Rocket Mass Heater repair?

Hello.  I built a RMH for my in-laws in 2015, we contacted Erica and Ernie and they set us up with a really amazing thorough set of instructions for our custom build.  They even did a phone meeting to answer some questions.  Great folks!  HOWEVER, recently we decided to repair our RMH.  Over the years cracks have formed and finally this summer they were too bad not to repair.  SO 2 weeks ago I began the process, based on what information I remember or have written down somewhere.  I routed out the cracks and mixed up some repair cob.  6 sand, 1 clay, and finely chopped straw.  AND it worked pretty good, but there are cracks that have since formed, though not as bad.  I am curious...if paper slip or paper clay would be applicable here.  I guess I could just add another layer of the 6 to 1 recipe, but wanted to make sure we had no cracking.

Does anyone have a recipe or insight(even if it is a slightly different direction) for such a thing?

Also, we never did put a top layer/skin on it.  What are you all using for the skin?  What is the recipe?  I have heard good things about lime plaster, but lime(the type I need) is really hard to get here.

Also, is there anything I can do to prevent more cracks down the road?  We had to use C and C ball clay or similar(grey/white powder from a pottery shop) because the clay we have around here is insanely full of rocks and organic matter...and we were told it would be terrible for cob since we would have to screen it to remove impurities.

Anyway, thanks folks, advice is appreciated.

Hey Will;
A Dry-bagged clay is perfect.
What sand are you using?
I only use a sakrete, commercial medium-graded sand, which I buy at Home Depot.
My common clay mortar mix is 1 clay to 3 sand; adding straw would make a good cob mix.

My current builds are of clay brick, but I have had a cob-covered J-Tube for several years.
I found that by adding a sheetrock fiberglass mesh at problem spots, I could control the cracking better.

There will always be a crack somewhere.
A covered coffee can with some cob and a wet finger, generally takes care of them quickly.

Thanks Thomas for the fast reply.

I am using masonry sand.  It is somewhat "sharp" but not terribly, being that this is close to the top of the clay/cob body I did not want anyone getting cut or scratched.  It is brown and black sand...but has a pretty wide range of sizes.  I did research on best sands and people were suggesting similar.

I found the straw worked really well for the clay/cob body, apparently so because it has been almost 10 years before repair of any kind was needed.  BUT the shrink rate between the new wet stuff and the drier clay/cob body is causing cracking.  AND I cannot get the straw fine enough to work well in smaller cracks.

I come at this from a different direction than some.  I have dealt with clay a lot in ceramics...and repairing leather hard stuff works really well with paper clay.  Which is just a thick (chocolate milkshake consistency) slip with toilet paper blended up(the cheaper TP the better).  It works well.

I think I will try this, and see.  I can mix up a small container and test it away from the fire part...and see.  BUT winter is upon us and I need this done...hence the reach out.

Our RMS is using 10 inch tube/stove pipe.  Has worked really well to heat a 2000 square foot home.  Has never caused issues until this year when we discovered the cracks, the loose brick(2 or 3) and the destroyed insulating material(perlite and clay) on the jtube upper chamber.  We replaced the perlite mix with a piece of fiber for kilns...and it has worked well, now the crack repair...just seeking "easier" things...

thanks again.

Hmm, 10" system? I'm wondering why I haven't seen examples of this around yet.

I remember from working on the cob floor for the yurt at Wheaton Labs we used horse manure in the cob mix. Apparently whatever fibres make it through the animal's digestive tract are proven durable, and I would think they would also be finer than straw so maybe good for small cracks? I believe it works with other ruminants also. Not sure exactly on the details, but could be worth looking into more...

Coydon Wallham wrote:Hmm, 10" system? I'm wondering why I haven't seen examples of this around yet.

If I remember correctly anything bigger than 8” gets to hot to use “normal” refractory brick.

So is it just the ducting that is 10”?

Coydon Wallham wrote:So is it just the ducting that is 10”?

Sorry, I should have 'splained better.  Yes all the metal stove pipe and ducting is 10 inches...as is the double wall insulated pipe through the roof.  The j tube is something closer to 8 inch.  As for "gets to hot for refractory brick", it does not, we are using kiln brick manufactured for HIGH fire ceramics work so there is some wiggle room.  Before that brick is destroyed the barrel would melt.  AND I have only ever seen it get red in a spot or two once...and that was due to a partial blockage in the accumulator area where the heat travels from barrel to mass.  A brick fell down and we had to fix that.

For mass cob, sand and gravel in your clay is just fine. My clay deposits are direct glacial till, with somewhere around 15% clay, 15% silt, 30% sand, and 40% gravel and rocks. It works great for cob walls when I just pick out rocks bigger than a golf ball. Organic matter could be an issue depending on its form. Deciduous leaves are bad because they leave planes of weakness in the cob. Thin stringy matter like grass or pine needles does not make a lot of difference though I try to avoid it. Large amounts of decayed matter would likely weaken the cob substantially and require treatment which would not be practical in large quantities.

Finish cob, of course, does need to be closely controlled for impurities.