28" homebrew cast oven in walled enclosure Belgium

[Kris S]

OK thanks David!

[Kris S]

Undecided whether to use a hebel block or P/V-crete as insulation for my door.

Any preference for on or the other? Same insulation characteristics?

[Kris S]

Time to throw in a chicken. Great work, you’ve done a nice job.

Done.

[Kris S]

Unfortunately I also cracked the oven. Now I know every oven is 'supposed' to have cracks so I'm not freaking out, just wondering if this a normal crack or not. It spans about half the dome, starting from the base at the back and running to the apex.

I might have overdone it on it's third firing, especially as I wsas just wanting to cook a chicken, but I read that you first need to clear the dome, so I did just that, and than let it cool down to put the chicken in.

I guess for a chicken I could just as well have stopped firing with half the dome cleared? That could have prevented the oven from cracking... Can someone confirm if it's a good practice to only clear part of the dome if you don't need pizza temps?

Anyway, pleased overall, these are the rough temperature data as measured by a laser temp gun:

45 mins: dome starting to clear
90 mins: dome completely clear, first sighting of crack
150 mins: glowing embers and floor temps around 320°C - 600 F
8 hours after firing I still had two big pans of tomato sauce (2.5kg approx) softly simmering inside
18 hours after firing (this morning) floor temp 80°C - 176 F and dome apex temp 93°C - 200 F.

it's only the third firing, and the first full temp one, so probably still some moisture in the oven/insulation and I only have a thin uninsulated ghetto alluminum door atm, so I excpect heat retaining capacity to still improve a little in the future.

[david s]

Top marks. My Son in law says he’ll never roast a chicken another way. I like to sit the bird on a rack in the baking pan and add 1/2 cup of wine and half a cup of water. That protects the bird from the intense heat of the oven floor when it’s popped in, eliminates any burning of the juices and leaves the right amount of liquid in the pan to make the gravy easily.

[david s]

For baking or roasting, to save time and fuel I stop firing when the dome begins to go white at the apex. This usually takes exactly one hour after lighting, in my oven.

[Kris S]

Well, I'm in need of a bit of reassurance (hopefully!) about the crack in my oven...

Upon closer inspection it seems the crack spans nearly the whole dome.
it starts at the bottom, right smack in the middle of the back wall staring me in the face, and is about 2mm wide, i guess.
it gets thinner the higher it gets and about 2/3 up it splits in two with both cracks going around in a square patch the size a of hand and then rejoin again. (I think the hand sized patch could be the same size as the lumps of homebrew I used to make the dome, so maybe it could have been the last lump of a batch which was maybe just a bit dryer than the rest?)
after that it seems there a thin hairline crack running the apex and joining a small crack which goes almost to the bottom again ending right of the entrance...

I tried to make a top view sketch.

[david s]

Hi Chris,

Any large refractory section is vulnerable to cracking, primarily because of the difference in thermal expansion. A one piece dome in particular, using wood as a fuel is particularly susceptible. For this reason most cast oven manufacturers offer their ovens in multi piece sections. This makes things more manageable and less weighty sections. For mobile ovens some manufacturers offer a single cast dome as they're less likely to have problems with road vibrations. When I first began to build ovens I used a single piece cast dome, but now produce a 3 piece dome. Unfortunately this requires much more work to create a mould, particularly if the sections are to be step joined. This is hardly worth the effort for a single cast oven, the simplicity of the single cast, in situ, over a sand mould has way more advantages. The single cast does however have a tendency to produce a crack, just like yours has, which runs vertically right opposite the oven mouth. This rarely gets any worse though, and the oven will not collapse.

The attached drawing shows where fine hairline cracks (you couldn't fit a playing card into them)have developed in my oven (14 years old and still working like a charm). Subsequently I redesigned my moulds and now cast them in 3 pieces with a step joint between the front and rear sections. The top section is quite a large diameter which allows it to get hotter and expand without stressing the other sections.

See top pic: one piece cast with hairline cracks
bottom pic: 3 section castings

I th ink you are over worrying, get into the joy of cooking

Dave

[Kris S]

Allright, thanks Dave, appreciate your feedback!

Apart from the bottom section I could indeed not fit a playing card into the crack.

Next weekend I plan our first pizza's, followed by some bread experimenting, finishing with slow meat roasting.

[Kris S]

First pizza’s yesterday!

I was a little curious to see whether the oven crack would grow, but it looks like it didn’t.

I prepared 6 dough balls the evening before and let them rest in the fridge overnight.

After 90 mins the oven was at pizza temp, the first baked in 3 minutes, after the third pizza I noticed it took quite a lot longer and the dough didn’t rise as much with the crust less airy.

I was doing other stuff at the same time between the first and finishing the third was probably 25 minutes, in which time the flames had died out.
So I decided to move the coals to the other side and build back to flames so I could bake pizza’s at the other side where the floor would be a bit hotter.

The next three pizza’s were okay, but the first was still the best and since then I was always struggling to keep the oven floor at 350°C or above. I guess 90 minutes is just about okay, but it’s better to saturate the oven just a bit longer so the floor stays hot longer.
And/or I should also build my fire bigger with more coals.

I almost forgot to take a picture.

After that I baked 2 breads @ +/- 230°C (they browned really nice but taste only “meh”, still learning all the dough stuff.

@ around 180°C put in some BBQ ribs for an hour, then wrapped them in alu foil, and 2 hours later took them out. Juicy, soft and loose from the bone. Very nice.

One little question: how easy/difficult is it to damage the dome with tools like the pizza peel, brush,… when I tap the inside of the dome with my knuckle it has a very hollow sound (Duh!), sound fragile, but I know the dome is a very sturdy design. Still I try not to touch the dome when brushing the floor, moving the fire and wood around, or raking coals out.

Also: I am stoked (Ha!) about the way the vent/chimney works: 99% of the smoke when starting the fire goes through the chimney, I have no ugly black soot deposit whatsoever on the front of the oven. something I have seen in some other ovens and really wanted to avoid. So big thumbs up for David S and others from who I copieda large funneled vent.
(I use the top down fire method which also helps ignite the fire more efficiently)

[david s]

Thanks for your comprehensive report Kris. An hour and a half is usually about how long it takes for the oven to go completely white, but an extra 20 mins or so of "the fire from hell" is usually required to get it really charged. Usually the floor is too hot for the first pizza and it needs to be cooked closer to the entry where the floor will be a bit cooler. Subsequent pizzas can be pushed further in and the oven will hit its sweet spot, cooking 2 min pizzas for most of the night. However you must maintain a fire on the side to keep the temp up. Also a new oven still purges itself from moisture for several firings and you will find its performance improves for around 5 or 6 decent firings. The under floor moisture is the hardest and slowest to remove.

[Kris S]

Here's an expense overview for used materials: oven itself (+ stand) about €480. Enclosure an extra €260. Total about €742.
Only thing left to buy is the paint.

	quantity	total €	remark
hollow concrete blocks 39 * 19 * 14 cm	36	36
25 kg bags prepared concrete mix	45	135
25kg bags prepared mortar	2	5
rebar for ground slab, stand and hearth slab		0	free / salvaged
wood for slab framework		27
fire bricks	32	64
25kg vermiculite + 25 kg perlite		35	+/- 2/3 used
25kg bags portland cement	2	7.5
25 kg bags sand for dome mould and homebrew	3	5
25 kg bag fireclay	1	17
+/- 10kg hydrated lime		0	free
1 bag 600g PP fibers	1	19	+/- 1/10 used ?
1 roll ceramic insulation blanket Insulfrax 25mm thick 96kg/m3 7320 x 610 mm	1	88
6" 90cm SS single wall chimney pipe	1	43
cement based boards 90 x 120 cm 12.5mm thick ( ~ hardiebacker)	5	125
wood for enclosure and roof		88
wire mesh for stucco	10m2	30
primer for stucco		9
more mortar for stucco?	50 kg	6
corner profile	1	3
alluminum panels for chimney		0	free / salvaged
roof tiles		0	free / salvaged
lead flashing around chimney		0	free / salvaged
bricks for arches		0	free / salvaged
OSB boards for various moulds		0	free / salvaged
		742.5
		stand + oven		481.5
		enclosure		261

[Kris S]

Could I use stainles steel to make a DIY log holder?

I can't weld so can't make an iron one, but have some Stainles steel laying around that I could cut and bend to immitate the FB log holder…

Can it withstand the temperature?

[UtahBeehiver]

SS does have a tendency to warp but can certainly handle most oven temps, that said I never needed to use one. At one time I had a old fire place log holder to use in the oven but found I never needed to use it. Maybe others do.

[Johannes]

hij Kris,

I see your in belgium , me to
i completed my build 2 weeks ago, and did the first real firing ( to pizza temp ) last weekend... also some minor cracks... but i don't think it should be a problem... i hope...
i see you wanted to make a fire dog? (log holder), i'm a hobby blacksmith/welder, so i'm gonna make me one from mild steel, i don't know where in belgium you are, but if you'r anywhere near sint-truiden, i't costs me about 5€ in steel ....so for some oven cooking tips i'l gladly make one... let me know
i was going to make em like this... but if anybody has a better design idea....