That's great news about the meeting. Congratulations.

Yes, I build my risers with the mix. I use whatever 8" pipe I can find for the inner, and prefer to use a 15 gallon drum, or "grease drum," for the outer. They are about 17" in diameter (if I remember correctly), so provide for a lot of insulation around the riser. If you use 10" around an 8" you will only have 1" of insulation, which is not enough in my opinion. Whatever you use for the inner pipe will burn out rather quickly, although I'd worry that sonotube might burn away before the mix was dry enough to support itself. I have not experimented with it though, it just might be fine.

I forgot to add before, regarding my drying technique, I'm sure one could take more care and dry it with less cracking. I'm fairly impatient! My feeling though, is that this shape is doomed to crack no matter how careful you are, which is why I call it a core and always wrap it well with cob. In order to make a core that was immune to cracks, I feel it would need to be cast in sections so as to have expansion joints.

Could the drying process for portland cement apply here? Portland cures over a 30 day period producing maximum strength-- IF the cement is kept wet. Of course, you never see people doing this when replacing a sidewalk blocks.

I honestly do not know the answer to your question Thomas. I do know the furnace cement is a different animal altogether from portland, and the container usually states that heat will help it dry. I'm sure there are others with more experience.

I should add that I do not feel the casting mix and method I share in the video is the ultimate end-all be-all core. It represents a fairly simple approach that works satisfactorily using materials that most should be able to locally source. If money were no object there are some great castable refractory products out there in a range of densities.

Hi Matt,

Thanks for sharing your video on casting a burn chamber. It inspired me to start experimenting with building one myself!

I would be interested to hear more about how you cast the heat riser as well! I imagine it would have to be done after the core has been installed? Or put in place (home, greenhouse, wherever)? How do you ensure the stability of the riser on top of the burn chamber? It seems like it would be unstable without something to act in place of rebar?

Finally, Have you noticed a significant performance advantage in your 'cast heat riser' over the 'perlite and clay slip' that Evans & Jackson discuss in their book?

Any input is much appreciated!

The industrial refractory industry has been changing over the past 20 years. Where firebrick was once the dominant material, castables and monolithic slabs have been gaining in use and effectiveness. Castables in particular have been developed which offer as much or more durability as firebrick, but with an installation time of a few days rather than a few weeks. Rather than a crew of masons laying brick at a rate of 500/shift, plus labor to get the bricks and material in place, the procedure is handled with a couple of carpenters and a pump.

The recipe above looks like it will work just fine in the environment described. AP Green or Harbison Walker puts out a product called Online 70. It's primarily applied as a shotcrete. We've used it as a castable repeatedly in kiln and boiler repairs. Another product is KS4 which is primarily applied as a castable material. These will both perform far beyond residential demands. Once the material is applied, we'll put some heat to it for 24 hours-a bigarse propane burner blasting all day and night to cure the job, at which point the vessel is turned over to the plant ready for use.

Refractory mortar differs from portland cement because of the addition of alumina and or crystalized silica. These allow the heat to penetrate rapidly through the material such that it heats uniformly. This prevents one side being substantially different in temperature which would result in thermal fracturing. The mortar would disintegrate rapidly with each heating and cooling cycle. Castable refractory material has these same heat transfer properties.

I just watched the video:
The fiberglass he shredded up-that's the crystalized silica I mentioned. If the material is already in the dry mix, it will be small particles rather than clumps. His recipe will improve the finer he shreds it.
Water-use as little as possible to get the material to hold together. In my experience, the mix in the video is on the wet side. Minimal water translates to minimal cure time.
Method-I see nothing wrong with what he has done in this video. Form build, mix, cast, burn...all looks good to me. I project he will get excellent service time from this casting. I would add 2 tips: [1] Mix well-you want as uniform a blend as possible. [2] Pack the material in TIGHT. In the places where you can only access with your hands, press the material in just as hard as you can. In the areas where you access with a tool, pound it in with a 2x4 or hammer. Leave no air pockets to weaken the casting.

SAFETY ISSUES
-When mixing dry refractory castables: WEAR A PARTICULATE RESPIRATOR OR N95 PARTICULATE MASK.
The crystalized silica is highly abrasive to lung tissue and a known carcinogen. You don't want to breath this stuff all day.
-In the video, take note of the gloves. You will be pressing HARD on the material to get it into the corners and pack it solid. The stuff is abrasive to your hands. Rubberized gloves will protect your soft, delicate hands. As an added bonus it lets go of the material as you work with it and cleans up well with just water alone.

I'm a foreman and sometimes field supervisor for an industrial refractory contractor. I'm not a mason, but I work closely with them (as in 'no room for car keys').

I'm thinking this out a bit more.
Sonotubes can be used as a form. They can be left in and will burn out without causing injury to the casting. The casting will handle those high temps as well as the abuse of flame contact. The burning of the sototube will actually help the casting cure before it is burned out and the flames reach the casting surface.

If you want to go to the extra effort, you can mix a small batch of portland into a slurry. Just before you pack the form, use a paint brush to apply a thin coat of this slurry to the burn chamber form, a little at a time just before you pack that spot. If using sonotubes, apply this slurry to the outside of the sonotubes. Here's what happens...as you pack in the castable material, the portland slurry is still moist. It will hold onto the refractory but won't be stirred in. The added moisture will be insignificant. Any flaws in the form material will be smoothed out, giving you a smoother casting. When you fire the unit, the portland won't survive the heat, but will serve as a sacrificial coating, protecting the castable for that first firing. The thin portland will simply flake off within the first couple of burns. You will have some fine cracks no matter what you do, but this thin slurry will minimize them. With a smoother casting on the surface of the burn chamber, you will gain durability, particular at the corners and higher abrasion points.

Exposed surfaces can be smoothed with a moist brush, moist sponge or trowel. Same idea as the slurry above, except without the portland. This is more for aesthetics than anything else. Makes the surface a bit less rough

Expansion Joints are probably not needed for this small size. Where you do opt to use expansion joint, packing them with mineral wool or fiberglass will help to cushion the squeeze as the 2 expanding sides come together. If covering the core with cob, you would be well advised to cover the core with a thin layer of mineral wool or fiberglass. You won't need much-you're not trying to insulate the space between the core and the cob, you just want enough to be compressed as the core expands so as not to crack the cob.

Hi Thomas. That's my video you linked to.

I just now caught this. Well done! I've seen Pros do sloppier jobs.

Ken, thanks so much for the thoughtful posts and the kudos. It's very nice to hear that I'm not way off in the weeds with this approach.

Jeremy, I just did the build and photos for the follow up riser video on Sunday, and am trying to find the time to get it edited and online for all of you folks who are asking. Frankly, it pretty much is what Evans and Jackson discuss in the book. I typically use just fireclay slip and perlite, mixed together until it just forms a ball which will "pop" when squeezed. I pack this in to a form made from 8" (in this case) flue pipe for the inner and a 15 (?) gallon drum or "grease drum" for the outer mold. These drums are about 14" diameter and 28" tall, so I'll cut the inner flue to 31" tall and build up that last 3" around the inner pipe with the mix. You can add the furnace cement and/or fiber reinforcement if it makes you feel better, but I feel it is unnecessary since the riser is a self supporting shape which should see no abrasion or movement.

As for support, yep, it just sits on the core mold. Make sure it's level and it should stand up just fine on it's own. At that point you should already have the core surrounded by cob, and can help seal/support the bottom of the riser with a bit more mud as you start to shape the transition to the mass run. Oh, also, that grease drum for the outer mold goes a long way to stabilize the whole thing, and it's protected from the hot core of the riser so should last a long, long time.

Hi Matt.

That's great info, thanks for clarifying and elaborating!

I look forward to seeing the video...I know they don't make themselves, so thanks for your time!

does it hurt anything to let it cure a bit before firing to avoid cracking?
im thinking let it cure for a week or so, then a small, cool fire the first day, get a bit hotter the next day and then really burn it day 3 and 4 after your week long cure

when ive poured concrete for other purposes ive let it cure for up to a month for max hardening before sealing and such, but im thinking that a week might suffice for this purpose...

Devon, I can't see how it would hurt, but I haven't tried so I don't know the answer. Sounds like Ken may have first hand experience. I've always just lit 'em up right away. He did imply that a drier mix would help mitigate some cracking. I've thought that, but once my hands are a sticky mess I usually get frustrated trying to mix and add more, probably too much, water. It works for me though!

Refractory is not concrete. You'll want to cure the cast in the manner for which it is designed.
Refractory moves the heat through the mass. Concrete is a poor conductor, heating unevenly.
Let it dry for a while before you burn, 24-48 hours is a good rule of thumb for this size. Longer won't hurt a thing. If not fully dried, several burn cycles would promote cracking rather than prevent it. If you want a more gradual cure, start a small fire, increase its intensity gradually, but get it roaring and keep it going for several hours, cooking the entire casting in one continuous burn. All of the moisture wants to be driven out before it cools or you will have an incomplete cure which will invite thermal fracturing. Burn it all day.

If you do get cracks, don't be alarmed. Some cracking will be expected. During the first burn, these will allow the exit of steam out of the casting. When the casting is heated, it will expand, shoving those cracks back together.

I was pondering other casting methods and wondered if you have considered casting non rectangular cores?
Angles are good in some places as they set up turbulence, but in other places you want to maximise flow and minimise dead spots.

Casting like that is typically achieved by making a positive mold out of a material that melts at a low temperature, or is dis-solvable in water etc. Typical materials are wax, plaster of Paris, or paper mache.

I was also thinking about the wear issue you've mentioned. Have you tried setting glazed tiles at the wear points? It's a much harder surface.

That got me thinking about the mold / casting process again... I wonder if sticking small glazed tiles face down on the mold before casting would result in a cast chamber that was lined with a highly reflective, hard layer of material. The weight of the liquid casting would probably be enough to force it between the cracks like a grouting. I'm just not sure how well it would stick.

Nick Kitchener wrote:
That got me thinking about the mold / casting process again... I wonder if sticking small glazed tiles face down on the mold before casting would result in a cast chamber that was lined with a highly reflective, hard layer of material. The weight of the liquid casting would probably be enough to force it between the cracks like a grouting. I'm just not sure how well it would stick.

If the only problem is not sticking to the tiles, you could put some material on the tiles beforehand that does stick to them. You know just small bits, splatter and such. it can mechanically lock inside the cast material.

It's a good idea Nick, but unfortunately tile doesn't last long in one of these. A few handfuls of furnace cement once in a while are my solution to any abrasion wear. As for the curves, in this one I was simply trying to show folks an easy way to DIY a highly insulated cast core with readily available materials. If you are looking for the ultimate in performance check out Dragon Heat's shippable cast core built on Peterberg's optimized J tube combustion engine. They have indicated pricing will be around $200, available in a couple weeks. It is a deal and a half in my opinion, and represents the cutting edge of rocket stove tech.

Matt, Could you provide a link to this "Dragon Heat's" shipable cast core? I couldn't find anything.

Also, Thank you for your inspirational designs!

-Colin

Thank you for the "thank you" Colin! You are welcome, it's my pleasure to share what I've been doing.

Here's the link to a discussion on Donkey's board regarding the Dragon Heat core.
http://donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=726

I have been scouring the world for rocket stove stuff the past couple months and of course this site, ernie and erica are ubiquitous, but Matt, I too was REALLY motivated by your cast! Great idea, now we are working on the "open source" portion of it, how to make it better?! I have pondered lining a cast core (calculating for the mass of course) with the thin fire bricks, but that starts to drive up cost immediately...

What would your inner dimensions be for a 6"? Psyched to see and hear about further developments.

Is there an easier formula for the core dimensions than the "C <(or =to) A,F, G, H, J,& K? (attached in case it doesn't ring a bell)

Oh, I learned that you can buy refractory cement (Heat Stop) dry, but it's still $50 for a bag. The structural stuff (nox-crete) is 15 more bucks a bag and you have to throw in $15 worth of stainless "needles" for the structural component... Fancy hi-heat cob!

Thanks Prescott, I'm really happy to hear that I've motivated you. That motivates me! 6" flue has a cross sectional area of just over 28". If you reduce the mold pieces such that the finished openings and tunnel in section form squares with sides that are roughly 5 3/8" instead of the 7 1/4", you'll have the same CSA throughout. Hope that makes sense. You shouldn't need to change any of the other dimensions of the core, and I'd stick with the full sized 55 gallon drum.

As for the other dimensions, oh man, that's a big topic. Basically though, that's good info. Go check out the "helpful hint" thread on Donkey's board to get a handle on the one spot that trips up a lot of stoves.

As for the materials, it's a slippery slope man. There's a million options out there, lot's of them would probably build a fabulous stove. I'm really into playing with the stuff that I can dig up around my house, and encourage folks to play with mud stoves in the yard a bunch before spending much cash. It's just good fun, and there's a lot to be learned by doing that. This technique I've shared, well, it's just a pretty good one that uses materials most folks have access to. Good luck on your build.

Here's the riser build video. It's short, and not that great, but it'll give you the basic info you need to build a well insulated riser that should last a long, long time.

I wanted to add a source for castable refractory. Your local certified chimney sweep should be able to get Thermix. I am not entirely sure about the ratios in the material but it is fireclay, perlite, and ceramic fibers. Looks very much like Matt's mixture. 2.5 cu ft (dry) goes for about $35 wholesale.

Has anyone considered using two adjustable elbows and a short piece of duct to make the interior mold of a castable burn chamber? There's gotta be something wrong with it. Seems too easy.

I'm planning to pre-cast a heat riser using Mizzou and perlite mix. I want to use a six inch dia. sonotube inside a ten-inch dia. sonotube to form the riser. This will give me an insulated, hard-faced riser with a six inch interior dimension, a wall thickness of about 1 3/4 inches and an outside diameter of 10 3/8ths inches. I have a 16" diameter water tank to drop over the heat riser. My question is, if I cut the tank so that the riser is two inches shorter than the tank, how tall a riser is optimal? The tank is presently forty-six inches long. Sonotubes are 48 inches, so I have options. Just wondering if there are optimal dimesions and proportions, before I go to cutting things up. Also, heater will be used to heat a greenhouse space, if that makes a difference.

I'm using Mizzou and perlite because they're both white and my theory is that the color will help to keep temps a little higher. Also, has anybody played with injecting secondary air at the base of the heat riser, specifically at tangents to the riser diameter in order to spin the flame and create an elongated flame front? Any feed-back would be most appreciated.

-rockpicker

Hey Rock, I'd recommend you do some homework reading Evans and Jackson or Wisner and Wisner before mixing your stuff. The math is crucial for your project's success. The space between the top of your riser and the barrel cieling must at least equal the cross area of the inside of the heater riser. It can be more but it will be cooler in the barrel risking loss of complete combustion. That cross area must(?) should not deviate throughout the entire burn and barrel path. My other concern as mentioned by Matt is the thickness of your riser walls (2") without the use of a steel column. He's recommending 8" and 14" creating a 3" wall and then not using sacrificial material like sonotubes.

Thanks for posting, it has really got me thinking now, reckon I'll be able to do this

I wonder if I could use same principle to make a clay oven core

Cheers

Matt Walker wrote:Here's the riser build video. It's short, and not that great, but it'll give you the basic info you need to build a well insulated riser that should last a long, long time.

Hi Matt!
Do you leave the metal, or is it there just for hold the shape when you cast?
In the first video, I understand that you remove the shape, or that it can burn if it is in wood, but I am not sure about the heat riser...

I am very glad that you remind us about the importance of insulating the core.
As I had seen some bricks' heat risers, I was tempted...

'' Bump'' Big Al

Thank you Allen.

This is all great! Another core will be born soon..

I know this is an old thread just recently bumped, but I am hoping some of the participants in the conversation are still paying attention. I'm reading this for the first time and something puzzles me. We use lots of perlite in the core because we want an insulated chamber that lets the fire burn hot. Then we're throwing in some refractory cement for durability. One of Ken Peavey's comments says that refractory cement includes alumina or silica, which basically acts as a good heat conductor, ensuring that the material heats uniformly (to reduce stresses from differential expansion, I presume).

Seems to me this is working at cross-purposes to the perlite. Will it not reduce the performance of the core? Matt, you say you have cast cores both with and without refractory cement in them. Have you noticed any performance differences that might be due to this?

Micky Ewing : Thats a very good question , I for one am very glad you asked it ! While Matt Walker is the best person to answer this, I bumped it to allow the material to be
reviewed.

It was bumped because I had originally mis-understood and mis-answered a question, and wanted help to back me up on a retraction of my mistake, For a very cheep way
to get around the need for self taught masonry skills ( no Matt I didn't mean you) a cast core is probably the best way to get around not using fire brick ! In this case the
refractory cement is necessary for increased durability.

I had mis- understood a third parties R.M.H. build plans missing the fact that the Refractory Cement was going to be added, I had announced that the combustion chamber
made that way would be too soft, and not durable enough !

I too would like an update on the Cast Cores Durability and further remarks on any repairs to his present 'Cores', and what he would do differently . For the craft!

As always, comments / qustions are solicited and Welcome. Think like fire, Flo like gas, don't be the Marshmallow ! PYRO - Logically BIG AL !

Micky, Allen, I would be happy to share my experiences and such regarding your questions. If you follow the link in my signature you'll find other members and myself discussing some current castable core builds, and some updates from me. I would love to carry on the conversation over there if you'd like.

Micky Ewing : In this case Matt W. means the signature line below the last line of Type ! - In my case the ''Tag line '' is look at 'similar Threads below' which is often ignored !

Mat Walkers 'Tag line " is - permsteading.com, there you will find a link to his discussion of his Rocket builds ! I Was hoping for a simple answer posted here !

For the Craft, Big AL !

Matt Walker wrote:Micky, Allen, I would be happy to share my experiences and such regarding your questions. If you follow the link in my signature you'll find other members and myself discussing some current castable core builds, and some updates from me. I would love to carry on the conversation over there if you'd like.

Thanks for the response Matt. I will definitely be checking out the forums at your website. But like Allen, I am hoping I can get an answer here. Breaking this thread to continue it at Permsteading.com would leaving everybody else here stranded and wondering where I went for my answer.

My apologies Micky, I started to type a response to both you and Allen and found myself writing a book. I'll do my best to answer your question succinctly. The mix is a composite, as is cob, and is made mostly of materials that are poor insulators. Adding perlite makes either composite a better insulator. As for the difference between a mix with or without the furnace cement, I haven't noticed a difference in the burn performance.

Bump....


How cool is this?! If anybody is still eyeballing this thread, I have a quick question.

I am designing an RMH to sit atop a suspended wood floor in my living room. So, I see potential here to build in some super duper insulation in the bottom of the core to make it wood-floor safe. Do you guys have ideas about the best materials to use for this?

I am thinking the wooden box form could be built a few inches deeper, and one could cut some fireboard to fit in the bottom, create some kind of mechanical adhesion on the surface (short screws drilled into the fireboard, left sticking out 1/4" for the cement to grip?), then a layer of the cement mix, perhaps with a higher perlite ratio, then the normal measurements of the core itself, with the 4" deep bottom and the form set in there on top of the insulation layers. Any other/better ideas?

Lauren Dixon : Have you been to rocketstoves.com to pick up your PDF Copy $15.oo of Evans' and Jackson's Great book 'Rocket Mass Heaters' ,
there is STILL No other book in any Language with as much Rocket Mass Heaters R.M.H.s, family information.

In one page (page 62), in one picture and two paragraphs the information you need is laid out for you !

You can go to ernieanderica.info , ( they are the moderators for the Rocket and the Wood Stoves Forum Threads here at Permies, ) and they
have a set of plans there for ' The Boney Convection Bench: Rocket Mass Heater, 8'' '. This is the exact model/build that they used to make their
' Construct a R.M.H.' video that they made with Callen Kennitt of Video village, that video will be available this month according to reports !

This is a little bit of ambitious build for a 1st build, but with the resources listed here and a little help from Ernie and Erica Wisner you should have a
show case build that will last you a lifetime ! Remember to make a test build outdoors before moving indoors ! For the Good of the Craft !
Think like Fire!, Flow like a Gas!, Don't be the Marshmallow ! As always your questions and comments are solicited and are Welcome ! PYRO AL

Bumped for a new fellow member ! Big Al !