Permies likes wood burning stoves and the farmer likes permies
  Search | Permaculture Wiki | Recent Topics | Flagged Topics | Hot Topics | Zero Replies | World Domination!
Register / Login
permies » forums » energy » wood burning stoves
Bookmark ""Cob" recipe for rocket stove?" Watch ""Cob" recipe for rocket stove?" New topic

"Cob" recipe for rocket stove?


Joined: Mar 27, 2011
Posts: 56
Location: Bremerton, Washington
I'm trying to sculpt a rocket stove for myself, and I'm flying by the seat of my pants.  I SO don't know what I'm doing.

I saw something about a rocket stove that showed mixing clay with perlite, but I don't know why.  Possibly for better insulation?  Maybe because it lightens the whole mix, for carrying around?

I've got some very nice white clay from the local pottery shop, and some perlite and vermiculite I've mixed in.  It feels nice and sculptey.  I'm not sure if I ought to add straw, though.  I have no idea what building properties straw is supposed to bring to the party.

I've rolled some tagboard into 4" tubes, and taped them together in a J shape to make a 16" tall rocket stove.  I'm just about to make this rocket stove today, and still have time to scrap the whole project and re-mix the mud with straw if that's what I ought to be doing.

Does anyone have experience building with a clay/perlite mixture? Will this be too brittle? 
Kirk Mobert

Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
The perlite/clay mix is for insulation directly around the heat riser, nothing more. You'll want it to be between 2 and 4 inches thick (more won't hurt).  It will be quite brittle, so you'll need something else to hold it in and protect it from any knocks.

As to cob in the general sense: The straw acts like re-bar, adding tensile strength, the sand adds compressive strength, the clay is the binder. You can use different proportions of any of the ingredients for different effects. Best thing to do is try mixes of different proportions and test, test, test. If a mix cracks, it needs more sand/straw, if it is crumbly either the clay (soil) you are using is too silty or you've added too much sand.

I'd say NOT to use pottery clay, ESPECIALLY the white stuff if you can help it. There's too much heat shock involved with wood fired gizmos. It's not that the pottery clay(s) can't handle a lot of heat, but that they can't deal with the up and down extremes, the quick turnaround that we see in stoves. Best to find clay rich soil and use that, not TOO clay rich either.
A lot of natural builders use the word "clay" and it actually gets folks off on the wrong track. We imagine a bag of potters clay, which is our main contact with the stuff. Unfortunately, it's the wrong choice and tends to add more problems than it fixes. You want CLAY RICH SOIL, which will be chock full of impurities, hopefully the right ones like sand, and tends to be more forgiving and resilient.

Build it yourself, make it small, occupy it.

Joined: Mar 27, 2011
Posts: 56
Location: Bremerton, Washington
Oh, how sad.   

I guess I'd better not even bother finishing the project.  I had no idea I was so wrong.

Wonder what I can do with all that stuff now?  Seems a shame to just throw it in the trash.
Kirk Mobert

Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
You could try it anyway, just because.
There's very little consequence of failure here, as long as yer playing in a safe way, outside or in a proper shop environment, etc.. You bought the stuff, may as well get on with it, yes?? And just 'cause a material may not be "ideal", doesn't mean you can't get along with the stuff. Just gotta make allowances for it.

You are going to need to mix the clay with as much other stuff (pearlite or sand or whatever)  as possible. The finished (cob) mix should be somewhere between 15 and 20 percent clay, maybe even a little less.
One of the best ways to go about making pearlite/clay is to turn the clay into slip. Mix the stuff with water and whip it with a paddle mixer till it's smooth and the consistency of a thick milkshake. The slip can then be poured over and mixed with pearlite. You want just enough slip that the mix want's to stick and no more, a handful in the hand will hold together when lightly squeezed but it won't be wet enough to drip. Remember to wear a mask when working with dry pearlite, the stuff under a microscope looks like crushed glass, and it'll do a number on yer lungs.

Cob is a MUCH heavier material, you want a WHOLE LOTTA sand in that white clay you've got.
Yer gonna need to run some tests. Make test blocks starting at (say) equal amounts of sand/clay and work your way up till there's OBVIOUSLY too much sand. With natural clay soils, 1 to 1 (sand/clay) or 1 1/2 to 1 is pretty common for a regular old, garden variety, cob mix. Oven cores can be in the 4 or 6 to 1 range (4 to 6 parts sand, 1 part soil), earthen floors can be in the 6 to 8 parts sand range and so on. I imagine that pure, white clay will require more sand per mix than all that.
Anyway, make yer tests, mark them well, let dry and test. As a general rule, bigger blocks will be better tests, provide a more accurate example of what to expect. Test each piece to destruction, bang on them, throw them in a fire, leave 'em in the rain, you want to know what to expect and how each mix will react or stand up to what abuse. If they crack when they dry, you need less clay, if they crumble, depending on what you need from the mix, more clay.

On sand: Sand provides the compressive strength of the mix, and so it acts like a stone wall (only smaller) held together with a clay mortar. The bigger the variety of grain sizes, the better. Smaller grains will fill in the holes between bigger ones, and it will all lock together like a jigsaw puzzle. Also, you want angular sharp sand, not smooth round sand to accentuate the jigsaw aspect of it all and have it lock tight to itself.
In high heat areas, DO NOT use quartz based sand! Quartz changes shape under heat and WILL crack your cob. Better to use black sand (like granite, pumice or other non-crystalline) or silica sand. Again, wear a mask while dealing with dry ingredients, especially silica sand.

Straw acts like re-bar in the mix, it provides tensile strength. It matters less on a bench than the walls of as house or some such, but generally the straw should be as long as possible. When choosing a bale, you can grab at the corners of a (candidate) bale, pull out some and check it. The straws should be around the length of your forearm.. 'Course as I said, it's a bench and doesn't really need a whole lot of tensile strength, so shorter straw is fine just don't expect REALLY short stuff (like rice hulls) to do anything for you. They say NOT to use straw in cob that get's high heats.. You're supposed to up the sand content instead.. I'll up the sand fer sure, but I personally like a little straw in the mix. It will tend to char out, but I just like the feel of working with it that way.. It just feels "right" and I don't think it does harm.

DON'T cover cob with cement based plasters! Portland cement, stucco etc. do NOT play well with cob or earthen materials in general and should be avoided. Also, portland based cement shouldn't be used in high heat areas, the stuff will crumble to nothing. If you've gotta use some bought, bagged material in heat risers or some such place, go for the high temp refractory. It'll be expensive, but worth it if you MUST do it.. I've found that most often, natural clays work quite well and little else is needed.
Remy Olson

Joined: Jan 03, 2013
Posts: 27
Location: Lexington, KY
@Kirk Mobert, this was a great overview of making cob for this application! I have gotten other input on other topics on this forum that got me to this point:

My riser is fire brick (yellow ones) and old chimney brick (red ones) both salvaged from chimneys, mortared with pottery clay and river sand (about 1:3) because that's what I had available. The base is concrete (to get it above ground level since it's outside) insulated by the following layers above: cored brick, cement board, perlite/clayslip boxed in by cored brick, cored brick, fire brick, and then another layer of fire brick which is the burn tunnel (so about 10" of insulation between cement and the burn tunnel, fingers crossed that's enough).

Tomorrow I go to work cobbing a base on which to set the riser insulation and eventually drum. So this description of ratios was useful. Thanks for any input!

Lex., KY; Zone 5b
subject: "Cob" recipe for rocket stove?