Homebrew castable build, Newcastle UK

[mesoiam]

So it's been almost a week of damp curing and I decided to demould to see if there were any gaps I needed to fill while it was still damp. I'm happier with the dome than I am with the flue gallery. I was a bit heavy handed in removing the wood from the chimney form and ended up creating a crack in the flue gallery while I was knocking the wood downwards. Looking at it again there is not much material at the front and back of the hole for the chimney to stiffen it, so the two sides of the gallery can 'do the splits' if you push the right and left legs away from each other. It stays put unless forced and otherwise seems solid enough.
I now have two options,
Scrap this flue gallery and mould it again Or build up support for the left and right of the flue gallery and proceed with it as is.
One drawback of remoulding the flue gallery is that I'm almost out of melt extract fibres (SS needles) so if I made it again it would have hardly any, I've got plenty of the other homebrew ingredients.

Thanks for the interest folks, hughjamton I used sharp sand in the homebrew.

[Nick J C]

Planning on casting mine tomorrow.. . Did you lubricate the wooden formwork?

[Robarb]

What would you say is the recommended gallery opening length? or essentially, what should it be no shorter than? For an oven with a 700mm internal diameter and 350mm height, the dome entrance height should be c.220mm. Given the gallery is a slightly larger half cylinder (than the dome entrance to make a decent door stop) would a height of 245mm and width of 490mm be too wide/shallow? Would this gallery entrance be enough room to create an opening for the flue?

Thanks

[mesoiam]

Hi Nick J C, I didn't lubricate the framework but the difficulty in removal was more to do with the homebrew curling around the woodwork and locking it in.
Robarb I couldn't comment on the theory of this, however some builds have very short gallery's indeed, achieved by moving the chimney back over the dome. Width wise I just went with something that fit into 5 bricks wide and it looked sensible enough.

[david s]

The shallower the entry the easier it is to work the oven, although there is then a risk of smoke escaping out the front rather than going up the flue. If the flue diameter is adequate for the oven size it will produce sufficient draw and if some volume is designed at the base of the flue for smoke to collect it will create a decent unobstructed flow for the smoke. Sharp angles are best avoided so the upside down funnel form works well. Limiting factors for gallery depth are also governed by build method. If doing it with brick then the thickness of a brick limits the overall design as well as the difficulty in creating compound curves. Casting allows far thinner walls and complex forms resulting in a much shallower design.

[mesoiam]

Hello again, it's been a while. When I last posted I had just cast and demoulded the flue gallery and it had a split which meant the legs could move away from eachother if pushed hard enough. After a bit of pondering whether or not I should just re-cast I decided to build some side stays to prevent such an occurrence. I built a short pillar on either side of the gallery, insulated by vermicrete and leftover calcium silicate board. I'm aware that this is not thermally ideal and may contribute to some heat loss, but hopefully the fact that they are only in contact with the gallery and not the dome and that they are insulated will go some way to mitigate that. The gallery is also insulate slightly from the dome by a thin layer of vermicrete, and there is the v shape cutout on the bottom firebricks to also aid in that.
One other question which took me a lot of thinking was, what am I going to do with the front of the oven? Other ovens have such nice openings on the front, but I've built the flue gallery right to the front of my concrete hearth/plinth...plearth.
I decided to cast a sill on the front which would serve as a base to add a decorative arch like a David S oven. I had some big concrete screws so sunk a few of those into my now well cured concrete and twisted some wire between for the concrete to grip onto. I did my best to make the mould an S-curve in profile so make it look a bit nicer than a cuboid just jutting out and shored it up with wood and ratchet straps. The mix was portland cement, prebagged ballast and polypropylene fibres, plenty of tamping down with a small stick. In the future I will probably grind off the right and left sides of this sill to get rid of the corners, that would have been a very complicated mould with compound curves so I thought I'd best sort it out later as it won't bring me closer to pizza.

[david s]

Looking good, that's a nice solution.

[mesoiam]

With the sill setting I set about making the decorative arch. I toyed with the idea of decorative stones and concrete polishing, even backlit glass etc but thought I'll just stick with what I know and have in stock. I have plenty of red bricks from building the plinth, I split them at the first and last hole with a bolster and made an arch out of the brick ends. I built this into an arch mould I had made which fit the shape of the front of the oven, a bit of extra mortar around the outside edge for strength and width as it will need to include the thickness of the blanket and vermicrete also.
added some wire mesh and some thicker wire, rebar would have been ideal but didn't have any. I added some loops of thick wire protruding from the back to act as ties to the oven. So the bricks have mortar between them, then reinforced concrete behind.
As the arch was setting, I added the ceramic blanket. This went OK, gloves and facemask were a good idea, the dust is nasty. I measured out enough blanket to go around the circumference, then did some relief cuts to allow it to curve inwards to fit the dome, add another piece to cap the top. The offcut triangles were used later around the gallery where it is slightly less critical. There are two layers of 1" blanket, On the second layer, I followed the same routine, but cut a few inches inches off the height so that my joints would be staggered. I didn't feel it was necessary to add any wire to tie it down.
On the next session I added the decorative arch, pleased with this as it makes the oven look a lot more appealing, got an ooOOoo from my wife at least. I wrapped the tie wires around some concrete screws which are driven into my 'side stay' pillars, a further wire ring went round the flue circumference and was pulled down tight, hopefully this will give the flue some extra support.
It has occured to me that I should have just wired the flue gallery down instead of building pillars, but at the time I needed to do something to progress, and hindsight is 20-20.
With the decorative arch in place I was ready to do the vermicrete insulation, this is the moment I have been waiting for, as it means I can stop adding water to the oven and let it dry. The vermicrete went on OK, ended up doing it in two sittings which ended up as a good thing, as it gave the lower layers time to stiffen a bit. This is 10:1 vermiculite, portland cement. Ok maybe 9:1 as I kept adding a little bit or portland for luck. It's a bit like wet rice crispies but with a bit of time you get into the swing of forming it. I used a piece of glossy wood in one hand and a medium sized trowel in the other, hold up the piece of wood against the outer wall, fill with vermicrete and pat it down lightly. Seemed like building an igloo, which is what I'm doing. I bought one 100 litre bag of vermicrete originally, ended up using another 2. There's a bit left over of the last bag, someone was asking on another thread, almost 3 bags for the 800mm ID oven to cover 50-60mm outer and 50mm slab underneath.
Anyway, hope you all enjoy the photos, so glad to have the insulation done. I will now let it air dry for 2 weeks. In the meantime I'm pondering oven door construction.

[Hattori-Hanzo]

Looking really good mesoiam

[Mullster]

Very inspiring mesoiam ! Another great oven coming together.
One thing I’m not sure of - the wire ties coming from the decorative arch - where/how do you ‘attach’ them to the oven?

[mesoiam]

Thanks folks. I have drilled some concrete screws into the top of my pillars, the wire wraps around these. There is also a ring of wire around the flue, the upper ones of the decorative arch also go and hook around this, the wire gets hidden by the vermicrete at the end.

[UtahBeehiver]

Now is the time to consider a breather vent in the apex of the dome before the final decorative coating. This allow any water that sublimates to steam to vent without building up pressure under the vcrete..Liquid water volume expand 1500 time when it converts to steam. Cheap insurance. There are many ways to accomplish, Gulf and I use breather vents from any auto store, less than $10 with a 1/2" pvc bushing set in the render.

[mesoiam]

Hi Russel, would you consider the pressure to build up under the vcrete or under the final render? I was going to leave the final coating until after the curing fires, then do a ridged final render with perforations under the lip, See sketch. I was thinking the vcrete would be porous enough to let vapour through with the risk being created by the final render being a seal. What's your thoughts on that?
Thanks

[WoodywWun]

Mesoiam, Thanks for the many detailed posts and your problem-solving journey. It has all been very informative and helpful. Just going back to your splitting issue, when you de-moulded, was there a moment during the time leading up to this when the home-brew was in a sort of mid-curing condition that you might have been able to scrape back any potential parts of the homebrew that might have caught later on? Or maybe these weren't accessible? Was there any part of your mould that (chimney support area?) that might have been kept as a separate part and removed early when the home-brew hadn't cured fully?

I'm just coming up to casting so any guidance or ideas would be great. The oven is looking great!

[mesoiam]

Cheers, the splitting was due to me being heavy handed with the removal of the flue gallery mould. I should not have hammered it when the castable was still quite fresh. My mould was a bit half baked to be honest. It was a shaped wooden cap to form the funnel, and then just filled in with sand below, if I was doing it again properly I'd think it through a bit better to consider the demould strategy. Make sure the homebrew cannot curl around and lock the mould in place at any points, make sure screws are accessible for removal, and dont make any parts thinner than they should be