Pizza Oven Base (Fireplace Part V)

Recently, we poured the concrete slab that the pizza oven will sit upon. Beneath the pizza oven are two wood storage alcoves, so it was necessary to build wooden forms. The slab that supports the pizza oven also serves as the ceilings for the wood storage alcoves. For reinforcement, I threw in some left over woven wire (mesh) fence before we bucketed in the concrete. After placing 8" of concrete, I worked in some horizontal rebar to further strengthen the slab. A sacrificial scrap of yellow foam insulation was used to "blockout" the space for the ash dump in the back of the pizza oven.

Just as the concrete was starting to set up, I laid out the floor plan of the pizza oven on the slab (top left picture in collage). The next day, I built a form to match the floor plan. Into this form, I poured a homemade concoction of perlite and cement that will help to insulate below the pizza oven (top right picture). This insulating layer is 3.5" deep. Most recipes on the internet say to use a 6:1 ratio of perlite to cement, but I found that the volume of the perlite decreases by a factor of two when you get it wet in the mixture... hmmm confusing. The ratio that I settled on was 4 cu feet of perlite (dry) to 50 pounds of cement (dry). This makes about 2 cubic feet of castable insulation and costs less than $15 (perlite: $8 , cement: $5 , water carried in buckets from the pond: priceless).

The manufacturers of the pizza oven kit sell "castable insulating refractory," which I presume is a mixture of perlite and exotic fireclay and will probably tolerate higher temperatures than my homemade brew. It must be something fancy, because it costs about $50 for 35 pounds, and only makes about 1 cubic foot. To be safe, I put 1.5" of the fancy "castable insulating refractory" on top of my homemade refractory. (It took $100 of that stuff to make a 1.5" layer - egads!) Instead of building another set of forms for this second layer, I simply loosened the screws on the first set of forms and raised them up 1.5". (the first layer was of course partially cured). The finished slab, ready for the firebrick hearth, is shown in the lower right picture of the collage.

So now I have a 6" layer of fireproof insulation upon which we can build the wood fired pizza oven. I am told that you can't put in too much insulation... it will reduce the time it takes the oven to come up to temperature, it will increase the time that the oven stays warm, it will burn less wood, and it might make using the oven on a cool summer night tolerable. These are all good things , so I'm trying to fit as much insulation as I reasonably can.

By the way, when I poured the homemade insulation layer, I got a little carried away and mixed too much insulation, so I parged the throat of the rumford with the leftovers. Here's a picture of the Rumford next to the pizza oven base.