Finally, the Oven Build Begins

I’m pretty patient, but waiting for the top slab to cure was pretty difficult. I removed the rickety form the day after the pour. The concrete was indeed smooth where it had been poured on the plastic covered plywood. Turned out I was very lucky that not only had the concrete stayed in the fragile form but removing the form in most places was pretty easy...it literally fell out of a couple of places. I tried to remove the ash slot void form, but it didn’t come out. I figured that I’d better wait for a full cure before I tried any brute strength techniques to remove it. I sprayed the concrete with water every day and kept it covered to make sure I got an adequate cure before continuing.

While waiting for the slab to cure, I had tried several brick patterns on the hearth mock-up. Although I liked the offset pattern, I’d seen several online recommendations for a herringbone design. The herringbone just looked a little intimidating to me.

After a week, I decided I could remove the ash slot void form and put the insulating pad/slab on top. From my mockup, I determined that I would need an insulation pad about 55" deep (from the oven edge of the ash slot) and about 45" wide. I found that the 2x4 ash slot void was almost impossible to remove. The wood had absorbed water and swollen considerably. I thought I could beat it out with a hammer...not! Next I tried drilling holes to weaken it enough to beat out...not! Ended up drilling, chipping, and beating the 2x4s for about an hour before finally getting it removed. Slot turned out nice, but I’ll always use foam for any future void form I may need in a concrete job.

Built another form on top of the slab for the perlite/cement insulation layer. This perlcrete would be mixed in a 5:1 ratio (by volume). The reason I used perlite is that it was pretty cheap ($12.54/bag) at the place where I got my block & firebricks and the each bag contained 4 cubic ft. I added five buckets of perlite into a wheelbarrow and one bucket of cement. I mixed the two dry ingredients together first and then slowly added water. The materials are pretty weird and you have to be careful to do more of a folding motion with the shovel. When I’d added enough water that I could take a handful of the mixture, squeeze it, and have it remain in a single lump--I was done. I used a bucket to transfer the perlcrete up into the form. I needed a little over that 4 cubic foot bag to fill the form, fortunately I had purchased a second bag...

I tapped the sides of the form and screed out the top level. After a day, the perlcrete had started to firm up, but it seemed too crumbly around the outer edges to remove the form. My brother-in-law (with actual masonry experience) stopped by and thought the perlcrete had been a bit too dry when I’d mixed it. So, we dug out about an 1"-2" along the form edges and refilled with a slightly wetter mix (still 5:1) which cured with much better “firmness”.

Mark, my brother-in-law, and his wife were willing to stay with us for a few days to help me get the hearth and some of the brickwork done for the oven...talk about relief! I spent quite a bit of time showing him pictures of WFOs and we talked quite a bit about they worked and what I wanted for this one. After he had the basic concepts down, we went out the garage and he discovered I had no masonry tools, so off to the hardware store to pick up several types of trowels, sponges, and some wooden wedges--I do enjoy shopping. Borrowed a 10" wet saw from a neighbor. Ended up getting a new diamond blade and a new belt to get it working to cut our firebricks.

Mark first set & leveled damp builders sand on the cured perlcrete pad. Next he started laying out and leveling the hearth bricks. It was easy for him to do the herringbone pattern and I’d recently read that it was best because it wouldn’t catch the edge of your pizza peel. We did a dry run for the layout of the oven chamber base. I wanted a short beavertail design so that I could easily pull out the coals and ash into the ash slot. This design also let me more easily access and use all of the hearth area. Once we’d both agreed on the layout, he started to show me how to properly set the bricks. We used ‘Sairset pre-mixed high temperature mortar and did not taper any bricks. I had read it would only take one, 5 gallon bucket for the oven...but by the time Mark & I had finished the fifth chain/course of bricks, we had emptied 3 buckets of ‘Sairset. I wish I had found the home-made high temp mortar info on the forum before I finished the build.

Mark & his wife had to leave us after finishing the fifth course, but he’d made sure that we’d talked about setting the chimney/flue placement. So, I guess I’m back on my own again...but now I actually think I know what I’m doing.