When dry, cob becomes rock-solid to frame a rocket mass heater

Cob is an amazing material – once it dries, it turns into a rock-solid structure that holds everything together, providing both strength and durability.

We take a closer look at how cob works its magic in rocket mass heaters. From starting as a water-soluble clay to becoming a solid rock, cob plays a crucial role in locking everything into place. We’ll show you how we apply it, rub it into a soup of clay to make it stick, and use it to hold everything, from the barrel lid to the heater structure, securely together.

Here’s what we’ll cover:

• How cob transforms into a strong, solid material
• Why properly applying cob makes all the difference in strength
• Locking the cob into place for long-lasting stability
• Aesthetic improvements after the functional build

Cob behaves differently than fire brick when heated and cooled. How do you prevent cracking and deformation if the inner fire brick core and outer cob shell are joint together forming "one solid rock"? I don't see an expansion gap being used in this build.

I see the benefit of cob holding the core structure together, but what about this method holding up after several heating and cooling cycles?

Appreciate your feedback permies!

Hey Michael.
Good question!
The method I’m familiar with is heating the rocket up as soon as possible, so while the cob still has some plasticity. This way it adjusts to the other parts in their expanded state and very limited cracks should appear.
If you can’t do it that way you need expansion joints. A lot of people here use some super wool between different materials. Often where metal meets cob.

Benjamin, thank you. Makes sense keeping the caveat you mentioned.

Going further, I wonder if using loose sifted wood ash placed in between the brick core and cob shell whilst progressing with every lift of cob would work just as well.

Hi Michael. I imagine the application of the wood ash would be rather complicated.
If you get the ash on top of the cob I don’t know wether the next layer would adhere properly.
Also in this particular build the core needs the weight and the bracing by the cob, so I don’t think ash would work in this case here.

So I understand the fire bricks are just loosely stacked against each other tightly without mortar or adhesive and the cob keeps the structure together. Am I correct?

Thanks

Not generally. The firebricks or inner layer of masonry of whatever kind is usually mortared, either with refractory cement as code dictates or with clay-sand mortar as many builders recommend.

Another possibility for the combustion core is a steel frame that compresses the bricks together,often with a thin layer of ceramic fiber in joints to seal and absorb irregularities. This is sometimes called a tension frame (I guess because the steel is in tension), but the important part is the compression of the bricks.

All clear. Thank you!

A layer of wood ash might work to separate the cob from the inner layer of bricks if you could keep the ash against the brick, but that is not a practical thing to do as it would just fall down in a heap, not stay in a vertical layer.

Hi Permies,

I`m not sure I should post in this thread however the question is related to cob.

A while ago I came across Carl Boehm`s build of a rocket https://www.youtube.com/watch?v=Oh47Jk3I7AU&list=PL9TTdzWoNjzd8_OVJSJlm2UAhTd_m_Ec1&index=1 and his solution of using materials (angle iron, superwool, fire brick + concrete slab) to construct the bell (12min 45sec into the video).

I`m comparing this solution to one I came across watching a Polish builder https://www.youtube.com/watch?v=S80JytfptFs&t=1965s who basically swapped the fire brick with cob (32min 10sec into the video). Instead of covering this with the concrete slab like in Carl`s build, he then covered with fire brick. The whole stove was later plastered. Would anyone be able to assess the durability of cob + fire brick right above the riser and if this would hold up against time and if there are any significant limitations?

Appreciate any feedback.

If you are using firebrick already, it only makes sense to use that as the first layer in the ceiling above the riser. Firebrick will be the most durable material you can get. Cob may be durable depending on the specific character of the clay.

Hi builders!

Kirk "Donkey" says he uses neolithic materials for the burn chamber in his batch box "Tiny House Rocket Mass Heater" https://www.youtube.com/watch?v=U7kqu1XvsXY&t=5s that is crushed fire brick, wood ash, cow dung and clay. Does anyone know the proportions he used?

Also, are there any proportions that should be maintained in terms of mass of his stratification bell vs the burn chamber or riser diameter?

Any help appreciated!

Keeping the thread alive with yet another question!

Looking at the batch box 6inch system dimensions as an example (8 10 /16 box width, 12 15/16 height, 17  4/16 depth), would the modification/ increase of width by 10-15% impact the performance in any negative way? Has anyone tinkered with the proportions to a tolerable extent?

The Wife is lobbying for a wider glass door to better resemble a fireplace insert for aesthetics, thus the question :]
I`m actually planning to do a 7 or 8 inch build, but the question is just about the proportions.

Merry Christmas to you all Permies!

Hi Michael;
Peter has allowed a modification to the published box length, making it easier to load long wood.
Any other size modifications should not be made.
Peter tried everything and only got consistent performance with repeatable emission readings when using the published numbers.
If you wish to experiment, we encourage it, but do not expect the out-of-the-box performance you would get by sticking with tried-and-true.

Consider building a Shorty core rather than a first-generation core.
Shorty utilizes an airframe to supply fresh air to the fire; her door allows the use of an 8x8 glass for a 7x7 viewport.
A  7" or an 8" core would use a larger door and a larger window.
 

That's true, but I think that 10% increase in width should not be detrimental.
My oven being an equivalent of 200 mm with different proportions (w=400 mm, h=260 mm) and rectangular (400x100 mm) riser burns, roars with double vortex so well and it's smoke free. I may have been lucky (I have tested it dry stacked before building), or clean smoke may be not sufficient indicator of clean burning, but at the same time majority of builders do not run meticulous Testo analysis and rely on the smoke visuals.
I think the large BBR can be more tolerant to changes, because power rises to the power of 3 of dimension and the drag to the power of 2, so bigger is more forgiving - it just burns like a blast furnace and cares less. Despite having different proportions I have afjusted the port size to match fi 200. In your case, increasing the width by 10% will increase the fuel load by the same amount but with the same port and riser. It may still affect the burning process to a lesser extent than for example loading the firebox full with very dense (0.8) wood versus very light (0.4).

Saying all of it I do not want to take any responsibility, because your approach, materials, fuel will or may be different than mine. I would recommend to build a dry stacked prototype first. It' s quick and fun. If it works - it will work even better if build with no air gaps and good chimney.