32” cast oven in Warwickshire

[Badbobby]

I’ll start my write up, mainly retrospective, to cover my build. Mainly to help others to follow and to allow wiser minds than mine to chip in and correct any rubbish that I write

I’m currently at the insulation drying out phase hence have some time to take a step back and document it all.

[SableSprings]

Welcome to the forum family...and congratulations on starting your journey learning to use your oven's unique cooking capabilities!

[Badbobby]

Like a lot of people, I came across this forum via links to threads that steered us away from the nonsense vermiculite yoga ball methods. So this forum has done a great service in this regard as I’m not the first and hopefully not the last!

I had though already been looking at a “proper” way of building an oven using a kit from one of the UK based suppliers. In the end, I like projects and, once I discovered that I could cast my own oven in my own time, I think I was not going go back. Part of my kit reticence was that they were all slightly different in approach, some things were or weren’t included. So decision was made to wing it.

First up, I was lucky to have what I felt was a purpose built area in which to build this. The photos show an L shaped brick structure that originally had a dwarf wall of bricks and decking on top to act as seating around a table. So I took out the decking and bricks, many of which I’ve reused (not as easy as I had hoped) to save a) taking them to the tip and b) I feel it’s the right thing to do re wastage/save the planet etc.

[Badbobby]

Once that was done it was time to nail down the size. I got into the idea of building a pizza oven because I bought a smaller relatively portable wood fired one during lock down. We loved it but it was quite an intense process of turning the pizzas every 20secs and then reloading just the right about of extra little logs etc. So I have wanted to build a decent sized one for ages. However, I knew I was going to move house so I didn’t want to build one and leave it behind! Settled on just under 32”/80cm in the end as a target size.

Working back from the interior size was the first head scratcher for me as I had to ensure I was happy with setting out. Lots of excuses to procrastinate. In the end, I decided that I didn’t want to pour a slab and went with the idea of using 4x 600x600mm concrete pavers. Then had to check that I was happy that I would get enough clearance from that for the thickness of the dome, blanket, vermiculite layer and the entrance. I reasoned it was just about ok – 80cm max interior, then 5-10cm cast thickness and similar again for blanket and vermiculite on each side.

Besides, I could freestyle as I went a long (part of the attraction for home brew vs expensive pre-cast kit). Meanwhile, I placed my order for bricks, refractory cement (didn’t use in the oven, but plan to use for my bbq next to it), clay and first batch of vermiculite. I was able to collect from Kiln Linings as I was kind of up that way with work. Massively over ordered bricks to hit the minimum order/fear of under ordering and they just fit in the back of my E63 wagon and without overloading the springs! Drove carefully home though…

[Badbobby]

End of May UK bank holiday was a perfect chance to get going with it, so was able to pick up blocks, sand and cement from B&Q over the preceding week. So I got the block base up, pavers on and vermiculite base insulation layer sorted over the 3 day weekend. I used plastic lawn edging to make the form work with the left over decking that I’d ripped out to make space.

So far so good, until I realised that I had made the vermiculite layer 10:1 not 5:1 cement! So in the almost dark I tried to guestimate how much cement I put in originally and doubled it and chucked it in! I’d made the mix very wet (not on purpose) which meant I could stir in the additional cement easily.

[Badbobby]

Left it for a week or so and then put some sand down to level and placed brick herringbone pattern. Cutting the triangles was a real pain as it was extremely hot in early June with full PPE on. More on it later, but I went 5 bricks wide at the front and time will tell if this is too wide. The bricks round the edge were a pain as the smaller ones just wanted to fall off the vermiculite layer! When it came to casting I kept spraying the sand with water and put temporary support bricks underneath to prop the slivers up.

Levelling the builders sand and ensuring that there were no lips was a pain. Better when wet. Kept redoing it as was not happy. In the end, i settled on any variations in height always running away so as not to catch the peel edge or the copper blow pipe for ember clearing. More useful tips accrued from the various other threads.

[Badbobby]

Once that was done, I created my gallery. I did a one piece casting, for richer or poorer. Making the formwork took longer than expected. I put a “door” in the back to help remove sand from the dome area, which helped take away pressure. The outside was made from plastic floor protection sheet, which I had a lot of from my kitchen renovation. Doubled up, it was quite sturdy, didn’t stick and could be cut in a worst case. It came out pretty well.

I used the tip of an old paint pot with some cardboard packed around it. I also created some buttresses, but you can't see these on the photos below. Might show up in later posts.

For the dome, cut out a plastic membrane shape to minimize sand onto the brick floor. Then used bricks to pack it out as much as possible. I had loads of builders sand as i knew i would be needing it to build up the brick outer leaf and the rest of the bbq next to it. Pretty sure it took about 5 standard bags. Are they 25kg each? Covered with newspaper, just because. I think it might have helped avoid the sandcastle collapsing. Sprayed the sandcastle a lot to keep moist as it was really hot when i was doing it.

Again, borrowed the idea of the the wooden guide, by jigsawing the 40cm radius into some ply. Very useful tip, as was the putting in a 40cm high baton.

[Badbobby]

For the casting, I followed the tried and tested home brew and the calcs we pretty much spot on. Hard to really gauge if I put in enough of the needles of fibres. As per all the advice, the lime is very nasty. I got a rash just picking it up from B&Q and getting it to my car! So it was very much a case of PPE (double gloved – washing up gloves under plastic builders gloves – no messing) to dry mix it in a tub in batches then add the water then the needles. Using a garden fork worked quite well.

I cast the dome first, then the gallery the next day. Removed a day later i think. Then a wet sheet over it for a while, which i dunked in water and re-applied daily.

[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....

[Badbobby]

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

[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!

[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]

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.

[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?

[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.