Clay Oven Construction and Deconstruction, Part 3

It is time to close the books on our first wood-fired oven, which we built ourselves last spring. The oven lived a happy life — it baked about 2000 loaves of bread for us in 2014 — but it has baked its last loaf! Rather than letting our first oven go quietly into the night, however, we want to share what we learned by examining some of the problems we experienced. You may want to check out part one and part two of our oven building series for more explanation and photos of the process.

After completing the clay dome in May 2014, we insulated the dome with a mix of watered down clay combined with sawdust that we got by the trailer-load from a nearby sawmill. Ideally, after the oven has been fired repeatedly, the sawdust near the dome partially burns out, leaving a “clay foam” in the words of Kiko Denzer. We believe that we may not have used enough clay in our insulation mix, however, because we experienced smoldering fires in the insulation layer for weeks, and eventually had to redo much of the insulation.

 

Note in the picture that the insulation at the top of the dome (from the final day’s work) seems to have had the correct proportion of clay and sawdust — it didn’t burn out! We replaced all of the insulation that we could reach, but we could not get under the hearth to the subfloor insulation, which also ignited. The smoke found its way up through cracks in the hearth and the oven smoked eerily for about a week as the subfloor insulation smoldered. It stressed us out quite a bit in early July as we waited to see what would happen. Would the dome collapse as the subfloor insulation turned to ash? It did not, and we continued to use the oven until Thanksgiving, when we roasted our turkey and other dishes in the dome. But by then we knew the oven was doomed.

Every week through the summer, more and more small chunks of clay were dropping from the ceiling of the dome onto the hearth. The dome was built in layers, and it was essentially delaminating, peeling back from the door to the highest point of the dome. But don’t be discouraged if you’re dreaming of building your own clay oven. We believe that we could build a better oven if we did it again, and we want to share some of our “lessons learned,” even though we have chosen to hire professional masons to build us a new brick oven (which we’ll be posting about later this week). The professionals can build so much quicker and better than we ever could, which frees us up to do what we do professionally: bake bread and grow produce.

So, things we would do differently: (1) We would not dry-stack concrete block for the foundation. We did see a working clay oven that used dry-stacked block as its foundation, but it was about half the size of ours. That smaller oven performed fine, but there are intense forces at work with such an extreme heating and cooling cycle. We think the materials’ expansion when heated, plus the weight of the dome, caused the dry-stacked walls of the foundation to spread out slightly, thus causing much of the cracking we saw on the inside of the dome. The gradual spreading of the dome could perhaps have been halted by a rigid support or buttress, such as an I-beam running down each side, bolted together in the front and back of the oven.  We also should have built the subfloor wide enough to fully support the downward pressure of the dome walls.  Amazingly, the glass wine bottles held up under all that pressure!

(2) We used one catenary curve to shape the sand form when building the clay dome, but we should have used several. In the picture, you can see the cardboard cutout of the catenary curve that represented the highest point of the dome. This part of the dome had no cracks in it, and in fact, some of the cracks veered away from it! But the lower part of the dome, near the door, was flatter than a catenary curve, and thus was not as strong. If we built such a large dome over again, we would make several cardboard cutouts, descending in size, to guide the sand form from the highest point of the dome down to the door.

And finally, (3) Our “pancake” method of building the clay dome did not work as well as we had hoped. During the demolition of the old oven in January 2015, we could see that many of the pancake layers never bonded, which caused the dome to delaminate.  You can see how easily the dome comes apart in this video.

Ideally, the dome would have been built all at once, but this was such a large dome that it was built over the course of three days, and some of the layers partially dried before new ones could be added. And so, the dome did not behave as a single, solid block of clay, but had internal weaknesses that accelerated the cracking.

 

One thing we were happy to see upon deconstruction is that the wine bottles that we used to insulate the subfloor were all intact.

So if you are thinking about building a clay oven for yourself, we would be happy to talk with you to share even more technical details and tips and know-how. We also want to give a big thanks to everyone in the brick oven Yahoo group, and Kiko Denzer in particular. Without the advice of the helpful builders there, our oven would never have lasted as long as it did.

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6 thoughts on “Clay Oven Construction and Deconstruction, Part 3

  1. Pingback: Meet our new bread oven! | Little Hat Creek Farm

  2. harlanpotlatch

    Hey, Heather, I just got this link from Eric Moshier, through the MHA, and am so glad to see the beautiful oven he built for you. I’m also glad you’ve posted your review and analysis — I think it will be helpful to others. With your permission, I’ll link to it from handprintpress.

    Reply
    1. harlanpotlatch

      And a question: what were you able to determine about the quality of the subfloor insulation? Did it survive intact, or did it collapse from lack of clay?

      Reply
    2. junipinyon Post author

      Hi there! So good to hear from you again. Yes, of course! Please spread the word any way you can. Thanks for the reminder, I have been meaning to post a link to the posts on the brick oven yahoo group too; I’ll do that this afternoon. The subfloor did survive! All of the clay-sawdust insulation had burned to ash (too little clay, perhaps?), but we were pleased (and surprised) to see that none of the glass bottles supporting the subfloor broke. However, we also used glass bottles between the foundation walls and the oven walls, and when the oven walls spread, those bottles did break.

      Reply

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