Substitute adobe bricks for fire bricks?

We're working on our first RMH for a greenhouse and there's no way I'll spend $400 on fire bricks.

We're looking at Erica and Ernie's Annex 6" plan and the Village Video DVD "How to Build RMH" with Ernie and Erica. This DVD is incredibly helpful. Unfortunately I don't even have an opinion on the 4 DVD RMH deal as we got the streaming through the kickstarter and it's totally worthless -- we have yet to be able to watch it all the way through and I've come to HATE Vimeo.

For anyone wanting to pay for material that's actually useful, get DVDs! The Village Video DVD is so well organized and you can quickly review chapters as needed -- IMPOSSIBLE to do on Vimeo. Streaming is ok to watch a movie (if you have 8 hours for a 2 hour movie), but not for learning how to do something and repeated viewing.

Sorry, got sidetracked, back to bricks. I did buy about 60 fire bricks, but now we're out and contemplating our options. There are numerous web pages on using adobe / cob for ovens (we have one too), but I don't know whether the RMH will be too hot.

We have several 50 lb bags of Lincoln 60 we could add to our local dirt (clay deficient) for bricks.

Any thoughts?

Thanks, Christine

An adobe with fireclay is a different beast. I can't think of a time I've seen that tried specifically, but it sounds promising.

Even more promising, though, is probably working up a castable material with that fireclay. Matt Walker just posted a new video about reworking the core of his indoor RMH. He talks for a bit about the virtue of the fireclay/perlite mix he had used. If the fireclay/perlite mix was going strong after a few years for him, you might find success with it too.

Here's the video:

I typically need about 70 split or half firebricks for a J-tube, they cost about $1 each, thus $70 total.

Brett, in Kingman it's $2.50 a brick (Home Depot) and they don't have half bricks. Since we have 60 or so bricks another 10 wouldn't break the bank. But how do you get away with using only 70 bricks? And where do you buy your brick?

Mike, thanks for the vid. That's interesting, perlite and clay holding up for several years in the burn tunnel. We happen to have a big bag of perlite.

This greenhouse is very likely to be moved. I remember hearing about shippable cores and it seems like that's the way to go for temporary installs. We also have a bag of high temp concrete.

And what about using an old Vogelzang firebox?

Here are a couple topics I found re. adobe bricks, maybe not such a good choice:

https://permies.com/t/18914/wood-burning-stoves/Clay-Bricks-fire-bricks-concrete
https://permies.com/t/41987/rocket-stoves/Substitute-adobe-bricks-fire-bricks#328922

Christine: Consider a firebrick J tube and a cast riser using fireclay & perlite . Matt walker at (broaudio) youtube has videos on pouring your own cast core and riser. I can highly recommend the cast riser ! mine is on its second season and is good as new !,including removing it from a cast core and resetting it on a brick J tube.

The "clay bricks - fire bricks" thread has incomplete information. Here is one that has much better info (look for Erica Wisner's post): https://permies.com/t/30551/rocket-stoves/Fake-fire-brick

Matt Walker's video with description of the three-year-old perlite-clay mix is a good testimony to it when done right. His house is/was almost completely uninsulated, so he runs the system much more than most, and that is probably 5 - 10 years' worth of use for typical installations. At well-insulated burn tunnel/heat riser temperatures (c. 2000 degrees) the clay actually gets fired into pottery or brick. What he shows is good soft (insulating) firebrick.

I feel your pain re firebrick prices - here they are $3 each for full or split brick. I have a resource of a huge old pottery kiln I bought for $500 back when I was trying to build a full-time pottery business, so that is around 500 insulating firebricks for a dollar each (plus a bunch of stainless steel sheet and banding from the exterior).

Thanks so much for all the info! The thread on bricks made my head spin. Matt's video on the core is great and I think that's what we'll do.

However, we're going with 6" and I've read the various comments regarding 6" systems on youtube, but I'm still confused, especially because we're also looking at the 6" Annex plans and the drawings with the bricks don't seem to be to scale. Our WWOOFer has been working on sketchup for a while, but I really want to make sure we do this right. Has anyone uploaded a file with the core dimensions?

Christine, don't rely too much on youtube. There's plenty of gibberish in there!

For dimensions, you can just use the formula 1:2:4 with 1 = say 1.3 x diameter or 8-9", and following centerlines. So 8+3=11" feed tube (from the floor), 16+3+3=22" burn tunnel (at floor), 32+3=35" riser (from floor). About 5 1/2" x 5 1/2" or 5 1/2"W x 6"H burn tunnel, you crunch the numbers. Then if you are using firebrick you need to find an arrangement that gives those dimensions with minimal cutting. You might need to fudge dimensions a bit...

A 4 1/2" wide firebrick plus a 1 1/4" thick split gives 5 3/4" for burn tunnel height. The rest are pretty straightforward, just run the courses around the burn tunnel with the extra lengths overlapping as necessary. Reverse directions with alternating courses so joints are staggered.
22-11 1/2=10 1/2" burn tunnel top, 2 bricks wide plus a split on edge gives 10 1/4" and the split can go by the feed tube to give a 10 1/4" feed tube height. You could add splits flat around the top to give 11 1/2" finished height and a nice surface.
add bricks around the riser base to complete the course from the burn tunnel roof (5 3/4+2 1/2=8 1/4"), then splits on edge (35-8 1/4=26 3/4"). 6 courses at 4 1/2" = 27" for a total riser height 35 1/4" from floor. Or you could cast the riser and make it whatever length you want.


BTW, this will only require cutting two bricks, one full and one split at the ends at burn tunnel roof height.

Satamax Antone wrote:Christine, don't rely too much on youtube. There's plenty of gibberish in there!

So Matt Walker is posting unreliable gibberish? I was under the impression that he knows this stuff! What am I missing?

And Glenn, thanks, but you completely lost me, just like the Annex plan drawings. After all we're not using bricks - which is why it's so important to do it right the first time.

So I'll start a new topic about the core.

Christine Baker wrote:

Satamax Antone wrote:Christine, don't rely too much on youtube. There's plenty of gibberish in there!

So Matt Walker is posting unreliable gibberish? I was under the impression that he knows this stuff! What am I missing?

Christine, you're not missing anything. Matt knows his stuff. I was just saying to be carefull about lotsa other "thingies" looking like rockets, but loosely based on the real deal. Using metal, multiple feeds, air entrances, feeders, no insulation, not respecting the gaps etc!

And if you want to talk adobe bricks, ask Kirk Mobert aka Donkey!

http://donkey32.proboards.com/thread/803/evaluating-6-dragon-burner

" After all we're not using bricks -"
Sorry, it sounded that way from your earlier posts. A cast core is even easier as far as forming and dimensions. A wood/plywood inner form can be made to any dimensions pretty simply, and then you just have to get the casting technique down. Practice is probably going to help a lot there - make up some test bricks so they will be useful if they turn out to be strong enough. Matt Walker's videos are good for this. There is also a good video of riser casting by David Eaks. https://www.youtube.com/user/david22234

Thanks for all the replies! I finally posted a new topic about the core dimensions:

https://permies.com/t/42121/rocket-stoves/Dimensions-core-Template-form

So I get the idea of making the bigger outer box to contain the outside of the core and the inner wood shape that will create empty space of the jtube. So how do you get the wood out after? Do you build it with something light (dry old redwood) and then burn it out?

I'm getting ready to build a rmh and was not looking forward to buying fire bricks or looking for old bricks, and I like the idea of making something integral with perfect dimensions. If you were starting from scratch, how much would it cost to buy enough perlite, fire clay and fire cement (and fiberglass if you think it's essential)? If the alternative is roughly 70 fire bricks and they cost me around $2.40/each here in Oakland, CA, does this come in much cheaper than $160? Or is the main advantage the perfect design and the lower cost is just added bonus?

Last question, after you make your core, would you then put more cob around the whole thing when you put on your riser and seat the barrel or are you thinking this is as insulated as it gets?

Thanks for all your work, and keep going fast, I'd love for you all to figure out the perfect answer before I build mine - Rick

Unless you cast your core in two pieces (with proper draft so the inner forms can be pulled out of the finished casting without damaging either), you need to make the inner form from something safely combustible or sacrificial. Wood will burn out nicely, it doesn't need to be specially light old wood. Don't use any pressure treated wood.

Cost of different materials can vary significantly depending on your region and local circumstances, so it's hard to give a good general answer on this. Perlite and fireclay will probably be significantly less than premixed castable refractory, but probably also not as durable. You can use a denser, stronger mix close to the inner surfaces and bulk it out with a very light insulating mix. You want lots of insulation all around the core.

It took us a long time to find Lincoln 60 locally (in Vegas):

http://www.aardvarkclay.com/

1 lb is $1.20, 100 lbs are $11.30. We bought 4 bags (several hundred pounds) and at the store they tried to charge us the 1 lb price initially (the lady didn't know what she was doing.)

They're in So Cal too, would think there's a shop like this in the bay area.

2 cu ft of perlite at the Home Depot in Kingman for $16.97:
http://www.homedepot.com/p/THERMOROCK-2-cu-ft-Perlite-Soil-Amendment-489701/202187623

For the cement I'd have to go outside and check what we got. Can't find anything searching for refractory, but at the Depot they sold us a big bag of some high strength cement that's supposed to be ok for high temps for around $10.

For fiberglass I don't know, kinda hate that stuff. We have a bunch of old insulation laying around, wonder if we could use that.

Christine; Look at landscaping & nurseries to find a 4' bag of perlite that will run you around $20 I too located the 2' bags at HD and bought one before looking at the nurseries and discovering the better supply source. Great find on the fireclay , super price even if you had to drive for it.

Christine Baker wrote:Thanks for all the replies! I finally posted a new topic about the core dimensions:

https://permies.com/t/42121/rocket-stoves/Dimensions-core-Template-form

Christine, that's wonderful! Very nice work by your Woofer there. The only change I would make is to reduce the width of your outer mold so that the finished core is 4" thick on the sides instead of the 5"+ you have now. No problem if you leave it larger, but it will use a lot more material.

Your "high temp cement" from HD is probably something you don't want to use in the core. I'm speculating, but it doesn't sound right. Look for the little tubs of liquid "furnace cement" like I show in the video, that will serve as the addition of sodium silicate and high alumina clay to your mix and make it function a bit more like a refractory in the sense that it will harden a bit before firing.

I came up with this technique because fire bricks were too expensive for me too. I also have found that the performance in my opinion is superior to fire brick cores, but it comes at the cost of durability. You must patch the feed occasionally over the winter with handfuls of the same mix. I find that to be an acceptable maintenance task, it's a very minor chore every two months or so, but if you choose not to do it your feed will wear to such a large opening that it won't work well any more. As well, when cleaning, you must reach in and verify that you haven't lost any big chunks, and replace them with handfuls of mix if you have. That's only an issue early in the build to my experience, but when things are curing you can loose chunks inside and if you don't repair it, eventually you will end up with failure. Once the thing is stable and in regular use, any of that settling should stop and it's only abuse from wood that will wear it down.

Rick, it's a core, not a complete stove, so it for sure need good cob surrounding it. Both to seat the barrel, and to hold the whole thing together. Do not try to use this type of core as a stand alone, it will fall apart for sure. They are fragile in their way, but can provide years of good service at incredibly high performance for very low cost with just a bit of regular maintenance. Good luck with your builds you two!

$20 for 4 cu ft would be much better, I'll ask around. We don't have much going on in Kingman, but maybe we can find a better deal in Vegas.

Since we're talking about the perfect mix to make the perfect core, could someone post the recipe (i.e. amount of fire clay, perlite and fire cement) here for what you're planning on using? If as Matt suggested it gets down to fine details, specific product names would be great

I've tracked down Lincoln 60 fire clay, perlite and a couple of options for furnace cement so now it sounds like it comes down to the exact ratios and questions about a couple of special ingredients.

In Matt Walker's videos, he used 1 50 lb bag of fire clay, 2 cf of perlite and a gallon of fire cement. I've seen another video where they added shredded fiberglass to the mix and another where they added water glass including a recipe for the water glass.

The fiberglass is easy to get, how much is worth adding? The water glass is doable, but a bit of a pain, is that a key ingredient or not so much? Is there any brand of fire cement that is best? Any to avoid?

Any other ingredients that would help this mix? Any different rations? Matt talked about it being brittle and requiring fairly regular maintenance. Anything to add or design to change to minimize that?

Something I am experimenting with for a cast core of insulating refractory (relatively soft & very lightweight) is adding fireclay in a stiff mix for a first layer troweled onto the inner form before casting the body of strictly light insulating mix. I have made test bricks with different ratios of materials which I will fire to cone 06 in my kiln (about 2000 F), and will post when I have results to give. This should give a more abrasion-resistant hot surface for the feed tube and burn tunnel... we will see.

If you have a way to fire the whole J-tube core casting, it should be significantly stronger in the feed tube especially, as normal operation will never let that most exposed area get fully fired and hardened. Even building a hot bonfire around the whole casting (work up to it slowly and cool it slowly) should help the durability. That is how pottery was made in primitive times, and even some traditional cultures to this day. Look up "clamp kiln" for a full description of the traditional process.

Rick; Plan on having 3 bags of fireclay on hand, I used almost 2 bags & maybe 3' of perlite, Don't short yourself have plenty on hand. I added apx 1 gal of refractory cement. A few handfuls of fiberglass is plenty. Using waterglass in the mix is a new idea. I used waterglass after firing by painting it on the walls of my feed tube and letting the high heat fire it into glass, it did work but just not well enough for my problem. I had built a cast core for my greenhouse last season ,using matts recipe. It could not stand up to the heavy ,multiple person loadings that it got and by the end of the burn season even with patching ,my 7.5"square hole was the size of a 5 gal pail,also a part of the roof of the burn tunnel caved in. For that heavy use location I rebuilt using heavy firebrick. If I was building a rmh inside an insulated home where the use would be easier , I would acquire enough (light) insulating firebrick to embed them in my casting on the feed tube side walls & floor and the roof of the burn tunnel packing my fireclay perlite around them and use all fireclay mix on the riser side of you J tube. The light firebrick is fragile also but not quite as much as a total casting and I believe would resist chunking better (chunks of wall falling in). Now using a 16 gal barrel & a sonotube as the inner form to make a heat riser with matts mix is a big winner! works great and can even be moved (very carefully) in the event of a core rebuild. The first one I built is running on my second core as I write this. I've included some photos from my rebuild. The first shows the size of the feed tube at the end of winter, second you can see that the riser side of my cast core was as good as the day I poured it. Hands down the cast core runs hotter faster than the firebrick core but for my application durability wins over speed.

So whatever mix I use, if I have a chance to kiln dry it, that will make a difference (big difference) in the durability? Sorry if that's a totally basic question, but I've never worked with ceramics at all before. I live in an area with tons of folks making pottery, I've got to be able to either find someone with a kiln or, my daughter took pottery classes at a studio, I'll see how much they'll charge me for kiln drying it. How long does it take? Hours? Days?

I get this might be a strange question, but how big of a difference would it make? For the most part, the core lives inside cob, so aside from the entrance to the feed tube, their wouldn't be a lot of wear and tear on the core, aside from the heat. If kiln drying helps it last longer in terms of dealing with the heat, that makes total sense, but if the only issue is helping the feed tube be stronger, it sure seems like that's more of a design question.

Thomas, thanks for the info, so a couple of questions. First, mine is going in a greenhouse as well, did you consider creating an air vent in the feed box so you could get an air source from outside the greenhouse so the fire doesn't suck the oxygen out of the greenhouse? I was also thinking of putting a T on the final exhaust pipe. During lighting, shutting down and burning at night, I vent outside, but if Im burning during the day, I'd love to get the CO2 in the greenhouse, so I'd have a cap I could put on either side depending on what I wanted. Does that sound reasonable?

Second, I'm hearing repeatedly the problem of the feed tube crumbling and needing some help. So, have you built your idea of using heavy fire brick for the feed tube and building a different core that attaches to that feed tube? What about using some steel pipe for a feed tube and embedding that in the mold?

Third, so the roof of the burn tunnel caved in, what happened there? There shouldn't be any physical contact, so was it just heat that made the materials fail? That sounds like this mix doesn't work then for making a core. Or was something else going on?

Last, I'm planning on making a 6" core, so was the three 50 lb bags of fire clay for a huge 8" or was Matt just way off in his video (he said use 1 50 lb bag and 2 cf of perlite and 1 gallon of fire cement)? I calculated I'd need 3.5 cf of material for my mold, so 1 50 lb bag sounded about right with 2 cf of perlite and the fire cement. Is there a lot of waste or stuff you have to discard?

Ok, really last, any recommendation for brand of fire cement?

Thanks for the info and I'd love to hear any thoughts - Rick

Rick; Your core will be too large to fit in most kilns, you don't need to. Pour it , let it air dry a few days ,then build a fire in it , don't go crazy and build a bonfire just burn it an hour or two and let it cool then do it again . Remember you are encasing it in cob so any cracks that might form will be covered up.

Rick; No on the air vent ,you won't suck up all the oxygen. I definitely would not vent inside and don't forget even below zero you won't be burning at night, thats what your mass is for. I have not tried adding light brick into a cast core yet, thats just an idea that I want to try. Other than a (P) channel Absolutely no metal at the core it just will not last. The roof started caving at the 90 degree corner and just kept crumbling , I did not know until my rebuild that it was leaking into my 55 gal barrel ! My core was an 8" so it was larger and I calculated that one bag was enough .... BUT I did run out in the middle of a pour and I think that was what caused some of my durability problems. If you are putting this in a greenhouse I would highly recommend building an 8" rather than a 6" and using heavy fire brick for your core and making a cast riser. It will not heat up as fast as a cast core but on the plus side for a greenhouse it will hold its heat longer and you will not have any feed tube durability issues. If you click on my name you can see all my topics with pictures from my original build and my rebuild this summer. Also see my post on experimental P channel , it works great although it's starting to warp already. Oh and lincoln 60 fireclay is what I used but I have heard hawthorn is another good one.

The air vent questions are a bit secondary and maybe the topic of another thread, but a good fire will burn up a lot of oxygen, I have a small greenhouse that is well insulated, so there's no easy air access and I would often be burning in the late afternoon/evening (that's often when I'm home to work in the greenhouse) when I'd rather have the greenhouse shut up. So getting oxygen in hits me as important. On the times I burn during the day or late afternoon, I'd love that CO2, so I still don't get why not get air from outside and have the option to vent inside. But that can be for another thread

So, you say go with heavy fire bricks, mainly because of the likelihood of damaging the feed tube? I'll be the only one running this heater and maybe I embed heavy fire bricks into the core build, on their sides, that would form the feed tube opening? That would seem to provide a strong edge for the wood to come into contact with and while I can imagine that the bricks would begin to move over time and start to weaken the core material they're set in, it seems that might be a lot less wear and tear on the core material. I could do the same thing around the opening for the heat riser if that would significantly strengthen the core.

In terms of the heat riser, I have a 1/4", 14 gauge steel pipe I was planning on wrapping with kaowool. I'll put a 30 gallon metal barrel around that with 4" or so of air space at the top. I don't want much heat in the barrel, I don't mind some instant heat, but I want as much heat as possible going out the ducting.

I don't mind paying for bricks and I don't have easy access to cob, so what about this option - I cast my core, I have a nice 14" wide by 16" high by 32" long core block ('m still thinking to do a 6" system). I build a wood box big enough for a few inches of sand and hardiboard on the bottom, a row or two of bricks around the core (pressed in tightly) and a piece of hardiboard (cementaceous board) on the outside to protect the wood. I put a layer or two or bricks on the top as well and I can fill in any gaps with sand. And I leave it like that, in the wood box. My goal is not to get a lot of heat at the barrel, I want most of the heat to go out the exhaust, into the ducting and into my mass. I'm guessing some heat would leak through the small gaps in the brick and box into the greenhouse, that's fine. A bunch of the heat will be stored in the bricks around the core to be released slowly, but ideally, most of the heat will go into the ducts. Sound reasonable?

The last thing that is stumping me is the connection from the barrel into the ducting. In the box I just described building, I'm thinking I can do some combination of bricks and perlite/fire clay mix to create a connection for a 6" duct. In the attached picture, it seems like it makes sense, but the air space isn't huge. The gap is about 3" - 4" at the widest point. I also ought to think of some kind of clean out for that area?

Thanks again for all the ideas and suggestions. One other piece about my system/needs. I'm in Northern California, and we get just a few nights/year in the high 30's, Most of the winter, it's in the 40's, and I'm only looking to keep the greenhouse close to 60 at night. But, the main thermal mass will be under an aquaponics bed with 225 gallons of water on it and I'd love to give heat to the water fairly regularly. It could be I'd run the system a lot in the winter, a good bit during fall/spring and a bit during the summer. If that helps provide context for what I'm looking to do. The greenhouse is small, 10' x 24' and only has single wall polycarbonate panels for insulation (but is fairly air tight).

Your 1/4" steel pipe heat riser may function okay with the kaowool wrapper, but it will definitely burn out soon at the base. People here and on youtube videos have posted pictures of similar heavy steel core components that have been destroyed within one heating season or less... and you don't want this failing in the middle of the winter.
METAL IS DOOMED!

The firebricks around the feed tube may serve you well, but there is no point in putting them under the riser - that area sees no abrasion or impacts.