Oven Performance

Brian, from Iowa, writes:

I live in Iowa, USA and have built a traditional woodfired
oven(cob). Makes great bread but uses lots of wood. I am searching your site for info on the plans/current operation, preformance of your present oven. How is it going? Any
thing on the blog besides the original construction?



My reply:

Sorry, running the bakery means we don't get round to blogging etc as often as we'd like. The oven is performing well. It bakes continuously, about 3 deck loads per hour, between 8 and 26 loaves per deck, depending on the size and shape of the loaves - 8 large cobs as opposed to 26 small pans. I am happy with the way it bakes, when I manage it properly.



We get our wood from a local sawmill, and use side off cuts around 30mm square in section and up to about 500mm long. These cost about GBP50 per crate, which measures about 2mx1mx1m and lasts about 3 weeks. One day's firing uses a volume about 600mmx500mmx500mm, which is enough to bake up to about 100 loaves.



On the downside: it took about 6 months to figure out how to manage it properly - briefly, it comes down to a rather uneven heat distribution, and the need to bake loaves evenly, i.e. not scorch them on one side. To do this we need to 1) heat the oven slowly, about 5 hours to come up to 240 degrees or so, this minimizes heat differentials and 2) move the loaves around after the first 10-15 minutes. The fire box measures about 180mmx180mmx180mm; a fire this size is more than sufficient to heat the oven - I have heated the lower deck (the hottest) to about 450 Celsius. The uneven heat is due to the direct heat transfer from the fire to the oven small differences in the flow of hot air around the oven can mean quite substantial differences in the temperature of different parts of the oven. There is no means of control other than simply loading more or less wood into the fire. Using this method it is a bit of a challenge to get oven to the right temperature and keep it there - but an enjoyable skill to acquire.



Lastly, this kind of oven is a complicated project to build - it took me about two months, and I had a blacksmith to do the metal work (by far the biggest part of the job), and a couple of experienced builders helping with the rest.

Blocks and mortar

The next step was to fit the blocks nicely and mortar them into place. For this we got Dave our neighbour downstairs to come and help. He is a builder and can do this kind of thing standing on his head. Ok, I've never actually seen that but I bet he could. I'd like to see it, for sure. I'm definitely going to think up a situation that would require upside-down brick laying.

So anyway, Dave pitched up back of nine with a load of sand and cement. First he built the outer walls and we mocked up the fireplace:

 

Next he set the fireplace and the combustion chamber floor:

All that stuff under the block is loose fill vermiculite insulation. After that was done we had lunch. After lunch we built up the front and the riser sleeve:

And that was all the blockwork done. Eight hours on the nail. I think I could have spent a week doing it by myself, easily, and it still wouldn't have been straight or level.

The Genie of the Oven

Having recognized my lack of experience in blockwork, I got my pal and neighbour Dave to come and spend a day working on it. Dave, being a builder, has plenty of experience with block and he sure did earn his money.

First though, the floor needed to be ready to receive the block. This meant a stabilizing sheet of ply and lots of screws, and several sheets of vermiculite board. You can use loose fill vermiculite for all kinds of things and it is very cheap. Vermiculite board is quite a bit more expensive but it has good compression strength, a rated operating temperature of 900 degrees celcius and provides excellent insulation. Guy and I spent half a day measuring, cutting, screwing down and figuring out how to get the vermiculite board to cover the required area without wasting too much of it. In the end it looked like this:

The last bits to go in were those two small dark triangles. The ones that look like the eyes, under a massive turban. As soon as I fitted them Guy said "Oh, it's a computer-generated genie."

It makes me irrationally happy to think of him down there, insulating the floor and keeping us safe from bakery fires.