Building an Earthen (Cob) Oven

As resident bread baker, I’ve wanted to build a cob oven for years. I finally got started in early 2010. Then I got a bad concussion that injured my neck, and I had to put the project on hold for a year. Now, at last, I’m back to playing in the mud. It is so much fun. If I could make a living playing in mud, I would switch careers in a flash.

Cob Oven

This is the oven with the final insulating coat. All the oven needs is the final plaster and cob around the door.

What I love about cob/earthen ovens (also called hornos here in the southwest) is that you can (sort of) make them from whatever soil you have available, or scavenge materials to make an almost free oven. The ovens are energy efficient (where wood is available) and can provide a cooking source if the power goes out. Plus you get that delicious brick-oven baked flavor!

GETTING STARTED
1. Start with Kiko Denzer’s Building an Earthen Oven. I read this for years, dreaming about my own oven, before I actually got started. Really. Buy the latest edition. If you’re in Albuquerque, you can get the older edition from the library.

2. Site your oven and gather materials. Kiko’s book goes into detail here, but some pointers: have the oven near your kitchen for convenience, door of oven should be facing away from prevailing winds (here that’s largely west and south), have a water source nearby in the unlikely event of fire.

MATERIALS
CLAY: Our soil is very sandy, so I bought clay from New Mexico Clay on Girard. I tried scavenging clay from the river and friends’ yards, but simply buying 25# of clay was easier and less expensive (considering all the gasoline I was using driving to spots to look for clay). Your soil may have enough clay in it already. Our soil is like beach sand.

SAND: You also need sand. For us, no problem. Your soil may also have plenty of sand. If you have to buy it, you probably don’t need much, just go to one of the big suppliers on Coors or Broadway. A more expensive option is to buy it at a Big Box Hardware store, but that works too.

Soil Test

Use the jar test to easily see how much sand vs. silt vs. clay you have in your soil.

To test your soil, fill a mason jar halfway with soil (save your precious topsoil for the garden!). Fill with water. Shake, shake, shake. The sand will filter out first, then silt, then clay. It takes what seems like forever for clay to completely filter out of the water, but you get the picture fairly quickly. More details are found in Kiko’s book (Do I keep saying that? It’s for a reason. Get the book!).

FIREBRICKS: I called everywhere looking for local firebricks. This is the one item I wanted to buy instead of using old bricks. You can use old bricks, but they may not hold up to the intense firing of these ovens. NM Clay had them for 6 or 7 dollars each, but lucky for us we have a brick manufacturer here in Albuquerque on South Broadway, Kinney Brick. The firebricks were just over a dollar apiece. Nice! When I have time, I’ll rummage in my notes to figure exactly what these are called -they have different kinds of firebricks. Folks at Kinney are helpful and friendly.

INSULATING, FOUNDATION & FILL MATERIALS: I used glass bottles, dryer lint, and cob for insulation; for the foundation, I used “urbanite” or broken pieces of concrete edging we had here on our property (we have more than we’ll be breaking up eventually in case anyone wants it). Concrete is very heavy, but it’s not hard to find rubble that people just throw out. Put out the word with friends. You can also buy a brick or concrete block. See Kiko’s book for other ideas. For the fill, I used broken up pieces of concrete waste from demo project here, plus a ton of ugly white landscaping rock.

STRAW: You will need straw for your cob (you could also use dried grasses, but straw is easiest). We always have straw bales lying around from various projects, and had some from insulating our goat hutches from the below zero wind chills we had this winter. The goats and chickens had torn up the straw bales and chopped them into pieces for us. Perfect! Plus the straw was mixed with goat manure -even better (see Kiko’s book or any other book that talks cob for the benefits of adding horse, cow or goat manure to your mix). You’ll probably need a bale of straw. I don’t know how much we’ve used, because we simply scoop it from the goat pen.

TOOLS: 5-gallon buckets (get them free from deli/bakery departments), wheelbarrow, tarp. The basic garden tools most people have on hand are all you need.

AMOUNTS/RATIOS: Kiko’s book goes into detail about how many buckets of this or that you will need. Buy the book or get it from your library. It’s well worth the $10 or $20. No, I’m not getting a commission!

Step 3. Dig your foundation, test your soil. Since our soil is very sandy, I didn’t dig down to the frost line. I  just dug maybe 4 inches to create a rubble trench foundation to secure the urbanite. It’s the clay in soil that causes so much expansion and contraction with water (never rains here) and freezing. If you have a lot of clay and want your oven to more permanent than temporary, then do a rubble trench to the frost line (18″ here in ABQ with climate change?)

Cob Oven Foundation

I dry stacked the urbanite (concrete curbing) and filled with rubble.

4. Create your foundation. Use any materials for this. You can build your oven much closer to the ground than I did, but as a farmer, my back is tired enough as it is to have to bend over. If it was on the ground, I would be worried about the dogs getting into the oven. Build your foundation “one over two” like you’re laying bricks. I dry stacked the urbanite, but you could also use some sort of mortar, whether commercial or earthen. Use your imagination. Fill the center with rubble. This is where being a hoarder comes in handy. I had been saving rubble from various projects since we moved to our new place, much to the concern of my partner (are you ever going to use that?). You don’t want to fill the center with earth or cob because you want a break between the oven and soil, so moisture doesn’t seep up and wick away heat when you’re firing your oven.

Cob Oven Bottle Insulation

Set your bottles in cob, fairly close together to create an insulating layer.

5. Add insulating layer. I use bottles set in cob. Put a thin layer of cob/mud mix over the fill, just to make it level, then set empty bottles as closely together as possible. I also used dryer lint and animal hair in the mud. Don’t have any bottles? Come down to my neighborhood, you can collect them from the sides of the road. Or ask your friends to save them for you. Or go to a bar and ask for empties. You don’t need many. Cob over the bottles, so that the bottles are just barely showing. I actually ended up cobbing over this insulating layer and creating a second layer because I wanted my oven opening to be at a more comfortable height.

bottle fill cob closeup

Here is a closeup of the bottles being set into cob. You can see the straw strands in the mud.

6. Creating your Cob/Mud Mix. Look at Kiko’s book or any other cob book for details on making cob (Ianto Evan’s Cob Building Book, Earth Bag Building, Building Green. All of these books are available from our library. My ratio is sort of 1 bucket soil (I ended finding an area of fill dirt that had a higher clay content than our native soil), 1 quart powdered clay, 4 huge handfuls of chopped straw/manure.

How to make cob: put the soil in a wheelbarrow or bucket or on a tarp or (use your imagination). Add water to make a slurry (clay slip). Think cake or thin pancake batter. Add clay. Moisten and adjust water as necessary. Add straw and work in till you have a much drier mixture. Think dry cookie batter. (Can you tell I’m a baker?) Again, check out a book on cob for more details. There isn’t one way to do this. You will figure out the right ratio for your soil as you go. Really. You will.

cob oven tree mixing mud

The work goes so much faster when you have someone to help. Here Tree is mixing up the clay slip for the insulating layer.

Testing the mixture for moisture/clay content. Make a cob ball (softball sized). If water is oozing out between your fingers you have too much water. Add clay or soil or straw. Once you have a well-formed cob ball, drop it from shoulder height. If it breaks apart, it’s too dry. If it oozes all over the place, it’s too wet. If it holds its shape and just slightly flattens, then it’s about right.

7. Building the base for your hearth. So now that you’ve cobbed over your bottles, create a ring around the edge of the foundation (use cob or bricks or whatever). I used cob. Into this ring you will pour sand or more cob to set in the firebricks. In the 3rd edition of Kiko’s book, he indicated that he was starting to use cob instead of sand to set the firebricks. I used sand, because, well, we have so much of it!

cob oven hearth base

I made a ring of cob “bricks” to hold in the sand base for the foundation. Note the concrete rubble I laid around the base of the foundation to hold the cob plaster. Also note that I started cobbing in the gaps in the foundation. Dont do this, just cob it in as you go.

8. The next step would be to set your firebricks, but instead, I have been working on applying a rough coat of cob/plaster to the foundation so that it looks more pleasing to my eye. I set pieces of concrete rubble at the base to serve as a foundation for the cob (so the cob wasn’t just sitting on dirt/gravel. I don’t know that this was necessary, but I felt like a capillary break between soil and cob was a good idea. I will mortar river rocks to the base using an earthen mortar to hide the concrete pieces sticking out. It took me about 3 hours to plaster the base with Tree mixing all the cob for me.

cob oven base firebricks

Notice the cob “ring” to hold in the sand base for the firebricks. Also note the slightly darker ring of cob in the center of the base. That particular batch of cob was wetter than I liked, but I didnt correct it. It “splooged” slightly and took longer to dry but worked out fine in the end.

My next step was to build a sand mound on the firebrick using damp sand. This went fairly quickly, but took much more sand than I would have thought. I used over 5 buckets.

cob oven sand mound

I ended up adding more sand to make the mound more round.

Immediately after creating the moist sand mound (this was like building sand castles at the beach!), wrap the mound in wet newspaper. In our climate, the paper dries almost immediately. I kept spraying it with water to stay moist while you put the “oven mix”/cob over the mound.

sand mound newspaper

The paper was tricky to apply in our windy weather. I soaked the paper in a bucket of water before applying.

The oven mix doesn’t have straw. It’s just a mix of clay and soil.  After you apply all the oven mud, you can wait and let it dry or go ahead and apply the insulating mix over the oven mud (the insulating mix is more of a straw-rich cob). I let the oven mix dry for a few days (I was also tired and needed a rest!). My next step was to carve out the door, and scoop out the sand. The door is not finished. I may make it just a little wider, but it was very neat to scoop all the sand out and have the oven retain its shape.

cob oven opening sand

Next step after scooping out the sand is applying the insulating cob mix.

Today, I applied the insulating mix, so the oven is almost done! I just have to make the door (really this should be done at an earlier stage), and then I will do a final plaster.

I will continue to update this post, adding photos, as I make progress on the oven and test it out. I had planned to do a workshop around building this oven, but since I’m learning my way through it, and I have only small windows of time to work on the oven, I decided to not have a workshop and simply share through the blog. Next oven, I’ll offer a workshop. Promise!

Happy cobbing!

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