The pattern for the oven was based on a Forno Bravo design which I bought. I laid it on melamine. I followed the plans exactly. the good thing about using the insulation is it is somewhat forgiving and if you take your time can be trimmed easily. I used a sawzall to trim it. I also used a sander (yes, an electric sander) to smooth out the final mold. It's very smooth.
Using 2" rigid foam insulation I "carved" the form for the pizza oven cavity. The floor area is 32". Because this is for home use I didn't want to make a huge oven - 32" should accomadate 1-2 - 10inch pizzas.
I used something like 5-6 sheets of the insulation. I made a simple circle maker to make the circles and the round arch.
I made the arch too small (notice the flat pieces under it) and this broke apart when I removed the mold from the cast. Make it all one continuous piece next time.
I had thought I would "pour" the cement, so I built a negative mold with spray foam - bad idea. I ended up just casting on the mold and melamine board. Worked great.
Be sure your cement isn't too wet when casting on the mold - too much slump slides down. You can see that a little bit on a front shot of the cast - notice how the bottom is thicker than the top, you want to avoid that as much as possible.
If you start and the cement is too wet, throw it back into the bucket and mix in a little dry cement - not an approved method but it will work.
The mold did not release from the cast as I'd liked. I wrapped the mold in food wrap film. I think if I used plastic sheeting (3 mil+) it would have released easier. I also should have caulked around the bottom edge of the mold so cement wouldn't creep under the mold and in the process made it harder to release the mold.
Good news is the mold is mostly intact and I have a better plan for next time.
As I have said un-molding the cast was tricky/hard. Getting it off the melamine board was straight forward. I had put long screws in the mold to hold it in place - bad idea, use short screws as they come out easier.
The cast weights a couple hundred pounds so lifting it up off the melamine required muscles and lucky I have a strong family!
I left it on it's back because I didn't like the thin edge on the oven side of the chimney. I made a cardboard form and built it up. I'll post the repair before and after.
I've built a few outdoor kitchens using steel framing so I thought I would use it for the base of the oven. The studs are 12" OC vs 16" and the top studs are 10" OC. I reinforced the front portion over the arch to help support the weight and the oven landing which will stick out approx. 8". I will have a couple of supports for the oven landing. This will make the front visually interesting too. It's also why the two studs above the arch are faced flat - the supports will be anchored to these.
The oven floor will be made from refractory cement poured into this mold. The thickness will end up being just a couple of inches (one bag) but I think it will work just fine.
I will be pouring a "slab" of vermiculite concrete on the base, then setting the floor and oven on the slab.
Here is a small time lapse of the casting process. If you notice at one point I scrape away the cement and re-mix it. The cement was too wet and was slumping.
The oven floor is 32" and I used roughly 7 bags of refractory cement
Here is a video of un-molding the form from the cast.