6" batch box RMH for bath/greenhouse with Cob bell

Some may be interested that I used cob for my batch box RMH bell. I have done a fair amount of home building using conventional materials, but I'll take cob over chemically treated plywood and vinyl siding any day! Cob is the most forgiving, natural building material I know of. I am also fond of curves, arches and anything other than right angles. After some challenges, I am very pleased with the performance of my cob Bell.

I decided to try building my RMH bell around my metal slipper tub. Having worked with Cobb before, I was well aware of the challenge of Cobb shrinking and causing smoke leak problems, especially where it met the metal surface of the tub. Fire rated caulk has remedied this, but leaky cracks appeared in the cob, especially on some of the more complicated curves. When I first lit fires in the batch box, I kept a ready supply of fresh, damp cob mix for filling cracks. These fixes worked, but new leaks kept showing up in other areas. I spotted them quickly and stopped the smoke before the smoke stopped me. These cracks in the cob are not surprising knowing that cob is not good at shrinking and expansion due to the heat of the stove. My RMH is in my greenhouse so the smoke is easily cleared in the smaller 12x12 structure. I am noticing fewer leaks as the heat hardens the cob. I am confident the leaking will go away completely.

My most expensive piece for my RMH is the 2x2 ft plate steel I use for the ceiling of my bell. I also don't seem to have cracks or problems with my one inch fire brick, the next expensive part of my stove. The rest of the stove was free, thanks to a large pile of subsoil I could use for making cob. Being from Alaska originally, I am used to working with gaskets around metal wood stove doors, so I was ambitious about using a gasket between the top of my cob Bell and the 2x2 plate steel. I wanted to be able to lift up the 2x2 plate steel and examine the condition of the fire brick and other potential concerns within the innards of the RMH. This means having to replace the fire rated caulk whenever I lift the plate metal and do an inspection. I've been able to cook and heat water for the bath on the plate metal surface so this is made having to replace the caulk worthwhile. Thanks to a Permies's post, the batch box door and inspection port door are simply lids from Pyrex dishes. I am in Central North Carolina if anyone is interested in seeing my RMH.

Great job on your stove Anni !
Never would have thought of taking a bath in a greenhouse while watching your plants grow could be a thing?
Need a cup of tea?... waters always hot and ready for a mug or two within arms reach.

Always nice to remind everyone that the 'bell' doesn't have to be a barrel, and yes, working with cob is so much fun because of its flexibility to create with many shapes and fit your space the way you like it.
As far as expansion joints and plugging up smoke leaks, many other products come to mind as well: superwool, stove gasket rope and even rock wool can be used.
With my double barrel system, I made the seal between the two with stove gasket rope and glued it to one of the barrel lips with high heat silicone. Took it on and off many times through the season without having to redo the whole seal.
Leaks on my dragon only occurred during a cold startup and quickly went away as the gasses found their way out the chimney. With the persistant leaks (away from really hot areas), rather than just smearing cob only to have it reopen shortly again, I made a fabric patch. Kind of like paper mache. Worked great!

I too used a pyrex lid for my batchbox door. It was further away from the flames than most people who used them and had them crack or shatter.
Does take a bit more caution but certainly a low budget option, especially if your the only one using the stove.

Thank you Gerry for your encouragement! How did you make your patch out of fabric? That sounds like such a good idea! I was thinking of just building the walls thicker because where the cob walls are thick there are no leaks.

I soaked a loose weave fabric like jute bags or heavy cheesecloth in warm water, then dipped them in a clay slip slurry and applied to the crack.
Once partially dry, I then applied a layer of cob over top to conceal the bandage.
This is a similar technique when your trying to make cob stick to a smooth surface.
Let us know how it turns out if you try it and it works for you.