Anyone with experience using cob on outside of RMH barrel?
posted 8 years ago
I'm about to get started on a rmh for my basement and workshop, and have seen a few pictures of rmh barrels that have been covered in cob, and I think that's the way I want to go. I like the aesthetic, and since i'm going to be moving around down there a bunch, it reduces the chance of rage when I accidentally bump up against it.
I've done a good bit of work with clays and plaster, and cracking when drying thin areas can be a big issue, so does anyone have any tips for the thickness or application of cob to put around the barrel to avoid cracking? I'm guessing the pictures i've seen have maybe 1-2 inches around them, but it's hard to say. I would like it as thin as possible for fairly quick warmup. I'm thinking about doing a bit of a hybrid design, leaving the very top metal so I can have a quick heating surface, but putting cob around the outsides.
Thanks for the advice!
posted 8 years ago
Use course and sharp sand in the cob.
The sand particles should be fairly large, NOT the size of table salt. More like the size of a bb pellet, but SHARP (from broken rocks), NOT rounded river sand.
We got a delivery of sharp sand and had to sift the fine stuff out of it using a window screen. We used the left over larger particles (the ones that don't fall thru regular sized window screen) in the cob mix. I used the resulting fines to make scratch coats of plaster, but it was kinda crumbly from too many fine particles. It should really be sifted again thru a finer screen to get two more sand sizes.
Make sure there's enough sand in the cob mix. If you have really pure clay, you can end up mixing almost three times the amount of sand into clay. Because our soil has no clay in it, we use a locally available pure clay (a bit of natural sand in it) and use a sand-clay ratio of 2.5-1. For finish plaster I use three parts of (very very fine and sharp) sand to one part of clay, plus half a part of fresh cow poop. It's a really strong plaster. Make test blocks using the sand and clay (or your soil, if it's got enough clay content) you have available, and in the thickness you'd like to have on the barrel. Press the mixture into a rigid mold and let it dry out thoroughly so you can see how much it shrinks. If the blocks crack, or shrink a lot from the sides of the mold, use more sand. Or use less water, if the mix is too wet it can cause cracking problems too. Usually the problem is not enough sand.
We plan to cover most of our stove's barrel in cob, and will use a volcano shape, much thicker on the bottom of the barrel and tapering to much thinner (probably 2") at the top. I think the gradual widening of the cob layer towards the will really help it not fall off the barrel as it dries. Will report back how well it adheres!
posted 8 years ago
That's the same sort of idea I was thinking with the mild volcano shape, so interested in hearing how it turns out. Thanks for the info!
posted 8 years ago
as you move to the cob thickness being less than 8" or so inches in thickness, I suggest you begin weaving corbels around the stove. I would use a two layer system with fiber free high grit fill 2+" thick against the barrel, almost a brick mix, and corbels stitched birds nest style around the barrel rather than the typical plaid weave. Corbels are extra durable and far less likely to be damaged by random bumps or fast temp changes. Still I would not make the entire lip less than 4" thick at any point, and keep the corbels back and separated from the barrel by 2+" of clay and sand mix. Something really gritty as per above.
Ive seen wraps of thinner composition, but suspect they are headed to early fail - years of life rather than decades. The drastic temp changes next to the barrel cause fritting. Clay particles containing trace moisture granulate as they are subject to temps of 600f+. Fritted clay cannot re-bond into a monolithic mass. Frit is the crumbles on a cob oven that wont reconstitute in water into slip. Imagine terra cotta with no aggregate. Fritting happens next to the barrel and thats why I suggest two layers- the inner layer may frit, but its connection to the woven outer layer is weak and insulative; the outer layer has fiber, which would 'burn' next to the barrel- but not if it is protected by the inner layer. Fritting is a prime clay oven fail. Weaving frit in with corbels seems reasonable. I have a project, linked below, that im designing with these thoughts in mind.
IM afraid that I dont know of a good video for corbelling technique, and if you dont know corbelling, its best learned by doing- IF you know a local cobber to your area, ask them to drop in.
Stitching direct from top and at perpendicular angles to your volcano alternately will help. corbel ends must be undifferentiated, so that there are no separate pieces. The corbels can be used to key a mantle in if you have flagstones; the back edge of the stone should sit cleanly on the corbels, and not the inner fill; The key above the mantle may have a compression ring with clay, stones, etc., to hold it down. Balancing it is key, I cant recommend a compression ring enough. I did this for a project in pdx, but cant find photos...
This is a half done piece which Ive used corbels in the manner described above, and which will get a stone mantle next spring when its workable again. not as hot as the RMH, but similar corbel construction.
HOld the press. after a review of the code that is being developed, some more looking at the book and a chat with erica wisner, there are some more to thinks here.
first, she says Ernie doesn't think much of the idea of wrapping the whole barrel in cob. The barrel neds to radiate heat and cool reasonably fast in order to maintain the downward convection cell that's established in the heat riser/combustion unit. That quick radiant heat is also a reward. 2" of fiberless grout wrapped in with tight corbels and plastered should be the minimum here, but again, not to cover too much of the barrrel in order to get that temp differential.
So a half wrap- ( which makes convection favor one side) on just a portion of the side, or leaving the top 1/2 of the barel exposed is really about making sure the downward current is maintained by heat differentials.
There's also a maintenance consideration. Sometimes it's easier to remove and replace the barrel than to access the inside through the manifold cleanout. For example if you get ash buildup on the top lip of the heat riser. E & E slopes this lip to mitigate it, but note it can still happen. I know of one barrel at least where removel may be problematic, but whether by foresight or habit the thing is overengineered and not likely to need replacemnet anytime this century. I highly recomend a modest overengineering of things when its possible.
A better compromise in erica's opinon is to bring the cob up on one side, or halfway around the barrel, to give some aesthetics but also maintain the radiant heat benefit. shes also contemplating making a radiant heat bell out of masonry, brick or granite. Thtas something im looking at on my designs as well.
she says this" The one thing you don't want to do is to insulate the barrel. Folks have with the best intentions, thrown a perlite layer in there "to keep the barrel hot," and it does exactly that. But you don't want hot gas trapped in the barrel; it is supposed to be cooling the exhaust so it downdrafts, not staying as hot as the heat riser. Insulated barrels can equalize in temperature with the heat riser, and then you get a stagnant pool of hot gas in the top of the barrel that gums up the draft. The gradient from 600 at the top down to 100-200 at the bottom represents the gas inside dropping from 1800+ down to about 600F, a significant enough change that it dramatically helps the system draft properly and push exhaust through the horizontal pipes."
So im eating my hat about the thickness. glad I asked before I built it. my hat might not taste great, but its better than building something that dont work at all.
posted 8 years ago
i was thinking the same .. either as the reburn? chamber with a cast iron plate in the top or cob cement or brick with a air gap or right now i am thinking is regular barrel with a erm 2 inch air gap and a single or double row core red brick with air vents on bottom to allow some air flow while catching radiant heat.