Thursday, June 9, 2016

Wind, power, and wood oven class.

Another long night of heavy winds which seems to be more and more common. We get a lot of good cover from our trees, but that doesn't protect power lines elsewhere. We lost power for around eight hours last night which is very inconvenient. Fortunately other than needing to throw out most of what is in the fridge, it's just that, an inconvenience. Sadly part of that inconvenience is not having our white noise machine that helps the kid sleep, so we had a long night.

Before that though, I went to an interesting class last night at the River Valley Market Co-Op that I'd intended to talk about today, so that's going to be below the cut.

The class I went to last night was on building earthen ovens, which is something that has sounded interesting to me for some time. The class was thorough, interesting, and had all of the information I needed to do decision making on building an earthen oven, and probably enough to figure out how to do it myself with a little bit of online research and experimentation added in. Before I get into the details though I'm going to credit the class properly

Earthen ovens in your backyard
Presented by Broadfork Permaculture and Ashley Schenk
Recommended other resources: and Build your own earth oven by Kiko Denzer

Ashley Schenk was the gentleman teaching the class, and he is knowledgeable and experienced. If one were to want to build an earthen oven I'd recommend a class with him, or hiring him to lead your work crew since that's something he provides as a service. I was frankly impressed with how well he knew the subject and how he handled a class of people who kept asking questions instead of letting him get to the material.

Let's get down to it.

Bottom Line Up Front: This is a really cool idea that isn't going to be realistic for us because of the combination of cost, relation to our weather, and specific time frame time investment. Something similar that allows for a different building method would be ideal, and I'm going to be looking into other options.

The process
Location: You need to choose the location for this oven carefully because of a number of factors. First and foremost, you aren't going to be able to move this oven. Even beyond the built onto a concrete pad problem it's going to weigh in around a ton or more. Factors that are important:

  • Fire will burn more easily with the door facing away from the wind
  • You need a safe distance from flammables (10+ feet)
  • You need a good amount of room to work at and around the oven since you will be cooking multiple things with it at a time
  • Access to water for cooking and safety
  • Access to fuel
  • Access to the kitchen
This is one place I might put one of these ovens.
Size: How big do you want this oven? Is it going to be for a single family use with some parties? Is it going to be for a community, or a village? This is going to affect cost, time, and skill. It will also heavily affect the fuel cost for firing the oven which I'll get into more later. Kiko Denzer's book has a great chart of oven materials to help make this decision as well.

Foundation: This is to provide both a stable base, as well as provide drainage away from the oven. The information he gave sounded very similar to a french drain, and I'd probably do an actual french drain for increased drainage given our area. The foundation recommended is a 6" - 12" concrete pad "floating" on a gravel and fill base. The gravel should be going down around 12" - 24". This is one of those things that is going to depend on your environment, where we are with deep frost heaves, 24" + would be wise.

Work time for the foundation is approximately 1 full work day with 2 - 4 people digging, packing gravel, tamping, framing, and pouring the concrete pad.

Wait for at least a week maybe two for the pad to cure properly.

The Base: The base brings the oven up to a good workable height that is going to depend on the person. Essentially counter top height so it's easy to work with. This can be built with any number of materials and in many styles. It can be built with a hollow base if you have a good solid foundation for the oven itself, or more likely closed and filled with rubble and clean fill. This needs to be built perfectly level and stable because it is going to be supporting the oven itself, and any imperfection at this point will affect the quality and stability of the oven itself.

This is also when you'd build in extra shelf area, or grilling areas on the sides of the oven. Good ideas I've seen are shelves off to the side to place baking materials.

Bottom Insulation Layer: This is to keep the heat in because otherwise the base would just suck the heat out of the thermal mass that provides the primary cooking heat. It forms a firm lip around the circumference of the foundation  of the oven. The material of this is clay slip which is one part clay and one part water mixed to a smoothie thickness. Once you have that mixed you add in perlite or vermiculite for further insulation. He warned, be careful to not breathe in clay dust.

This takes about another day or more depending on how many people you have helping, and your skill with masonry.

Sand Layer: Much like building a patio you then put in a layer of fine sand, and level it perfectly.

Hearth: This is the base of the cooking area. You can use a variety of materials for this ranging from slabs of soap stone to heavy duty fire brick. Again, the thicker your hearth, the higher the heating cost, and starting time, but the longer you can cook. He recommends for residential uses having the hearth be 2" fire brick because it is a good balance of charging and discharging.

When placing the bricks instead of placing on the sand and sliding, put the brick butted up to the one next to it and slide it down, you don't want sand working even a little bit between the bricks as they have to be perfectly butted to each other.

Door Arch: You are going to want to build the door arch now if you're going to be using brick so it can be meshed with the next insulation layer. Fire Brick is recommended for this because it is more resilient to dings than clay. You are going to be dinging the door arch because it's regularly interacted with using tools. This is a specialist thing I'd look up since my notes on it are Long.

Shaping the baking space: Mark a circle on the hearth to denote the interior space of your baking area. You then use damp sand to build the form of the interior. The baking space is going to be a parabolic shape, and to build this you use a cardboard or plywood form to increase the accuracy. Once you have built this you are going to layer two layers of wet news paper over it to make removing the sand without damaging the next layer easy. When working with the sand having one person with a spray bottle keeping the sand damp as their whole job is recommended.

Inner Thermal Battery Layer: This layer fills the same niche as the hearth in terms of acting to retain the heat for cooking, and protect the rest of the structure from thermal damage. This is built of one part clay, and 2 - 3 parts coarse sand. The clay should be a pudding consistency before you mix the sand. This is hard work to mix, and must be mixed thoroughly to build with. This should be applied in a 2" - 4" thick layer similar to the hearth. It should be of even thickness top to bottom, and packed in firmly to eliminate any air bubbles.

Top Insulation Layer: Mix clay slip similar to the bottom insulation layer, then mix in straw. This will be the insulation layer similar to what kept all of the heat from going into the base except to keep the heat from going out into the air. This doesn't need a lot of clay, the clay is just a binder for the straw in this case. This will be applied in a 4" - 10" layer. He recommends 4" - 6" that seems to be optimal for his experience. Start from the bottom, build a ring layer and pack down, build the next ring, and so on. This helps prevent slumping and uneven thickness. When it does slump cut from the bottom and move the material up.

Coarse layer: Mix one part clay slip and one part sand and apply in a rough coat for the first part of weather protection.
Fine layer: Use ideally lime sand plaster and apply a smooth finish coat to the outside for weather protection.

Clearing: Once all of that is done which should be a day of work with significant manpower help you carefully clear the sand out stopping at the damp news paper.

Firing: Once the sand is out start by lighting a small fire, and then over the next 4 - 5 days lighting slightly larger fires each day to dry the interior of the oven space. On the fifth day light a full fire to properly fire the clay that forms the oven.

Roof: In this climate it is recommended to build a shed roof over this oven to keep the earth work from getting battered by rain and snow which will preserve the life span. When building this insure you don't have wood over the thermal plume from the oven because the temperature of air coming out of the mouth of the oven will be 600 - 1,000 degrees Fahrenheit.

Door: You need two doors for this oven. First is a metal firing door that has an air inlet at the bottom, and an air outlet at the top. Second is a hardwood baking door that fits tightly and keeps the heat in the oven with no air outlet because when you are cooking and baking there won't be a fire inside.

Cooking and Baking:

  • Build a large fire in the oven and burn for 1 - 3 hours
  • Remove ashes and coal
  • Bake pizza or other high temperature foods first
  • Plan to bake many dishes throughout the day, as the temperature of the oven slowly cools.
Maintenance: You are going to have to re plaster every 2 - 4 years, and every so often depending on the oven and environment you will have to take the entire oven off the hearth, and rebuild the earth outer work.

As you can see this is a work intensive building process that will require a lot of people, and isn't going to be anything like free. If you can find clay for low cost which I'm assured is possible that will help. Using recycled materials from other projects or other places would also help.

All in all this sounds like a great baking oven that does what I want it to, but I need something I work on building a bit every day. If at some point in the future I have much greater leisure than I do today this is something I'd love to do. I'm hoping that at some point I'll be able to help someone else build one so I can watch this process and learn. If you want to know more about how to do this, or have time to do so, use the resources I listed, there's a lot from my class notes I couldn't put in, and I didn't even take notes on everything! Sounds like a very fun process with a great result.


    1. Our neighbors have one, and it's just wonderful! I'd love one, too, fully knowing tat it's quite a project.

      1. I also love what you can produce with it. For our situation it's just not ideal between the weather, and the restrictions time wise on how it's constructed. I'm going to be doing another post soon on masonry ovens and how they work for constructing them.