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RMH: Cast in-place riser tube materials formula

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I've watched a few of Erica's and Ernie's seminars on RMH. I've heard the question posed as to whether it's a good idea to use sawdust and clay as medium for a cast in-place riser tube insulation. I understood Ernie's response to be that the sawdust would take nearly a decade to completely burn-out. My question is: has anyone tried to use bio-char in place of sawdust in a clay mixture as it's already burned-out? I imagine that this would be an excellent recipe for a cast in-place riser. Thoughts?
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Location: latitude 47 N.W. montana zone 6A
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Hi Joel; my cast riser was built using fire clay and perlite with a small amount of refractory thrown in. I used an 8" sonitube (cardboard concrete form) as a sacrificial inner form and a sixteen gal grease barrel as the outer form . The sonitube burns out on your first fire and you are left with a smooth super insulated riser . No need to use sawdust or bio char , perlite is a good insulator and fire clay just gets harder as you heat it. I put some refractory in mine ,but i don't think it was needed. I'm enclosing pictures of the riser as it was built and one after it had been running for 3 months.
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Posts: 1958
Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
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We're looking for a solution to this question too!

Perlite, vermiculite and pumice are not available here as of now. We just used ashes in our first rocket stove, though they have settled a lot and we have had to keep refilling them. Since the top of the barrel is open because we put a huge pot on top for cooking, it's easy to refill.

Biochar is not all burned out already. Charcoal is an excellent fuel and has been produced to be used as fuel since ancient times!
Joel Francis
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Hey Thomas,

Thanks for you're reply. As I have never actually built one of these things, I value your experience. I like the suggestion about the sonitube. I would need to go dumpster-diving at a construction site to get some. I can see the advantage of using an inert insulating material such as perlite. It probably fills the same role as grog or temper when making pottery. The only down side I see is that I can't locally source perlite or vermiculite, so I would have to go buy it.

I have heard many references to using perlite and/or vermiculite with clay to for an insulated riser, which I'm sure works great. My interest in the bio-char as an insulator is mainly because I have some, and I'm looking for ways to use it. I've built the 1G toucan TLUD stove Paul featured in one of his videos, and now I have several gallons of raw, crushed bio-char. In the video below, Ernie's friends at Aprovecho describe making insulating refractory brick with clay, sawdust, and water, which then needs to be fired in a kiln. The need for a kiln is the main reason I wouldn't just make the brick, because I don't have access to a kiln, and I'm not willing to make a kiln. But, the only draw back from using the Aprovecho recipe for a cast in-place riser that I've heard is that the sawdust takes a long time to burn-out (and that it tends to crack when drying), so the full benefit of insulation is not realized until much later in the life of the heater. This then leads to my question of why not use bio-char instead of sawdust.

I can appreciate the tried-and-true advice to use perlite/vermiculite as the insulator, but something seems off if I have to create demand for a mined material that needs to be shipped to me when I could potentially replace the material from local, renewable resources while gaining the heat benefit from its production. I'm sure I'm not the first to have this question. I'm often disappointed to realize that my great ideas have been thoroughly documented in the 1800's. I guess the real answer is to try it myself and report my findings. That may take a while to get around to...
Joel Francis
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Wow! didn't realize my question was relevant to someone living that far away from Louisiana, USA.

I also had concern about off-gassing of charred material within the clay binder. The thing about char is that all of the volatile gasses have been boiled-off. Char is only useful as fuel in the presents of oxygen. The clay should provide an effective boundary between the char and the outside gasses except for those few pockets at the surface, so some minor pitting should be expected on the surface. But, also realize the environment of the riser is starved of oxygen because of the fire.
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I had a potter friend who made a large (walk in) kiln. He used a standard recipe for DIY castable refractory which had sawdust in it. After very few firings it started cracking and spalling and all the stuff spalling off contaminated his glazes. He had to scrap the kiln at a big loss. He thought the problem was caused from firing it in place with the heat all coming from one side rather than firing it as bricks in a kiln.

After watching that disaster unfold I'm very leery of casting a soft insulative mix containing sawdust in place. The clay perlite mix seems to be the standard riser material. But. i wonder how it holds up over time. Reread "lessons learned" in Art Ludwig's case study in "the book". If you're building it to last fifty years you have to think differently than if you're expecting to replace it in five years.

Look at how they cast the riser for the stove at http://www.rechoroket.com/Home_files/EnglishBook.pdf I'm thinking of plastering a durable hardface on the inner tube then packing the insulator around it. Use a split tube instead of a bucket for the outer form, so it wouldn't be tapered. And make it in interlocking segments short enough to be fired in my kiln before I install them in the stove

How good does it have to be? How long will a standard clay perlite riser last? Do I even need to jump through these hoops or is the burn tube and elbow going to burn out first so I'll have to rebuild the rest of the stove first and I'll have to replace it then anyway?

I see these stoves encased in a beautiful cob sculpture and I think "It's going to be awful hard to raise the hood if you have to replace a spark-plug in that thing".
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Location: Okanogan Highlands, Washington
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We definitely encourage leaving the barrel accessible, not covered in cob. We have a class exercise handout on decorating the barrel using various compatible materials - available by request for those interested.

Regarding using biochar for insulation: My impression was the whole point was to use it as soil-building material for carbon sequestration. (I have my own speculation about the popularity of the biochar process; I think it's an attractive / viral meme for several reasons, most of which have nothing to do with the explanations that are sold with it, and everything to do with humanity's susceptibility to certain fascinations.)

The carbon will burn out if it's used to insulate a high-temp flame path, just not necessarily all at once, and there might be a point where anything remaining doesn't burn out because it's insulated from the fire.

I saw some examples of charcoal-clay "vernacular insulative ceramic" bricks, made in a rocket cookstove project in Uganda, and fired in a kiln before use.
Here's a couple of links to projects that may be related:

I think what I originally saw was this PDF summary of the project, over a decade ago now:

There are a lot of other PDFs linked from the same menu on the Approvecho website: http://www.aprovecho.org/lab/rad/rl/stove-design/category/1

Bear in mind that the Approvecho cookstoves are generally quite short, risers about 12 inches in many cases, or up to 2-3 feet in a few large-scale cookers. They are also more accessible for repairs or brick replacement; and in many cases are designed to be portable and used outdoors. They are not burned as hot or as long as a whole-house heater. So making metal shells and filling with insulative ash or burn-out materials has been the most widespread version, with insulative ceramics researched mainly in places where clay is more readily available than metal parts.

We have stayed with the perlite-clay version mostly because it seems to work well. Although it is not super-durable even when made excellently, it is about the cheapest effective material, and the most durable cheap material.
(I like the idea of smearing a hard-coat liner on the inner form, I would recommend something like earthen plaster or a refractory cement that's compatible with the clay or whatever binder is used for the rest of the insulation.)

Ianto Evans uses perlite for insulation around his stoves in workshops. I've been told that sawdust, straw, and other gap-forming materials were used with clay, with disappointing results. The material tended to become crumbly on firing, and though that could just be poor pottery technique (firing before completely dry), most of the insulative materials and gap-forming burnout materials also hold moisture and complete drying is a big part of the challenge. An extra step of firing in a home kiln, to high-grade for the materials that don't become crumbly, would be both time- and energy-intensive.

I have come to believe that the insulative kiln bricks widely used by hobby and professional potters for making kilns, and the refractory insulation used by industry for lagging, are probably reasonably priced. This is considering the value of time and energy it would take to re-create them at a home DIY scale, and the energy savings that would result from doing larger-scale production in dedicated facilities. Like ovens, an occasionally-used home kiln will be far less efficient on fuel than a commercial, regularly-used, large-batch kiln. If you're going to fire bricks or refractory tubes at home before building, it's probably going to save time and money to just order some bricks online and have them shipped.

Except maybe in that Indian Himalayas situation. I could see shipping being extremely expensive in that case.

The certification and ratings of refractory products probably do add to their cost, as would liability insurance for such commercial producers, but I think it might roughly offset the additional time and energy costs of making your own.
The energy costs of processing refractory cements also add to their cost - which I think is why Ianto avoided them. Embodied energy, uncertain chemistry (some refractory cements are far less toxic than Portland cement, but it's a new learning curve for each one), plus having to actually shell out money for a project, would all be a real turnoff for Ianto.

We do find that perlite-clay insulation around the heat risers, either as a stand-alone or (more durably) around a brick liner, is pretty darn effective.
We will also use refractory blanket (rock wool, cermic-fiber blanket) where it's available, as it insulates very well with a thin layer. Think I've described the other options we routinely use elsewhere on these forums, with thicknesses and all.
Vermiculite and pumice seem to be less effective - clay fills the spaces in vermiculite, and it doesn't bond well at densities light enough to remain insulating. Vermiculite in a refractory cement might work out better.

We are still very interested in improved solutions. I have been talking about the cast-around-a-cardboard-core for some time now, and tried it on the burn-tunnel castings we did last year with some satisfactory results. Delighted to see a tall, working heat riser. I would be tempted to make one in sections, like a Grecian column, so that if it did get bumped your prospects for salvage and repair are better.

My understanding with clay /ceramics is that there are several weak points before you get to the final, hardens-with-heat ceramic set:
1) Water: trapped moisture will cause spalling or small steam explosions wherever the heat finally finds it. In insulative materials, this can happen as a series of small spalling explosions as the heat penetrates each successive layer of material. Commercial refractories are often held at around 100 F (65 C) to dry them thoroughly before raising the temperature above 212 F (100 C) for firing-in or curing.

2) 600-800 F, or up to 1200 F (roughly 400-600 C - this is my general recollection anyway so look it up if it matters to you):
I've heard reports that some clay materials can go through a weak phase at this temperature, one reason why kilns are heated slowly and gradually. If material is heated only to this temperature and no further, such as in certain parts of a fireplace back, you may see irrigation or crumbling of the materials. This is about the point where Portland cement gives up, also, so materials that contain impurities like lime or hydrated minerals might go through a second, steam-spalling phase in this range.

3) Ceramic firing-in: usually in the range around 2000 F (1000 C) or so. When brought slowly to this phase and held for enough time , the clay bonds into stone-like hardness. Vitrification (glassy-ness) happens in here; glazes or other inclusions can change state in interesting ways. In some cases, the hardest firing is just before the melting point, which makes me think that 'sintering' (fusing of particles as they melt at the edges, but not all the way through), might be what's happening in the extreme case of ceramic hardening.

4) Melting point: Many common clays like brick-clay and terra cotta, known in the pottery world as "low-fired" clays, will melt if their temperature exceeds a certain point. For brick clay I think it's around 2600 F, but it would vary a lot. Firebrick is made with fire-clay grog (essentially a fire-suitable clay that's fired once for use as sand or aggregate in the brick works), then the whole brick is fired again. They are rated for higher temperatures, can handle that 2600 F in most cases. But if you did get them hot enough they would melt too - maybe up around 3000? depends on the brick, check the rating if possible.
High-fired clays like porcelain will not melt at temperatures that would melt terra cotta, and will need somewhat hotter firing to achieve the most desirable hardening and effects. Porcelain contains talc, as does soapstone. It would eventually melt, if raised hot enough; everything does, which is why the earth's mantle is liquid.

-Erica W
Posts: 245
Location: near Houston, TX; zone 8b
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I am elaborating on the melting point issue raised by Erica with regard to terra cotta clays. There is another factor here which is the rate at which these clays heat up. Even kiln-fired terra cotta chimney flue liners will crack apart if their temperature is raised by more than 50°F in one hour.

If you make your heat riser based on this type of material, it seems logical that eventually it will fail because it is heated up too fast.

The heat riser does not need to be structural. So, use a material which can just sit there and take the heat.
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