Tag Archives: mixing cob

Building an Earth Oven Part I: Foundation, Dome, and Structure

An earth oven is an oven made of cob (a mixture of clay, sand, and straw) with insulating features (firebricks, bottles).  It is an extremely efficient and sustainable method of doing any baking you might need to do. One firing of your earth oven can allow you to bake different things for hours (pizzas, breads, casseroles, vegetables, etc), and it takes only a small amount of wood to heat.  We fire ours by simply picking up deadfall sticks and branches, cutting them up, and that’s all we need. An Earth Oven is fully sustainable to build and to cook with, and you can locally source literally all of the materials for the oven (and in fact, minus fire bricks, you can probably harvest everything you need from your own land or the land around you).

The completed earth oven

The completed earth oven

An Earth Oven allows you to connect deeply with the earth itself, encouraging you to slow down, root, and ground.  It is certainly a nice example of “slow time” and “slow food”, as the oven takes a few hours to fire before you can bake in it, and it requires some learning.  With that said, the rewards of this approach way outweigh any challenges you might experience as you learn and grow.

 

The other thing about your oven that is rarely discussed is that it is truly a sensory experience.  Your earth oven will allow you to bake any number of delicious creations–and when you bake in it, the smell of the bread permeates the clay itself, and it has this incredible smell, unlike anything you’ve ever experienced.  Even on cold days, being near your oven will keep you warm (and your oven will stay warm for literally hours–possibly all night).  It is just such a wonderful thing.

Fire in the near-complete earth oven!

Fire in the near-complete earth oven!

Earth ovens are a wonderful kind of “hearthspace” where we can cook foods for ourselves and loved ones, use natural materials to build from the land and honor the land, and create a very sustainable and low-input cooking method that is a joy to make and a joy to bake in.  In this new series, I’ll offer full details about how to build an earth oven step by step!  This series will have three posts–today’s post covers materials, preparing the foundation, preparing the base, and creating the dome.  Next week’s post will explore insulating the oven and finishing the oven through a mosaic/cob, finish plaster, and options for doors.  The final post will explore how to cook in your earth oven and some tips and tricks I’ve learned in my first year with the earth oven.

 

Preparation: Materials

In order to build your earth oven, you will need to prepare some materials and do some prep work.  Earth ovens are not expensive to build–most of the investment is in your own time and labor.

Materials for building:

  • Stone and urbanite: You will need some kind of base for your oven; most people use local stone or even urbanite for a strong foundation.  Urbanite refers to waste materials like broken bricks, broken concrete blocks, etc.  You can use urbanite to “fill” the inside of your base and create a stable foundation.  I have also seen people build ovens on old large stumps or platforms.
  • Subsoil / cob: First, you will need access to your subsoil (that’s the soil that is not full of organic matter, but the soil below it). You will need quite a bit of subsoil, so make sure you have easy access. Once you have access to the subsoil, you will want to do some soil testing (which I explain in the link) so that you can be sure you have good subsoil to work with.  To access subsoil here, I found a tree that had gone down and uprooted itself–the roots and ground below it was full of all of the subsoil I needed.
  • Straw:  You will need a bale of straw for your cob.  Straw adds strength, can provide insulation, and can help hold your structure together.
  • Sharp sand. You may also need some sharp sand (known as builder’s sand) depending on what your cob tests reveal. You can get this from a landscape supply or building supply place.
  • Sand: In addition to the sharp sand as an additive, you will need sand to build your sand dome.
  • Newspaper: You will need 3-4 newspapers for your dome.
  • Bottles: You will need at least 20 wine bottles for insulating your base. These can be foraged from recycling bins, saved by friends, or even found at local restaurants. Depending on your structure, you can also use smaller recycled bottles to help with insulation (I did this, but some people use an only straw).

Tools for building:

  • Wheelbarrow or cart: You need something to move materials around your site.
  • Shovel: A shovel for digging subsoil and moving cob around.
  • Screen.  To prepare your subsoil, you will need to screen it of larger items (stones, debris, etc).  For this, you can build a simple soil screener (this also works great for screening compost!).  Make your soil screener large enough that it will sit over your wheelbarrow or cart that you will be using to move it.  I built mine with scrap lumber and 1/2″ hardware cloth.
  • 5-gallon buckets: A few buckets are super helpful.  They can move cob from where you mix it to your oven.  You can fill some with water to rinse your feet.
  • Tarps:  A small tarp will allow you to mix your cob.  You might want several, especially if you are doing the work yourself (I found that when I was doing the work myself, I would gather materials – screen cob, gather rocks, etc, on one day and then build on the next, so tarps were able to keep my cob screened and protect it from the elements).  You also will need a tarp to cover your cob oven until you can have a structure built for it.
Source of clay / subsoil - a tree that uprooted during a storm

Source of clay / subsoil – a tree that uprooted during a storm

Cob screen in action - this was built with scrap lumber and 1/2" hardware cloth

Cob screen in action – this was built with scrap lumber and 1/2″ hardware cloth

Other Tools and Materials:

  • Pizza peel (I created one from an old aluminum pan and an old rake handle)
  • Firing door and baking doors (more on this in an upcoming post)
  • Shelter structure

We’ll cover these last three in an upcoming post.

Preparation: Oven Size

You can decide how large of an earth oven you want to build.  There are many measurements in the Build Your Own Earth Oven which I used as a reference.  Here’s a simple diagram of the oven and dimensions that I used.  These allow you to figure out how large of a foundation and base you need and will determine the overall structure.

For example, to get good airflow, I created an oven that was 27″ in diameter, which included an 18″ high dome.  Your door should be 63% the height of the oven, or 11.5″ high.

Your oven can be larger or smaller, of course, depending on your needs.

Preparation: Selecting a site

I think it is really important to spend time selecting the right space.  Your space needs to drain well (especially if you are in a temperate climate that has snow/ice).  Your space also needs to be protected from the elements as cob is a natural material that will weather quickly if left without protection.  Your space should also align with your landscape (see setting intentions with nature).

Your oven should be somewhere easily accessible.  You will need to tend a fire every 20-30 minutes for four hours in order to have an earth oven hot enough for pizzas or other baked goods.  Thus, you don’t want to put it so far away from your house that its hard to tend the fire (but not close enough to cause any insurance issues!)

For my earth oven, I decided to build in a little nook on the edge of the property that leads into forest.  That provided both shelter from the elements (and it will get a permanent roof this year) as well as being pretty close

Preparation: Clearing and Foundation

To build your earth oven, it begins with a good foundation. Any cob structure should have “a good hat and feet”, and the foundation is the “feet” part of that equation.  Here is step by step how I built my foundation.

I began by measuring out my foundation – 46″ based on the height of my oven.   Then I removed all of the plant material (as described in this post); I worked to replant any material that needed to be replanted, etc.

Clearing the space

After clearing, I dug down 1.5 feet and added a gravel base for drainage.  Its hard to tell from these photos, but we are on a slight incline, so I made a drainage area to the left of what you see here.

Foundation of gravel

Foundation

Preparing and Mixing Cob

The next step is to screen and create your cob.   I went to my site with my wheelbarrow and put a 5 gallon bucket at a time in my screen.  Using a rock, I worked the cob through the screen. (More details about making cob here).

Screening cob

Screening cob

After that, I moved it to my building site and prepared to mix the cob.

Subsoil screened and ready to mix!

Subsoil screened and ready to mix!

To mix the cob, you will need an old tarp, two or more buckets of water (I like to use warm water!), and some happy feet.  First create a well in the center of your cob and, just like making noodles or dough by hand, then start mixing the cob with your feet.  You can use the tarp to flip the cob and keep working it.

For building, you will want to also add handfuls of straw (I cut the straw up so it’s a little shorter) and work that in.  Ideally, you want something with a good consistency that sticks together and isn’t too wet and crumbly.  You can add more water or more soil as you mix.  I have more detailed instructions on mixing cob in an earlier post.

Mixing cob

Mixing cob

You can also get others to help you–friends, children, or even geese.

Goose helpers

Goose helpers

Building your base

In addition to the cob, you will also need something to build your base up that is fireproof and stable–rock or brick is a very good choice for you. For my foundation, I used a mixture of cob along with locally foraged stones and urbanite (old concrete bricks that were broken and strewn about the property.) A good foundation and good base are critical to the success of your oven. You want to build your base high enough that it’s comfortable for you to fire and use the oven.  I kept mine pretty low cause I like to sit on the ground when I work!

I began by mapping out a circle (using a string and some chalk) and building the outside of the circle up using the cob as a mortar.  On the inside, I added my urbanite brick pieces and filled in all holes with smaller stones and cob.  Here’s the first layer.

Building the base.

Building the base.

As I worked, I continued to build up the stones on the outside.  Since I also planned some decoration, I had stones I had already added mosaic to that I wanted to include.  These pieces were made of mosaic materials leftover from both from doing my mosaic bathtub a few years ago + broken pottery and plates that I save.

My mosaic stones

My mosaic stones

It took me two building sessions, but the base continued to grow.

Base after adding mosaic stones

Base after adding mosaic stones

You can see that I’m building up the inside of the base as I build up the outside of the base.

Base before insulation layer

Base before insulation layer

At this stage, I have the base at the top level and as high as I want it.  You can see I am using a piece of wood and a level to check to make sure everything is level as I work. The next step is to add the insulation layer to the base.  The insulation layer is made up of a straw-rich cob combined with wine bottles.  Bottles offer insulation, which keeps heat from getting absorbed by the stone below.  Insulation is really an important feature of these ovens–investing the time to do insulation will allow your oven to be used in colder temperatures, hold heat longer, and heat up sooner (meaning you use less wood to make that happen).

Thick layer of cob in base

Thick layer of cob in base

I started this with a thick layer of straw-rich cob. Straw itself is insulating as it traps small amounts of air in it as it is worked into the cob.

Next, I layered the bottles with more cob in between each to create a solid foundation.

Bottle insulation

Bottle insulation

Here’s the bottles with more cob added

Bottles getting covered

Bottles getting covered in cob

After the bottle layer, I used the board to smooth out the layer and wait for the base to dry about a week before continuing.

The base is done!

The base is done!

Except that we have powerful raccoon activity, and that night, someone tried to dig up my base to see what was in the bottles!

Coon activity

Coon activity

I repaired the cob and then started putting a tarp over it at night to deter the raccoons until my base was a bit more dry.

Laying your firebrick

Firebrick layout

Firebrick layout

To start your hearth, you want to put down a fresh layer of cob so that you can set the bricks in carefully.  Slide the bricks against each other and make everything perfectly level.  To make an oven that is XXX” inside (which is enough to cook a few breads and two small pizzas, perfect for small groups or families) you can use fifteen firebricks in the following pattern (the two firebricks in the back were redundant and I removed them later).

Notice here that I’m also planning a lip to pull my pizzas in and out of the oven.

Circle for dome

Using a string, I made a circle to show me where to build the sand dome for the oven itself.   Now I’m ready to build the actual oven.

Creating Your Sand Dome

I think the most labor-intensive part of this entire process is building the dome and cobbing the dome. Everything else can be done in stages, but this really has to be done all at once. This part is a great time to invite some friends or family to help you if you are doing this on your own.

Start by mixing 1-2 large batches of cob (with straw reinforcement) and tarping that while you build the dome.

To build the dome, you will want a ruler (to test the height of the dome), newspaper, and a few wheelbarrows of sand.  I mixed my sand right in the wheel barrow and got it to a consistency where it would build up well.

Mixing the sand

Mixing the sand

Building the dome

Building the dome

So then, start building the dome, using the guide you drew.  I stuck a metal art ruler down the center of the dome so I would know exactly how tall it was (with a goal of 18″ tall).

As you get further along, you can take a small board and smooth and shape the sand dome.  PUll out the ruler, and you are ready to build!

The completed dome

The completed dome

Go head and coat the whole sand dome with a layer of thick wet newspaper (this creates a barrier both to keep the cob from sticking to the sand and gives you a sense of where to dig out later.

Now, you want at least a 2-3″ thick layer of cob all around the dome.  You dont’ want to press the cob into the dome, but rather, shape the walls downward with your hands.

Building the oven

Building the oven

Keep working your way up.

Oven build continued

Oven build continued

A lot of ovens have some kind of door area. We used old bricks to build our door area (planned out in advanced).  To keep the door balanced, we added two pieces of packing foam that we cut to the right size.  These were later pulled out when the structure was dry.

Building the arch

Building the arch

The arch proved a bit tricky–we layered sticks in between the sand dome and the foam arches and that provided enough stability until the oven dried.

Here is our completed first layer of the cob oven.

first layer of cob finished!

first layer of cob finished!

At this point, you will want to give your oven some time to dry.  We put a little pavilion over it and allowed the summer sun to dry it out.

Digging out the oven

The next step is to dig out your oven and light a few small fires to help dry out the inside of the oven. You can just use your hands, a small trowel, or a stick.  You want to be careful as you dig out so that you are not digging into the cob itself (hence why the newspaper barrier is so useful).

Digging out the oven

Digging out the oven

I used my shovel to remove the bulk of the sand and then got in there with my hands as I got closer to the walls. I let the oven sit another week after I pulled out the sand, and then I lit a small fire for a few days to get the inside dried out.  Here’s the very first fire.

First fire

First fire

Alright! That’s over half of the oven build.  In my next post, I’ll show how I added layers of insulation, a final plaster, and also decorated it with more mosaic pieces that were salvaged from broken pottery.

Goose helpers and the druid builder!

Goose helpers and the druid builder!

One final thing I want to say now is this is an incredibly good way to connect with the element of earth.  There is nothing more grounding than having your feet and hands directly in the soil, shaping it, honoring it, and getting to know it.  I really enjoyed my time building this and felt incredibly grounded afterward!

Building with Cob, Part II: Soil Tests and Mixing Cob

Happy feet mixing cob!

In a meadow under the summer sun, a group of dancers laugh and fling mud.  Beneath their feet, clay, sand, and water become mixed together, creating a sticky earthen blend that sticks to their feet, their legs, and, after some play, faces and fingers! This is a cob mixing party, one of the best times you can have with good friends. After the cob is mixed, it is added by others to the bench and more soil is added and the dance continues.  In last week’s post we explored some reasons to consider exploring natural building as a potential way to build sustainable structures and be more attuned with the energies of earth.  In this week’s post, we will get into how to test your soil and how to make some cob!

One thing I want to share about cob–you don’t have to build big things, like houses or ovens, with cob.  You can also build really small things–candleholders, paperweights, primitive statuary, and so on.  You can do an earth plaster on a wall in your home, or build a small cob bench overlooking the woods. I think the underlying practice with cob is simply to work with the earth in this very earth-honoring and embodied practice.  This post explores how to test your soil and make cob, which you can then use to shape your world!

Preliminaries for Cob: Testing Your Soil and Quality of Cob

Soil Horizons and Your Subsoil

Soil horizonsMaking cob requires you to understand a bit about soil horizons and how soil lays on the earth.  If you dig a hole in the earth straight down, you’ll see that soil show up in layers (horizons).  The first layer, the O layer, is an organic layer–where dead and rotting organic matter can be found.  This is the layer that is created when leaves fall and rot, creating dark, rich, humus.  This is what we want to grow plants in, NOT what we want for natural building.

The second layer, the  A Horizon, is the surface layer.  It is usually less dark, but still contains nutrients and organic matter. It usually appears lighter in color. Again, this is for plants, not for cob.

The third layer, the B Horizon, represents the sub-soil.  It is here where we find clay, sand, and silt; our basic building blocks for cob construction.  You’ll notice another break in your soil as you go down–in my region, the soil gets quite orange, representing the high iron content that we have here.

The fourth layer is the C Horizon, or sub-stratum, where you get quite rocky before hitting the final layer: the R layer, bedrock.  We don’t really want that for cob either.  Depending on where you live, the bedrock may be very close to the surface or dozens of feet down, so you may never see it.  Here in Pennsylvania, however, you can get a good look at the C horizon less than a foot down!

The amount of O and A Horizons you have is based on your own soil ecology as well as the long-term land use and land history. Parts of the world were stripped to bedrock by glaciers.  Other parts have a 15 foot A Horizon due to long-term patterns of beneficial animal herd grazing.  The same is true of the B, C, and R layers–the depth of these layers is based on a lot of land history factors spanning back tens or hundreds of thousands of years.

The good news is that in many parts of the world, clay and sand are fairly abundant and easy to get to with only a shovel!  You can learn more about soil horizons in your area by looking at recently dug up areas–a fallen tree that has taken the roots with it as it fell offers one such opportunity; new construction into a hillside offers another.  Or, you can simply get out a shovel and start digging–the secrets of the soil horizons will be revealed to you with a bit of sweat equity.

The Soil Jar Test

To find out how much sand, clay, and silt you are working with in your subsoil, you can perform a simple soil jar test.  Dig down into the subsoil and get yourself a good cupful of subsoil.  Break it up well if it is compacted as much as you can (this might mean letting it dry out for a few days in the sun and then breaking it up that way). Place this in a quart mason jar and fill with water to the top, leaving about an inch or so to shake it.  If you have animal helpers, this is a good time to enlist their help. Shake it very well.

Soil jar after shaking well.

Goose inspection of the jar. All is well.

Now, let your jar sit somewhere undisturbed or 24 hours.

  • The sand (a large grain particle) will immediately sink to the bottom, within a few minutes.  Mark this with tape or a marker if you can, or mentally note where it is.
  • A layer of silt (a medium particle) will settle on top of the sand in about 30 min.  You should .again mentally note where that ends.
  • Over the next 24 hours or so, the clay (a vey fine-grained particle) will settle out of the water.
  • You will also see any organic matter floating at the top of the jar.

24 hour later, all has settled.

You can look at these ratios as a way to determine if you will need to source some off-site materials to make an effective cob blend (2 parts sand, 1 part silt/clay).  As you can see from above, I am blessed in that I have an excellent ration of sand to clay/silt, and the cob from my land is almost perfect without any additions.

Any organic matter will settle on top (or float) but if you are using subsoil and you dug below the A horizon, you shouldn’t have much of that. (As an aside, you can use this same test for garden soil on the surface and it will tell you how much organic matter is in your soil, which is a very good thing!)

Clay Ribbon Test

Another good thing to do to test your clay in your subsoil is to do a ribbon test.  This gives you a simple test that lets you know how pure your clay is and how it will hold up over time.  A good example of how to do this is here. You mix up your cob as usual and work to create one of those clay snakes (like you may have done as a kid). When you have it mixed, you see how well it can be worked (bending) without breaking.  The higher the clay content, the more bendy it is.

Making cob at my PDC in 2015!

Soft and Sharp Sand

Not all sand is created equal and it is very good to know what kind of sand you have in your subsoil. Some sand has very soft edges; you can think of beach sand here. The waves and water over a long period of time have softened the sand to the point where it is smooth. It is possible that the sand in your subsoil is like this–it is very soft because at one time, it was on a beach somewhere! Sharper sand contributes to a stronger cob. If you are doing major load-bearing building projects, you might consider adding some “builders sand” (which is a sharp, coarse sand) to your mix.

How to Make Cob!

Without further delay, let’s mix up some cob!

To make your own cob, you will need the following tools:

  • A shovel to dig out subsoil, wheelbarrow
  • Subsoil
  • Straw (chopped up), aged manure, or other grassy things (this adds strength)
  • A mixing tarp (any tarp will do, at least 6′ across  so you can move the cob around on it
  • A water source (hose, bucket, etc.)
  • Some happy feet for cob dancing (you know you want to!)
  • A large wooden screen sizes (use 1/2 or 1/4″ screen; see “tools” below)
  • A rock or small board to help sift subsoil

Tools

Probably the only tool you will need to make is your screen sifter. For general cob applications, you will want a 1/2″ screen for coarse/building cob. For finer cob applications (finish plasters, earthen candleholders or statuary, etc) you will want a 1/4″ screen. I made my screen by making a simple wooden frame out of scrap 2×4″ board. Then, I used a good amount of staples to staple my hardware cloth (1/2″) to the frame. The process took about 30 minutes, which mostly involved cutting and stapling the hardware cloth.

Screen with soil

It is necessary that you screen your cob in most locations–you don’t want those happy dancing feet to step on sharp rocks, sticks, or other stuff.  If you are taking your cob down to the 1/4″ level and have really rocky and uneven subsoil, I suggest starting to screen it at 1/2″ and then rescreen it down to 1/4″.

Making Cob

Dig your soil. The first thing to do is to dig out a good amount of subsoil.  I usually mix two medium wheelbarrows full at a time.  If you mix too much at once, it becomes unwieldy, particularly if you are mixing it yourself.  You can see how rocky our subsoil is!

Wheelbarrow full of subsoil

Screen your cob. Now, screen your cob. To do this, break up the hard chunks as much as you can with a shovel.  Wet soil will not screen.  Really dry soil (as in, you haven’t had rain for quite a while) may get hard to screen as well, so there really is a sweet spot for soil moisture (experiment, you will see what I mean). Put a few shovel fulls of cob in your screen and then start moving it around. After you break up the big stuff, you can use a rock or small piece of board to really push the soil through the screen. Once you’ve screened all of the cob, place the stones and other debris in a bucket, and continue with more subsoil.

Using a stone to work out the last bits of subsoil/clay chunks from stone

Make your cob. Once your subsoil is all screened, you can dump it into the mixing tarp. Make a well in the center of the subsoil, and just like you’d do making a dough, place water in the well in the center.  Don’t overdo it, just fill up that well.

The well with water

Now, start mixing the cob together with your feet. As you mix, grab an edge of the tarp and pull part of the soil over on itself. Add more water. Mix with your feet again, and continue the process–flipping over the cob, adding water, etc, until all the cob is firm yet pliable. How wet you want your cob depends on the application. If you are using cob as a mortar for a stone wall or brick rocket stove, you will want it much wetter. If you want to make bricks and build with it (like an earth oven), you will want it more firm.

Work it!  Mixing in the straw.  Sprinkle lightly to prevent clumps.

Optional: Add straw. At this stage, if you want your cob to have extra strength, you can mix in some straw or other grassy matter. This addition is excellent for building cob ovens, walls, and so on. The straw will suck up some of the moisture in your mix, so you may have to add a bit more water till you get a perfect consistency!  For fine applications, a lot of cobbers actually use dried out horse or cow manure–the cellulose stays in the plant matter as it moves through the animal, giving a really nice soft strengthener.

In my photo here, I am using this cob as a mortar for my greenhouse back wall, so I have added straw to help strengthen it.

A good mix!

Create anything with your cob! Now, you have a wonderful building material that you can do anything with!  If you don’t have anything to build yet, consider not adding the straw and instead, making some primitive statuary, cob candleholders, paperweights, and so on.  I love the way that some cob statuary and candleholders look on an altar!

Ready to use!

Cob and cobblestone wall ongoing in the greenhouse!

The photos in this blog post show two different locations–at my permaculture design certificate program, where the soil was more brown/gray and then here in PA, where we have beautiful yellow-orange iron-rich soil.  One of the other delightful things about cob is that it reflects the land where it comes from–we can truly see the colors of the land through this practice.

I hope this post was inspirational to you and you consider experimenting with this amazing building source!