Wood Fired Oven Build (1/2)

(Part 1 of 2)

The following is a condensed account of our 32″ x 36″ Alan Scott barrel vault wood-fired oven build. We are sharing this for others interested in pursuing a similar project, but – just a warning – there is not enough information here to get you through a whole build of your own. If you do want to build your own, see below for suggestions on helpful resources.

It took about a year of intermittent weekend work to build the whole oven – a timeframe that does not include the initial cement pad, which we poured in December of 2011:

..nor does it include the “bread shed” we built adjacent to the pad, using home-milled Ponderosa pine from our mountain property:

It then took five more years before we managed to commit to an oven design and find time to build it. During this time, we considered various oven types and sizes, asked lots of people lots of questions, spent quite a bit of time reading internet fora, and baked in friends’ and acquaintances’ ovens. We definitely wanted an oven optimized for bread, not for pizza, and wanted enough thermal mass for multiple bread loads and/or other types of cooking during the falling temperature. We also prefer to make things on our own, rather than getting kits or having other people make stuff for us.

These preferences tilted the choice against the popular Mugnaini and Forno Bravo ovens, and towards the classic Alan Scott bread oven design. Unfortunately, the detailed Alan Scott plans are no longer available; Alan Scott passed away in 2009, and his Ovencrafters website, which sold the plans, was taken down a few years ago. A description of an AS build is included in The Bread Builders but it is not as detailed as are the actual plans, and it would be difficult for an inexperienced person to successfully build an oven using only the information in the Bread Builders. We managed to get a copy of the detailed plans for the larger 42″ x 48″ oven from a generous friend*; the plans, the book, and several suggested improvements from internet sites (mostly this one and this one) got us through this build. We also found many excellent suggestions in Richard Miscovich’s book From the Wood-Fired Oven. *More recently, I have found others sharing the AS plans informally online, including people on this site.

In 2016 we finally got started, making a plastic-covered plywood form around which to construct the oven base:

We made the oven base using busted-up pieces of old concrete salvaged from the dump and other sites (also known as “urbanite”).

Building the arch.

Almost finished with the base.

Once this base had set, we removed the form and built a new plywood form to hold the oven hearth slab and the vermiculite/concrete (“vermicrete”) insulation below the hearth slab. The unique thing about the Alan Scott design is this hearth slab, which is suspended over the space below it, rather than sitting on top of the oven base. It is held in place with a framework of rebar, and the ends of the rebar are supported by the top of the oven base. A narrow space between the hearth components and the oven base provides room for thermal expansion of the hearth during oven bakes. Oven designs like this that take into account the movement of the different components as they heat and cool are less likely to crack and more likely to give more years of trouble-free use.

2×4 supports for the plywood form.

Plywood in place.

This form included a cutout area for the ash slot; the two small blocks in the image below indicate the height to which the vermicrete layer would be poured:

We covered the plywood with plastic to make it easily removable, then added 6″ wire concrete reinforcing mesh, with some strands bent up, to provide a connection between the vermicrete insulation and the hearth slab that would be poured above it:

Next we mixed and poured the vermicrete (6 parts vermiculite to 1 part Portland cement)

Mixing the vermicrete

Vermicrete poured; wires showing above it are bent, to bind the hearth slab to this layer.

Above the vermicrete we made a wooden 5″ high form in which to pour the hearth slab. We cut holes into the form for the rebar cross-pieces that sit on the outer frame. (This rebar lattice, embedded in the hearth slab, supports the whole oven). The vertical wire pieces emerging from the vermicrete were tied to this lattice. Once the wooden form was patched to protect it from leaking concrete, we poured the slab. The slab was made from a blend of 3:2:1 crushed rock:sand:Lumnite, mixed fairly dry. We covered it with plastic after pouring to ensure a slow, even cure.

Pouring the hearth slab. Note wooden form with rebar poking through and suspended on top of oven base.

We covered the slab with plastic to ensure a slow, even dry.

Then it was time for the hearth (note: for clarification, the “hearth” is the part of the oven on which the bread is baked. The “hearth slab” is the section under this that provides thermal mass for the oven. The thicker this hearth slab, the longer it will take for your oven to saturate with heat, and the more wood this process will take; also, the thicker the slab the longer your oven will remain hot and the more baking you can do in one firing). Anyway, back to setting the hearth. For this, we used food-safe firebrick from HarbisonWalker International (“HWI”) (these guys have one of the most user-unfriendly websites I’ve ever seen; however, they were incredibly friendly and helpful once I talked to them on the phone).

The hearth turned out remarkably level:

We next laid out the arch and mocked up the walls and throat of the oven…

Laying out the arch shape

Mock-up of the first five courses of the back wall, plus the arch form with marks indicating placement of each arch brick.

Cutting custom shapes for the constricted oven throat.

Detail of the oven throat.

Mockup of all vertical sections of oven body.

More detail of mocked-up oven body. I took these photos partly as a reference to use while setting the bricks.

Detail of oven throat bricks. Each brick was custom-cut; we labeled them to keep track of placement.

…and then got going mortaring everything in place, using HWI’s food-safe OvenZZ mortar. We added in a home-made metal brace to help counter the lateral force of the arch bricks (we got this idea from Miscovich’s book); the front section of this brace is the angle iron lintel that will support the firebricks that angle down from the arch to create the oven’s throat.

Sides of oven completed.

Back of oven built up 3 additional courses.

Detail of oven side and throat.

oven throat from above.

Metal corset to counteract lateral thrust of arch.

Next it was time to build the arch. For this, we made a plywood arch form that we set in place using firebricks and shims. Once the mortar for each arch row had set we moved the form to the next row. We used wooden shims between the firebricks to help with even spacing.

First course of the arch, showing plywood arch form and stickers for even brick spacing.

Detail of the stickers used to keep the bricks evenly spaced.

Second course of arch bricks.

Interior view after two courses of arch bricks.

With three courses of brick the arch was complete:

Next came the most challenging part. In order to help the oven retain heat, the door height (10″) is lower than the oven arch (16″), so we needed to angle the next bricks down from that arch height to the height of the angle iron lintel. This is the part of the build for which the AS oven plans are the least helpful. We found the information on this website to more than make up for the shortcomings of the AS plans (thanks, BrickOvenTampa!

The first step of this constriction process involved custom-cutting bricks for each spot and mortaring them into place, using a piece of 1″ square metal tubing for temporary support.

Shims on the back of each brick adjust them to the appropriate height.

Cutting the second row of bricks to connect with the main arch section of the oven.

All set in place. Looks good!

The second part involved more custom-cut pieces of firebrick to close up the gaps on both sides, and making firebrick wedges to create a level surface across the oven’s front. This front section will become the back wall of the chimney.