Bells:Cob/steel instead of firebrick.

Can cob replace the firebricks and refractory slabs that are used above riser level in a bell?
When cob surrounds ducts, drums or bricks it is generally considered to be a reliable barrier for exhaust gasses.
Cob is also excellent as thermal mass.
If it could be counted on in the hottest parts of the bell, that would go a long way to reducing the number of firebricks needed.

So can cob replace the firebricks and refractory slabs that are used above riser level in a bell?
I imagine the answer is the usual  "It depends", especially since the nature of cob is site specific.
Fireclay is not cheap where I live, otherwise, but I think I could use it to make a cob that could withstand the heat above the riser.
Steel, on the other hand, is cheap.
The tops of 55 gallon barrels, as thin as they are, reliably withstand the heat from a riser.
The sides of barrels can be safely encased in cob.
Having access to the inside of the barrel is important to most RMH builders, which might explain why I can't seem to find examples of cob encasing the very top of a barrel.
The 4"Mini Mouse  heater is arguably an example of this, but the mud hat is not directly against the top of the barrel.
Can the top of a RMH barrel be covered with cob without reducing the lifespan of the steel,  or does the steel need to radiate heat directly into the surrounding air in order to survive?

In a typical barrel set up, coving the top in cob would ruin a great access point, but capping a bell with steel and sealing/concealing the steel with cob seems like a good way to reduce the cost of thermal durability in the upper reaches of a bell.
If cob over steel is as thermally durable as steel by itself, we can eliminate the need for firebricks in the upper parts of a bell.
The space above a riser could be formed from half a barrel, split lengthwise.
This offers an arched bell roof that is almost 2'x3', with the peak about 11" from the base.
Placed on top of a bell made of clay bricks or even concrete block, a wooden form could be used to shape a rectangular block of cob around it, sealing it to the bricks.
Coating the entire bell in cob could be made easier via slip forming.

Capping an RfMH  with steel and cob should work, if the cob doesn't reduce the thermal durability of the steel.
Let me know what you think.
Have you topped a RMH barrel with cob, and if so how did it work out?

Hey William;
I'm sure that the thin metal encased in cob above a riser would spawl away quickly.
At that point, you are counting on the cob being self-supporting (it should be... but)
Much easier to use Tbar angle iron wrapped in 1/2" of superwool and then a full sheet of 1" superwool overtop, with hardy board and plain clay bricks dry stacked above.
This gives you access to the riser, the inside of the bell, and firebricks are used for the core and on the walls above the riser level.

I'm not very comfortable with special materials.
Not that they are bad, but the more precious they are the harder time I have using them.
Because of that and some concerns I have with ceramic fibers, I'm preoccupied with other solutions.

Hey William.
Maybe that thread is interesting for you:
https://permies.com/t/205912/batch-box-RMH-bath-greenhouse

Since a flat barrel lid a few inches above the riser top has proven to be durable, I think a half-barrel bell top at least 6" above the riser should work too. A foot of clearance would be even safer. It might be beneficial to add a sheetmetal spacer an inch above the barrel at and around the riser location to allow the barrel to radiate heat away from the hottest area.

I would definitely install an access/inspection port that would allow you to look at the bell top in this case, until it is proven that such a system is durable.

I have built wood-fired pottery kilns entirely from cob which reach up to 2000 F for several hours continuously. The clay at the hottest points does shrink, crack and distort, but holds together for years. These points are surrounded by 2000+ F temperatures and variable atmosphere for hours on end. An arched/domed cob ceiling of good native clay with a separate cover layer of cob would probably be durable, as it would be subjected to much less intense conditions. Of course, clays vary, so you would want to test your clay for heat resistance and stability.

New to Permies, Rico in N Georgia, my involvement up to today  has been reading .... inspired by all the website and specifically your fine, thought provoking ideas William

I remember an amazing RMH build by Donkey that used only materials available to anyone...I think he mixed cob with wood ash and grog?  It was a lovely bit of art with a casserole lid for the door.  Donkey built that one at Wheaton labs if memory serves

Perhaps that RMH could be your starting point for your build, using no special materials.  Good luck and please keep us posted on your journey

Rico Loma
Bill Mollison fan for 35 years

William,  I did find that heater, it was built by Donkey in the Red Cabin.  Video shows some of construction process, Donkey might have additional info on his pro boards site. Anyone else have an in depth view of this masterpiece?
And how to make grog? I only remember it having ingrediants like crushed brick, clay, wood ash

Here is the thread about that build, with lots of good info. It, along with Erica Wisner's great post in this thread, reassured me when I was starting to despair over the possibility of building a rmh without very fancy, hard-to-get or expensive materials. (Note, though, that I haven't actually built one yet.)

This topic is very timely for me, since I've been pondering exactly these questions lately. Right now I'm thinking about a double-skin bell, with the inner skin being old red clay bricks (possibly set on their side, to reduce the number of bricks needed) and the outer being cob. Would this work? I'd like to minimize the number of refractory bricks needed, so would prefer to not have to use any in the bell...

I built my bell from old red clay bricks laid up on their sides, covered by an outer skin of cob (about 6"/15cm). I did use firebrick for the top third and ceiling; red clay brick may not last as long, but if given generous clearances to the riser top should be good for many years.

6" is the upper limit of cob thickness I would recommend, as that takes several hours for the heat to start coming through and heating the room. 4"/10cm would probably be the best balance for mass and heat transmission considerations.

You folks are great, lots of ideas tp onder

To ponder............apologies

Glenn Herbert wrote:I built my bell from old red clay bricks laid up on their sides, covered by an outer skin of cob (about 6"/15cm). I did use firebrick for the top third and ceiling; red clay brick may not last as long, but if given generous clearances to the riser top should be good for many years.

6" is the upper limit of cob thickness I would recommend, as that takes several hours for the heat to start coming through and heating the room. 4"/10cm would probably be the best balance for mass and heat transmission considerations.

Thank you! Good to know that it's doable.

I've also been thinking about making part of the roof of the bell (not directly above the riser) from metal, to get some immediate heat. Like William, I'm a bit sceptical about ceramic fibers, so I've been toying with the idea of using a sand-filled groove as an expansion joint. The idea was to find a box or other container made of suitably thick metal, cut it in half more or less, and set it in a brick or cob groove filled with sand. See picture. Any idea if this could work?

I think a seal as you describe would work as long as the metal edge was sunk deep enough into the sand. Not sure how much is necessary.

I don't think the top is the best location for a radiant panel, though; it would be sending heat up to the ceiling, not out into the space where people could quickly feel it. I would set an access panel into a face of the bell, to serve two purposes: instant heat directly to the room occupants, and easy access for inspection, maintenance and repair of the combustion core without having to dismantle brickwork. A steel angle frame anchored to the masonry, with a flat steel panel screwed into it, works excellently in my experience.

Glenn Herbert wrote:I don't think the top is the best location for a radiant panel, though; it would be sending heat up to the ceiling, not out into the space where people could quickly feel it. I would set an access panel into a face of the bell, to serve two purposes: instant heat directly to the room occupants, and easy access for inspection, maintenance and repair of the combustion core without having to dismantle brickwork. A steel angle frame anchored to the masonry, with a flat steel panel screwed into it, works excellently in my experience.

Hmm, that's true. Can you get a good enough seal around such an access panel without ceramic fiber or similar? Would like to manage without it, if at all possible.

I would use standard fiberglass rope made for stove door sealing, with furnace cement as needed to mount it to the frame.