14 June 2013

Build a Clay (Cob) Oven in Your Yard!

How to build a wood-fired pizza/bread oven using local natural materials

If you are looking for a small project to get your hands (and feet!) dirty testing out some natural building skills, then building a wood-fired oven is a great place to start.  If you have a little help, it takes just a couple days to build, then a few weeks to let it dry out (during which time, you can sculpt your oven to any shape), and then you're ready for a pizza party!!

What is a cob wood-fired pizza oven??  Well...it's a baking oven that is heated by lighting a fire inside, the fire warms up a thick clay oven wall, and the clay wall remains warm for hours after the fire is pulled out.  So you build the fire in the same oven area that becomes your baking space.  The beauty of this type of oven is a) the oven is simple to build using local, natural materials and b) the oven temperature remains very even throughout, with no hot or cold spots.  Plus, it's a fun project to do with a bunch of people and you can celebrate your accomplishment with a pizza party!



So, what do you need to know to ensure that your oven project is successful?


FIRST, get this book:

this book contains all of the information you
need to successfully build a cob oven.  Really!

SECOND, decide what size oven you want to build.

The appropriate size for you will depend on how you intend to use your oven.  Here are the variables effected by size:

  1. The larger the oven, the more materials you need to build it.  For example, an oven that is 36" wide inside takes about twice as much clay, sand, & straw as an oven that is 24" wide inside.  And more materials translates to more building time as well.
  2. The larger the oven, the longer it takes to heat up.  For example, a 24" wide oven takes about 2 hours of fire to heat up, whereas a 36" oven takes about 3 hours.
  3. The larger the oven, the more mass, soooooo, the longer the oven stays warm.  This means you can cook in it longer each time you fire it up.  Especially if you a good insulation layer on your oven.
  4. And obviously, the larger your oven, the more pizzas you can bake at once!  (Or whatever you are cooking...)
So, think about how long you want to wait for your oven to heat up, how long you want the oven to stay hot (larger oven for pizza party use, smaller oven for personal use), and think about how big of a project you want to take on (do you want to build a small oven over 2 days or go for a larger oven and spend longer to build)??
Typical sizes are 22-1/2", 27", or 36" (these sizes work out well with the size of standard fire brick).  Of course, there are mini ovens as well as massive ovens, but those are mostly for special use applications.

THIRD, decide if you will build a roof over your oven.

A roof will help protect your oven from the elements, and allows you to bake even when the weather is sucky.  Rain, especially, will erode a clay oven over time.  You can either allow that, replaster your oven every year, put a tarp over your oven when it's not in use, or....build a roof to protect it.  If you decide to build a roof, those materials will be in addition to those listed below.  Build your roof so you have plenty of room to stand underneath, and to clear any smoke out.  I recommend at least 7 feet of clearance under the roof.

FOURTH, gather your materials.

You will need the following materials to build your oven:

  1. Clay:  Clay is your essential ingredient, because it is the binder that holds all the materials together.  When wet, clay is sticky.  When it dries, it is strong & hard.  You can use clay-soil OR you can purchase dry, bagged, pottery clay.  (see video below on how to test your soil for clay content.)  If you are using clay soil, you will need to determine the proportion of clay in your soil (it may feel like it's 100% clay, but it rarely actually is...usually there is sand in there as well).
    HOW MUCH?  The amount of clay needed depends on the size oven you are building.  Here I am talking about total clay, so if you are using soil with clay in it, you will calculate the amount of clay based on the percentage of clay in the soil  (So, if your soil is 50% clay & 50% sand, then every bucket of soil = 1/2 bucket of clay & 1/2 bucket of sand.)  So total clay needed is about 25 gallons for a 22-1/2" oven, about 35 gallons for a 27" oven, and about 50 gallons for a 36" oven.
  2. Sand:  Sand is your aggregate.  It reduces shrinkage of the clay as it dries and it adds total strength to your oven walls.  You need to use angular sand, not smooth sand or silt.  Concrete sand is pretty cheap & works great.  I also use sand to build the form for the oven (this sand is taken out at the end and can be used to make plaster if you finish your oven that way).
    HOW MUCH?  Plan on about 300 to 500 lbs of sand if you are using clay soil with at least 50% sand content; if you are using bagged pottery clay, double the sand.
  3. Straw:  Straw is used to create an insulating layer for your oven.  It is also helpful to stand on the bales as your oven gets tall.  Make sure your straw is clean, dry, and mold-free.
    HOW MUCH?  You need about 2 to 3 strawbales for a small oven and 3 to 4 strawbales for a larger oven.  If you plan to sculpt your oven into a fun shape, make sure you have ample straw.
  4. Firebrick:  This is what I like to use for the floor of the oven, because they don't split in the heat of the fire and they have extremely squared edges, so they make a really smooth floor.  Typical firebrick are 4-1/2" x 9" x 1-1/2".  You can lay out the bricks for your desired oven size to see exactly how many you need, but below is what I use as a reference.
    HOW MUCH?  I use 15 firebrick for a 22-1/2" oven (12 for the floor + 3 for the door opening), 22 firebrick for a 27" oven (18 for the floor + 4 for the door opening), and 37 firebrick for a 36" oven (32 for the floor + 5 for the door opening).
  5. Water:  you will need a running water source to wet the clay binder.  (and is helpful for clean-up)
  6. Newspaper (optional):  I use this as a layer between my sand form & the first layer of clay...it lets you know when to stop digging out your sand so you don't accidentally gouge your oven wall.
  7. Stones or brick or urbanite (optional):  I recommend building your oven up on a base so you don't have to kneel on the ground to tend your fire & bake.  A comfortable height is typically 24" to 36" off the ground, but choose whatever height is comfortable to you.  You can use any kind of masonry material that is available to you, and you can make cob (clay, sand, and straw) to make a strong mortar.  Just make sure that your oven base is very stable.  Once the base is built & dry, don't forget to fill in the center (with something sturdy & non-compressible), so you have something solid to build your oven floor on.
Note: I highly recommend collecting extra material than you think you need so you don't run out of anything mid-stream.

How to test your soil to see if it has clay in it:


And this is my tools list when doing an oven workshop:

  1. buckets - I like to have ample 5-gallon buckets; you these to transport and measure your materials; to me, 5 buckets is a minimum, but if you are working alone, one bucket will work
  2. tarps - I like to have 2 tarps, but one works; 10' x 10' seems to be a manageable size
  3. shovels - if you are working alone, one shovel is fine; if you are going to have a bunch of people, have at least one shovel for sand & one for clay (more if you will have lots of helpers)
  4. sifter - if you are using clay soil dug out of the ground, I find it easiest to sift it roughly through a 1/2" screen to remove any rocks & to break up the clay and make it easier to mix; I like a table screen that fits over a wheelbarrow
  5. a wheelbarrow is useful to transport material, but is not essential


FINALLY, build your oven!

STEP ONE: Build your base

I like to make a shallow foundation filled with gravel to help keep the oven from moving with freeze-thaw cycles in the ground.

You can build your base with any masonry material - this example uses old cobblestones from the streets of Philadelphia, built with a cob (clay, sand & straw) mortar mixture.

STEP TWO: Fill in your base & build the oven floor

Fill in the base of the oven with a non-compressible material, like tamped gravel.  Then add a 4" or so layer of sand that extend just above the sides of your base.  Tamp and level that sand.

Then build your oven floor with the firebricks, making sure they are nice & tight to each other.  Tamp your bricks and make sure they are nice & level.  Spend some extra time here...the nicer your oven floor, the fewer nooks & crannies that will cause you potential headaches when you are baking.

STEP THREE: Build the form for your oven cavity
Basically, you are building the most boring sand castle ever: a nice dome.  The dome width is the diameter of baking space you want.  The height of the dome will be 75% of whatever your width is.

Add a layer of newspaper over your sand so that when you dig out the sand at the end, you know when to stop digging (before gauging your clay layer).

STEP FOUR: Build the clay mass layer of your oven
 
The first layer of your oven is cob without the straw, so just sand & clay.  You want enough clay so the mixture is sticky, but enough sand so that this layer doesn't shrink (and then crack) a lot.  For most types of clay, that means between 20% and 25% total clay content.  I like to make this layer 4" thick, all the way around your sand dome.  I do not worry about the door at this point...I carve that out later.

STEP FIVE: Add the insulation layer
Your insulation layer is mostly straw, with just enough clay to act like hairspray to hold the straw together.   The idea is to keep the heat inside the oven for as long as possible, increasing the efficiency.   What to do: use a clay mixture that is 50% clay & 50% sand (if that is the proportion in your soil, then you can just use your soil), wet the clay/sand mixture so it is quite soggy...like a chocolate milk shake, then add lots of loose straw, until all of the straw is coated with clay.  Then smoosh it in place.  I use about 6" thick of insulation.

STEP SIX: Cut the door

I like to sculpt the door in the insulating layer.  The straw has really good binding properties, which make it easy to sculpt a nice arch for your door.  If you are not doing a chimney, then the height of the door needs to be 2/3 as high as the inside dome.  This allows fresh oxygen-rich air to come in the bottom of your door opening to feed your fire and the exhaust exits the top of the door area without smothering the fire.  If you make a door ahead of time, you can use that as your template for sculpting & cutting your opening.

STEP SEVEN: Let the oven dry for a few days, then pull out the sand
There are a few strategies for taking out the sand.  I like to carve out the door the day the oven is finished, then pull out about 1/3 of the sand, and then let the oven sit for several days to let the clay mass layer dry out a bit.  Then you can pull out the remainder of the sand without risking any collapse of the dome.  But don't worry, if you pull out the sand too soon and part of it collapses, you can just patch it with the same materials.

STEP EIGHT: Sculpt your oven as desired
Use clay, sand, and lots of straw to make a firm, cob mixture that you can use to sculpt your oven into any shape that makes you happy.  If your oven has begun to dry out when you start sculpting, be sure to wet down the surface before you add your sculpted pieces, otherwise they will not bond to what is existing.  You can also put mosaic tile on your oven, or plaster it with clay or lime plaster.  (This photo shows a lime plaster that is fairly weather-resistant.)

STEP NINE: Let your oven dry out completely & then have a pizza party!
Fire the oven for 2 to 3 hours, with a nice hot fire.  Then let the fire go to embers.  If you made a fairly large oven, you can distribute the coals around the back perimeter of the oven to keep it hotter longer.  Otherwise, pull all of the fire out (into something non-flammable, non-meltable)  and clean the oven floor with a damp cotton mop or cotton T-shirt.
Put the door on for about 20 minutes to let the temperatures come to equilibrium, and then bake pizza, bread, casseroles, cookies, pies, and anything else your heart desires.  As the oven gets to around 100 degrees, you can even use it to culture yogurt.  And keep a well-fitting door on the oven to keep the heat in.




Still not clear?  Watch these videos that show you the step-by-step process for building a cob oven.

Building the floor of the oven & the sand mold for the oven cavity


Building the thermal mass layer (that will heat up when you build your fire) & the insulating layer that keeps the oven hot longer.


Digging out the sand form & baking your first pizza.


Happy baking!!

189 comments:

  1. Question for you - for the sculpted layer on top of the structure, you say to use "clay, sand & lots of straw." Do the ratios not matter here?

    Also, and presumably "yes" because you've done it here, it's ok to use a strong finishing agent like lime plaster on the final layer? This won't have a greater chance of cracking with weaker layers & permeability beneath it?

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    1. Ratios do matter a bit for your sculpting. I like about 25-30% clay content, and then enough straw that you can still work the clay really well, but have the fibers there for structure.

      For the lime, yes, it is ok for this application because the clay below is very thick. The problem you describe is what I have experienced with lime plaster over clay plaster (which is thin, maybe an inch or so). When doing lime plaster on the outside, I recommend at least 2 layers of plaster, about 1/4" to 3/8" thick.

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  2. Thank you for the great detailed instructions! Question - I have seen other instructions for cob ovens where it is recommended to put a layer of empty glass bottles below the oven for insulation. Is this necessary? Why don't you include the glass bottles in your instructions?

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    1. You can definitely put some kind of insulation underneath your oven floor, I have just found that most of the heat escapes up, not down, and so to me it is optional. But it probably would improve your performance to some degree.

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  3. excellent blog, very informative. thank you :) i have just built the base for mine and will work on the rest in the next few weeks

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  4. Hello, can you make suggestions for what kind of clay to buy if one lives on a gravel bed? At the fireplace store they sell some kind of masonry clay. Do you have any idea how that would work? Or pottery clay? I do have the book but havnt got the clay part figured out.
    Thanks so much!
    Jean in Mt

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    1. I have used pottery clay before, and that works fine. You can purchase bagged dry clay and rehydrate it with water...that's usually cheaper and a bit easier to work with than buying wet clay. Hope that helps!

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    2. Awesome instructions. How many bags of dry pottery clay would you estimate for a 36" oven?

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    3. wow, I have not idea...my preference is always site clay. The time I used pottery clay, it was because the person we were building for had some that they wanted to use up. You might contact Kiko Denzer who wrote the Cob Oven book...he may have done an oven with pottery clay. Sorry I don't know the answer to this one.

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  5. Great article. Can you use a layer of cob with straw for your base layer instead of sand for insulation?

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    1. I'm not sure I understand your question. If you mean the sand dome, that comes out in the end...it's just the formwork...so it can be made of anything that will hold its shape. If you mean the first clay layer, that wants to be a high thermal mass material, so clay + sand, so that it will hold the heat of the fire for a long time. The final layer is the straw + clay layer and that is your insulation. Hope this helps clarify.

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    2. I think Megan is referring to the sand beneath the firebricks. If so, the reason for using sand there is that the sand is loose and easy to make level, and easy to set the bricks into. It would be harder to get the bricks to make a nice even floor if you were laying them over cob.

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  6. we just completed a cob oven in Saskatchewan. we set the mosaic tile in to our final smooth layer of clay/sand, but they are falling out. should we have used adhesive or put another layer of clay/sand on top and then polished the individual tiles afterwards? I found your site very useful.

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    1. Sorry to hear about your oven. Sounds frustrating. Unfortunately there are a bunch of variables that could cause the failure, so I'd need to understand a bit more about it to help you problem-solve. 1) were the tiles textured on the backside or smooth? 2) can you describe exactly how you attached the tiles? 3) did you wet the previous surface before you put on the last layer? 4) did the tiles begin popping off right away or was it after a seasonal cycle (ie, did the oven go through a winter uncovered)? Hopefully we can sort out an answer for you!

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  7. Hello and good mornyng!
    We are yn the plannyng stages of buyldyng and earthen oven here yn western Mayne. The oven wyll be anywhere from 40 to 140 feet from the hygh flow peryod of ryver flow. Checyng the clay content today. Generally fayrly sandy around here although there ysd a nyce clay swath up the ryver banc.
    My questyon at thys tyme ys what would you suggest for gravel foundaton depth?
    We love your ynstructyons and have also just ordered Denzers booc for ynspyratyon. Oh man - good luc readyng thys!!!
    Thanc you ahead of tyme for readyng and answeryng thys. Holly Mae

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    1. I usually just do a 12" deep gravel depth under the oven.

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  8. oops - sorry- eye forgot to mentyon that my computer ys not typyng letters correctly so eye am ymprovyzyng.

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    1. This comment has been removed by the author.

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    2. tri moreyMay 29, 2016 at 1:58 AM
      Oh my gosh those letters! Ha ha ha. Only another Mainer. I'm in Maine too and the reason I am posting here, is because I really want to build one of these, but I understand they are a bit water soluble. We get between 8 and 9 months of winter here. I'm in "The County" (Aroostook) where we face 6-12 foot snow drifts. I have seen entire houses go under and people have to be dug out just to get out of their homes from their second story windows. We get an actual "mud season" that lasts a month... even up to two months in our wettest year, though it's been fairly dry the past couple. Does an oven like this even stand a chance in our climate? Hate to do all that work just to watch it turn into a really impressive mudball. A roof isn't an option... not so sure it would hold up through a winter anyway and same with the tarp. Any suggestions or ideas would be great. Perhaps I should settle on another building material. Really had my heart set on cob though. If anyone else, say in Canada north of us has had any luck with these, I'd love to hear from ya. Thanks a bunch. OH! I almost forgot... I'd heard that adding horse droppings to the mixture and letting it ferment for a few days before use helps with waterproofing. Any experience with this and do you think this might be a viable answer for an oven (I have only seen this method used for houses). I don't want my oven to smell bad enough to gag a maggot in a gut wagon either and I'd only be using this on the very outside of the oven, for obvious reasons. :/

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    3. Clay is not water soluble, but rather, prone to erosion when exposed to weather. If you cannot build a roof to protect your oven, then for your extreme climate I would not take on this project. There are ample examples of clay ovens in very cold climates, including to your north in Nova Scotia. But as with anything, you need to protect in a way that is appropriate for your climate.

      Regarding additives, horse manure adds fiber to your mix. What you are thinking of is cow manure, which has an enzyme that increases clay's ability to withstand weather. But nothing is magical, so cow manure alone will not give your oven a super-power to withstand your climate in the long term. Neither type of manure causes maggots or smell.

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  9. I started experimenting with my soil and am having a few problems, I think I have pure clay. It's mostly red and grey and won't break down easily even when I run over it with a tractor. Shaking it in a mason jar does nothing but make it easier to form by hand to mold. I used a paint mixer on a drill to break it up but takes hours for a small amount. Any suggestions? Rob from virginia

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    1. Not sure your question. Are you asking how to make your clay workable? If yes, then you can dig it up and soak it in a pit of water for a few days. If that's not your question, could you please clarify? Thanks.

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    2. Ok, got it, soak in water. So since it seems to be pure clay would the mix be 25% clay to 75% sand?

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    3. You have to test, I can't say for sure. Most clay soil isn't actually 100% clay and different clays have different stickinesses. Since both are variables, you have to test your materials to find the best proportions for your project. If you are just building an oven, then err on the side of higher clay content...ie, slightly stickier mix...for the thermal layer to make it easier to build. If you are building anything more structural, then make test bricks with various proportions, let them dry, and then break them to see which is strongest.

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    4. Thanks, made test clay 2 days ago using 25, 50, and 75% sand to pure clay. Total failure, my clay was to wet so I will do another test today.

      From reading your blog I made the following calculations, 32" diameter, 24" high with a door opening of 18". I will be using a chimney but am still trying to figure it out. Does that sound right?

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    5. Yes, those proportions sound good, and if you build like that you don't need the chimney. The height of the door allows fresh oxygenated air in at the bottom and exhaust air out at the top. The trick with a chimney (besides building it) is closing off the oven from heat loss once you are in baking mode. Good luck! Sounds like you are off on the right track!

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  10. Hi Sigi. Thanks so much for your videos. I decided on a whim to build a pizza oven in my backyard and I've watched them about 742 times each. Give or take a hundred views.

    I've built my base and am now at the cob mixing phase. I've discovered I will definitely have to go out and dig about twice as much clay as I thought I'd need. I'm also having trouble deciding what the mixture should look/feel like. At the moment it seems like 1 part clay to 1.5 parts sand might be right. Is there another test other than forming a ball of cob and dropping it to the ground to determine if the cob mixture is good? Thanks again. ~ karen!

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    1. the mixture should feel kind of like pie crust dough...sticky enough to hold together, but not so so sticky, and a bit on the dry side (wet enough to work it, but not so wet that it's gooey). If too wet, you just cant build vertically too fast, because it will start to slump on itself. If that happens, you can just wait a few hours or a day and then continue. For exact mixture, it's impossible to tell you what is right for your particular soil make-up. But you can make test bricks, let them dry, and see which is the strongest. But if this is just an oven, then you can just wing it. As long as you have enough clay that your mix is sticky, you are good, and too much clay will not be the end of the world. :-)

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    2. Thanks. Good reference points for me. I just like to have more than one idea to go by. Makes me feel safer. Especially when building a 500 lb semi-permanent object in my backyard. :) ~ karen

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  11. i also want to build a pizza in Australia so please if any provider of this service visit building insulation toowoomba.
    thanks

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  12. Thank you for this wonderful resource! I am building a cob oven with my third grade class at a Waldorf School in Charlotesville, VA. I have done all of the tests in Kiko Denzer's book, but am still feeling uncertain about what soil/sand mix ratio is the best. We have red clay that seems like pure clay. I have made bricks and balls at a variety of ratios, they all seem strong and dry hard. They all pass the drop ball test too. I was going to use 2.5 soil to 1.5 parts sand. Does that sound right?

    My next question is - can I spread the building process over three days? I would like to build the dome the first day, do the first layer of cob the second day and do the insulating layer the third day. Is it possible? Can I tarp it with plastic over the nights to keep it damp and workable?

    My third question - what if the temperature drops to freezing overnight? Should I also put a blanket over it to prevent it from freezing?

    Warmly,
    Dominique

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    1. It's difficult to recommend an exact proportion since there are so many different types of clay, and the exact makeup on clayey soil can vary greatly. If your soil is 50% or more clay content, then you probably need more sand than you are proposing. For the clay most common up here (a bit North of you), when the soil is 50% clay (do a shake test to find out), then I use at least 1 part sand for each part soil...meaning 25% maximum total clay content in the mix. That said, I would use the strongest test you made that had the most amount of sand in it.

      For timing, you can spread the project out over as many days as you like. The only challenge is keeping the sand dome intact, so you may need to touch that up a bit before your 1st layer. But otherwise, you'll be fine. In terms of freezing...you don't want wet clay to freeze at all. Water expands when frozen, so it will expand inside your cob and kindof explode apart. The cob takes weeks to dry, so I would keep that in mind in terms of your timing. If the temperature just drops to freezing and comes back up, a blanket or bubble wrap + a blanket should keep it from freezing.

      Hope that helps!
      Sigi

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  13. Hello again,
    Well, we finally have fattened or thickened the insulaton layer over say the top 3/4 of dome and it is actually drying. We scooped the sand out two days ago and stuck an electric heater, one of those little brown square ones you may have seen, set at 50 degrees. The dome did not fall down but does have cracks inside already, maybe 1/4 inch in a couple places. The reason i was scared it may fall down is because i put one small plastic bag in an area where the door is because i ran out of newpaper. Well the dense layer splooged quite a bit in that area, leaving above the door thin, which i didn't really understand till being just about finished with insulation layer (the first one) oh ya i told you that. The oven i s under a canopy and we put sides up on it at night running the grill on low nights that it goes below freezing. sort of crazy. never knew it would take this long to do this. far from my house ect. just takes time....
    We want to had some sculpting cob. I am assuming I should wet it a bit and maybe put some sloppy cob on for make it stick. I think also i will try and fill the cracks inside at some point before firing it up, when ever that is.
    My fear of course is adding another layer of wettish straw cob and at the same time trying to get it to dry because it is NOVEMBER IN MAINE!!!! Can one add straw clay for sculpting in the spring, even after possibly using it? This thing is a bit of a monstrosity! But i like it!
    Any input other than good luck?!
    Thank you for your input.
    Holly Mae

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    1. definitely you can do your light straw mix in the Spring. The only downside is that when you heat it up, it won't stay as hot as long, since no insulation on the outside. It sounds like your cracks are fairly significant if they are worrying you... So you can go ahead and fill them. You might use a bag filled with frosting-consistency cob (no straw) and cut a hole in one end of the bag, then squeeze the mixture into the holes, like a pastry bag concept. For temperature, you just want to keep the cob from freezing, so heater or blankets or whatever you have on hand to accomplish that is fine. As it dries out, you can build a mini fire inside to help finish up the drying process. All of the moisture has to escape before the oven walls can heat up significantly, so a low low low fire over several hours can help you significantly with this. Does that help?
      Sigi

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    2. yes, yes this does help. thank you. To clarify though, there is a layer of insulation over the dense cob. I did not sculpt into that. It is drying, has a ways to go but it is drying. We still have heater running in the enclosed area un der the canopy 24 / 7 and did two days ago start a small fire inside. Fun to see!
      So even though there is a layer of insulation i want to add a bit more in the spring and sculpt that. Too much insulation? Hope not. Hoping to wet the present layer in spring and add insulation and sculpt that. Hope that will work. Will the heat from the oven this winter affect how the cob added in the spring sticks i wonder.
      Great to see your wrkshop photos! Wish i was there!
      Thanks Sigi, Holly Mae

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  14. Hi! i am from canada, quebec. It's all frozen now, but i am starting my oven plan for spring.
    I saw your excellent videos on how to build the hay oven, which i plan to make. I do have some basic questions :

    Could i take "clay like" material from my property? it has a soil composed of clay in a percentage unknown... could i just use it, mixed with sand in a proportion that "feels good"?

    Also, on part 3 of your video, you seems to put fire right after the digging of the door... Did you give time to the oven to dry completely before setting fire or it is the fire that dries the clay?

    and at last, could it be a good alternative to use light bendable wood branches as structure instead of sand?

    thank you very much !
    Francois.

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    1. sorry i'm not good with blogs! i just read that you have a chapter on clay soil!
      thanks!

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    2. no worries. Let me know if you still have clay soil questions after you read through that. Also, to answer your question about drying the oven...yes, absolutely, step #9 above is to allow the oven to dry completely. The risk is additional cracking from the clay drying to quickly. The form for the dome can be anything really. The only caveat is that I would figure out a way to make it smooth-ish inside. Otherwise you will have turbulent air currents that could create odd exhaust patterns or uneven heat transfer. Hope that helps! Have fun building your oven!
      Sigi

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    3. ok! thank you very much, i passed too quickly on your step-by-step, it is well documented. it seems pretty straight forward, i cant wait to begin building it. Right now, it is 30 Celsius below zero, so i will wait few months!... if you don't mind, i contact you again then if i have questions!

      Best regards, and have a great holiday
      François

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  15. I love your videos! What is your recommended height for the inside of the dome for a 36" diameter oven?

    Thanks!

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  16. Thanks, glad you enjoyed the videos! The height of the interior is 75% of the interior diameter, so the height of the interior for 36" diameter is 27" tall.

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  17. I enjoyed the strategy to make the oven in home. I were thinking for buy a wood fired pizza oven for me. How will that If I brought Portable pizza oven for my home, rather then make it my self?.

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    Replies
    1. I can't speak to purchased ovens, since I have only used hand-made pizza ovens.

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  18. Hello again! i wrote to you in december, i was planning the build of a cob oven... in the middle of quebec's winter. It's now all ready!. The place is done, (roof, firewood, etc.) and the base is done. I made a 6"x6" lumber base that is a little more of 4 foot x 4 foot, at good height. I found salvaged red bricks to do a base with encapsuled bottles, and a baking surface of refractory brics (the only thing i bought so far) so it will make a decent room for pizza, i hope!

    I am writing today because this morning, i finally found my "clay mine", i dug a *lot* of natural, very gray and slippy clay from a small pond. (My kids even made some figurines and bowls that we hardened to red in the fire with it) My problem is to determine the amount of "not clay". I have read Kiko's book, and i am still not sure. Some parts seems pretty pure, some other got small gravel + sand into it... should i mix it all (there's a lot) to figure out? or should-i average in samples?. not sure. And also, i believe the whole mud is pretty wet and sticky, like nutella at some point, should i wait a little to make it dry, or adding sand would help to attain good texture?!

    at some point, i tend to use it as-is and see what happen. After all, everybody says that the second build is better

    ;)
    a lot of good and fun question, no stress about it.
    Anyways, i have friends coming next week end to do the dome, so we'll figure out. I'd love to send you picture to show you, one time, maybe.

    all the best,
    François.

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  19. UPDATE :

    I finally built it with sand form, wet thermal layer clay, wood chips+clay insulating layer and pure clay finish layer = i let it set for two weeks under a roof before diggint out the sand, so it when i did it, overall clay texture was hard and perfectly in form... I wish i could send you picture!

    all the best, François.

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    1. Nice! I'm so happy to hear your oven was a success! :)

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    2. François, are you baking yet? Your oven sounds nice. I built one in southern Arizona and have been having so much fun with it. I started a Facebook group and would invite anyone to join and post pictures. Sigi, I have already linked to this page, as it is extremely helpful. https://www.facebook.com/groups/mudovens/

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  20. Thanks! hi again Mrs. Koko, how are you!
    it's been 10 days since i dug out the sand, and i can still "feel" moisture in the cob wall, especially in the back wall of the dome... wondering... how do you tell it's completely dry? i set up few shy fires only to warm the whole thing, but i fear to proceed to a hell-fire and make crack in two... any suggestion?

    thanks in advance!

    All the best, Francois.


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    Replies
    1. It usually takes several weeks to dry completely. BUT, if you have lit small fires already, and those have not caused cracking, then you can continue to set those small fires to get the drying time down to a week or even less. What happens is that once the inside feels completely dry (ie, not damp inside) and you set a larger fire, the first thing that happens is that any remaining moisture is driven out of the cob before the oven will heat up properly. So your first big fire will take care of the moisture. It definitely won't crack in two! Worst case scenario if you set a larger fire too early, is that you get some internal cracking on the inside of the dome. And you can always patch those up with your same cob mixture (only mixing it wetter). Does that help?

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    2. Thanks again for your knowledge, yes it does help. Help me understand that i am too impatient to cook with it!. If i could, i would sleep there to watch it dry! ;) I was guessing that with this amount of clay, it would take long to dry. thank you to confirm it. I dit have a good number of "drying" fires and it did well. There was a 3/8" crack near the brick door, but i did as you suggested : it was filled with pure clay. The patch just disappeared under the black smoked coating. I guess the next step is to make bigger fires and get things going!

      I take a picture and post a link next time! Thanks again, your blog is really helping,

      All the best, François.

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  21. Hi,
    Our friend's cob oven is raining sand on their pizza! We were not sure if the ratio of clay/sand/straw was correct? Is there some way they could resurface the inside walls of the oven with a new (and better) mix of clay/sand/straw, so that the sand doesn't get release when heating and baking? Hopefully there is some way to fix this!
    Thank you!
    Amy

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    Replies
    1. I have 2 thoughts here. First, is to make sure that all of the sand from the sand dome was completely cleaned out of the inside of the oven. You can do this with a damp sponge, brushing the inside surface in circular motion, and cleaning the sponge often as you go. Second, if it is the cob mixture itself, then you can simply make a clay plaster and resurface the interior of the oven. See http://buildnaturally.blogspot.com/2011/08/clay-plastering-strawbales-second-coat.html for instructions (but eliminate the straw). Dampen the existing surface before applying the plaster, then let the plaster dry completely before firing up the oven again. Let me know if that works!

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  22. Hi Sigi
    Thankyou for your wonderful videos. I have watched them over and over again whilst we have been building our cob oven. We live in the south of France and our house is built out of the traditional cob method and is still standing after a hundred years.
    The question I would like to ask which is bothering me greatly is : when we put on our first inner layer after the sand dome our clay balls were wet but kept a firm hold whilst in our hands and when positioned in place on wall. So I have only put my insulated cob layer on thinly not like you had shown in video because I am worried that my inner layer will not dry out. I am relying on the sun very heavily to dry it. Then maybe in a week or two I will put another thick layer of straw cob on.
    My children keep telling me - when we pull the sand out the walls will collapse and I am now starting to worry myself. Am I fussing or do I need to worry?


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    Replies
    1. You can add the insulating layer in whatever timeframe makes you happy. For removing the sand, what I like to do is to remove about 1/3 of the sand soon after building. Then keep the door area open and that allows some airflow inside. As the cob dries, the sand will also dry, lose its shape, and fall down into a dry pile. As you get those dry piles, just pull them out, until the whole oven is clear. Usually takes about a week depending on weather, etc. I would definitely build your insulating straw layer before you pull the sand out. But other than that, if you pull the sand as it dries, you don't need to worry about a collapse.

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    2. Thanks Sigi. We have lovely sun at the moment so the cob is drying quite quick. So I will take some sand out and let the warm air flow into the opening.

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    3. perfect! Then as the sand dries and falls, you can just sweep it out. Could be a fun daily task for the kids...sweep out the dry sand... :)

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  23. Hi Sigi
    Today I did what you suggested and pulled just under half of the sand dome out.
    All seems fine apart from a few cracks. I assume reading other posts I will fill the gaps with pure clay once oven is fully dry. I think I will have to send my skinny son in for that job.
    I am leaving the other half of sand in until I have at least got another layer of straw cob on the back half of oven. I am now discovering all the lovely sculptures people are making their ovens into. Thanks again.

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    Replies
    1. Hi Susan,
      I think I wasn't clear...I would have waited to remove any sand until your entire outside layers are complete. So either...don't remove any more sand, wait until your first layer is at least leather hard, add your insulating layer, let it dry, and then continue your sand removal. OR, keep your plan for removing sand, let the oven dry completely completely completely (many weeks!), then add your insulating layer on over a totally dry oven.

      Hope that clarifies. And yes, your skinny son can fix the cracks inside...or you can just leave them if they don't bug you. Make sure to brush/wash all of the sand out from the surface inside or your first few pizzas will be crunchy.

      All the best, Sigi

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    2. Ok understood, but the bulk of the sand is still in the structure of the dome I have taken out the entrance and some more. I have a chimney with our oven and I have just cleared sand back to the beginning of the dome.
      Before I took out the sand I did put a 2 inch ish layer of straw/sand/ cob on. Is this enough insulation?
      I have started doing another 2 inches on the side of insulation as my outer layer i did a couple weeks ago has now dried. Only reached half way up oven wall so far. Shall I stop and not continue?
      The structure it's self is solid and firm. Now wondering if I should have left alone.

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    3. You're all good, no worries. Really clay is super duper forgiving. Just work slowly, and the inner form will tell you what to do. In terms of how much insulation...I like to have 1.5 times the thickness of the mass layer. The more insulation, the more heat stays in (assuming your mix is really straw coated with clay, not just cob with lots of straw). hope that makes sense.

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  24. Hi,
    I was going to use the bagged clay, but I wanted to verify the type of clay. I realize you said pottery clay, but would raw clay or fire clay be suffice? Or is it specifically pottery clay?
    Thanks

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    Replies
    1. any un-fired clay will do :) The only differences are color and stickiness. As a totally general rule of thumb, the lighter the clay color, the less sand you need to add. Definitely test various ratios of clay:sand to find the strongest that does not crack.

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    2. One more question I forgot to mention. The clay is sold in 50 lb bags, would that be enough for a 27" oven?

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    3. I'm not really sure how to give you a sense of quantity, because I never use powdered clay to build an oven (just for plasters). Sorry!

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  25. I lime plastered my oven on sunday and now its wed. it is lightly raining and the light wind keeps blowing my plastic off, has it been long enough to just let it be out in the air? I did not build a roof. I live in Eugene, OR. and of course we are headed into the rainy season. I also wanted to know when can I do the second coat?

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  26. The lime cures in a chemical reaction with CO2 in the air. The speed of that reaction is directly correlated to how warm it is...cooler air causes the lime to cure slowly, and the reaction stops altogether at around 40-degrees F. So you'll have to judge the curing of your plaster visually. It should feel very very hard. And it should be rather white in color (it gets whiter and whiter as it cures). In normal summer weather, you can apply a 2nd coat after a week. If it's cooler, you need to adjust the time between coats (ie, wait longer). Sorry I can't be more specific...

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  27. Sigi, I passed by a countertop and flooring place today that had a "Free Granite" sign out front. I stopped and picked up a piece that would be big enough for the entire floor of a mud oven. I already used firebricks for our oven, but want to make another oven for a friend. Do you think a slab of granite would work as an oven floor, or do you think it would break, or even possibly explode?

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    Replies
    1. It may work great, or it may break. Granite has veins in it, and those veins can be weak to pressure or temperature. You could try making a fire on the slab outside the oven, and see if it breaks. If it does break, you could fill in any cracks with clay...but granite is crystalline, so you may have jagged little bits to address. Most of the time you can see the veins. So if your slab is pretty uniform, I'd say your risk is pretty low. Does that help at all? Sorry I don't have a more definitive answer for you.

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    2. I ended up too worried about the granite breaking, so bought firebricks. I was able to get a section of very large-diameter drainage pipe to use as a base (we filled it with chunks of broken concrete, rocks, gravel, sand, etc). It is a very tough plastic. I don't think it will be a problem, since most of the heat travels up, not down, and since it is just the edges of the base. Here are some pictures of the finished base and floor: https://plus.google.com/photos/106162920569324161970/albums/6095763259435587025

      We did not set the bricks perfectly--they are smooth and level enough, but there are some gaps. I did not think it would be a problem so left it, but if you say take them out and redo them, we will follow your instructions!

      We will be covering the pipe with cob (may staple chicken wire or burlap or something to it to help), and I want to use a lime plaster for the oven and base. I have not done lime plaster before--do I need to buy some now and start slaking it, or is there something at the hardware store I can buy ready to use?

      Thanks for your inspiring and helpful article! Now that we have great weather in southern Arizona it is cob building time!

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    3. What an awesome oven you made! I love it!!

      So your base is completely fine. It won't get hot from the fire, and it if you coat it in a cob layer to the outside, it's totally protected anyway. I don't think you need chicken wire or burlap...just apply a fairly thick layer...maybe 4" or so. Whatever feels stable as you build. And add plenty of straw to knit that cob together. It will be very stable.

      For your fire brick...it's kindof up to you. If there are gaps, it's not really a big deal, since they fill in with ash from the fire over time. If there are big bumps between the bricks, it might be slightly annoying taking pizzas in and out. I would fire it up, make something in the oven, and see if it bugs you or not. If not, it's all good :)

      For lime plaster, it's a bit more technical...you can purchase type S lime from a hardware store, mix it with 3 parts sand to 1 part lime and add water to make a very thick plaster. Apply no more than 3/8" thick at a time or it won't cure properly.

      And thanks for sharing!

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  29. Hi Sigi my name is Catherine and I'm building a cob oven for a project at school. If I could get in touch with you that would be great. I would like to interview you on the process and how it turned out. My email is classencrew@gmail.com
    Thank you!

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  30. Hi Sigi,
    Thank you for sharing your knowledge of natural building! During step #4 the clay mass layer it begins to slump on itself and become larger at the base. This causes the layer to separate about 1/3 of the way from top in a horizontal fashion soon after the layer is complete. It is doing this before it is dry. We have tried dryer mixes and this did not help.
    Thanks

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    Replies
    1. Hi, I'm not clear what your actual question is...? I've never had the thermal layer pull away from the sand dome. It typically will slump and get fat at the bottom, but not actually lose contact with the sand. The thick part you can do any of the following remedies:
      1) simply let it slump, and then shave off the excess
      2) buttress the bottom (I've used a heavy canvas cloth tied around the base like a belt to hold it in, or you can use stones or bricks or anything that will keep it from splushing out)
      3) build more slowly...if you are building somewhere reasonably warm, you can built 12" in the morning, then 12" in the late afternoon, and then complete the remainder the following day.

      Does this help? If not, please clarify your question and I will do my best to answer it.

      Thanks, Sigi

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  31. Hi , we build a pizza oven last year where we used a layer of salt under the fire bricks this will retain the heat better then sand.(according to some Italian pizza oven builders) . for the first layer over the sand/newspaper we used pyrocreate this is normally used to build kilns and withstand temps of up to 1400 C. this stops sand dropping on the pizza nut expensive to use. Other tip is not to use a blower to speed up the process of heating the oven it will damage the oven I have seen it happen.
    Regards Rudy

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  32. Hi Sigi,

    Thanks so much for the site and blog.

    My oven is built and insulated, my next task will be to apply lime plaster. My cob seems like it might be just a bit too sandy. The strength of the dome seems fine, but when I brush the inside with my hand there are spots that seem to always dislodge a little sand. My worry is that I will have some sand spall onto my food. I have only had a couple of small fires, so I don't know how the inside will change as I really fire the oven.

    I figure I have two options: break it down and rebuild with a higher ratio of clay, or go ahead with the plastering and fix the standing oven if there are problems with sand.

    Two questions...

    Is the break down and rebuild as easy before and after plastering? I want to use the same materials if possible, especially for the cob.

    How do I fix sandy spots on the inside? I've read both that thin layers of clay slip can be used and lime plaster. I can't get my shoulders in the door because my oven is small, but I can reach most of the inside dome.

    Thanks,
    Curtis from North Alabama

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    1. Hi Curtis, and first of all, congrats on building your oven! Now for your sand problem. I would say that if the dome feels strong, then you will not need to knock it down and rebuild. But instead, address the sand by adjusting the surface. I wouldn't recommend lime on the inside, because your fire can get hot enough to turn it back into calcium oxide, and then you have a mess on your hands. Instead I would try 2 things. First, if you think your clay is pretty much dry, then start by lighting a larger fire in the oven. Light it a bit toward the back (not directly in the center). You want a big enough fire so the flames are at least 3/4 as high as the inside of the dome. Let this burn for an hour or two. It is possible that this will completely solve your problem, as it will kindof fire (like pottery) the inside of your oven wall. Once the oven totally cools down, dust off any remaining loose sand, and then see if the surface is stable (ie, no sand coming loose). If that doesn't quite do it, then try this... With the oven completely cooled down, dampen the inside surface really well (ideally spray it with water at least 2 or 3 times), until the inside feels pretty saturated. Then make a plaster, using the same clay that you used for the cob, but with a bit more clay. Then used that mix to apply about a 1/4" or so of plaster around at least the top part of your dome (where sand is likely to fall. Then let that dry, and rebuild the fire as before. That should get you where you want to be. Let me know how it works out! Best of luck.

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    2. Hi again Sigi,

      I took your advice, fired the oven long and hot several times, and for most of the interior, it has hardened up and doesn't seem to be a problem. There were still places where the walls felt overly sandy, and I think now that it was not only too much sand in the mix (I did the mix myself out of bagged fireclay - so it was a bunch of small batches), but I had not packed the cob in place well. These places became fairly deep holes in the dome, but your suggestion to wet the surface and fill in with a less sandy mix is what I have been working on. I did this slowly over the course of several weeks. It was actually kind of fun to come home from work every day and get to play with mud. When I would find a sandy spot, I would get out my water bottle and start squirting, the real problem areas would become very soft right away. I've got a much better dome now and have been lighting some small fires. I hope to get some good cooking practice in this fall. Thanks very much for the quick reply and the great advice!

      Curtis

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    3. That makes me so happy to hear! I'm glad you had fun with the process and have a spectacular oven. :) Enjoy baking!!

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    4. oven is working great, pizzas are fantastic, I've roasted a whole chicken and smoked a couple of pork shoulders.

      I lime plastered it a couple of months ago. Last fire I got some pretty big cracks in the plaster. Not too worried about it, I will fill them in when I get the opportunity. Is it possible for enough water to get in there to ruin the oven? Cracks are about 1/8". I can keep it covered with tarp in that case. But I'd like to not have that big ugly tarp sitting in my yard all the time.

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    5. yes, absolutely cover it. Anytime water can get in faster than it can get out (evaporate), you will have a build up of moisture below. Did you put on at least 2 coats of lime plaster? And is it over an insulating layer? I would recommend repairing the cracks before replastering, otherwise they will just redevelop in your new plaster.

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    6. Only one coat, and it's over an insulating layer. When I fire it really hot, the outside is warm to the touch. The inside keeps its heat pretty well. I have a just a little bit of lime paste left, probably enough mix up plaster and fill in the cracks, I will slake a batch over the next couple of months and put on more coats in the spring after patching cracks. Long term plan is some sort of roof, but I haven't figured out how I want to do it. I built it on the ground to save money. It's kind of a pain to cook and tend the fire at that level, so I may end up knocking it down after a year or so to put it up higher. Thanks so much for the advice, this site, the videos and the blog have really helped me out with this project!

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    7. So glad this all helped you! If you decide to keep this one, then filling/patching cracks plus a good second coat is the way to go. If you build a second oven, then plan on 2 coats of lime right off the bat (or build that roof). Sounds like you are getting great use out of the oven, even with it low to the ground. :)

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  33. Curtis, I don't think you have to rebuild. I suggest brushing the inside with a small hand broom or other soft brush to get the loosest sand, then don't worry about the rest. While baking the food is not touching the walls or top, so there is not much reason for the sand to come loose. You could always fire the oven good and hot for a few hours, then remove the fire and wipe the floor clean like you would for baking bread. Then let it sit for a few hours and look to see how much sand actually fell during that time. I am guessing it will not be a problem and that you will be enjoying baking very soon!

    For the record, my oven is over a year old and has been used quite heavily (usually once a week). It has no lime plaster and has suffered water damage plus some large cracks have developed. But we did a party and baked 20 pizzas the other night, and I don't think anyone had sand in their food.

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    1. Awesome advice! Thanks for posting :)

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  35. I am ready to put my final insulating cob layer on my oven and I have two questions - 1) my first layer cracked significantly overnight but I THINK I repaired them, and now I re-wet it and covered it tighter while I mix my insulating cob, was my mixture too clay heavy? and 2) I wondered about adding perlite to my insulating layer, I read something in Mother Earth news that this would help retain heat -from what I gathered I would add the perlite roughly like straw, have you ever heard of this? Thanks by the way for the videos, I've been 4 years in the "I want to build this" and collecting materials stage and now getting close to the "let's have pizza tonight" and it's thanks to people like you that are willing to share your knowledge :) oh, and I guess a 3) is there any reason I cannot decorate the outside with things like a grotto? bits of glass, etc.?

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    Replies
    1. if your clay cracked significantly enough to need repairing that fast, then it almost assuredly has too high a clay content. And the cracks will probably expand as the clay continues to dry out. What you will probably need to do is patch the cracks from the inside of the oven after you build the insulating layer and after you pull the sand out. You may even want to widen the cracks so they are "v" shaped and you can really pack enough clay into them from inside to fill the cracks. Otherwise, you run the risk of fire licking through the cracks and burning out the insulating layer. Which then comes to your second question. Yes you can use perlite, and it insulates just as well as the straw. And in your case, I would actually recommend the perlite because it won't burn out through any cracks (which straw can). In order to maximize your insulation, you will want to be sure that you are just coating the perlite with clay slip...not just adding perlite to cob. Otherwise you are really just putting more thermal mass on your oven. Good luck!! And have fun building.

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    3. one suggestion that may help. after making the V shaped marks also score the area around it making Xs. Then use vinegar to help bind new clay to dry clay inside. I use vinegar sometimes when doing pottery. when i leave a mug too long where it gets too dry and need to attach a handle to it. I score area where i attach the handle. Then make a slip(Watery clay) but instead of water use vinegar. then i spread the vinegar slip on scored area and attach handle. It helps bind the wetter handle clay to the drier mug clay. I also use it if a crack appears in the bottom of mugs to seal it up.

      So it may help to score the area around the cracks and inside them. Then spray with vinegar or make a vinegar slip. Then attach clay. Not sure if it will work on the pizza oven but it has done miracles when I go away for a week and come back to dry mugs with no handles yet. hope this helps.

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    4. thanks for sharing your pottery secrets!

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  36. Sigi, The other day I was looking at the straw wattle erosion control tubes they have at a construction site and wondered if that could be used as the insulation layer – my concern is that the stretchy tube is made of a synthetic, maybe some type of plastic. What are your thoughts?

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    1. I'm not really sure. I've never used them for anything, so can't speak from experience on this. I personally would avoid plastic completely, given the temperatures that the oven gets to. You could always try it and redo it if it doesn't work the way you want.

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  37. Looks like they are also available in tubes made of jute. I will look for an opportunity to try this out! http://jindaram.trustpass.alibaba.com/product/145912679-106983733/wattle_tube_for_erosion_control.html

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  38. I have a question. I am in the middle of making my pizza oven. I was wondering if it matters how wide the door is. I know the height of the door is 2/3 the height of the inside of the oven but wasn't sure about the width. Thanks for all the great info. Your website has been very helpful

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    1. Great question! Width doesn't really matter too much, at least for the airflow feeding the fire. I usually make the width just wider than whatever pizza peel will be used in the oven...this way it will be functional but the opening is the smallest size that will work (which means less heat loss during baking). Hope that helps! And enjoy your oven when it's done. :)

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    2. At what point do I need to cut the door before it's too hard to cut. I am starting the straw layer today but won't be able to finish it until next week. I hope that's ok. Also I don't have a pizza peel yet. What width would you suggest.

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    3. Easiest to cut while the clay is wet or damp. If you let it dry out, you will need a chisel...-ish
      For peels, it's really a personal preference. I use one that is 11 inches wide, but there are definitely wider ones. Really depends on what you are going to make.

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  39. I have another question. The first layer of clay/sand has been done for 2 days and today there is some significant cracking on the outside-(I haven't finished the clay/sand/straw layer) the crack goes from the outside right through to the sand dome. Would this be because there wasn't enough sand in mixture? I am planning on filling those cracks from the outside then once I take out the sand I will fill any from the inside as well. Is it normal to have some cracks on the outside. one crack is probably over a 1/4 inch. I'm kind of worried about this.

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  40. Just finished my Thermal layer on my pizza oven. Going to do the cob layer tomorrow and an outside clean layer the next day. Looking good so far. Your blog, videos and the this comment section where extremely helpful. Thank you!

    a suggestion for people having cracking. With pottery you want to dry pieces evenly and slowly to prevent cracking. the thicker a piece is the higher probability of cracks forming. I cover with plastic to slow down drying. With a cob oven having 4 inch thick walls for two layers so 8 inches or more total is going to have a high chance for cracks. You want all the layers to dry at close to the same time. If outside is drying fast while inside is drying slow then cracks will form on outside. Probably why its suggested to dig a 1/3 of sand out so inside is drying close to outside. Problem is you still have 8 inch think wall so the center of that wall is drying slower than the outside. If its hot and sun is beating down your outside layer is probably drying way faster then the center of the wall. If you are in a warm climate and/or its windy then I would try covering it with a tarp and/or shading it. Let the outside clay dry at a closer rate to inner clay. You could also spray outside layer with water every day or two to keep outside damper and closer to inside of wall. it will take longer to dry and start making pizzas but it could help reduce cracks. You could also cover it during day and remove at night. Maybe after it has gotten pretty solid remove tarp for rest of drying. I'm going to do this so I'll try to come back and let people know if it made any difference. Hope that helps.

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    1. so true, especially when there are different thicknesses all drying out. With the oven, if you build the insulation layer (high high straw content) right away, and then slowly remove the sand inside (ie, not all at once), then the clay dry-out speed is moderated and prevents cracking. Same concept as what you describe with the plastic. Definitely would love to hear how your oven goes!

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    2. I got some cracks the other day in my 1st layer. I noticed that it seemed to be from the clay sagging or slumping. I know the clay was probably too wet when I made the first layer. Also my dome may have been to vertical for the first foot. So my thinking is with gravity the wet clay sagged. As it did this it pulled apart the top layer causing it to rip apart at the top. I also noticed after a day the top of the dome was way thinner. Luckily it was so wet I mashed it back together. I also pressed a little straw into it to bind it. I figure it wasn't enough straw to insulate. As i put the insulating straw layer I noticed it started ripping at top again. Probably from the weight pulling it down. I noticed at this point that the first layer was only an inch thick. I took some more clay/sand mix and added it to the top and tried to make it more solid then quickly added the straw layer to help bind it. Hopefully this works. Won't know until i pull some of the sand out tomorrow.

      So I would say if big cracks are forming early on the top of dome it could be that the initial clay was too wet and/or maybe the sand dome should be more rounded. The wet clay is probably sagging down pulling the clay at top apart. I would definitely make sure the clay is much more dry next time. I got too impatient and didn't feel like waiting until it dried some more before applying

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    3. Wow, I have never had the clay flow like that. It sounds like a) the mix was super duper wet...like WET...and b) that there is not enough sand in the mixture. The total clay content should be around 25% to 30%. The rest is sand. The mix should be very stiff. Like cold cookie dough. Matters were probably exacerbated with the insulating...if your thermal layer was that wet, then it's better to wait and let it firm up and stabilize before you add the insulating layer. Otherwise you are just adding even more water to the already sticky situation. Also, if you are getting that kind of sagging and movement, I would NOT remove your sand yet. Let it all dry out and stabilize before you do anything else. Just let it sit for several days (or a week)...as long as it takes for it to all feel firm.

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    4. Also, the issue is not the shape. It should be nearly vertical for the bottom 1/2-ish of the dome.

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    5. Good to know about the shape being ok. I put 3 buckets of sand to 1 bucket of pottery clay so I should have had plenty of sand. I know i just mixed it to wet and got impatient about letting it dry a bit. For my final layer to cover straw i mixed it better and i should have had this consistency for the first layer. So mixing clay to wet and my impatience caused the problem. I'm definitely letting the oven dry a few days before I take sand out slowly. I'll probably have to patch the inside but I still have clay left. Will report anymore issues and how it turned out.

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    6. It's also possible that your clay is super sticky and could have handled even more sand. I've only had this be the case when using pottery clay...one pottery clay I used took 12 parts sand to 1 part clay. Not sure why it would be the case, but it has been my empirical experience. If you let it dry slowly (as you wrote above), your risk of cracking will of course be minimal. And you are correct, you can always patch the cracks. Just be sure to work the clay deep into the crack. I'm sure it will make yummy pizza/bread regardless! :)

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    7. thanks for the help. I did use reclaimed pottery clay that was given to me. Will give an update in a few weeks.

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  41. Dan, I have little experience with clay, but when I made my oven my first layer had a lot more sand than clay and was rather solid. I tried for enough water and clay to hold it all together, and basically tried to make it as firm feeling as possible. I did not get cracking on my inner layer, but later developed big cracks on my outer layer that seem mostly related to thermal expansion--the cracks widen significantly when the oven is hot. Thanks for sharing your experience and insights!

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    1. You're welcome. :) Thanks for sharing yours as well

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  42. Hi Sigi,

    Thanks for sharing your knowledge, the information here is so useful! I just finished a cob oven, and now I'm wondering when it's the best timing to do the lime plaster. Should I wait til the insulation and the 1st plaster layer dry? Or I should do it before they are fully dried?

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    1. I always wait until each layer has dried out before applying any finishes. The reason is that the clay shrinks as it dries, and that shrinkage could cause your plaster to crack. Once it's all dry (and you can use small fires to accelerate the drying process...just know that that increases risk of the cob cracking), then you dampen the surface with water and go ahead with at least 2 coats of lime plaster. The best way to really waterproof the oven, though, is a roof :)

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  43. Sigi, I have had some people ask me how much I would charge for building them an oven. I have only built two so far, and I have scrounged for free materials and recruited volunteers. I am having trouble gauging how much to charge if it were just me doing all the labor. There are certainly a lot of variables that would influence cost of materials, and the labor could also vary based on complexity of design, access to sand and clay, etc. But have you seen examples of people offering a basic cob oven for a set price? Just looking for any examples that could help me in setting a price. There seems to be a fair amount of people who would like a cob oven but who do not want to build one.

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    1. I think it would be great to offer a cob oven building service! I would also say that instead of looking outward, at what other people charge, I would think about what you want to offer as a package and what feels like truly fair payment for your work. Perhaps you offer the basic oven at a fixed price, but any sculpting is charged on an hourly/daily basis. Or perhaps you get paid a fixed fee for the oven, but then you can also offer a workshop and supplement your labor and income in exchange for teaching. Just be sure that you charge enough!! Building an oven is a lot of work. Don't short change your value! You might also contact Kiko Denzer who wrote the best book on cob ovens (link at the top of this article). He has built many many ovens and may give you more practical advice.

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  44. Hi Sigi! Great project,
    I am currently in Africa Tanzania. We are a bunch of volunteers there would love to build an oven! my question is, is there anything else you could use besides fire bricks as the floor? ..Hope the blog is still going (2016)

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    1. yes, there are other things you can use. Firebrick is useful because it can withstand the heat of the fire without cracking. If you are ok with cracking, you can use regular bricks. Or you can build the floor with clay, and let the heat of your first fire cure the surface. You may want to use a platter or something to cook on if you use clay as the floor (at least initially).

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  45. This comment has been removed by the author.

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  46. Thanks for this excellent blog and videos. I was surprised to see old time music friend Pete Hobbie (and others) in the video. Anyway, I am building a cob oven based mostly on your instructions. I plan to use bottle insulation beneath the base sand, and wonder if I need to do anything structural to protect them from crushing, or if they will be ok embedded in coarse gravel?

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    1. If you know Pete Hobbie et al, you might also like this video: https://youtu.be/RYWsY-I-mVk of them all playing together.

      Regarding your question...you don't need to protect the bottles from crushing, but do embed them in sand (you can do coarse gravel below and then sand above). Be sure they are open ended, though. If they are closed up, and then heat up, they may explode themselves (from expanding hot air)

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    2. Thanks. That is what I thought. I used to play with those guys all the time at Paul's cabin. Where did you build that strawbale house?

      BTW, I can't believe I've never met you. I was was executive director at Cacapon Institute for 15 years, and still work there part time.

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    3. Project was in Capon Bridge WV. (I live in PA.) Pete et al participated in a bunch of natural building workshops that I led to build a small strawbale cottage in the area.

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  47. One thing your blog does not show is laying the firebricks that extend out from the opening.

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    1. You lay them out the same way...tamping in the sand bed. And then build the cob mixture around them to keep them tight

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  48. Do you need to have insulation under the firebrick?

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    1. I find that it doesn't make much difference in the oven floor maintaining heat, so I do not put insulation below the firebrick. But it wouldn't hurt anything to do so.

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  49. Have you made an oven with 1.25 inch fire bricks? I am having a hard time finding any that are 2.5 inches thick. I don't know if they would be sturdy enough, especially for the ones that extend out of the opening.

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    1. excellent question! And your answer is yes, you can use the 1.25" fire bricks. At the front, just be sure to support them really well so they are not cantilevering. Otherwise, you are good to go :)

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  50. Ok, I have another question. I am at the point where I want to start making the oven itself. I have done a shake test and I came up with 25% for my clay content (first line was 1.5 inches, second was 0.5 inches higher). This would seem to imply that I should do two parts dirt to one part sand if I want 20% clay. I also made some test bricks with different ratios. I did a pure dirt, a one part dirt to one part sand, and a one part dirt to two part sand. The one to one seemed to be the best, although the two part sand brick was also pretty good. By my calculations the 1:1 is only 12.5% clay. I suppose I should have done a 2:1 brick but I made the bricks before the shake test.
    Anyway, my question is what do you think my ratio should be? Should I err on the side of more or less sand? Thanks for your opinion and for the info on this website.

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    1. So there is no way for me to assess your ingredients and say exactly what the ratio should be without being there and touching them. Every type of clay has different stickiness, and I've used clay that used over 90% sand and clay that used only 70% sand. And everything in between. I would do more tests. You want the strongest mix (that doesn't break, that you can't compress with your hand, and that doesn't dust sand off when dry) and that has the most amount of sand (while maintaining those characteristics listed above). hope that helps.

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    2. Thanks for all your help. I got the oven done and have made pizza and slow roast pork already. It was really enjoyable to build, I didn't have any issues with cracks. Now just to build a roof over it so it can survive the winter.

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    3. That's awesome! Thanks so much for letting me know your success. Enjoy that pizza!! :)

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  51. Hi Sigi-

    You did a workshop here in Madison a few years back and I built my oven last year! I got a lime plaster coat on it this summer. Hoping it will make it easier to get through the winter! Thinking of building a shelter of the top now to really protect it.

    Two questions:

    1, Any advice on using the oven in the winter in cold place (like Wisconsin)? Will I crack it?

    2. The oven is great, but a few repairs were needed due to charring on my insulation layer (this was before I put the plaster on). Is it possible I have some interior cracks I don't know about? There is some pitting of the interior surface of the dome, and a little flaking of the cob. I could add another layer of interior cob? Suggestions?

    Thanks-
    Claudio

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    1. Hi Claudio,
      So lovely to hear from you and I'm thrilled to hear you built your own oven! :)

      For protecting your oven, your basic principle is that you want to make sure the clay is dry when it freezes. The best way to do that is several coats of lime plaster (at least 2) and a roof. If you can't get the roof over the oven by winter, then you can cover the oven with a heavy duty tarp (not the blue ones) after you use it (since it's dry after you've had the oven hot). If you want to use it in winter, simply uncover, use as normal, and then be sure to recover it as soon as the oven cools down.

      The charring insulation is a slightly bigger issue, and yes, it indicates a crack on the inside that's big enough for flames to lick through. It would need to be bigger than a hairline crack...ie, big enough to stick a coin into. If it's burning into your insulation layer, then these cracks need to be filled. I know of one example (not one of my ovens) where the entire insulation layer burned out. (Of course the solution then is to patch the outside of the thermal layer and reapply the insulation. Simple solution, but kindof a pain.)

      The flaking isn't necessarily a problem, though it is a little unusual. Ultimately the clay interior should fire itself and get really hard. And then no flaking can occur. You can definitely add a cob/clay plaster to the inside. Just be sure to wet down the inside surface really well before you try to add new material. And if the inside surface is very smooth, you will need to rough it up in order to create a "key" for the new material to attach with the old.

      Final advice...not sure how big a fire you are building, but it doesn't need to be huge... Once you have it going, the flames only need to be 1/2 to 3/4 as high as the dome (max). And never should it be so big that the fire comes out the front of the oven.

      Hope that all helps! Let me know if you have follow-up questions. And have fun baking!

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  52. HI there
    I built a cob oven last year (not quite up to the standards that you have on your site, and not nearly as large!) but it worked out pretty good, with a few exceptions. I have some cracks that were created and it seems no matter what i do, the cracks keep reappearing in the same area.

    What is of greater concern is that the fire won’t stay lit. Could I possibly need a higher entrance? It’s about 11 inches high, and the oven itself is around 18 inches or so. The smoke will come out of the chimney that I made, but it also “pools” out from underneath the chimney. The oven creates a great amount of smoke. Initially it burns pretty good but it doesn’t take long before I’m fighting to keep it going. It’s possible that I’m using too big pieces of kindling or i’m just not letting it get big enough before pushing it to the back of the oven. I tried building it far in the back of the oven and lighting it that way and also tried keeping it near the entrance, but the fires continue to snuff out.

    I’m not keen on rebuiding it again, i’m hoping that with some modifications i can get it so that there is a nice roaring fire for a good hour or so that will get the oven really hot.

    One other thing, should i put another layer of mud on top of it again? It may repair all of the cracks and imperfections.
    If there are any suggestions you could offer, I'd certainly appreciate it!

    Thank you

    Peter

    https://www.dropbox.com/home?preview=IMG_1434.JPG
    https://www.dropbox.com/home?preview=IMG_1436.JPG
    https://www.dropbox.com/home?preview=IMG_1437.JPG

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    1. Hi, so I don't build with a chimney. I design the oven to have the proper proportions (as noted above in the article) to create good draft for the fire as well as maximizing heat transfer from the fire to the clay. So I can't speak to your chimney issue at all.

      For the fire...if it is not staying lit and you are using dry wood etc., then you don't have enough oxygen feeding the flame and/or the configuration of your oven is causing the smoke to extinguish the fire. I would start by increasing the height of your door so that it is 2/3 the height inside your oven. Most often, this will fix the draft problem. (Although I again can't speak to how your chimney configuration is effecting the draft.)

      As for cracking...if your substrate is cracked, then you will almost always see that crack telegraph into any additional layers. The crack would need to be filled an you'd need some kind of bridge material with fiber in it to sort of "tape" the crack. This isolates movement so it doesn't appear in subsequent coats.

      I would re-read the article above, get the book listed at the top if you don't have it already, then figure out what you did that is radically different from the descriptions. That will give you starting place for fixing (or make it clear whether it would be less work to start over).

      All the best to you.

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    2. Thanks for the reply, and actually I think I have it solved. I was using wood that was far too big, and I was likely being a bit impatient. I took my wood, and made really small kindling out of it. I started a small fire, and then continually added more pieces every 10-15 minutes. Once i did that and established a good base of embers, I was able to increase the size of pieces. I had a really good fire going last night, the inside of the oven was turning white hot, and i cooked a piece of toast in about 15 seconds! My biggest concern is the cracking. The inside of the oven is fine but it's the outer layer. As the oven heated up, the cracks got bigger. I thought I'd easily be able to patch it up today, but as the oven cooled, the crack shrunk again. I was actually hoping it was going to stay bigger becuase I think it would be easier to patch up. As it is now, it's a significant crack but too small to be able to really shove any clay into.

      I'll try to patch it up and see what happens. I think a rebuild is eventually in the works but for now, i'll let it ride.

      Thank you!

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    3. Glad you sorted out the fire :)

      For the cracks, they will always be bigger when the oven is hot because the clay actually expands with heat. So on option, is to squeeze wet cob (with screened aggregate so nothing is large) into the cracks while the oven is still hot. then you can patch above, ideally with either burlap strips to kindof "tape" the cracks or with a very high straw content (again to bridge the cracks). You can plaster over all of that once you've sorted out the cracking.

      Best of luck!

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    4. Ok thanks. I'll give that a try. One last question and i'll stop bothering you...should the outside of the oven be warm when it's fired? It's certainly not hot to the point where it would burn you (except where the cracks are, it's hotter in those areas) but it isn't cool to the touch either. I'm wondering if an entire new layer with high straw or wood shavings would hurt. (just need to find some more clay)

      thanks!

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    5. no bother :)
      The outside of the oven is most efficient if you include an insulating layer with high high straw content (called "light clay straw"). See step 5 above. If you don't have that insulation layer, or if there isn't much straw in it, then you are dissipating some of your heat energy unnecessarily.

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    6. Thank you. I actually do have an insulation layer but i'm thinking i didn't use enough straw/wood shavings. I'll think about that.

      Thanks for all your help from PEI!

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    7. ah, ok, so sounds like more straw is in order. But definitely get the cracks sorted first, otherwise you could end up smoldering the insulation when you have the fire roaring. (Or you can just fix your cracks and accept a little less efficiency.) Let me know how it goes!

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    8. Will do. Thank you for all the help/suggestions!

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  53. Hey Sigi,

    This is a great post. I appreciate you sharing. I have the Denzer book and have been reading it and re-reading it, learning lots.

    At my home in Vermont, I have a brook in the backyard with a pure clay outcropping. To give you an idea about the nature of this clay, it is blue/gray and usable out of the bank for firing pottery. I recently brought my students to my house to harvest some to bring to school to use. We sculpted it into bowls, plates, sculptures, and fired it in the kiln and it worked! Because of the ferric nature of the surrounding soil, the clay, after being fired, turns a terra cotta red, and it is very hard.

    So... A couple of questions:

    1. How much sand should I add to the clay before sculpting my first layer over the sand dome? The Denzer book is pretty quiet on the subject of "pure" clay and how to use it. The book talks mostly about clay in the soil, not pure clay, so far as I can tell.

    2. How many gallons of clay do you think I'd need for a roughly 30" diameter oven?

    Again, thanks for the post and I hope to hear from you!

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    1. Sounds like some lovey clay you have there! Nice. So the reason why the proportions are not totally clear in any book worth its salt, is because there are so so soooo many different kinds of clay. Each has different properties, specifically, varying stickiness. The stickier the clay, the more sand you need. The more expansive the clay, the more sand you need. Roughly, most clays want about 70-75% sand (including any silt or sand in your clay). But you need to do some test bricks to see what proportions will be the strongest to work with.

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    2. Thanks so much, Sigi!

      The clay I can harvest is immediately sticky and roll-able into worms that hold form. That said...

      What would be your best suggestion on how to test any bricks I make?

      Should the ratio of sand to clay be by weight or volume?

      Again, many thanks. I am looking forward to doing some testing!

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    3. for your test bricks, make a variety of different proportions. Usually the optimal average total clay is around 25-30%, however it can be as high as 50% and as low as 7-10%, depending on the clay. So make a range of proportions. Make the bricks about the size & proportion of a small loaf of bread (think banana bread). If they are too flat, you can get a false read for strength. Then you are going to break them by dropping them (without throwing) from about waist height. Whatever breaks is weak. You want the highest percentage of sand (lowest clay) that doesn't break. Hope that helps.

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  54. Ben, I wish I had clay like that! One thing I learned, which was not obvious to me at first, is that the sand should be coarse. I was living in an area with very fine sand and I had to find a riverbed of coarse sand, or you can just buy coarse sand easily from a cement factory (I think they call it masonry sand). You could measure your ratios by either volume or weight--whichever would be most consistent for you (if by weight you have to be careful about consistent moisture levels, so most people use volume). It's not rocket science--get a yogurt container or something and scoop one part clay plus 2 parts sand, mix well with a bit of water, then form a "brick" (approximately). Next 1:3, then 1:4, then 1:5, then 1:6. You will probably have a good guess just by the feel of mixing them together. You want something that feels firm and strong and not crumbly. Since you are mixing more materials compared to your first brick you can also take a gob, make a ball, and drop from a consistent height onto pavement. Too much clay and it will kind of mush; too much sand and it will kind of explode. Maybe you find that 1:3 and 1:4 both seem pretty good and you could try 1:3.5. If you want to get fancy then make your bricks in a wooden mold so they are all the same size and then you can measure how much shrinkage they experience (you want minimal shrinkage).

    I'm really jealous of that clay! I moved from endless fine desert sand to endless limestone where I can't seem to find sand or clay!!

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  55. I have a 4 year old cob oven that has produced many dozens of pizzas. In general the oven is holding up very well under the roof that we have built. But I do have 2 issues, 10 there are 2 cracks in the struck.. they let out steam and dont seem to have any affect in the integrity of the structure. Should I take any action to fill in the cracks. 2) the surface plaster has begun the crack and peel away from portions of the oven. I think this is mostly a matter of aesthetics and I am concerned that if I try to pull it off and replaster I might harm a very well working oven. Any advice on what I can do?

    Thanks,

    Rich

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    1. If you are losing steam, you are losing heat...which means the oven is not getting as hot as it could and not staying hot as long. Kindof like baking in the kitchen oven with the door open. Also, there is a high probability that the cracks developing now will get worse, especially if your plaster is peeling off. (That's a sign of a bonding break between the plaster and the oven...which could mean you are getting water behind the plaster.) So, I would fix the cracks... This involves widening the cracks into a "v" shape, dampening the clay around the cracks really well, and then applying new plaster into the cracks. You might want to use a squeegie bag...like what pastry chefs use...to really get the clay deep into the crack. Let it dry very slowly so your fill has the best chance of staying bonded to the existing oven walls.

      hope this helps

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  56. Hi Sigi,

    Greetings from England! I have nearly completed my cob oven, the design of which incorporated many ideas from your blog and from others I found on the Net. I would like to finish the oven with a more weather resistant layer than the clay cob (I would like to avoid building a roof if possible). Having read your post on lime plaster, I don't think this is the way to go because the clay plaster on top of the straw cob is quite thin. I was thinking of putting on another layer of clay plaster (1:1) and then using a clay paint when it is dry. My question is: How weather resistant would this be and would it do away with the need for a roof? BTW I live in NW England, which has quite a wet climate but little frost/snow in winter.

    Many thanks for your imparting your knowledge to the greater world - I hope you can help me out here!

    Regards,

    John

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    1. clay plaster will definitely erode over time and likely require annual replastering. Also, if the clay is damp/wet and then it freezes, it will kindof explode off the oven (because water is larger frozen than liquid and the water freezes inside the clay). So, I would either build a roof, or do 2 to 3 coats of quality lime plaster, or I would tarp the oven really really well after each use to keep it dry.

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    2. Hi Sigi,

      Thanks for the prompt response. My intention is to make the oven sufficiently weatherproof so that with the winter temperatures we experience here it is unlikely that freezing will be a major issue.

      Would your reply imply that you don't recommend using clay paint on top of clay plaster because that would also erode at the same rate? Also, from your post on lime plaster, I understood that if lime plaster was applied over clay it would crack quite quickly. Is this not the case on a cob oven?

      Kind regards,

      John

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    3. clay will definitely not waterproof your oven, it will definitely erode and definitely get damp (if then also freezes, the clay will fail). Lime over just thin lime plaster is not good. However, lime over thick cob is fine. The reason is that there is less measurable expansion/contraction (due to changes in moisture) when you have a lot of clay behind the lime. They are totally different scenarios. In order of what will work best in the longterm, here are your options:

      1. build a roof (this is the only guaranteed way to protect your oven)
      2. shape your oven so that it sheds water really really really well and then apply 2 to 3 coats of high quality lime plaster
      3. shape your oven so that it sheds water really really well and then apply clay plaster with cow manure in it and seal it with linseed oil (or other hardening oil) -- note that this option has a failure risk within the first year

      Hope that helps clarify

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  57. Hi, I am a bit confused about the type of firebrick used. I see people refer to both insulating (light) brick and conductive (heavy) brick as "firebrick". Which do I need for the oven? I've been reading about these and rocket mass heaters simultaneously and am confused.

    TIA

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    1. either will work. Dense firebrick is more durable, especially with a wood fire on it. More info here:
      https://en.wikipedia.org/wiki/Fire_brick

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  58. Hi Sigi,
    My name is Patricia. We live in Spain. We built our cob oven following your excellent video tutorials, however significantly smaller than yours.It measures 44cm in diameter. It works well in that it is easy to light a fire in it and it has great airflow. I can easily get the temperature up to over 600 fahrenheit (350 Celsius). But the problem is that the temperature falls drastically when I start to cook in it and I just can't get to cook a whole lot of things in it. I can do bread and I then dry herb, or pizza and dry herb, but not much more than that. My question is: perhaps the oven is just too small? Another question is: can I add another layer of insulation now that the whole oven is completely dry? We really enjoyed making it, thanks for the inspiration!!

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    1. Hi Patricia,
      Sounds like you made a lovely oven! And yes, what you are experiencing is the limitation of a small oven. Picture that the thermal mass is a battery. The smaller the battery, the faster it charges (ie, the smaller the oven mass, the faster it heats)...but also, the smaller the battery the faster the charge dissipates (ie, the smaller the oven, the faster it cools down). You could help this by adding additional insulation, and yes, you can add more insulation any time, even after the oven has been used (and is completely dry). Hope that helps! I love that you are using your oven!!

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  59. Hi Sigi,
    I am begnning to plan for my oven for next Spring and am confused about estimating the quantities. Kiko Denzer in his book mentions that for a 22.5" oven I'll need 8 five-gal buckets of mud and 8 of insulation. If soil is 50/50 clay/sand then I'll need a total 8 buckets of sand and 8 of soil for a 3:1 mix. I'll also need 4 buckets for the sand form. So 12 buckets of sand would weigh about 900 lbs if each weighs 75lbs as Denzer suggests. You mention only 300-500 lbs.. I don't mind having some extra but don't want to waste material or get more than I need. Can you help?

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    1. I'm terrible at quantities...I generally just dig a pit, test the soil, figure out what the cob recipe will be, and then start building... Sorry!

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  60. Thanks. Got it. My other question is about the soil test... i did the worm test and the soil holds well and is very sticky, as if it has a good amount of clay, but the jar test is not clear: about 1/2 inch settled pretty quickly (some sand and coarse soil), then about 24 hours later about 2 inches of very fine soil settled (silt?) but the water remained very cloudy... after a week there is a small 1/8" settlement of whitish sediment (clay?) on top of the second layer but still coudiness in the water, although it began to separate into fairly clear water up top and a clear delineation of darker brown water in the middle, as if more clay was slowly settling...hard to explain...how should I interpret? Please note I am in Northern NJ. I'll try to post some pics.

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    1. any sand and silt drops out quickly. Whatever settled after 1 minute or so is all clay. Not sure what the whitish sediment is, unless you have 2 different clays in your sample or high mineral content in the water. Your best bet is to make some samples of whatever material you are using the clay for...plaster, cob, adobe, etc. Use different proportions of sand & straw in your samples. And find a mix recipe that is strong without cracking or dusting. That will be more informative than the shake test in your case.

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  61. Thanks, so then it seems I have about 50% clay.... I also made a number of test bricks with different proportions... I assume these need to dry several weeks before testing them?
    One more question: after you fill your base with gravel and add a few inches of sand.. won't all the sand start dripping down through the gravel and eventually the fire brick floor will collapse? do you put anything between the gravel and the sand?
    Thanks again for all your help.

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    1. yes, let your test bricks dry completely before testing. As for the sand, no it does not drip down through through the gravel. At first a little, but then it locks in place. But if you were worried about it, you could always lay some filter fabric between the gravel and the sand.

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  62. Hey Sigi

    Thank you very very much for this article. It is amazing that you have put up such a detailed explanation of how to construct the oven out of cob. Very helpful for everyone over the world :D

    We are currently working with a 36 inch oven in india and we were wondering if its possible to use another material rather than firebricks? Can we use clay itself? If so we would have to maintain the floor of the oven regularly with clay and water. We wanted to know your opinion on the matter.

    Look forward to hearing from you.

    Dom

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    1. sure, you can make the floor of your oven with clay. It may crack a bit from the heat of the fire, but you can use the ash to fill the cracks. The one thing to watch out for, is make sure that your floor mix isn't too sandy. Otherwise you'll get sand in any bread or pizza baked directly on the surface.

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    2. update on the clay base : we made a 45x50 block with 2 layers of clay + straw = 3" thick and then a top layer of clay (pure + refined )= 2" thick. The clay obviously cracked like the australian desert floor. We found the best way to fill the cracks was with sand and ash from bbq's. We are about the cover the filled cracks with thin a layer of clay.

      We wanted to know your opinion our progress

      thanks

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    3. Nicely done! For cracks, I would just fill with ash from your fire coals. don't add sand or you may get sand in anything you bake directly on the surface (like pizza). And I wouldn't do the thin layer of pure clay, or you're going to be fixing cracks all over again.

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  63. Yo Sigi!

    We're building in the foothills of the Sierra Nevada, Colombia.

    Tons of clay in the soil. Access to sand by the river. But no firebrick.

    Can you throw out some alternative hearth suggestions?



    A high sand cob recipe? Ash & clay? What did our ancestors use before precision cut firebrick?

    Thanks!

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    1. Hi, just answered this exact same question above. Hope that helps answer your question too.

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  64. Hi, great article and info! Got a question for you, we are in Belize and they make their tradition oven with fire ash and clay and cement. You ever heard of this? Just thinking it might be good to add (hardwood) ash into the mix as will be using clay ( no firebricks) on floor and also in first layer?

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    1. I have never used ash as an additive, but you might experiment with it. I would avoid the cement, though. It will impede the bonding of the clay and could therefore cause cracking

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  65. Hi Sigi Koko I am from INDIA, Mumbai. I have been following up on ur videos on YouTube, I really admire your work. I am planning to build my cob oven, but a bit larger one 40" diameter . I have completed my foundation, insulating sub floor and laying of firebricks. I live near seashore so we have sandy soil but I have found clay on beaches seems like pure clay .. has anyone ever used this kind of clay it's dark gray in color..And and have also purchased dry fireclay 4 bags approx 160kg.. what ratios should I use pls help
    Regards Silvan Richard Dsilva-

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    1. there are many different kinds of clay and what defines clay is that it is sticky. The color just indicates which minerals are present. You have to do tests to determine the exact ratios you need for strong cob with your clay...that's because every type of clay is a bit different.

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  66. Hello, I appreciated your knowledge sharing in word and videos. I'm planning to build an oven have two questions, so I’m interested in your opinion:
    1. I bought a one-piece slab of travertine from an estate sale to use for the oven floor. Do you think it's a bad idea?
    2. we're thinking of building a firebox UNDER the dome... meaning that there would be two stories/floors for the oven. The lower would be built directly on the ground (travertine slab as the base, and old red clay bricks as the walls) as the wood burning compartment. The upper story would be where the food is baked. The floor here would be fire bricks, and the dome would be sculpted using clay. The brick floor for the upper compartment would have an opening (about one third the floor size) in the back to allow heat to rise from the firebox under and into the dome. I have seen this design used in north African countries and I guess the benefit would be the ability to add wood as needed from the bottom while baking in the dome. Thoughts? Pros/cons? thanks, again

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    1. Hi, I'm not sure I totally understand your construction. I've certainly never built a double-decker oven, so my first suggestion would be to try it! Worst that happens is that it doesn't work and you either will tweak it or rebuild it. As for the travertine, I wouldn't use that in direct contact with the fire, as it will have a high probability of cracking from the extreme heat.

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  67. Thanks Sigi I have completed the thermal layer of my oven and removed the sand form and it's holding pretty good should I wait for the layer to dry naturally or can I make small fire to speed up the process . And by the way I used 3:1 ratio .. should I wait to put my insulation layer ,. One more question about insulation is hay sand and clay mix better than sawdust and clayslip.?

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    1. If your mass layer is holding and no longer damp, then you can go ahead with the insulation anytime. (I actually install the insulation right away, and then slowly remove the sand as the whole thing dries.) You can build a small fire inside to speed it up, but I would stress the small part...just enough to create some warmth. For your insulation, whether you use hay, straw, sawdust, or woodchips all performs about the same...as long as you aren't adding too much slip.

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  68. Hi Sigi,
    Your blog is so helpful! I was wondering though, how you adhere mosaic tile to the finished oven? I saw from an earlier query that someone tried putting the tiles directly into the finished oven, but the tiles fell out. I'm glad I read that, as that is what I was going to do. So, for Plan B, what would you recommend to be the best way to adhere the mosaic tiles? Thanks for your help - it is greatly appreciated! With Best Wishes, Cynthia

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    1. I would plan on 2 to 3 coats of lime plaster over your oven. And before you put the final plaster coat on, use the lime plaster to make a sticky mastic to glue your tiles to the previous coat of cured lime plaster. Then use your finish coat to grout between the tiles and plaster whatever is not tiled.

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  69. Sigi, finally Spring is here and I started preparing to build the oven (base completed and all brick tests done last October).
    Regarding the lime plaster as my finishing coat, I've had the putty mixed and in buckets with the recommended inch or so of water "aging" in my basement since October. When it cones time to mix to make the paster with sand, do you also add the short fibers (manure, straw, etc) recommended by Denzer and other sites I've see? You don't seem to use fibers in your mix. And if I use manure, can it be fresh horse manure or does it need to be dry?
    Thanks!

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    1. I only ever mix sand with lime...1 part lime 3 parts sand. So I can't speak to using fiber with lime...

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