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Refractory Cob or cob-bricks? Forges, rocket stoves...

Posts: 19
Location: Oshkosh WI
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Hello Group--been out of the loop for a while, but this pandemic has me without a career at the moment.  Passing the time and occupying my mind with permaculture and garden reading....

Years back I messed around with rocket stoves, and even built a nice little one from 5" square steel.  I gave it to my brother who occasionally uses it for outdoor cooking.  I've been thinking next time, I would like to build a nicer model, and have come down to either going the 5-gallon pail route, with perlite concrete, or building a brick unit.  

I built a small wood forge years back simply by lining a roasting pan with clay mixed with wood ash.  It seemed to hold up ok, but since it was in a permanent shell, I suspect it didn't matter if the material was brittle or not.  

Can anyone direct me to some info on making high-temp bricks?  Ideally, something that one could source from the earth.  We have plenty of clay and sand here.  Reading up on cob pizza ovens, it looks like they use a pretty standard cob mix--including the straw.  Because the straw is encased in clay, I suppose it cannot burn?

It seems like I could make a cob mix, but mix it maybe 1:3 with perlite and a bunch of wood ash to help kind of cement it together.    I wouldn't bother, but with fire brick costing ~$5 a piece locally, this could get expensive quick!  And it would be fun to start from clay and do it in a really primitive and ecologically sound way.

I thought I could build some rudimentary brick molds from scrap wood, fill them carefully with the cob mix, let them sun dry for a while, then stack them to fully sun dry before use.  

Anyone build something like that? A brick rocket stove with homeade bricks?  I'd love to have one in the back yard for cooking.  Maybe integrate it into an oven in the stack to hold a dutch oven.  It would be a good way to get rid of the branches my old apple tree keeps shedding.

Hope everyone is staying healthy during this unsettling time!
Posts: 1343
Location: Westbridge, BC, Canada
building solar woodworking rocket stoves wood heat greening the desert
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Hi John,      Welcome back!

I remember posting my findings somewhere on permies of the high temp homemade bricks I made a while back. If I find it I will include the link.

Donkey over at proboards has done a fair amount of building rocket stoves using all natural materials. One that comes to mind is the cyclone tiny-house-rocket-mass-heater at Wheaton Labs. He used clay, manure, grog and wood ash I think.

I just finished tearing apart my Perlite/clay heat riser the other day and it held up very well. It certainly would not have been good in an area that gets any kind of abrasion though.
I do remember that I found it very helpful to add some fire cement to my homemade bricks as everything else I tried was just too soft or tended to crumble way to easily.
Right now those bricks are installed into the back end of my batch box acting as a base for my heat riser J -tube-Batch-Box-Conversion

The straw encased in the cob (especially the area closest to the fire) will mostly burn out but in the process leaves voids that can add insulative value.
Every batch of clay is different so there won't be one fail safe recipe to follow...only guidelines.

I added your thread to the rocket stove forum as well. Perhaps someone there will also be able to help you.

EDIT- Found a bunch of other stuff but not my own information...oh well.

Found this comment by Donkey using charcoal as insulation
A thorough explanation of Donkeys process as of 2014 of making bricks: dear-donkey-master-cob
and one more by another fellow who explores sodium silicate and other high heat materials: diy-refractories
Others: Making Insulative Clay Combustion Chambers
Primitive Technology
John Kestell
Posts: 19
Location: Oshkosh WI
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Thanks so much for the info!   I live near a large lake here in WI, and their is a bank of heavy, slick, red clay that I think would be perfect for the project.  I've been scribbling down ideas---maybe build the guts of one out of fired clay flue pipe, then cover it in cob?  Or as I mentioned in this post, prepare bricks and just stack them up?     I have a "normal" home at the moment, so if things didn't go well, or if it didn't hold up great, I still have a functioning oven and stove.

Still, it would be very cool to build a little rocket stove with a small oven.  Someplace to do a little outdoor cooking, burn some of the branches that are constantly falling from the apple and lilac trees...

I will look into the links you provided.  I wonder if I could source a bit of fire cement locally?   Or if it would make sense to carefully sun dry the bricks, then fire them to make them good and stable?  

I'll read up--I'm certain their is a lot of info, I've seen some cob pizza ovens, thermal mass stoves, etc that would be a good jumping off point.  Mine would be the most basic system--an updraft burner, maybe 3' tall, with some kind of trivet or platform to put my big 7 gallon (old beer keg) pot on, or a little oven that would hold my dutch oven for making some bread....  

Thanks again!
Posts: 766
Location: Central Virginia USA
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Hi, I'll just mention geopolymers,  search the donkey forum, there's loads of stuff there Karl is a primary expert, but there are several others playing with them as well. Interestingly enough, your original clay ash formula  has many of the ingredients of geopolymers in a somewhat more primitive form.
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