New home-brew cast oven in Scotland

[Minnie]

Hello! After much reading and faffing, I have made a start on my new oven. I will share my plans and material quantities. As it it lockdown, I haven't been able to find either melt extract fibres or polypropylene fibres. I am going ahead anyway.

I have already got a problem with the insulation layer. I made it 10:1, which is weak, and will now need to reinforce. At present it is in a frame which I had planned to remove, however it is not going to be possible.

To those who have gone before and those who offer their advice, massive thanks!

Here is a link to a picture folder: https://drive.google.com/open?id=1Ws...4BNvCzcLPu1Hkp

Oven plan

Base

75mm reinforced concrete slab sited on timber frame, dug into ground by 40mm and concreted in

Beneath Dome

Sheet insulation material cut to shape (largely to support the mould for the VC)
Vermiculite concrete layer: 150mm (I mixed 10:1 - now will need to reinforce)
Firebricks 64mm thickness
Dome to sit on top of bricks

Dome specs
radius: 430mm
Depth of dome 80mm
Height of flu entry 62% of dome height (26.66cm)
Dome entry depth: 34.5cm (1.5 bricks)

Sand form, wet newspaper lining

Cast homebrew refractory mix 1:1:1:3

Wet cure of dome: 7 days

First insulation layer:

100mm vermiculite concrete, 10:1

Second insulation layer:

150mm rockwool type insulation

Third insulation layer:

50mm vermiculite concrete, 10:1

Weatherproofing:

render (dyed a nice colour) + roof

Flu:

6" insulated flu 1m high
Edit: flue dimension

[aaronpizza]

david s I'm a newbie around here, no posts yet as I'm reading the thousands of posts, I'd love to hear your feedback of this summary - I have very similar plans.

[david s]

First up, I think it is a pity folk build their ovens on timber stands. If left in the weather I couldn’t see it lasting 20 years. If kept out of the weather I guess about double that. I’ve seen enough ovens in castles built with local stone that are hundreds of years old.and still probably fire ok.
A 10:1 vermicrete is really not strong enough to support much,let alone heavy dome and floor bricks, but it’s really easy to carve so you could cut half of it out and lay a 75 mm 5:1 vermicrete layer replacing it.
A 430 radius results in an 860mm dome which should have a 6” (150 mm) diam flue pipe. A 90mm diam flue will result, as you’ve no doubt found in a lot of smoke escaping out the door, particularly on light up.

[Minnie]

Hi both,

david s Thanks for the feedback. I'm interested in the reasoning for the wood not lasting long. It's treated timber that will be boxed in, also, if we did decide to move it it would be reasonably straightforward. The materials are largely coming from things we already have, but will investigate getting a different flue. If I didn't, would changing the depth of the entry make a difference to the draw? And yes the VC is going to be reinforced but there will still be some compressibility.

aaronpizza I'm a completely novice builder/oven maker, but a few possibly useful things I have thought about/discovered during the first stages of building it are:

- how to efficiently cut bricks. They are extremely hard and you need the right cutting disks
- challenge of detailing at joins, eg between dome and entry, dome and floor, preserving continuous/contiguous layers
- minimising thermal bridging between layers.
- supporting the flue
- levelling everything
- there is never enough reading to do before you commence
- the purpose of the oven: balance the thermal mass needed with fuel use and time to heat up, and the internal volume
- finding certain materials has been a little challenging. Fireclay and bricks I found on eBay. If builders merchants weren't closed, I would be able to get firebricks, and hydrated lime and cement much more cheaply, but as it is I've had to have them delivered
- figuring out appropriate ratios for oven size/flue etc: something to do with volume oven to flue diameter but i don't know the correct formula.
- curing of dome is more risky without the polypropylene fibres, so I'll have to go slow

Edit:
- i'm also not sure of the best distance between bricks, of course we want a smooth surface to cook on but also need to allow for some expansion. The angle of the bricks relative to the entrance also strikes me as important, in order to minimise opportunities for the peel to catch.

all the best,
Minnie

[UtahBeehiver]

David S is correct on the flue size, you need a 6" ID chimney. Changing the entry will not affect the draw, a longer length chimney "might" but most likely not. As you have noted, the 10:1 vcrete is crumbly, suggested is 5:1 under floor and up to 10:1 on dome. Even by keeping the ring around the floor insulation, there is concern that the 10:1 vcrete will not support the weight of the dome and you could settle and crack the dome. Rockwool insulation is really prone to moisture problems so reconsider using it. It appears that standard building foam insulation is under the vcrete floor. With the vcrete on top it probably will not see temperatures that will affect it but have you checked the compression strength of the material. It needs to be at least 75 psi at 5% compression.

[Minnie]

Thank you. Yes, I will scrape back the VC and replace it with a 5:1 mix. I'll double check the compressibility of the insulation boards. I'll also consider the fire blanket solution, now I've stumped up for all the rest it may not be such a huge additional cost. I've seen others on here have used it.
I have checked and the flue i have is actually 6" - that was a typo - but thanks!

Another thing i have noted: the amount of water you add to the cement mix alters both the final strength and the volume, especially with vermiculite. For a 5:1 mix, what volume of water is best?

[david s]

Hi Minnie,

The reasoning behind the unsuitability of timber is its expansion and contraction with fluctuating moisture content. If stored indoors this is greatly ,but not completely reduced. If stored outdoors but not in the weather, somewhat worse and if left in the weather the movement is extreme. Heavy brittle refractory does not like movement and adding wheels exacerbates the problem, I know this because I have a mobile oven sitting on a wooden trolley that I roll on and off my trailer and transport it to do functions. Having said all that there are plenty of ovens on movable stands, but the longevity is immeasurably improved if the oven sits on a permanent masonry stand.