A couple of years ago, I decided to try and keep a bit better track of my oven’s performance. In particular, I was interested in seeing how much wood I was burning compared to how much bread and other cooking we were getting out of it. My data is neither consistent nor precise, but the exercise has been useful, if only as a good excuse to focus my attention on what I was seeing and doing.

I’ve posted my “oven journal” separately; it includes specifics of each oven firing, including how much fuel I used, how long I fired, what we cooked, and how long the oven held the temps, but being as I was always doing several other things on bake day, the records didn’t have enough hard data to be useful in any scientific way. However, I did learn some things, which I’ll try to summarize here, with some anecdotal information which I hope might be useful.

Four things stand out:
1. The effect on fuel/firing cycles of water content and weight (density) of fuel.
2. The value of insulation, and the various insulating properties of different materials
3. The effect of multiple firings on do-it-yourself, hi-temp insulation materials, and
4. An idea for further limiting conductive heat loss by including a “thermal break” between the dense masonry and the insulation

1. FUEL: Weighing my wood before and after drying it out in the final heat of the oven allowed me to compute how much water I was able to drive out of a load of wood. One 35lb load of “seasoned” firewood came out of the oven 9% lighter than it went in. That means it lost 45 ounces of water – nearly a quart and a half! Imagine how long it would take to mist that much water into your burning oven fire, and how much it would cool off and slow down the fire.
2. FIRING TIME: How long you fire the oven makes a big difference in how long it holds heat. Every half hour is significant. Related to this is the fact that the weight, or density of the wood is what determines how much heat you get out of it. A small pile of dense oak will burn longer and put more heat in your oven than a big pile of soft pine or cedar (assuming they’re both dry dry dry).
3. INSULATION: When I started paying more attention and taking notes, I would frequently end up with temps of 250 almost 24 hours after starting a fire. Then I started to notice that my oven didn’t seem to be holding heat so well. One day I noticed a small plume of smoke or steam rising from the back of the oven. There was a hole! “Aha!” I thought. “A critter has been making a nest in the warm insulation. I’m going to have to fill some holes.” When I finally got around to it, however, what I found was not a critter in my oven, but insulation that had burnt to ash and collapsed – completely.
This particular iteration of the oven (it has gone through more re-designs and rebuilds than I care to mention) was insulated with about 1-1/2 inches of “sawdust-clay,” followed by about 6” of “straw-clay.” Both materials are simply made of straw or sawdust mixed with clay slip (your clayey subsoil thoroughly crushed and mixed with water to a viscous liquid (thicker or thinner according to your preference). Straw-clay, being made of larger, springier stuff, is typically open and loose – great insulation, until the straw went from carbon to ash. Without enough clay to form a rigid connecting “foam” structure, the ash collapsed, and I lost insulation value. The layer below, however, of sawdust mixed with a higher proportion of clay, burnt out to a soft but rigid layer of fired clay-foam.
I repaired it with pure sawdust and clay, thinking it would last longer and insulate well enough. However, the oven did NOT hold heat as well as it had previously. Since I wasn’t prepared to replace the newly replaced insulation and plaster, I just had to swallow my pride in the oven’s performance and continue. However, performance HAS IMPROVED over time! Greatly. Clearly, repeated firings have caused more and more of the sawdust to burn out, so the insulation is getting lighter and more insulative.
4. THERMAL BREAK: Since the insulation is tightly packed against the hot masonry, however, it occurs to me that it would make sense to design in a non-conductive air gap between the dense masonry and the light insulation. Since doing the journal, I’ve tried this idea on several new ovens, using a layer of cardboard or paper machier, which will burn out and leave a void. But they were ovens I built for other folks, so I don’t yet have info about performance (my own or anyone else’s). When I do, I’ll post it.

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