32” cast oven in Warwickshire

[Badbobby]

Hi Badbobby. We're right at the beginning of our pizza oven build and have found your thread and instructions really helpful. At the moment we're trying to find materials and gather them ready to start the build. We're in Cheltenham, so not that far from you I think. Do you still have your excess materials left?

I’m very sorry, I didn’t see this. I sold most of the bits piecemeal on eBay. The fire bricks is a pain as the minimum order is way more than you need! I have built a bbq area out of my excess though.

Update: I have started up the oven this week - a few smaller fires to drive out the moisture. But I’m very glad I put in the metal escape valve. Once it was up to temperature there was a huge amount of steam coming out and I assume that the mortar would have cracked otherwise.

i really would recommend putting in a valve.

Still needed to put on render top coat and paint…

[rayturf]

Hi Badbobby. We're right at the beginning of our pizza oven build and have found your thread and instructions really helpful. At the moment we're trying to find materials and gather them ready to start the build. We're in Cheltenham, so not that far from you I think. Do you still have your excess materials left?

[Badbobby]

If anyone wants to pick up some excess materials, I have the following:

- 12 fire bricks
- 1.5 rolls of 25mm fibre insulation
- most of a large box of stainless steel needles
- burn out fibres
- 25kg bag of clay

Just let me know and I’m sure I can agree a reasonable price. South of Coventry.

[david s]

It will crack a bit from expanding steam pressure, so that's why a week of sun and wind is preferred.If applied over blanket then the blanket acts as an expansion joint which lessens the problem. Even if it cracks a bit it doesn't matter because it will be held inlace between the inner and outer shells.

[Badbobby]

Congratulations - that's a really nice looking build. I am also at the slow cure stage of my build which I started after a roughly 10 day moist cure of the dome and flue. From what I can gather, holding in the moisture for at least 7 days (I went with Dave's advice of wrapping in cling film) will increase the final compressive strength of the mix versus a dry cure but in any case the cement will reach its final compressive strength after 28 days. My vermicrete was only a few days old when I started the curing fires but by the time it see's any significant heat from the dome it will be at least a week old.

Having said all that, I do now wonder if a higher compressive strength increases or decreases the risk of thermal cracking. There's virtually no structural load on the homebrew domes, so does a stronger compressive strength help or hinder when it comes to thermal cracks? Any experts out there?

Thanks for the comments. I see you’re enjoying it so much that you’re building a second. I have a feeling I’ll be “consulting” (doing) on others for my friends next year…

[Badbobby]

Because you’re not after strength in the vermicrete layer, it only needs to be firm enough to apply the stucco/render on to. The more cement you add to it the more you reduce its insulating capacity. Because the mix takes up so much water ther is a lot of free water in it so I see no need to damp cure. It is eliminating this free water that’s a more important issue, so I give it a week of drying in the sun and wind, keep off any rain.

Thanks David.

I was wondering whether the verm layer is more prone to cracking etc from the heat via the cast dome. Shame it's raining a lot in the UK! Sounds like i can start the fires this weekend. Very excited!!

[david s]

Because you’re not after strength in the vermicrete layer, it only needs to be firm enough to apply the stucco/render on to. The more cement you add to it the more you reduce its insulating capacity. Because the mix takes up so much water ther is a lot of free water in it so I see no need to damp cure. It is eliminating this free water that’s a more important issue, so I give it a week of drying in the sun and wind, keep off any rain.

[Badbobby]

@ David S - can you confirm how long i need to leave the insulation verm layer to cure prior to running the week of fires? To clarify, the homebrew cast dome had a week of wet curing and it's about a month or so in total since i first cast it.

My hope, based around a 2 week holiday i have booked on 12th August, is that i can leave it a week and then get on with starting fires before i go away. Ready to have pizza on my return!

[david s]

I'm not 100% sure on tha, but it follows that greater strength should be more resistant to cracks developing and progressing. Unusually though, Sth American pottery that can withstand direct flame from stoves is pretty weak and is fired to around 500C, only just above sintering point. But the clay's very open body probably has more to do with its thermal shock resistance than its strength.

[bamforp]

Congratulations - that's a really nice looking build. I am also at the slow cure stage of my build which I started after a roughly 10 day moist cure of the dome and flue. From what I can gather, holding in the moisture for at least 7 days (I went with Dave's advice of wrapping in cling film) will increase the final compressive strength of the mix versus a dry cure but in any case the cement will reach its final compressive strength after 28 days. My vermicrete was only a few days old when I started the curing fires but by the time it see's any significant heat from the dome it will be at least a week old.

Having said all that, I do now wonder if a higher compressive strength increases or decreases the risk of thermal cracking. There's virtually no structural load on the homebrew domes, so does a stronger compressive strength help or hinder when it comes to thermal cracks? Any experts out there?

[Badbobby]

Then onto the blanket and vermiculite layer. Took the advice and bought perlite as well and mixed together 10:1 with cement with abuot 3 of water. It was just the right mixture, so thanks all.

Blanket (2x 2.5cm layers) was fine without the wire. As with everyone else, the insulation layer was best built about 10cm high then calling it a night. I think i was going too thick at around 10cm, albeit it created a very stable footing. I reduced it down to 5cm as i got higher.

I put in a breather pipe at the top rear. Aluminum 2cm diameter pipe with holes drilled in it. Maybe 10cm long. Seemed a worthwhile endeavour.

The flue is 6" and i cut in 4 tabs with a metal blade jigsaw. With some trial and error this got it pretty vertical. I had plenty of space in the cast hole as i had packed out the paint can with a lot of cardboard. I actually packed it out with more at this stage. Perhaps it'll go up in flames! I also then put metal lattice sheet around the flue, spaced with card where i have insulated up to hold it in place. Hopefully that gives some stretch.

The arch has brick ties cast in it, which are then in the verm layer . I had a large gap at the floor level so stuffed it with verm mix but ignored to the side and top.

So that's where i am now.

First question - i the dome cast has had a month of drying, but the verm layer has only been on since Friday evening. How long does that need before i start the slow fires. Do i need 2 weeks for the verm to dry or is that mainly the dome? The dome was cast a month ago. I'm hoping i can get started as i go on holiday on the 12th for two weeks so i'd love to cure it before i go!!! I would really welcome a steer.

I'll do a UK order list when i have a chance.

I plan to finish with lime render and paint. I could create a cantilevered cover, but we'll see.

[david s]

Excellent work. Thanks so much for the detailed documentation. Your 32” oven will require a 6” diameter flue pipe. I hope you cast the hole for the pipe just a little bit bigger to allow for the stainless pipe expansion. Because you’ve cast the gallery pretty thin, this is important. You should also hold in the moisture for at least a week to enhance strength. Wrapping the whole oven in cling wrap works well.

[Badbobby]

Arch casting. Pain in the rear. Got their in the end. I think if i had my time again i'd cut some polystyrene to shape rather than curving thin fake wood.

I put PVA on the bottom and sealed the joints and the screw heads. Then promptly didn't pay attention and grossly over watered my mixture! Live and learn. I recovered it a bit with more aggregate, but rather a shame.

Came out ok, polished ok and the colour (using mortar dye) was ok. Used brick ties as rebar and also sticking out so as to tie into the verm layer. I wonder if they're too thick... Time will tell.

All of the above lifted from cleverer people than me on all the other build threads!

[Badbobby]

Then time to build up the bricks so that i could get the levels correct for the concrete arch.

[Badbobby]

Photos below of the removal - undid the door in the back of the gallery mold to remove some tension. Came out easily, thankfully. Used some homebrew putty mixture - pain to put on and cut myself on the steel needles a bit when i tried to force it in with my fingers. Mainly was a pain to adhere.

I think the entrance is too wide, but i'll find out soon enough....