Finally decided on 32in castable dome

[Mullster]

Having the hearth poured

So again went for a company that brings the concrete from the roadside truck to your project. This time they were an hour late which clashed with a work meeting I had so I even had to let them do the leveling. This felt weird like I was cheating but I had no choice and the guys were really interested in the project and asking me loads about it. They did a fine job and I trowel smoothed it as soon as I could.

My mistake

The day of the pour it was windy and sunny and I didn’t cover the fresh concrete. I believe it started to dry too quickly which is why I’ve ended up with a hairline crack on the surface of it. I covered it later that night and left it covered and watered for several days after. The crack hasn’t developed or widened and I’m confident that structurally it’s all ok. Also the oven floor will completely cover the crack so no cosmetic problem either.

A few pics below

[Mullster]

Sourcing materials in the UK (mid-lockdown)

So it’s been a bit challenging at times getting materials and I’ve struggled with the decisions around firebricks for the floor and the clay for the home brew mix.

In the end it was relatively easy to get sand, cement and hydrated lime from a builders merchant (and cheap too) and I had already sourced a large bag of vermiculite off Amazon a couple of months back when I was going to do a gym ball dome like I’d seen on YouTube (before I learnt so much on this forum!)

Clay
The clay is still not sourced - found several sites with links people on here have sharedaaronpizza andPoochdog12 thank you. But I still haven’t figured exactly which clay to buy so any experience would be welcome there especially from the UK contingent.

Firebrick
So I done so much looking for the bricks. Vitcas website has them at what appears a good price (less than £2 a brick) however by the time you get to shipping costs it’s up beyond £3 a brick.
Also they are 3” depth and I’m getting concerned about the overall height of the Oven floor relative to where I will be stood.
Instead with some advice from DavidS I looked into storage heater bricks as a resource that several UK oven builders have used. There’s a fair old trade of them second hand (and new if you prefer) and the price is good. The bricks are slightly larger and squarer so I need fewer, they are 2” thick instead of 3 and I have a seller at £1.50 a brick so I think I’m going to plump for those. For whatever reason haven’t committed to it yet. (This project is ridiculous - I make far bigger decisions at work much more easily!)

So that’s up to date - today I’m hoping to create the vermicrete insulation layer for underneath the floor. I’ll share some details of that later.

[aaronpizza]

Fireclay https://www.bathpotters.co.uk/powdered-fireclay

Super nice company - I called and they where happy to chat and advise.

I ordered my bricks and blanket from Vitcas (we won't speak about the price haha) However for new bricks they seem to be the cheapest.

[Nick J C]

Hi Mullster,

Following your progress closely, as you are also in UK, and I am just setting out (Selco order for base supplies arriving next week!). I havent got as far as buying supplies for the dome, but I can see the issues are those of cost for firebricks, and ceramic products (blanket +/- a base sheet), along with supply issues in the UK for some of the other components.

Have you decided not to use any reinforcing agent in the castable dome? DavidS suggested the use of stainless needles, also called melt extract (along with the polypropylene fibres for air release which have no structural role). I cant for the life of me find anywhere in the UK that stocks them on online search - any clues yourself? The other possiblity is what some of the commercially made domes have 'ceramic needles/pins' - which I guess would be good as they would be less likely to hurt your hands - cant find them either though. I would be a bit gutted to put so much effort into making it to the gold standard spec, and not use some sort of reinforcing - there must be something that could be used as a cost effective alternative - ?stainless nails - thoughts

Re: firebricks - was there a reason you decided against the heater bricks?

[david s]

As moisture under the floor is an issue, providing an escape route for it is prudent. You can do this preferably by casting some holes in the centre of your slab. Having already laid the slab you can drill some either from the top down or the bottom up. Use a small diameter pilot hole to reduce the concrete bow out when the drill is nearly through. Foamglas is great because it is not water absorbent, cal sit board best insulator, but vermicrete at 1:5 is the cheapest, but requires extended drying (see attachment)

Vermicrete insulating slab copy.doc.zip

[Mullster]

Nick J C I think in some of the lost posts I discussed this with DavidS and we concluded that you can get away without the steel Fibres as long as the burnout ones are still there so that’s my plan.

And I have decided to go for the heater bricks - partly for the cost but also the height - I’m getting worried about the overall height of the floor.

Look forward to seeing your progress too!

[david s]

If the working height of the floor concerns you then it is way easier to build a step up than it is to dig a hole. In practice you get used to whatever working height you have.

[Hattori-Hanzo]

Fire clay seems to be in short supply at the moment in the UK

I believe any powdered clay will work but "ball clay" keeps coming up and that's what I went for in the end.

Here are some clay suppliers I've found.

https://www.valentineclays.co.uk/product/aa-fireclay

https://www.bathpotters.co.uk/powdered-fireclay

https://www.claymansupplies.co.uk/Ca...dered_clays/11


I've yet to find a small amount of stainless steel reinforcement needles so will probably have to omit them from my home brew.
I have found polypropylene fibres though on ebay.

https://www.ebay.co.uk/itm/Concrete-...-/141736144766


As for the height of the oven, it seems to vary.
elbow height seems to be recommended for the cooking floor, any where from 40" to 47" seems to be the range.
I'm aiming for around 44" for mine.

[Mullster]

Cheers guys!

Some more progress today.

Measuring on the hearth

So it was cool today to get drawing out the plan on the hearth - was a relief to find my scale drawing had served me well - no nasty surprises of miscalculating - so the 32in dome is still the plan!

Used a stick with various hole positions to slot a pencil in for drawing the circles. Used a size 3 or 4 drill bit at the centre point which I think worked better than another pencil would have done as it dug into the surface slightly and stayed in place while I drew the circles.

Mould for the insulation layer


Took me much longer than I expected to build the mould. I couldn’t figure out with the materials I had an effective way of making a rounded mould so just went for straight edges in the end. I figured that if absolutely necessary I could always cut some away later if it was going to be a problem.

Vermicrete layer

So first experience of vermicrete. Mixed at 5:1 with a small trowel. Found it strange to work with at first but got into in rhythm after a while. Got through a 100ltr bag and still have about an inch of depth to finish off - never mind! I covered it over.
I assume I’m ok to simply add to it tomorrow. Would it be a problem if it was next weekend before I finish it?

A satisfying day - getting there now.

[Petter]

Nice to see that you're back on track. Keep up the good work!

[KennyR]

Hi Mullster.
I'm new here and also in the UK, Preston to be exact.
I've just read through your thread and will be following intently. Great work up to now and if you find a source for the SS Fibres then please post it up and also which burn out fibers you go with.
Best of luck and regards.

[Nick J C]

looking good, I poured my slab yesterday so am weeks behind you. I have found an industrial supplier of SS fibres, but not heard back from them yet as to whether they would supply a small quantity - perhaps I should buy a truck load and sell on in small quantiites! Just looking at your vermicrete base, and having read around this a bit, wondered whether I may be better off using thermalite blocks - they seem to have almost identical insulation values, and are dirt cheap and easy to work with. This has been looked at by a number in the dim and distant past, (see link) https://community.fornobravo.com/for...rth-insulation and all I can see that is a downside is their tendency to suck in water - david s is vermicrete better is this respect to go beneath the firebricks? From my reading vermicrete seems to also be quite sponge like when water is around until protected.

[Nick J C]

Re: Stainless steel fibres: Found a UK supplier willing to supply small quantities: £9/kg plus VAT plus delivery. Just put my order in. https://rockbond.co.uk/products/rock...-steel-fibres/

[UtahBeehiver]

Interesting question on the Thermalite, it appears to be an aerated concrete product similar to the the Aussies call Hebel. There have been a few builds using Hebel as a base structure but uncertain whether used as floor insulation in lieu of v/prete or CaSi. I think that David S did some experimenting with aerated concrete. I briefly looked at the Thermalite specs but could not see anywhere what the max working temp of the product was.

[mesoiam]

Re: Stainless steel fibres: Found a UK supplier willing to supply small quantities: £9/kg plus VAT plus delivery. Just put my order in. https://rockbond.co.uk/products/rock...-steel-fibres/

Excellent, thanks for posting that Nick. Will check them out straight away, I was going to proceed without needles but with a sense of unease.