Rocks inside a cob wall

[martinpolley]

patamos Thanks for the tip! As well as the round rocks, I have plenty of broken or odd shaped ones that I can play jigsaw puzzle with . Also, there are several house renovations going on nearby, so I can go dumpster-diving for material. (Actual bricks, on the other hand, are kinda thin on the ground in my part of the world. Most construction here is reinforced concrete and CMUs.)

[patamos]

Ya,

I get my clay bricks from old chimney tear downs. The thin slabs of mortar are usually portland-based. I use them and various concrete chunks here and there in shaping the benches and transitions. Dry-stacked urbanite is a great way to get out of the ground. As lousy as concrete is in high heat cycling applications, reusing it as mass fill can be a satisfying, at least for what we are sequestering from the waste stream…

With the jigsaw and cob mortar combo, just keep in mind that the cob will shrink a wee bit as it dries. So make sure the hard chunks of rock etc sit in a soft bed of cob mortar. If they are touching other solid material - even a pebble wedged between hard chunks… you may get a bit of shrink cracking. Not a big deal in your application. But more important where flue runs are involved.

[martinpolley]

patamos So far, I've done it without any rocks touching each other (and with a good handful or more of cob under each rock where it sits on the dried cob). That seems to be working pretty well so far—I haven't seen any cracking.

If I do add a heated bench later, I'm thinking of going low-tech and building the whole thing out of cob and rocks with channels inside, without any flue pipe. Maybe use adobe bricks to bridge the top of the channels. It's outdoors, so I don't think CO is an issue. Any tips? Is coating the inside of the channels with water glass a good idea? Or overkill for an outdoor thing?

[patamos]

I'd say the water glass is over kill. Adobe brick flues are fun but can be slow compared to starting with galvanized pipe as a permanent form.
As you build the flue use a mix of fine sand and clay (with a bit of screened horse manure) to close off the corners and round off the seams. Apply clay slip to the substrate first and keeping your hands a bit wet with clay slip will help things to shape smoothly.

have fun

[martinpolley]

patamos So if I just have the exhaust running into one end of the bench and out the other, in a straight line, with no bends, do I need to worry about the internal shape much? Do I need a chimney where it exits the bench?

[patamos]

A straight flue run = less resistance to flow, so that reduces the need for chimney height.
The issue will be on cold start up when expansion pressures up the heat riser are minimal.
You might want to experiment with chimney lengths once it is mostly built. Try a worst-case-scenario start up with inexperienced fire starters using slightly damp or green wood…

Have you considered using Matt Walker style half barrel bells to bring the flue gasses back towards the heat riser? This makes for the possibility of adding a bypass damper for start up. One idea there is having the gasses run first through a flue pipe to the far end of bell. In longer bells this can help even out the temperature. Alternately, you could stack a line of bricks along the centre line of the bell and have the gasses run along the front half first (where you may want the most heat to be radiating into gathering area) then turn around and come back along the back side. This makes for a sort of hybrid flue/bell harvesting.

[martinpolley]

Thanks patamos—some interesting ideas there. I think I'll try the suck-it-and-see approach that you suggested and see how it goes. Anyway, it's going to be a while till I get to the bench part—there's still lots of work to do on the oven itself.