Finally decided on 32in castable dome

[WoodywWun]

Just found this thread! I'm on a lockdown build as well and going through the same decisions. So far I've got a house-brick stand constructed, with a nice little arch, which I'm pleased with! Now the serious stull...

I'm gearing up to pour the base slab this weekend. It's 75mm thick, reinforced with a bog-standard cement mix about 1m square.

Wish I'd joined the "Thermalite" debate earlier as I can see possibilities here but they are very thick and using them would probably throw out my height calculations. Did anyone get to the bottom of how good an insulator they are? They are ridiculously cheap but I've only found 100 mm thick ones. The "FB" ceramic-based boards for insulation look the bee's knees but what a price! I may go the in the Vermiculite cement direction for underfloor insulation but it's 100 mm minimum thickness that is required so it's going to throw out my height calculations.

I'm going to go the extra expense of fire bricks for the cooking surface and the Vitcas folks seem to know what they are doing.

As for the dome, well I'm humming and harring on this as well! My original intent was to use half house bricks (as seen on many YouTube videos) held together with a good refractory mix of mortar. Then I started to get some negative views on the long-term durability on these as they heat and cool. Any views? The cost of proper fire bricks is out of the question for me but is obviously the best. So what about the cast refractory dome idea? Any info on how durable they are or how much heat they hold compared with any type of brick?

Looking at the dome insulation, the ceramic-based blanket (again from Victas) looks like a good solution as they have good insulating properties and would be fairly easy to put on. ...or are there home-brew alternatives such as another layer of fire clay/lime/cement sand mixes.

I'll render the final covering with a standard cement mix.

Sorry to pitch in so late but any learning experiences, info or ideas would be great!

[Stan74]

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.

I second that, thanks for the heads up. It says on the site there are 3 sizes, 12, 25 & 35mm, does anyone know the recommended size to use ?
Good luck with the builds Mullster & Mesoiam, I'm loving the updates, tips and links etc

[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.

[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.

[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/

[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.

[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.

[Petter]

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

[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.

[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.

[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.

[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]

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

[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?

[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.