Second oven build? Just for pizza this time…

[Kris S]

Hi all!

It’s been just over 4 years since I built my smallish homebrew cast oven at 70cm – 28” internal oven floor diameter, see here:
https://community.fornobravo.com/for...losure-belgium

I’m really pleased with the oven, great for pizza’s (the original reason for the oven), various types of dishes and also good for overnight retained heat cooking because I’ve insulated it as much as I could.

I hate to let all this heat go to waste so I always prepare a few other dishes afterwards when the fire is out. sometimes a few sourdough breads first, always a stew or two, finishing with an overnight slow cook, which means more preparation and some more work later in the evening/night.

Okay, it’s not ‘work’ of course, it’s fun to do but I notice my desire to not let the retained heat go to waste really stands in my way to fire up the oven more often for just enjoying a few pizza’s and then nothing afterwards.

So I was thinking of maybe building a second oven only for pizza’s, this oven would have the same internal diameter, but would not need to have the same big layer of insulation on top, or thickness of fire bricks (the ones I have now are 8 or 10cm thick!).

With less insulation and thinner fire bricks / tiles the oven would reach temp faster, and I could try my hand at an igloo style oven (no enclosure but a waterproof render over the dome) which I find aesthetically more pleasing. I’d use less wood, would be baking pizza faster and the oven would cool down much faster so I wouldn’t feel bad about wasting heat.

I could even fabricate a mobile stand on wheels with the oven on top…

I would appreciate some input/thoughts about:

• What thickness floor tiles would be advisable
• What thickness under floor insulation
• What thickness dome wall – previous oven had walls about 5 cm – 2” thick, I’m a bit scared going thinner, especially given the big cracks I have in the dome now.
• What thickness/type of insulation on top (2” fire blanket, or only 2” vermiculite/perlite/cement mixture?)
• Also: what would be the expected time until the oven reaches pizza temp?
• And what would be the weight of a trimmed down oven like this?

Thanks in advance for any input!

[Kris S]

While searching the forum for more information I came across this interesting piece of info from David S:

My oven is 540 mm in diam and there are times i wish it were larger, although there are probably many more that I'm glad it's small. Remember that the fuel consumption is relative to the volume of the oven chamber so a small oven is far more fuel efficient. If you increase the diameter of the oven by just 20% then the chamber volume is increased by around 70%. This also of course means that the larger oven will be massively heavier and dearer with only a small gain in internal size.Regarding heat up you would think that a smaller oven would heat faster. In most cases this is not so because it is the relationship between the wall thickness and the chamber volume and consequently the size of the fire it can contain, which determines how quickly the oven will heat up. My oven has 50 mm wall thickness but still takes an hour and a half to get to pizza temp, two hours if I'm doing a big party. Most oven owners only cook one pizza at a time because it takes longer to prep a pizza than to cook one.
I sometimes use an L shaped piece of stainless around 150 mm wide, placed to one side. If the live fire is contained in it then the pizza base nearest to the fire is protected from burning and I still have 490 mm of oven width, plenty for a 30 cm (12") pizza. I usually cook 9" pizzas though because it's easier to make and handle the bases that size.The height of the oven door can be a restricting factor for a small oven when used for roasting. Mine will just fit a 5 Kg turkey, a bigger bird wouldl be too tall to fit through the door.
I regularly cook a loaf of bread and a couple of baguettes (how much bread can a family eat in a day), firing the oven for one hour exactly from light up and using surprisingly little wood. You couldn't do that with a large oven. Likewise you probably wouldn't bother to fire up a larger oven to only cook two or three pizzas, but we do frequently.

https://community.fornobravo.com/forum/pizza-oven-design-and-installation/other-oven-types/393365-minimum-size-for-a-castable-wfo-dome

I can follow the reasoning (I don’t doubt David's knowledge on this subject!), but it still surprises me!

Now I’m back at the fence, contemplating building a second oven or not. A smaller oven would not heat up faster, in fact may be slower than my oven right now (mine can reach pizza temp in about 1 hour).
Yes, it would consume less wood, but that’s not too big an issue for me since the 28” is already on the small side, (I once measured I can heat up the oven, cook 10 or 12 pizzas all with 10kg of oak).
t would cool down faster though, not leaving me uneasy about the ‘waste’ of leftover heat.

On the other hand I shouldn’t worry about it too much and remember I already tailored the oven at 28” especially to be flexible in use: good for pizza and also decent for retained heat cooking. Kinda like a dual use.

But also: I might just be looking for a reason to start a new mini DIY project and try my hand at making a somewhat waterproof oven without enclosure. I’d also have to learn how to weld if I decide to make a metal ‘trolly’.

[david s]

Even if you weld it yourself a steel trolley will cost way more than a masonry stand. As most ovens are out in the weather, provided the outer igloo form is waterproofed on the outside, flue pipe to oven properly sealed and any cracks where water can enter are sealed, you should have no trouble. We live in the tropics and ovens out in the weather don’t usually need any special drying fires like new ovens do, although this year we’ve had 3 metres of rain already and our annual average is only 1 metre. This has meant drying fires for both my home oven and the one I have on a trailer which are both out in the weather.