Burying the Evidence of Shoddy Brickwork

Now that my oven dome is cured (water all driven out), it’s time to put on some insulation. Basically, I need to bury/cover the dome with a 5 to 7 inch thick layer of perlcrete. Ideally the perlcrete should be a 10:1 mix (10 parts perlite and 1 part cement) for the dome insulation.

In many ways, it was a good thing that I needed insulation over my oven build...didn’t want someone wandering by that knew about brickwork...or more importantly point out how much I still didn’t know about laying brick. Creating an insulation coat of perlcrete over the entire oven sounded pretty easy at the time (and a great way to hide/bury the evidence of my shortcomings as a brick layer). As with all my projects, once again, I was able to make the job more difficult than it needed to be.

I decided that in order to make a consistent thickness of perlcrete over the oven dome, it would be a good idea to use wire mesh placed over the dome at a set distance. When I laid on the perlcrete, I could keep the thickness of the insulating layer fairly even just by keeping the wire in the center of the layer. Subsequent methods by Gulf & UtahBeehiver (Forno Bravo forum) would have been much better, but this worked for me at the time.

First, I built quite humorous (and flimsy) forms around the oven perimeter using bricks and wood scraps. I poured a 3-4 inch thick layer of standard redi-mix concrete a minimum of 4" out from the oven base and then embedded wire mesh into the concrete. When the concrete set, I cut, shaped, formed, and secured the wire over the dome. I was pretty happy that this wire mesh would give me a good thickness guide and add some strength to the insulation cap. Into the concrete I also embedded a leaning piece of ½" rebar on each of the front opening sides to add a little more buttressing strength for the front arches.

What a pain! The perlcrete does not want to “stack” up the sides of the dome. Some will hold behind the wire, but mostly I put it in place and it falls out of place. In addition, I had read that putting a layer of diatomaceous earth (DE) between the dome bricks and insulation layer would help provide expansion slip (as the dome expanded and contracted during firing/cooling cycles) and actually fill minor cracks in the dome mortar joints. However, that DE layer also kept the perlcrete from adhering in any way to the oven bricks...bummer! I also could not get the 10:1 perlcrete mix to hold, so I backed off and ended up using a 6:1 or 7:1 mix that at least I could work with...just a little frustrating!

Decided that it was really just the vertical, lower half portion of the oven that was the problem...so...leftover plywood to the rescue. I set pieces vertically along the base of the oven and then I could simply drop the perlcrete into the space between them and the sides of the oven. The wire mesh didn’t seem to impede the perlcrete as it was dropped down and the loose insulating perlcrete mix appeared to encase the wire nicely as it filled the gap. Once I got to the portion of the oven where it started to “dome” significantly, the perlcrete finally would stay in place. With this technique, insulating the oven dome went quite a bit faster...until of course we ran out of perlite and were due to head out on a trip.

Oh well...I set up a canopy again to keep the rain off and figured the perlcrete would just dry and cure until we got back from our trip...but right now, I need a beer!

Starting the Cure

Final oven specs - 19" wide opening, 11.75" height, internal 20" high, 39" widest internal, depth is 42" (cooking door brick inside edge to rear wall), 11" inside entry brick to edge of ash slot, 3.5" x 22" ash slot, and door height to dome height ratio = 59% . (My original target for the opening/dome height ratio was the “prefect” 63% ratio, but additional 0.75" at opening height was not in plan specs–I think, along with others, that the 63% is a guideline and the igloo/Montreal/beavertail style shape changes the flow dynamics. Bottom line is that when I fire it up I’ll find out in a hurry about if all’s well.)

After cleaning up the area and returning the tile/brick wet saw to our neighbor, we decided to make our first curing fire into a party. So, mid-morning on September first we made a few calls to friends and folks involved in the building process to the initial lighting of the oven. After dinner about 7:30 pm, we poured a little White Lightnin’ on some newspaper and put a match to it while our dozen guests watched (and I’m sure wondered what the big deal was...). I fed the fire an occasional twist of newspaper to keep some flames going and after about 30 minutes the dome was registering about 200F (IR gun reading). No problems showed up and the chimney/vent system seemed to pull the smoke up & out just fine.

I’d made some whole wheat bread in the house oven, sliced it up into little strips and put them on a baking sheet. Added salt & pepper, a slice of pepadew sweet pepper, plus a few shavings of Parmesan drizzled with EVOO and put ‘em back in a 425F oven to crisp up a bit and meld the flavors. Brownies for a sweet touch of dessert and our first WFO party ended after a couple hours and a lot of laughs (and obviously no pizza).

After a week of gradually increased firing times & heating of the oven, I got a center top section that started to turn “white” where the soot burned off (~700-800F). Continued for several more days bringing the oven chamber up to high enough temperature to clear the soot off the inside surfaces. My cooking floor bricks were the last to come up to temp...I’m sure it was because the perlcrete was still getting rid of the last of the water in that bottom insulation layer. Several little cracks open up on top of the dome during firing...pretty small and fairly normal from what I understand. Initially when the oven curing process started to actually expand the dome, those cracks would be steaming. Amazing how much water needed to get driven out of the bricks, mortar, and hearth insulation during the curing process.