Cob Bread Oven, Back Yard Sourced!





Introduction: Cob Bread Oven, Back Yard Sourced!

About: I am a computer nerd (CS degree by accident), bake bread, outdoors, build airplanes, balloons animals, photography. I have too many hobbies.

I was in Philadelphia for school for 4 years. Loved it. Great city, great bread. They say it is all about something the Schuylkill River water that makes the bread so great. Either way I decided I wanted to learn how to make sourdough! Why? I have no idea. Leave it up to me to move to Philadelphia, a city know for its great bread, and decide to learn to make the bread well know in the city 3000 miles away on the other side of the country! But hey, that's me!

When I moved back to my square state out in the middle (does it really matter which one? I learned in Philly that they are all the same.) I became even more obsessive with it and soon decided I wanted a brick oven. You know those brick oven's that look kinda like you your house, only smaller, and you light a wood fire on the inside. But then you get to put beautiful breads, whole pizzas, and all kinds of goodies that come out all blistered, browned, and perfect!

Then reality set in. My wife and I have only been planning on living in this current house for a few more years, and there was not way she was going to allow me to blow two grand, or whatever it would cost on a really nice brick oven! Even a more bohemian amalgamation was going to cost more than my very cheap (and my wife's frugal) sensibilities would allow! So I started looking at Cob.

Now we will get into what exactly cob is soon enough, but this is about my oven, and how I got to where it is. So I started looking around my house, and I figured for a few hundred bucks I could make a cob oven. Then I went from Mr cheapo, to just a little crazy, and started looking at how cheap could I build it?!!?

I bet I can build it with only the things I can find on my property!!!

Now you may think, because I live in one of the big square states out in the middle I live on sprawling acres and tons of natural resources. Well you would be wrong. I live in a regular house, on a regular plot in town, on Broadway to be exact! Now it is half an acre, so it isn't as small as say the 16 foot lot we owned in Philly, but it certainly isn't sprawling. Just a nice back yard.

And there it begins. Me, obsessed with building a bread oven, in my back yard, using only the supplies I could find in my back yard! Let the fun begin!!!

Step 1: Back Yard, in the Raw

So lets see... what do you need to build a cob oven?

  1. Clay
  2. Sand
  3. Straw

I pulled up a soil survey map of my county off the good ole inter-web, and I found that much of the soil around my town is clay. OK, so most of it was loam... but everything is some form of loam! But much of my loam's were high in clay. So clay is the biggest thing we need! Great!

So I went out to the back yard, and dug a hole. Got down a foot or so a sure enough it looks like clay! Great! We are doing great so far!

Now, if you want to get exact you can do some simple tests on the dirt. Basically put some dirt in a jar or water and shake it up. Sand falls down fast, clay last. Measure how much of each and you know how much clay or sand you have. I did this, but in the end this was not an exact oven by any means... and I pretty much just eyeballed the whole thing.

Sand, oh boy, that could certainly be a problem. There really isn't much sand around here, and most of that I know of is river sand... not the best for Cob (don't want nice rounded off sand, too slippery. You want the course jagged stuff so it will stick better). WAIT! When we first moved in we hauled in a huge truck load of sand to help level and aerate the soil (I knew we had clay soil!) And they dumped it all up at the front in a dead spot! Sure enough, all the sand I could want. Buckets of the stuff. It was also the spot were anything from the front half of the house washed out to... so there was quite a bit that had washed out to here as well. Great! That is the sand down!

Now for the straw. How the heck am I going to grow straw? Well, I chopped down some weeds, and grass and called it good. Great, so now we have straw. Now... lets see where to put this little baby!

Important points:

  1. I was about to find everything I needed in my back yard.

Step 2: The Mound

So I went on a little walk in the back yard (all 50 foot of it or so) and I had this mound of dirt. I have seen it before, and have no idea what it was from. Looks like the last owner had some dirt brought in and pilled up the leftovers here. It was not very big, just a few feet off the ground, but I thought it would make a perfect starting point. At least I would not have to get all the way down on my belly to get in it!.

Important points:

  1. I wanted it off the ground

Step 3: The Design

So I went back and forth on how to make this. The traditional method is to make a mound of sand. Well no way I had that much sand, but maybe dirt? But that seemed quite labor intensive, esp for what I was wanting.

Then I found my method!

First a little about my oven design. If you are wanting a oven that will stand the test of time, last years, bake dozens of loaves, huge pizzas, and really add value to your house... keep lookin. This one ain't it. Now if on the other hand you are camping for a few days, and want to make some homemade sourdough bread in an oven that you hope will last the week. This one is perfect! Why in the world would you want to make something so temporary in your back yard!?!!? Well, because I can. And more importantly because I can for not only cheap, not only with things I can scrounge, but with only things I can find on my property! It may not be your oven, but I love it. (And one of these days I will love several thousand dollar brick and mortar one even more!)

So image this. Take a sapling (a young tree... one that bends really easily). Bend in over and stick both ends in the ground about 3 foot apart. You have an arch right? Do that again with another stick that is at 90 degrees to the first. Keeping doing that, and put several on each side of the original largest ones, and now you have a dome! There is your dome mold for the inside of your oven! Cover with mud and you have an oven.

The great side of this design is you can build it, fire it (slowly at first) and start baking bread by the next day. It may crack, it may fall part, but it will work. The wood framework just burns away with your first firing. It is also very fast to put up.

On the down side it is difficult to make very large. And the inside of the oven will not be very smooth. It is also more difficult to get the door just right.

Overall this was clearly the design I wanted to go with. I had my materials, I had my location, I had my design... it was time to get dirty!

Important points:

  1. Wood frame
  2. Cob on top of wood
  3. wood burns away

Step 4: The Base

I mentioned earlier there was a mound of dirt where I started. I added some rocks around the edges to help support things, and leveled out the top with a high clay cob. How much clay? A good 60+% in my mind. and no grass. I kinda figured it would crack or something, but it has held up OK. I used an old piece of board I found to help level it. As this was going to be my baking surface I wanted to make sure it was as perfectly flat as I could get it. After it was level and smooth I got the wood wet and smoothed it out even more.

Also, make this part bigger than you think... once you put the oven on top it will end up being smaller than you think.

Important points:

  1. Higher clay content
  2. make it smooth
  3. make it bigger than you think

***************************NOT HOW I DID IT (but wished I had)*****************************************************

In the end the oven just sits on the ground. This makes a huge heat sink into the earth. There is just no way you are ever going to get this thing fully saturated with heat. It is all getting sucked down and out. If I were to do this again I would layer the floor with old glass bottles. The trapped air would help insulate the floor, and keep much of the heat from escaping. Then pile a few inches of mud on top of the bottles to form my base as above.

Step 5: Cob? What the Heck Is Cob?!?!

Cob? What the heck is cob? Cob is what you would think of as adobe, only in the mud form. Adobe is cob made into bricks. You use cob to make adobe bricks, and then you use the bricks to make adobe structures. In the case of cob building we are skipping the brick part and just making our structure with the cob itself.

The general formula is (VERY ballpark numbers) 40% clay, 60% sand, and the rest straw. Just enough straw that there is straw going through it all.

Clay (40%) You really want to try and get down below silt and organic matter and into the actual clay. It is usually not hard to find most places. The cleaner the better. I really thought this was mostly going to be clay, but clay really is a smaller portion. It is really the binder, but too much is in many ways worse that too little. If you use too much clay it will crack and fall apart.

Sand (60%) you don't want river sand. This has been rolled and is nice and smooth. You want rough sand with an edge, that will bite into the clay.

Straw How much? What kind? I am not sure how much it matters. I have read everything about the reason in an oven from "it does nothing past the drying, just burns up". To "it forms carbon fiber rods that are stronger than anything else you could put in the cob to give it strength". I'm not sure how much I buy the "space age technology in the hands of the caveman", but its kinda a neat idea. It does help things hold together and give it strength. So mix some straw in there. I got my straw mostly in the form of cut grass.

Water is another issue. You want to add just enough to hold things together. This isn't the mudpie you made when you were 3, too much water will cause problems. So we are making mud, but with just enough water to hold it together.

As for mixing... stomping with your feet really is the easiest way to go here. Watch out for sticks, rocks, or even hard straw. It is not good for the cob, and will hurt your feet!

Important points:

  1. Clay 40%
  2. Sand 60%
  3. Straw to make strawy
  4. Water, just enough to hold together

Step 6: Your Framed!

OK, these pictures really do not show it very well. I left al the leaves on. The leaves actually help to form a nice flat smooth area on the inside of the oven (I have a better method for doing that, we will get into that at the bottom).

I do want to give you a warning on the leaves. While the leaves can help with making a flat area, you want to be very careful to make sure leaves do not get incorporated into the cob. While a nice small piece of straw will hep give it strength; a leaf will cause a shear line, and cause the cob to break off.

So again make a frame by sticking the end of stick in the dirt around your formed base. Some of the sticks you see are ones stuck in the ground, but some of them are just woven between these anchor sticks. In fact I managed to find a long grapevine that I was able to weave into a lot of the spaces it the frame.

Don't forget to make the door. The door should should be 60% of the height of the dome (really 62% but this isn't an exacting structure). Don't forget as you are building this the top of the inside the dome will be the top of the outside of the frame. Depending on how exactly you do the door this could be the opposite.

Important points:

  1. Large saplings stuck in the ground form the frame
  2. Smaller branches and vines form the rest of the frame
  3. Door is 62% the height of the dome.

***************************NOT HOW I DID IT (but wished I had)*****************************************************

I really wanted to do this with all nature and found things. Next time I do one of these I will get a lot of newspapers and cover the whole thing with a thick layer of the newspaper. I think using newspapers you should be able to make the dome much smoother, and this will make the inside of your oven much smoother. Remember, it really does not mater how much you put in here. It is all going to burn off.

Step 7: First Layer

The first layer of cob is a fairly standard cob. You really want to pack on a good layer here, starting at the bottom and working your way up. You really don't want to pack it down super hard compacted, just together in one large mass. You want to make this layer a good 3-5". You also don't want to stop and let it dry. The structural integrity of the cob is reduced from areas that are allowed dry and continued with fresh.

This is going to be the real integrity of your oven. We will do other things, but this layer is by far the most important part of your oven.

If you are doing the "I am just making a oven for a few days while I go camping and then I will never see it again" oven. Then you are done. Start the firing slowly and slowly add a bigger and bigger fire. It should take about a day or so to dry out till it is ready to bake. But if you are going to continue to improve it keep reading!

Important points:

  1. 3-5" thick
  2. Do it all at once
  3. don't get any of the leaves mixed in with the cob
  4. You may be done, but more improvements ahead!

Step 8: A Little Insulation

Give the cob a few days to a few weeks to dry. Then you will want to add an insulating layer to help hold in more heat.

This next layer of cob is pretty much like regular cob, only A LOT more straw! Really there is likely more clay than normal. In fact it has very little structural strength because it is more straw than clay. This layer is just there to add a lot of pockets (formed by the straw) to hold air.

This is also the point I had to break with the "only on my property". I was using a lot of grass, but I couldn't get enough off my property. But next door there was a vacant lot where they cut the grass and did not bag it. So I went next door and raked several trash cans full of grass. It worked perfect! So yes, I broke my rule of only getting it off my property, but I could haven gotten it off my place, I just didn't have any at the time available.

I actually did two or three layers of the insulation cob.

Important points:

  1. Insulation layer
  2. more grass than clay

Step 9: Fire Fire FIRE!!!!

So give it a few days to dry still. If you fire it and there is still too much water in the cob you will convert that water to steam and cause the oven to crack and fall apart. So, while you can fire it right away, and it may do just fine, you are better off taking your time and letting it dry some on its own.

After I burned out some of the leaves and easy to burn things I actually put a candle in there a few days before the first big fire. In fact I keep any old candles my wife does not like the smell of, or is just done with. Anytime I have not used it in awhile I will burn a candle in there the day before I am planning on firing it up. This will start the drying process.

Then I will start a series of small fires in to help dry out as much as I can before I start the big fire.

Once you are ready to start a big fire start by building a fire in the mouth of the oven. Once this fire is going good push the whole fire deeper into the oven. Often during this start up time I have found a leaf blower, stationed several feet from the mouth of the oven, can help get things going faster. It is not needed, and I understand it is kinda cheating, but it can help. Especially if you are having friend over and just flat out didn't get the fire in the oven soon enough!

You will also notice a lot of smoke coming out from around the sides in some of the above pictures. This is where the sticks we used to form the structure where burning up. After the first full on fire this decreased, and I added more cob in these areas to help seal in the structure.

I took an old 1x12 that I had and cut it to fit the door. It isn't a super tight fit, but it works to hold the heat in. Just make sure you soak it in some water before you light up the fire!

Important points:

  1. Start your fire
  2. make a series of small drying fires
  3. A leaf blower can help at times.

Step 10: The Bread

This is the first loaves I baked in my cob oven. They certainly aren't as pretty as the ones I pull out of my oven in the kitchen, but they sure tasted great! Also a pizza. It was great! Loved the smokey flavor it added!

This style of oven does not get as hot as I had hoped. I think this is mostly because of loosing heat through the floor as we discussed earlier.

I also started off use whatever wood I could find on my property. Mostly limbs that fell. I have found that real logs work much better. They have a lot less sand and grit on them (that gets in the oven), but they also burn hotter and longer.

In proper brick ovens you can take all the fire out of the oven and you still can cook. This one you really need to have some fire going in order to bake. You also need to rotate you loaves in order to bake evenly.

Important points:

  1. It makes some great bread
  2. Real hard wood logs work best
  3. I keep fire or coals going when I cook, and rotate

Step 11: Taking Care

Now there are some great advantages to a cob oven, but there are also some negatives. The biggest negative is that is it basically just, well, mud. If you don't take care of it next time it rains you will truly end up with just a pile of mud!

There are fancy lime treatments that you can do, but I don't know about how to do that.

You can't do something like cement. It is very important for the oven to be able to breath. And by breath I mean the water in the walls need a way to escape when the inside heats up. If this water can't escape it will just build up steam and explode! OK, so it may not really explode (it could) but most likely it will just cause large cracks and chunks to fall off the oven. Either way things you don't want to happen! So cement it out.

One of the best things you can do is build a tin roof over the oven to keep the rain off, but that was not in the budget.

So in the end an old tarp was my solution! OK, so it was an old tarp, then another new tarp that was brown so it wasn't quite as obnoxious as the blue one was. I won't say it is the best solution, but it works.

One of the down sides it is not only keeps the rain out, but also holds the moisture in. So it really is a good idea to light a fire in there every so often, just to dry it out. Also don't forget to go slow if you haven't fired it up in a while. You don't want a rip roaring fire when it is full of moisture. You want a small one to help drive out the moisture slowly, then build you big fire after it has dried out. Again this is where old candles can help.

Important points:

  1. Keep the rain off!
  2. I just use a tarp and some stakes.
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    43 Discussions

    Actually, Oncer, cob is much different from Wattle & Daub (whixh is also a great name for a firm of lawyers representing parrots & people with double chins). Wattle is a woven mesh of skinny saplings and such. After it's in position, the daub is applied, hence W&D is the ancestor of lath & plaster. Cob is made of soil, clay, straw and perhaps other squishable materials; they are bunged together into a moldable condition that doesn't require wattle or other supporting structure. In short W&D is one material applied to another, and cob is one material that is molded rather than applied. Both are excellent building materials, used within their limits.They are literally dirt cheap and like adobe and rammed earth they are being revived to some extent by sustainable builders. Pretty soon we'll be seeing hay-bale houses again. I think some still exist in the prairie states, which had Great Plains grass by the long ton and damn few trees.

    4 replies

    Thank you... I didn't know that (Wattle & Daub).

    Cob took me a while to understand what exactly it was, my mind kept going to adobe.

    Here is the difference (in case someone out there does not get it)

    Cob you take the cob (straw, clay and sand) and build a structure.

    Adobe you take cob and form it into bricks, you then use the bricks to build a structure.

    Tlan: As I noted earlier, this is a good explanation. One reason many people are confused about this is that most if not all adobe houses do not look as if they are built with adobe bricks. That's because, in all example I've seen, the house exterior is finished with a smooth layer spread over the bricks, as is done with stucco. Rest assured that behind this mask is a lovely wall of adobe bricks!

    "Wattle and daub" is an expression used in Jamaica (among many places)

    to describe a type of building technique ... "wattle" are overlapped / intertwined branches or small diameter wooden laths. once the desired structural shape is achieved, it is sealed with the "daub" ..which is a mixture very similar to what you describe.... Some are sealed by the application of a plaster-of-paris coat on the exterior.. large roof overhangs prevent excessive water splash onto the walls ...

    That's seems right. Good explanation.

    Hi, have been researching this clay oven thing for years in Jamaica, there is coconut husk / fiber available..... could this be used in place of grass ??

    I have looked into this (friend wanted to do it). You are better off if you make a fire pit, and the flue is a chamber that holds the pottery. Make sure you insulate all around REALLY well. You can't have too much when trying to get that hot. But you can the block up the chimney to increase the heat in the firing box.

    Kinda picture this... two of my ovens stacked one on top of the other (the bottom one can be a dug out hole in the ground... it loosing heat is not that important) with a hole in the back connecting the two. that way you can really crank up a big fire, and get that center chamber really hot!

    Good luck.

    You would need to make some modifications to the airflow as pottery needs to get much hotter. Now cob, absoutly you can make a kiln, but you will need a way to get increased amount of air to the wood, and usually have a separate fire and pottery chamber. This (not my design, but it is was all done right) would only get up to about 1200 MAX! And you would really have to work to get it that hot. Usually it is going to be half that.

    Beautiful~ I sure want to build one of these~ Thanks for sharing such a wonderful tutorial~ Have a splendorous baking experience~


    1 reply

    Very inspiring, am venturing out into being part of building one in Scotland in the fall x

    Cob is great stuff. In England, many medieval houses are built of it and they're still standing today. As for venting, future builders will do well to find a real pizzeria with a wood-fired oven to see how it's vented. When such is at cooking heat, the flames do a mesmerizing slow waltz, the wood burns very slowly and the inside of the oven is coated with white ash. There is usually a closure to the front--just a simple flat piece of metal, not a hinged door, which is used a control venting.

    4 replies

    I got old 1x12 and cut it to the rough shape of the hole. Just make sure you soak it before you put it in the door!
    Thanks for the comment, there are plenty of improvements you can make on this design if you want to make a lasting high quality oven. But to spite the limitations I imposed with the design and raw materials it has served me well for 2-3 years now, and I look forward to using it every time.

    i'm lucky to have a friend from Northern New Mexico who built an adobe orno in my back yard. when i visited one of the pueblos i saw an orno used for demonstrations. the wood door was wrapped with several layers of burlap which was soaked in water before being used. the burlap offered a little 'cushion' which acted as a gasket.

    That is smart. I may have to try that as mine has some very large holes around the sides of my door. Thank you.

    Wattle and daub! Wattle and Daub! Sorry, just thinking what a mediaeval parrot might say instead of pieces of eight...

    The wide picture of the oven is without the finish layer. The closeup picture has the finish layer on it.

    3 replies

    beautiful. I assume you used a sand mound for the form?

    The walls look quite thin? How thick are they? (Any issues with heat loss. Or just keep a fire burning and you are good?)

    And here is the big question.... what did you use for the final cover? I tried just leaving mine exposed, and it became quite clear that it would not take too many rains to totally wash it out. Just one of the reasons I haven't really even tried to place a finishing pretty layer.... it just washes away anyway. I know cob can be quite resilient, but the rain really is my issue. Fired brick, metal roof, lime treatment, and I'm not sure what else all just didn't seem to fit what I was wanting. Yours looks to be raw cob exposed to the elements? Other than moving to the desert, how have you protected it?

    Yes, I did use a sand mound for the form. The picture may look as if the walls are thin but it must've been the angle. I'm including a couple more pics for you to see. It's about 6 inches thick which includes the base layer, the insulation layer, and the finish layer.

    I do not have an issue with heat loss due to the insulation layer in the dome, and glass bottles neatly tucked into more cob that formed the base below the fire bricks. It takes a good 48-72 hours (depending on outdoor temps) to go from 700° F down to "room temperature" inside the oven with the biggest drop happening in the first 24-36 hours. It's the benefit of this type of oven.....cook pizza, breads, then meats/vegetables, soups, stews, fish, baked goods such as cookies, cakes, or pies, finished with further drying out your next round of wood in a very low 150-200° oven. It utilizes the oven at all temperatures.

    My final cover is essentially the same cob mixture with more water added along with fresh horse (or cow) manure. The manure naturally adds VERY fine chopped up pieces of straw instead of the longer pieces used in the original cob mix. Mix it up so it's more of a slurry (like heavy cream or pancake batter consistency, and smear it all over. Once it starts to lighten up during the drying process, take a smoothing tool (plastic yogurt lids are great!) and give it a good rub all over. It'll smooth out and get a nice "shine" to it. You'll know what I mean. You'll see the golden flecks of straw really shine through. I didn't do the greatest job on my finish plaster but it looks decent, though rustic. I still use a plastic drop cloth and a blue or green tarp to keep it covered anytime rain is in the forecast. We had a freak rainstorm last summer that did "melt" a portion of my oven and I had to repair it. A total pain.

    Yea, that looks more right, it just looked too thin to insulate at all in one of those pics.
    Yeah, I loose a lot more heat that than. I have a good insulation layer over top, but there is zero insulation in the base, and it all gets sucked out quite quickly. Did you pack your bottles in a high straw insulation type cob, or just regular cob? I know mixing it with pearlite or something similar would help a lot as well, but I didn't anticipate just how much not having an insulation in the floor would matter. Wow, it really makes a big difference.

    I like the plastic lid idea... never heard that one. I did have a much smoother finish when I first finished it (not as nice as yours, but it didn't looks shaggy). But Oklahoma get pop up thunderstorms all the time, especially this time of year. I had left the top off and it all washed away. I re-insulated it, but have just never redone the finish layer. Maybe I'll do that sometime. But it pretty well stays under the tarp all the time (too many waking up at 2AM running out in the rain to cover it up! 2-3 times of that and you just leave the tarp on!)

    Thanks for the input... one of the best things I have found about writing an instructable is that other people teach me more about my project that I didn't know! And really, beautiful oven.,,,,,,,,,,