Cob Oven: A Making Of, in Photos

(TL;DR: photos of cob oven construction below / timelapse video / basic how-to guide)

I visited my friend Dan Paluska in late August at Monkshood Farm and Nursery in Stuyvesant, NY.  Dan and I, apparently in parallel without knowing it, had both been looking into cob, that mysterious and alluring construction material that’s greener than anything you’ll find labeled “green” – it’s literally a mix of clay, water, sand, and straw/sawdust, or whatever you can find to suit the purpose.  “Green construction” might sound like a newfangled concept, but hey – this is how people have been building for thousands of years, and it works.  Beautifully, actually.

Anyway, a few chance emails later, we realized we we both wanted to get our hands dirty with this “cob” business we’d read about everywhere but had never actually seen in person, and Dan thought an outdoor oven would be a good addition to the farm.  So we both did a little research, I wrote a little guide you can find here, and we took a weekend and did it.  The guide is pretty exacting in some parts, but we didn’t have any way of reading it on the fly (neither of us thought to print it), so we used the basic ideas (and even ignored some of those) and the oven came out great just the same.

There was a swimming hole near the farm and we took a bunch of buckets (and a bunch of people) and had a clay digging / swimming party.  We got a couple bags of pre-moistened sand, but also used some extra sand from the driveway.  Some of the sand in the dome went in totally dry, and we used a cardboard base and a bit of white plastic tarp instead of newspaper to cover the sand dome and help it keep its shape.  While we were mixing the cob, we had a little trouble sometimes finding the right consistency – sometimes we’d use the right ratio of clay to sand, but due to the clay and/or sand that we were using, the texture would feel too sticky or too grainy.  So we had to do some adjusting and, in the end, realized that 3:1 is a good starting point, but considering that you’re pretty literally using the earth under your feet which varies from place to place, adjusting as you go to get a good texture is the most important consideration of all.  (The last couple pictures are of minor cracking that we spotted the morning after building that first layer, and I think those are from there being a little too much clay, without enough or the right sand to hold it all together.)

I wasn’t able to stay long enough to see the first layer of cob dry / help get the outer insulation layer of cob onto the oven, but it’s a process very similar to the first layer.  Also, we completely succeeded in the “getting our hands dirty” objective – so dirty that I couldn’t take any pictures in the middle of the process, but these pictures at least should help visualize a little bit what’s written in the guide.  Also, here’s a little timelapse video that shows us building, including the middle part that I couldn’t photograph, like stamping in the stuff to mix it.  It was fun.

Although I haven’t been there yet to try the finished oven out, Dan says it’s great.  Briefly, as far as the end product is concerned and in case you don’t know anything about this type of oven, you start a fire in the back of the oven and let it fire for an hour to a couple hours.  The cob soaks up and holds the heat, and after you’ve cleared out the inside of fire debris, you can cook pizzas, breads, etc. – works like any oven.  I’ve heard of cob ovens reaching and maintaining 700 degrees.  I don’t know how capable ours is, but the purpose was to use it as a pizza oven, and it sounds like it was a success despite our off-the-cuff approach.

Cob Oven Proposal

Here is the rough proposal / guide I wrote that my friend Dan and I used to construct a cob oven.  Cob is basically an earthen mixture (clay, sand, water, straw or sawdust, with variations) that has been used for thousands of years as a construction material.  Mixed accordingly, it can have great thermal properties, eg. soaking up and slowly releasing heat (and sunlight heat energy!), or allowing cool air to similarly pass through.  Someday, I want to build a house of it.  Google “cob house” and prepare to have your mind blown.

Anyway, for now, Dan and I thought an oven would be a good introduction to cob, as its thermal properties make it a great outdoor oven option on the cheap/free; plus, it’s totally earth-friendly and all natural, and fun to build.  With this guide and maybe a little additional research, you should be able to make one, too.  Photos forthcoming!  Also, this is not edited or polished to be published.  I just felt like putting it up anyway.  (Note:  ultimately, we needed less detail/steps and less materials than this guide proposes to make our fully functional oven; we didn’t refer to this guide during the actual construction so we cut a few corners and still came out with a great product.)

The oven, after first cob layer

 In order to build an outdoor, wood-burning oven as cheaply and naturally as possible (while keeping durability and efficiency in mind), the process is designed around these ingredients:

 

  • Clay / subsoil with a high clay content
  • Sand
  • Straw (or sawdust)
  • Water
  • Empty beer bottles
  • Old cinder blocks / urbanite, or bricks, or stones
  • Some firebricks, or more bricks (preferably the old red clay type) with at least one flat surface
  • A large tarp
  • Newspaper
  • Cardboard (only as much as the mouth of the oven is big)
  • Scrap wood and a few nails (for oven door)

 

Time required:

A bit less than a day of construction + a couple sunny drying days during cobbing

 

Cob is (for our purposes) a mixture of water, clay or clay-rich subsoil, sand, and straw. High thermal mass cob is a mixture of around 25% clay and 75% sand. The most insulative cob is a mixture with as much straw in it as possible while keeping a form-able consistency. By building an oven of both types of cob in two layers, we can achieve high heat efficiency and long cooking length.

 

Foundation: old cinder blocks, or stones, or bricks. Between three and four feet high (think of a good working height). Around two feet in diameter would provide space in the oven for around three medium-sized loaves of bread or one or two personal-sized pizzas, so we can make it a bit bigger if we wish.

The foundation’s outer ring of stone is filled in until the top ~4”, leaving a small pit at the top of the foundation.

First into these top 4” is a flat, thin layer of wet (but of form-able consistency) sand. On top of the sand are the beer bottles, laid on their sides, as a layer of insulation. The space around the beer bottles is filled in with more wet sand, and this layer is then covered with another thin layer of wet sand, made as flat as possible. This top layer of sand may need to be a little thicker if the oven floor bricks are uneven on their bottom face.

The firebricks or red clay bricks are laid out as the floor of the oven on top of this last layer of sand, flattest face up. Laying brick outward from the middle using the “kissing” brick-laying technique and then tapping the bricks flush ensures as flat and tight an oven floor as possible, which is important for heat insulation and cleaning. (A couple bricks may jut out in one direction as a lip to make loading and cleaning the oven a bit easier.)

Once the oven floor is done, a dome of wet sand is made on top. The dome’s center is at the center of the oven floor, and the dome’s height should be 75% of the dome’s diameter.

Working inwards from the outer edge of the dome (the circumference can be drawn using a string from the center of the oven floor), the wet sand dome is built until it is of the right height. The dome is then covered with strips of newspaper.

High thermal mass cob is mixed in the tarp. A ratio of 3 parts sand to 1 part clay is best. If subsoil, it should be certain that there are no organisms in the soil (no topsoil), as decay and rot can lead to some cracking. The clay and sand are folded and mixed on the tarp dry first as they’re difficult to mix when wet. Then, water can be added while the cob is stomped together until it is sticky enough to be form-able but not too liquidy – the consistency can be tested to be right by forming the cob into a ball and rolling it in the palm. The ball should be just sticky enough to keep its shape, but dry enough that it doesn’t pancake while rolling in the palm. If the ball is dropped, it should not go “splat.” The cob is then formed into small “loaves” or balls, which can be easily pressed into place during cobbing.

Also, an arched mouth of the oven is made by arranging and cob-mortaring bricks around a piece of cardboard cut and measured to represent the mouth opening. By a few accounts, the height of the mouth should be ~60% of the dome height so as to best draw cool air when firing. In one account, an experienced builder stated that he built his mouth at “a critical 63%” of the dome height.

The mouth of the oven can be put in place (over the brick lip if one was installed), and the thermal mass cob balls are formed around and pressed against the sand dome to create the inner oven wall, at a thickness of ~4 inches. This layer should be given a some time to dry.

After some drying time has been allowed, the sand can be dug from inside the oven. The newspaper strips over the sand dome provide a guide for when to stop digging as you’ve reached the cob wall!

Insulative cob is made as the outer wall layer. Thermal mass cob is made in the same way as described above. On the side, straw is chopped into smallish bits. It should be certain that the straw is straw rather than hay, that is, dead and dried, for the same decay/rot consideration mentioned before. (Sawdust can also be used). Handfuls of straw are then dropped and gradually stomped into the cob. Large clumps of straw are harder to integrate into the cob, so good practice is to “sprinkle” straw in front of you as you walk around in the cob and stomp it in gradually. Best consistency of insulative cob is a very high straw content while still form-able.

This layer of cob is applied over the thermal mass cob once that’s had some time to dry. A good thickness is between 2-4”. This cob is applied in the same way, but small finger holes can be made in the cob as it is applied. Subsequent cob “loaves” can be pressed into these holes so as to “lock” the straw together and better the overall structural integrity.

Using the scrap wood, make a little wood door with a handle to hold the heat after firing. The piece of rounded cardboard from earlier can be used to get the size right.

 

I’ve read that a natural plaster can be applied to the outside to help protect from the elements, like a lime-based plaster that would breathe and not cause cracking in the cob. This is also good for larger structure construction. For now (and even post-plastering), propping a tarp over the oven (especially before rains) is boranj!

 

Band Over Boston

They did this study. They isolated a group of people over time, and they monitored their abilities at crossword puzzles, right, in relation to the general population. And they secretly gave them a day-old crossword, one that had already been answered by thousands of other people, right. And their scores went up dramatically, like 20 percent. So it’s like once the answers are out there, people can pick up on them.”

 

So I had this project for a year called Band Over Boston.  (Ignore that quote for now; it’s not really important.)

It was a computer set up in the back room of In House Cafe, a little cafe at the corner of Harvard and Commonwealth Aves in Allston.  It had an iTunes playlist loaded full with local Boston (and suburban to Boston) music, and it was always connected to the In House WiFi network.  iTunes was open and the BOB playlist was set to “share over network,” with downloading disabled.

What this meant was that anyone in the cafe with their laptop open (which was generally a lot of people) and iTunes booted would see this playlist called “Band Over Boston” and could go in and browse, library- or jukebox-style, and listen to tons of local music, for free, at will, on their own computer.  I had flyers set up by the cash register advertising it and explaining how to find it; by the end of the project’s run, I was refilling a few dozen flyers about once a week.

The advertisement flyer I used outside of the cafe

As I was a local musician myself, the support and cooperation of the local music scene was
paramount to me, so I did not add anything to the library without permission, and I reached out to countless musicians to see if they wanted to take part.  Shortly after I announced the idea and started putting the word out, I had about 150 bands and musicians not only submitting their music by my request but getting in touch out of the blue, excited about the project.  I also gave some interviews and talked about the idea with press outlets like Northeast Performer Mag, Evolver.fm and Boston Band Crush.  The concept (plus some monetization ideas) also won a $1000 business concept competition at Boston University’s School of Management.  Two friends, Joan Cerdas and Patrick Coman, helped with some of the graphic work, design, and blog writing as the project unfolded.

The obvious intent here was to bring local music to a new audience.  Although music is reveling in myriad new ways to deliver music to people outside of the tired major label pipeline, some of the best music I’ve ever heard was in a little bar in front of a little stage with as few as 20 other people next to me in the audience.  I mean, and at the risk of tired simplification, Nirvana was that annoying band in the garage next door to someone at some point.  There’s great music being made right down the street, and I wanted more people to know they didn’t have to spend $80 to get into a big arena to get their socks knocked off by a guy slinging a guitar; they could be standing close enough to have his sweat slung onto them instead (awesome, right?!).

But the idea was not only to bring local music to a new audience, but also to bring a new audience to the cafe and, ultimately, other cafes and WiFi hotspots.  By advertising the project on social media and a few blogs and starting a little music review column (excerpts of which can be found on my music writing page), I spread the word not only about BOB but also its cafe home.  I had a website up with a description of the cafe and an embedded Google map pointing out exactly where it was.  Local music as a draw to a local, independent cafe.  I was soon hearing from people who worked the counter there that new customers had come in and asked about Band Over Boston, as well as musicians visiting the cafe to ask how to submit music.

My next step, in short, was to be to reach out to other cafes to expand the project to new WiFi hosts.  I would then print small logo stickers (something like the beside logo) and have them in a window at each cafe, as well as designated cafe “profile” pages on the BOB website, to visibly tie the project to its physical spaces in a non-intrusive way.

I also wanted to have a proprietary player programmed, so I didn’t have to run it through iTunes but so it could still be visible by iTunes.  I had a deep feature-set planned for the program and a parallel mobile app that would enable easy interaction with the local music scene based on what you listened to and what you gave a “thumbs up” within the player.  Features would have included pointing you towards buying music you liked directly from the musicians, adding you to email lists, and adding your favorite bands’ upcoming shows to your calendar of choice, among a number of other “one-click interactions” with your local scene.  Users could also just browse by what was being listened to or what was hot at the time if they wanted an entirely new experience (there would be a splash page chart), or browse “celebrity playlists” with a local spin – playlists created by local musicians – and other spotlight playlists by locals like cafe workers, politicians, public service workers, educators, etc.  The mobile app could add a plethora of GPS features; after all, we’re talking locality, here!

Not only does all this make a large local music landscape more digestible for the listener, it also provides a new tool to local-level musicians.  Musicians could find new bands they may not be socially or otherwise connected to that are similar enough to share the bill with.  And by checking out other cities’s libraries along a potential tour route, musicians could see what bands and, more broadly, what music was hot where to help them plan.

On the other side of things, I wanted to deepen the relationship between the music and the cafes (or other WiFi hotspots) by creating, with the cooperation of the WiFi host and select bands, certain deals, like “happy hour” specials (buy x between the hours of y and y2 and get album z for half price!), “buzz points” (get a free small coffee if you recommend the cafe or your own local music playlist to x number of friends), pairing deals (a free coffee if you buy a certain album or a certain number of albums), flash sales, recurring free song download periods, etc.

Ultimately, I wanted to take the project beyond Boston, and beyond local music ecosystems and tap into the broader music landscape.  I wanted the program to look at what (inter)national or major-label acts you listened to and suggest local bands based on those, for example.  And there would also be of course a local music library for every major city and its surrounding suburbs, and umbrella features would add a new dimension to traveling (or touring, for musicians) by suggesting local bands in other municipalities to you based on your hometown faves.

So why all the above “wanted to” and “would have” phrasing?  Alas, I was working on BOB when I was playing a lot of music and not making very much money, and I did not have the time or funds to pursue the project to the heights I envisioned.

Why am I writing all this, then, about a dead project?  First off, one of the goals of this website is to serve as a kind of EPK, and this is I think an important part of my musical past.  Also, in part because I believe in the underlying “hive mind” idea behind the quote that opened this little piece (from the movie Waking Life), that once ideas are out there, they’re part of a collective awareness – granted, that’s a little obvious if they’re written up online, but for both these reasons I wanted to put this idea and some of its heretofore un-publicly-discussed future concepts down in a public space.  Similarly and also, because I still think it’s a great idea and maybe, just maybe, someone will see this and start a similar venture in their city, or use some of these ideas and expound upon them to make a truly great tool for pairing people with their local music scenes and their local businesses.  Yes, there were some business elements baked into this idea that I could probably hoard, copyright/patent and pursue, but I want most to see the principles and the core motivations and values for the project reach their full potential, notwithstanding my own personal schedule or ability – and besides, many minds working together are greater than one alone.

So whether I begin the project anew myself, with a partner in one city, in parallel with a partner from another city, or whether someone completely different takes the reins and follows through on this or a completely different platform with the same core values, here’s hoping localization is the way of the future.  Because with an ever-thickening web being spun around us, it’s that much more satisfying to remember and appreciate just where you are.