Earth Oven pt 3: the Oven Dome

After the Storm

Last week, I talked about the work I and others did to make the Firebox portion of our Earth Oven out at Knight's Rest Retreat.  Today, I'm going to further expound on that in the visual, and show you what it was like building the baking dome portion, which is the actual cooking area of the oven.

This

Is

OVEN-DOME.

2 Muffins go in, but only 1 Scone emerges victorious!

Anyway, now that I've gotten the cheese outta  my system (well, mostly anyway - there are a couple of quiches still awaiting trial runs in the oven to do some more timing/temp tests!), on to the meat (roasted) of this article.

Building a Positive Mold

In order to get our dome built on top of the firebox, the first thing we had to do was figure out how to get the actual dome shape built. We set the brick work for our chimney while we were making the final decision, and let our bricklayer do his magic while the rest of us debated about method.

We decided it would be easiest to build up layers of our cob over a positive mold of shaped dirt, which could then be dug out once the cob was dry and solidly stable.

Once we'd decided how we wanted to proceed, we built a simple wooden form in the shape and size of our intended door and set it in place.  Then we started building our dome mold at that point, packing the dirt in well and making certain it was absolutely level where we were going to be putting the brick arch support of our entry door.


Once we had our door frame mold in place to use as a guide, our bricklayer went to work again, setting our brick archway in place.  This was a much shorter process than laying the bricks for the firebox, but was still labor intensive enough that I was glad I wasn't the one having to do the actual work.

After he'd placed all the bricks into the archway except our keystone (the one at the very top of the arch), things started getting mortared into place. The keystone went in last, and then the mortar was allowed to set while we all took a short break.

After the break, we moved on to rearranging the excess dirt so that our dome mold would be uniform in height and properly align with the door.  It was a few hours of work, which included turning the top layers into a thick muddy slurry so that it could be properly smoothed out.

When we finally got everything smoothed and evened out, it was obvious this oven was going to be even larger than we'd originally anticipated.  The height of the dome had been lowered, but the overall surface area to cook in was going to be much larger than was first planned.

We needed a means of keeping the cob mix from sticking to the positive mold while the dome was being formed.  Our solution was simple: newspapers that were going to be recycled anyway were pulled out and put to use.

A double layer of newspaper was placed over the entire dome (except for the doorway arch), and then a thin layer of straight mud was put in place as the bottom layer of what would become the interior of the dome.





The kids, of course, were more than eager to "help play in the mud"

A helper (right) and I (left) applying the initial mud coat.






Once that was finished, it was time to start mixing our cob to form the dome. Cob has traditionally been a mix of clay, sand, and straw.





Think of it as a sort of rudimentary cross between the basic material for bricks, and a primitive cement, all rolled into one.






 Yep, you guessed it.  Trevor's kids wanted a chance to play in the mud for that part too!  Thankfully it was still pretty warm outside in late September and early October here in Oklahoma, so hosing them off at the end of each day wasn't an issue!

While it was still messy work even for the adults and older youths helping out,





 
at least we weren't bad enough to need actual hosing down before going in the house to clean up!





Yes, building the dome of the mold took longer than expected, and it was well past twilight by the time we'd finished putting the layer of straight mud over our newspaper barrier and adding the first layer of cob.





Then we needed to allow some time for that layer of cob to set before we added another, but that would require working in the dark.




We called a break for the evening to allow things to start drying, and met again a couple of days later to start slathering on further layers of cob.

As I mentioned a moment ago, traditionally cob is formulated from a mix of clay, sand, and straw. We used a ratio of 70% clay from the high quality clay banks on the property (there are advantages to having a stream running through the middle of your acreage!), 20% sand from the one of the sand banks of the same stream, and 10% high quality grout mix that we happened to have on hand.


We mixed up our mud - that clay/sand/grout mix - in a small bucket until it was the consistency we desired.  We originally intended to mix our mud completely by hand, but that proved to be far too labor intensive when we considered Cordless Power Drills and Drywall Mixers!

Jen, Nessa, and Rhi
The Dirty Girls of Knights Rest

Once our mud was mixed by one of the guys, he would bring it over to our big mixing bucket - a repurposed and somewhat banged up small feeding trough.  The mud would be dumped from its small mix bucket into our larger mix bucket, and we would then add a 50% straw content to add support and help hold the structure of the cob over the long term.  This portion was done completely by hand, as we didn't want the straw tangling itself around the mixer bit - and we only had one mixer bit available for use.

This also proved advantages in other ways.  Since our Mud Guy could only mix up so much on his end at any one time, we had the lag between mud deliveries to mix in straw, and place the now ready cob onto whichever portion of the dome we were currently concentrating on for that layer.


 After getting the initial 2 layers of cob in place, the thought crossed several minds simultaneously that adding some extra internal support structure might be a Good Thing over the long term.  To this end, we decided to place a single layer of chicken wire between the cob already in place and what would be going in on top of this.
 

We also decided to add a small, non-functional secondary "chimney pipe" so that later on we can do some serious Sculpting work and make the oven look like a Troll smoking a pipe.  (That part hasn't been started yet - I'm still working on the small scale practice version!)

Me and My Shadow

At this point, we had our positive mold in place, our door arch built, a thin layer of mud without cob mix, 1 thin layer and 2 thick layers of cob, then a layer of chicken wire, and 2 more thick layers of cob over the wire support structure.

And it was back to work placing thick layers of cob mixture over the entire dome.

At this point it was decided to pay a bit of extra attention to the area where the dome was being built up around the base of the chimney.  We wanted to make certain we had a good seal between the cob and the brick, so that we wouldn't have to cope with excessive heat loss or unexpected drafts of air through the cooking chamber.

As a group, we spent 6 days of laying in a couple of layers of cob each day, and allowing for a day or two of drying time between sessions.  We wanted to give each successive set of layers time to dry somewhat, independently of the whole, so that we could locate and repair cracks without them becoming deep fissures that ran through the whole thickness of our dome structure.

Our final day on the project to build the dome itself, there were only 3 of us out there getting muddy - Jen (the owner of Knight's Rest) myself, and my buddy Vanessa.  It's mid-October, and you can see from the photos that it was still warm enough for shorts and t-shirts!



Laying the final layer of cob turned out to be the most time consuming of all the cob layers.  We wanted to make certain it was smooth, uniform, and was visually appealing.  A lot of smoothing, both by hand and with small bricklayer's hand tools later, and we finally had things looking the way we wanted them to!


Our oven build was complete... almost.  We still needed to give it a week to dry out completely.  Then we had to dig out the dirt used to form the positive mold for the dome.  And build several fires, each one successively hotter and more intense than the last, in the cooking chamber itself to cure out the cob completely and to burn off the remains of the newspaper we used to keep the mold and dome separate.

We're still doing test runs in the Oven, to see how large a fire we need for various temperatures, and how we need to adjust recipes for timing so we don't end up with burnt or undercooked foods.  While under-cooking can be salvaged easily enough by simply putting our cooking vessel back into the Dome for a bit longer, burnt bottoms aren't so easily fixed.

Next week, I'll discuss some of the difficulties, tips, and tricks that we learned during this build, our plans for the Oven Mark II, and extra considerations to be taken into account for different areas.  Bon appetit!