(continued from part 1)
At this point the core of the oven is complete, and we’re moving out to the external layers.
First we added a course of pavers to make a level shelf at the top of the urbanite:
Then we built up the front and tiled it to make a mantle:
Next we started building up the front support columns (see picture captions for details):
The front arch was made the way we made the interior arch, using a plywood form to hold the bricks in place until the mortar was dry:
We then continued building up the support columns on both sides of the arch and filled in the area above the arch to create a level row upon which to set the subsequent courses of flue bricks. This required cutting bricks into custom-sized wedges to fill in the area above the lower parts of the arch.
Narrowing the throat of the flue required some more custom cutting and fitting of firebrick; after these pieces were in place, the rest of the front facade was easy to complete using whole bricks.
The next two pictures show the completed chimney throat and front facade before and after the installation of the chimney flue support collar. We installed the collar using thin-set mortar and expansion bolts set into the brick.
The next step was to add 2.5″ of cladding (plus an additional 1/2″ on top of the arch) to the outside of the oven, to provide additional thermal mass. (A note about this cladding: It was hard to decide how much thermal mass to add; an oven with more will stay hot through multiple loads of baking without having to re-build the fire. However, more thermal mass also means that each firing requires more wood, plus more time, to get the oven up to heat and nicely saturated. Ovens used primarily for pizza don’t need thermal mass; ovens used for commercial bakeries are best built with lots of it. We didn’t really know how extensively we would use the oven, but the idea of baking multiple loads of bread, then pies, then roasts, etc. over several days using the falling temperature from a single firing of the oven sounded appealing, so we went with an intermediate amount, enough to allow us to bake several loads with the stored heat but that we hoped wouldn’t use ridiculous amounts of wood.)
First we had to build a wooden form into which we would pour the cladding. Note wire mesh reinforcing support.
The cladding was made of a mix of 3 parts rock (3/4″) to 2 parts sand to 1 part lumnite. It set up pretty quickly, so we mixed it in small batches (5 gal rock/sand, 1 gal lumnite, 1 gal water per batch) and had some kind, hardworking friends who generously offered to help us mix, pour, and tamp it in place.
We left the form on for a couple of days to facilitate a slow, even drying, then removed it with much anticipation. We were quite happy with the results:
Before proceeding it was really important for the cladding to be completely dry, so we built several small fires over a week or so.
Meanwhile, we further insulated the bottom of the oven by adding a layer of ceramic fiber board insulation (again purchased from HWI), held in place with plywood, which was in turn held up with a couple of horizontally- oriented 2x2s bracketed and bolted into the oven base. This was not very easy, what with gravity not being on our side and the space being cramped. But we made it work.
The next step was to weld the frame for the oven’s shell:
We set the rest of the flue at this point. We used a double-wall stainless steel insulated flue made for wood stoves.
We added a little cubby hole (made with an electrical outlet box) to allow us to access our two thermocouple wires from the front of the oven. We snaked the thermocouple wires down through a hole drilled in the concrete oven base, across the underside of the hearth area, and then back up at the midpoint of the hearth area into holes drilled the plywood and ceramic fiberboard insulation under the oven. The holes were drilled to two depths up into the oven floor – half-way through the hearth slab and just below the firebrick – in order to let us track the depth of oven heat saturation during firings. This information, in combination with infrared thermometer readings of interior surface temperatures, provides a thorough heat profile during firings, telling us not only how hot the oven is, but also how long it will be able to retain that heat.
We covered the cladding with two layers of ceramic fiber blanket (also from HWI), and then covered that with chicken wire to keep it snugly in place.
Finally, we surrounded the whole oven with a shell made of recycled corrugated metal. It is common at this step to fill the space between the blanket insulation and the oven shell with vermiculite or similar particulate insulating materials. However, we had not noticed much heat loss through the blanket while burning the initial drying fires, so we decided that our insulation was sufficient without vermiculite.
Then we added a roof, flue cap, and silicone base flashing, plus a door made from a ceramic fiberboard/plywood sandwich and we were done!
The oven was finished on 7/7/17, after 7 years thinking about it and a year of weekends working on it. We were excited to start it up!
Here’s a bit of what we’ve used the oven for: