Cob fireplace, bottles, cans and bricks.

Dear permies,

I want to build a fireplace under an outdoor shed.
The goal is both open fire for Dutch Oven, cast iron kettle cooking and or bbq nights, you know, the good stuff.
On the end of the existing open shed, no walls just over roofed, this fire place is about to emerge.
The idea is to make a round-ish cob wall, place a roof with overhang on the wall and build a chimney on top of the roof. (Haven’t got a clue on the chimney construction yet, ideas are welcome)

Foundation is layed by some urbanite (smashed bricks) covered with cob.
Since the height of the wall is going to be close to 2.5 meters, I m going to need a fair amount of cob.
Reducing the total amount by adding bottles, cans or bricks in the wall is very tempting.
Since the cobwall is esentially a fireplace, pretty big temperature changes are to be expected.
Adding different materials in this wall, all having their own expansion/ shrinking rates concerns me slightly.
Especially given the fact that the wall is loadbarying for the roofstructure as well as a fair mass itself, I don’t want the risk of the walls falling down due to big cracks.
Hurting my toes or worse, spoiling my meal.
I can imagine that the base of the fireplace up to say 1 meter, takes the biggest temperature swings and therefore should be 100% cob. Above that I would love to use bottles and cans, even if its just for the beauty of the end result.

Hope anyone has some thoughts about this?

Thanks in advance and love to hear from you guys and girls.

Kind regards, Ivar

Hi Ivar,

sounds like a solid plan. Check out some of the cob fireplace/walls they have at cob cottage company like this one

https://www.google.com/search?q=fire+cob+cottage+company&rlz=1C1DIMA_enUS711US711&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi9kdHz1rngAhX1IjQIHaSxCFIQ_AUIDygC&biw=1920&bih=921#imgrc=91MJkZFET9AhVM:

you can bury all sorts of waste into your cob walls to take up room as long as it won't decompose or lose its shape over time. My first house I buried a broken toaster and waffle iron in the walls. extra urbanite is especially helpful for taking up room in cob walls.

For your chimney I would just find used 8-10" single wall chimney pipe and use it as a form for a cob chimney. You can get it set in place, then just cob around it.

https://cowichancoboven.weebly.com/

Good luck and post some awesome photos of your project!

A 2.5M wall is not too high for cob, just do it in layers.
I encourage people to install steel posts to hold the roof prior to cob construction, its gives shelter to
work under [ rain or sun], catches water you can use during building and gives you something to work on whilst waiting for the cob to dry.

Think about using different materials at the immeadiate fire spot, IE Steel plate, firebricks.
Also think of a Rumsford shaped firebox to maximise your output.
Another concept is to look at Rocket stoves / ovens.
I have buried many bottles in walls, I have a network of collectors.

Dear Daniel and John (and other permies)

Thanks for the reply, I have checked out the Rumsfort fire box but that doesn’t cut it for me.
The cast iron kettle is 40 cm in diameter and the grill grid is 120 cm x 60 cm. So the shallow fire box has to be at least 60 cm deep, inner size.
As you can see on the pictures, I have started the first batch of cob on top of the urbanite. I might decide to put some more cob on the wooden deck to ingrease the size of the of the actual pit, inner size.

That aside, my main concern stil is, the different expansion/ shrinking rates of the different materials I want to use in the cob wall.

Since glass and cob are both made of sand I expect not too much difference in those but still.

Daniel Ray, what a beautifull kitchen you guys made, big up!
Bit bigger of a project I have started but who knows in the future I might be able to pull something off like that.
Question, is the firebox below just heating the oven by radiation or are the fumes passing through the oven compartment?

I have made a temporary roof out of plastic because I ‘m not sure about the roof yet. I might opt for extending the current shelter roof.
I would much rather build it completely out of cob, chimney and all.
Hoping to seal the outside with boiled line sead oil for weather protection.
Given the fact it tends to rain quite a bit in the Netherlands that might be a bold or plain stupid choice.
It would look awesome though, even if its just for three years.

Ill udate when I’ve got some work done.
Pics are not spectaculair yet.

Thanks in advance.

Be good!

Update some more pics.
After almost a year finally got some work done.

Just to be sure I added some rebar-ish metal I found somewhere. The idea is that it would probably give some more strengt in the wall because the desired shape I have in mind is some what bottle shaped. I want the wall to lean over a bit before leaning back in.
Since I had the rebar laying around I fiffured It would hurt to put it in.

If any one strongly advices against it for good reasons its not too late to take it out yet.
Given the pass of the building tempo plenty of time for that.

Another thing I wonder about is the bottom grid, an old table surface made of a fine mazed metal grid.
I put it in there to build the fire upon, below the ash collector area.
Also I thought that it would give the fire plenty of acces to oxygen for a good burn. Now I m thinking isn't it too much oxygen for the fire. In other words would it in this setting be almost impossible to make a nice and low fire for slow cooking purposes. given the fact that the fire can maybe too easy suck in all the air it needs for a big intense burn...

Maybe I should put in a cob slab with just some holes for air intake instead of the all area open grid.



Kind regards.

Ivar

Thanks for the update Ivar. The rebar outlines what you have in mind really well. I don't think you'll have any issues with it as its buried deep inside the cob.
As for air control, you could cob in a curved wall to fill in the gap below the grate, then cut a hole which could have a simple adjustable damper for those times you want things just to simmer. Something like a cob oven door but hinged or a slider plate if you want to get more fancy. Keep us updated as we all like pictures!

Gerry Parent wrote:Thanks for the update Ivar. The rebar outlines what you have in mind really well. I don't think you'll have any issues with it as its buried deep inside the cob.
As for air control, you could cob in a curved wall to fill in the gap below the grate, then cut a hole which could have a simple adjustable damper for those times you want things just to simmer. Something like a cob oven door but hinged or a slider plate if you want to get more fancy. Keep us updated as we all like pictures!

Thanks Gerry for the reply.
Would you have any suggestions on what the minimum thickness of the wall should be in order to be safe?
Normally I would think at least 30 cm but given the fact this wall is reenforced with rebar I tend to think that it could be a bit less tick.

Any suggestions on that?

Kind regards...

30 cm seems like a reasonable thickness but I think as you go upwards, it could slowly taper off like a regularly build cob wall does. The top cold maybe be half of that if not less.
Not sure if you've mentioned whether or not your using any kind of firebrick or insulation around the fire area to keep the integrity of the cob as intact as possible as it is quite a high structure and needs a solid foundation.  
I'm just guessing on all of this as I have not done what you are attempting but I have built several rocket stoves out of cob before that have cracked and crumbled due to the heat so just want to make sure you stay safe and have a unit that can be enjoyed rather than repaired all too often.

Thanks for the reply.

I did not use any fire bricks or whatsoever for insulation, I figured that the mass of the cob would do that trick since the base of the structure is at least 1 meter 20 by 90 centimeter.
I did use some bricks in the base to fill up space. Being a lazy cobber. Or efficient, I always mix up these therms.
Since the project is moving on slowly, each time I add a new layer I fill in cracks of previous layers. Most of them don't reappear after drying.
Haven't stoke fire yet so I'll see what that does to my cob.
I'll keep ya'll posted.

Kind regards from the Netherlands.

Slowly but surely progressing...