Making a Cob Oven: The Oven

The oven floor was complete, the stone support wall was ready, and finally we had time to start on the oven proper.  The general plan is to make a sand mound and pack the cob around it, let it dry a bit, and then cut out the door and scoop out the sand.  Jennifer had researched this for years and had several books on earthen ovens.  If you get only one, I'd suggest "Build your own Earth Oven" by Kiko Denzer and Hanna Field.

First, we brought several wheelbarrow loads of sand around and started piling and packing it into shape.  We cut a slender stick to the desired height and put it in the sand at the high-point.  In the end, we could not get it quite as tall as we wanted, but it was close enough.

It is suggested that you wrap the sand mound with newspaper in the end to help it keep shape and to keep the sand from coating the cob oven interior when you dig the sand out.  In our case, it was windy, the paper blew all over, and we gave up on that idea.  We also had issues with the sand slumping on the back side.  We had some river sand that was a bit coarse and round and I think it does not pack as well as the finer crushed sand.  To resolve this, we would build up a bit of sand, then we packed some cob around it, and continued up until the sand's angle of repose was within tolerance.  We took a picture of the good side, so you can just imagine the other!

On the outside of the sand, you pack a layer of dense cob about 3-4 inches thick, working your way up to the top.  There are tons of resources on how to make cob, and it seems there is a wide variety of mixtures that will work.  In our case, we used about 1 part clay to 4 parts sand, and then mixed in a modest amount of straw.  The straw will likely burn out on the first good firing, so it is mainly to hold it together while it dries.  Jennifer asked around at the local Potters's association and we ended up with a large amount of scrap clay.  We let that dry, broke it up with a hammer, and then added water to reconstitute it.

The sand came from the rock store, and the straw (not hay) from the local farm supply store.

To mix the cob, we placed a large glop of clay in the center of an 8x8 'blue' tarp, and then poured 4 equal sized clumps of sand around it.  Then, kick some sand into the center and start stomping and twisting.  When the mixture is flat, pick up one side of the tarp and pull it backwards to fold the mixture back into the middle.  Repeat for a while.  When the mixture seems right, we sprinkled straw fairly thick on top and stomped and folded some more.  The basic test is to make a fist sized ball and drop it from shoulder height.  If it splatters, it is too wet and you need more dry clay powder or sand or wait a day or two and let it dry out.  If it crumbles, more clay or water.

Don't wear yourself out all at once, it takes a lot of the stuff!  It is easier to do two smaller batches than one overly huge one that you cannot easily roll over with the tarp.

Once we had a tarp full, it was time to start with the cob.  We picked up double fist sized clumps, squished it together a bit, and added it to the wall.  Press down and in and force the layers together by sticking your fingers down in the cob to weave the straw and mud together.  Texture the outside well to allow the next layers to attach.  We will be adding an insulating layer as well as a final decorative and protective cob layer on the outside, so also make sure you calculate the width such that you have room on the outside to add more layers.

And finally we are done wit the first layer.  We pull out the height-measurement stick and put the final cap of cob on the oven.  One of the important ratios is the height of the inside of the oven to the height of the door.  If the door is too high, heat will escape too quickly, and if it is too low, it will not burn well.  The proper ratio is too have the oven door about 62% of the height of the oven.  So, if the oven is 26 inches tall, then the door should be about 16 inches high.  Jennifer sculpted the door into place but we had to wait a week or so to let it dry before we cut it out.

When it was finally dry enough I cut out the door with a sheet-rock saw.  It would have been better if I made the opening a little more shaped like a funnel, with it being wider on the outside than the inside, but it works well enough in practice.

Next, time to remove the wet, heavy sand.  The first part is easy, but then you tend to run out of buckets, and the final bit of cleaning is easiest if you have a sturdy child that you can lure into your oven and get them to clean it out.  Maybe with gingerbread cookies or something!

Be careful of the inside walls, the cob is still quite wet at this point and will not hold up to abuse or heavy scraping.

In case you wonder how many buckets of sand you need, here is an idea.  Note we also have a tarp over the cob to help keep the rain off so it can dry properly.

When the cob is finally dry enough (leather hard or there about), the second layer goes on.  This layer is made of clay slip and sawdust.  It is not very strong, but it will dry somewhat fluffy and the air pockets (and wood, which will become carbon smudges and air pockets when Jennifer really gets it hot) provide good thermal insulation.  This layer is about 1-2 inches thick, and it dried fairly quickly, cracking as it did.  It seems safe to ignore the cracks in this layer.

You should reserve the area right around the door and build it up with the dense cob so that the insulation layer is never right next to the door opening.  We will do a better job of this next time we build one, and even with the insulation near the door opening we are doing fine so far.

The second layer is done and we are excited to finish it up.  Spring has sprung, the sun is out, and we are on our final batch of cob.  We used a lot of red clay in this mixture, and plenty of straw to try to make it strong and beautiful.

I'm mostly in charge of the back side of the oven and of carrying cob to Jennifer.  She takes care of the front side.  We put on a layer about 2 inches thick and this time we make it smooth on the outside, whacking it with hands and a paddle to make sure it all sticks together well and is a pleasing shape.

Then the decorations start.  Jennifer spent around 10 hours on this, and Angelina helped with some sea life sculptures around the bottom.

It turns out beautiful!

We had originally thought about putting on a final layer of lime plaster, but we liked the color and texture of the red cob layer, and were ready to be done, so we skipped that part.  We can always add it later if we want, but I don't think we will any time soon.

We still could not light the fire until everything dried out for a few weeks, so I got back to working on the counter top slab.  Now that the mud work was done, it was time to clean it up and varnish it.  I used the hand plane to make it flat and smooth, and in the places where there was a larger blemish, I used a sharp wood gouge to make smooth cuts.

Then, lots of sanding, both with an orbital palm sander for the flat parts, and hand sanding for the concave parts.  I had a quart of 'Spar' marine varnish from back when I thought I had time to play with a sail boat, and figured if it could handle salt water, it could handle being in an outdoor kitchen.  I added about 1/3 extra volume of paint thinner to the first coat in hopes it would penetrate farther into the wood and provide a durable finish.  The second and third coat I put on without diluting it.  I sanded between each coat, and in the end it seems to be durable and pleasant to use.

Finally, Jennifer agrees that we can start a little fire in the oven.  Just a small one, just to see how things go.  It has been over a year since we started, and we all sit around and watch this small fire growing an growing as we cannot help but to keep adding twigs and shavings.

To be continued....

Making a Cob Oven: More Rocks

Fall was nice.  Jennifer and I got married, we had our annual Apple Cider Party and Thanksgiving Eve Pumpkin Pie Party, and suddenly it was Christmas and we were in Georgia.  After a good time with my folks hanging out and making wood crafts in Dad's shop, we came back home to Washington.  When the weather cooperated, I started on the rock work again.  The goal was to build a nice strong foundation for the cob oven, a fire pit next to it, and a supporting wall for the counter top.

I sunk another truck load of rocks into the north west corner of the structure and started building a circular wall about 4 feet across.  I built a wooden form to support the arch so we could have a door in the oven support structure to store firewood.  I used another piece of plywood to support the base of the oven floor and filled it in with scrap metal, cement, and rock.  The solid part of the oven floor ended up being around 6 inches thick.  Later we will layer insulation and fire brick on top.  I calculated to have the fire brick floor right at normal counter height for ease of use.

Here's a top-down view showing the scrap metal being buried with purpose!

And finally the oven floor support is done!

Next I started building up the circular wall again, as well as the counter support wall.  When the oven is complete, we will get it hot and then let the stored heat cook the food.  So, we want to insulate it as best we can.  For the floor, we will put in glass bottles (beer bottles mostly) and pack a mixture of clay and sawdust around them.  The air pockets and glass will provide insulation.  On top of that goes a small layer of sand to level things out, and the firebricks will go on top of that.  I cut a 4-foot pole to use to measure the wall diameter, and used a level to keep things, well, level.  Near the top of the oven support I used the river rocks we had set aside.  They look better in my opinion and it is easier to get a nice cement finish when working with smooth rocks.

I also started building the fire pit using dry-stacked hard fire bricks backed by stone and concrete.  The fire bricks are fairly expensive, and you could probably find them used with a bit of effort, especially if you can make some potter friends.

You might remember the large slab of Fir wood destined to become the counter top.  We let it dry out in the sun over the summer and then moved it under the roof when Fall came around.  My son Jovan and I used an axe and hatchet to get the slab somewhat smooth.

Next, I bought a $45 electric hand planer and a $60 manual 2-handed jack plane.  I didn't have a lot of hope for the electric planer, but it actually did an admirable job of smoothing down the slab.  It probably took 2-3 hours of planing, and in the end, it was dull.  The manual says you cannot sharpen the extremely small blades, but it turns out you can if you are careful!  The slab abides, waiting for its time to shine, and supporting the brick cutting saw in the meantime.

Once the oven floor support walls were finished, we put in the bottles and mixed up the sawdust and clay mixture.

And packed it in tight and level!

Next, it was time to place the counter top.  I used a chainsaw to cut out an opening so it could go around the support post and slid it into place.  It didn't fit, so I had to take it down and do it again.  Fortunately, it was a lot lighter now that it had dried for almost a year.  Wooden wedges were driven in under it to support it against the stone work, and I screwed the wedges into the underside of the slab to keep it stable as the wood expands and shrinks with the seasons.

I also finished up the fire pit.  I placed some of the bricks perpendicular to the wall and let them stick out a bit to act as grate supports.  I think it might help keep the whole wall better connected to the concrete and stone backing as well.  If I ever do it again, I think I will probably key every other run instead of every third run in order to have more flexibility with grate height.

We are anxious to get started on the oven, but we still need to do the brick floor.  So we lay the brick on top of the sand (which is on top of the bottles and sawdust & clay).  We pack them tight, sliding them down against each other so sand cannot get between the bricks.  This will be our cooking surface, so it needs to be done well.

And finally I extend the river-rock wall up to be level with the bricks, taking special care to make the front (future oven entrance) as smooth as possible.  It is OK for the other parts of the wall to be a bit jagged, I think that will help support and keep the cob oven walls from shifting.

Next time, we start on the oven!

Making a Cob Oven: Out of the ground

The foundation is complete, and now it is time to start making some more visible progress.  In addition to scavenging rocks, I started getting some longer poles as well.  These were typically 6-10 inches in diameter and 8 to 14 feet long.  If you can find trees that grew in a dense stand, they are usually tall and with fewer limbs.  If you are lucky enough to have a national forest nearby you can normally get a wood permit to allow you to harvest 'dead and down' trees.  For this type of work, green trees are desired, so this means trees that were knocked down somehow and are still fresh.

The logs will last and look much better without the bark, so my son and I removed the bark using a draw-knife.  It took us about 10-15 minutes to clean a log of most of the bark, and later I cleaned them up a bit better and smoothed the knots with a chisel.  I dried the logs in the garage.  The large ones were never light, but they certainly were easier to work with once they dried out a while!

In addition to the structure, we wanted to add a wooden countertop.  The kids and I cut off a 6-foot chunk of a large downed fir tree.  I cut a good slot in it with the chain saw and then started driving wedges.  It was fortunate that we had started moving Jennifer in already, because her chainsaw was large enough to cut the log easily, and she had a bucket of extra wedges.  In the end, I had to make more wooden wedges on site to finally split the log.  And then it was too heavy to move, so we split off the outside slab again.  The log was on the uphill side of the road, so I backed the truck in against the bank and managed with some luck, some ramp boards, and some brute force to slide it down the bank and into the back of the truck.  It took around an hour to drive home, and by then the adrenaline had worn off, and I was getting a bit tired and stiff.  I tried to slide it out of the truck and could not budge it!  I had to cut small roller logs and use the crow bar to wedge them under.  It sat in the yard for a month or two slowly drying out and getting lighter!

When we had enough of the logs, we started putting up the structure.  I used 12-inch lag bolts to attach the logs together.  I notched them a bit to help make them fit nicely.  The plan is to make a shed roof, so there is a low side and a high side.  We did the slow side first and put it up as a single unit, but that proved quite heavy, and for the high side, we did it post by post with ropes and bracing to help get it done safely.  I was very glad to have Jennifer's help in this stage, because it would be quite difficult and dangerous to try to do this by myself.

The vertical posts have holes drilled in them, and their lengths were carefully calculated based on measuring down from the batter-board strings.  Since the metal posts were not perfectly centered in some cases, I had to drill off-center holes.  And since the concrete pillars were not exactly the same height, I had to cut them different lengths.  Mostly this worked fine, but I mis-calculated one of the low sides and now the roof beam will forever be a few inches out of level since it seemed to minor to take everything down and do it again!

I did not want water to wick from the concrete and stone up the posts so I added a small piece of treated 2x4 lumber between the pillar and the beam.

Once we had the verticals and rafter support beams up, we added the beams to complete the upper structure and braced it off.  The braces were made from smaller beams connected with some timber-lock screws, which are cheaper and easier to use than the larger lag bolts.  So far, they seem to be sufficiently strong to do the job.  We tried to do all of this pretty quickly since the structure is weak until we get all the beams connected and braced.

The roof system is supported by 4 x 8 fir beams from the local lumber yard.  Fortunately Dad was in town for this part and no one let him know he should be retired from this sort of work already so up he went!  We screwed these down into the log with some long decking screws.  It seemed better than just using nails and was easier to accomplish than nailing.

Finally, the beams are up!  We will add 2x4 cross-supports to hold the roofing material.  For that, we had some local help from our good friends Ellen & Reed.  We wanted lots of light, so we went with poly-carbonate roofing, which hopefully will remain clear unlike older clear fiber glass roofing.

By this time, the garden harvest was coming in with great abundance.  Jennifer made pickles and preserves and jam for days on end.  We took a break from the kitchen project for a while and enjoyed the late summer!

But the weather will turn again, then fall is a good time for concrete work!

Making a Cob Oven: Setting the foundation

In the spring of 2017, life was busy.  I was getting used to being single again, and at the same time getting ready to be married again.  It was a good time for some heavy labor and a large project to help focus my thoughts on the better parts of life.

I had pondered building a good pit barbecue  in order to make some good Southern pulled pork for a number of years, and my new love Jennifer had been thinking about building a cob oven for even longer.  We agreed on a nice location next to our back porch and combined our two plans.  We would have a covered area, around 9 x 12 feet dimensions, with concrete and stone foundation, pole and beam construction for the roof, with a fire pit and cob oven.

As luck would have it, the winter of 2017 gave us a significant ice and wind storm up near Mt Baker and there were trees (and rock slides) down all over the place.  When I woke up at 5 in the AM unable to sleep, it was a perfect time to drive up and grab a load and be back in time for work.

I grew up building things with my Dad, and I just cannot help but dig real footer when I start on a structure.  In this case, since we planned to support a large stone and cob oven, it was well worth the effort I think.  My kids helped me place the batter boards and measure the diagonals in order to true in a square framework to build against.  The nice thing about this arrangement is that you can easily take down the strings to work on the walls and put them back up to check your measurements.  The boards should be far enough outside the building area that they are not in the way.

I was thinking we would end up with an earth and gravel floor, so I also dug a french drain on the uphill side in order to channel any water around the structure and foundation.

I dug about 2 x 2 foot  footers for the corners, down about 12 inches and filled them with rocks and concrete.  Trick of the trade:  square off the corners and sides and bottom of your footers so that the eventual block of concrete and stone is square.  That makes it less likely to want to roll in the future.  When nearing the top of the corner pier, I set in a large steel bolt that will later be used to hold the structure's vertical support in place.  Use your batter board strings to set these as close to centers as possible, it will make everything easier if they are on center.

Fortunately, there were plenty of rocks available.  When scavenging for stone, you should NOT get stones from in or near streams.  It is generally against the law and it will often erode and pollute the stream.  River rocks do look nice though, so I bought 6 yards of mixed cobbles from the local rock store.  Mixed cobbles are around $40 per yard, where sorted river rocks are closer to $500 per yard.  The kids and I sorted them, using the smaller gravel to fill the french drain and for related projects.  The larger ones (about fist size and larger) were set aside for later use.

Once the four corners were complete, I dug some trenches to be the foundation of the 3 walls on the lower side of the structure.  The dirt from the trenches and french drain is piled in the middle to become the floor.

I use cement for mortar instead of 'mortar' mix.  Cement is stronger and is what I am used to using.  To make it, you get a bag of Portland cement (94 lb bags typically, lift with your legs!) and a bunch of sand.  I think it works out to about 5 bags of Portland per cubic  yard of sand.  The ratio is 3 parts sand, one part Portland.  Mix it dry in a wheelbarrow with a hoe and then add water.  When you are filling trenches with rubble, you can mix it pretty soupy, but when you start laying rock walls then you need it to be fairly stiff in order to hold in place while you set the stone.  When building up walls, it is easiest to do 1 or two courses of stone at a time, so build wide and let it dry for a day and then put on another layer.  It takes practice in order to make it look nice, but even with modest skill you can at least make it strong!

Coming up next...out of the ground!