Using Cob for the outer layer

I was wondering if anyone has had experience using cob for the outer layer of their mason heater. My husband and I built the inner layer out of firebrick last fall and used it through winter. I would like to start the outer layer this summer, hopefully using cob with a plaster finish. Working with these materials is new to me. I have had a really hard time finding descriptive information on how to build a mason stove so needless to say it has been a learning experience. I know cob is used frequently when building a rocket mass heater but I have not seen it in the building of mason heaters.

Any experiences, stories, suggestions, recipes would be much appreciated. In addition, any advice on the plaster finish would be helpful as well.

I am so happy to have been referred to this website. I have found many of the posts to be very helpful!


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I’ve faced a couple of heaters with earth/cob and have been documenting some tricks to help it go smoothly. I’ve been making a mix of what I consider “thermal cob” which is essentially concrete grade sand with a little bit of road base gravel and just enough subsoil clay to hold it together. I mix it in a large mortar mixer with paddles because the mix is so stiff its daunting to attempt it by foot or by drill and paddle. I pack it around the heater like I would around the sand form of an earthen oven. I still have used cardboard up to this point for wrapping the heater core prior to this earth facing, but i’m not sure its necessary… i’ve found the cob shrinks away from the core as it dries, leaving its own slip joint.

I then fire the heater as hot as I consider reasonable for several days to really charge it up and look for cracks before putting on a finish coat of plaster. I’ve also used linseed oil to seal it like an earthen floor, but this is best done during the summer months when the oil has time to cure and not applied during the winter when the heater is always warm and prevents the oil from polymerizing properly.

There’s not a lot of tricks to an earth facing, its really quite straight forward… however, there is one head scratcher… the mounting of the doors. Ash/soot cleanout doors mount easily enough straight into a thermal cob facing. Big heavy hinged firing doors seem treacherous just to lag screw into cob. I’d love to hear what others have done for mounting doors, but I’ve come to frame out the door area(s) with brick that are pinned together and wired around the back of the heater. This detail has taken a bit of work and I’m not sure I’ve perfected it. I can share photos of that part of the process if its of help.

I appreciate the information. I too was trying to find creative ways to attach the door. To get us through winter I built a frame for the door out of brick. Unfortunately I mortared the bricks directly to the firebrick (this was before I knew about keeping space between the two). Of course the brick frame pulled away from the firebrick, so my husband had to wire the door to the heater to keep it from completely falling off. It was quite unsettling. I plan to redo it this summer. The doors are very heavy and I too have my doubts about screwing into cob. I’d love to see the pictures of your door.

What has been your experience using a regular cob mix of clay soil, sand, and grass? Did it hold heat?