New member, first oven build

[Michaelh]

Hello all
I'm new here having been browsing this site for a while for tips and pointers, I've decided to get involved.
So the plans for the oven, I've almost finished the 'stand' which is brick-built with oak beams for support. The oven will sit on one half and on the other half I'm going for a slab of granite.

The actual oven is going to be a refractory cast, which having not used the stuff, is where the majority of my questions probably lie.
Approximate plan for layers
From bottom up: 50mm concrete paving slabs, followed by 100mm vermiculite, light layer of sand, 70mm fire bricks.
From inside out: 30-50mm 1400c cast, 50mm fibre blanket, heatproof chicken wire, 50mm vermiculite, 20mm render, paint.

For the mold I have a gym ball which I can do right next to the oven. I'm not sure if it's the best thing and have read sand will be easier. With the gym ball I can if needed(?) make an external mold from bendy ply but can't really do that from a sand form so it's whether that's needed or not.

Any comments and advice welcome, thank you

[UtahBeehiver]

DavidS is our resident cast oven expert. I suggest you look at his posts and threads and reach out to him directly. I do recall he advocates a sand form over a gym ball.

[david s]

If you planning on using commercial castable refractory here is a well documented build.

https://community.fornobravo.com/for...and#post396954

If you use a gym ball as a mould you will have to move the casting to its new location and they are heavy. You will also need to build formwork so that you can cast over the top half of the ball. Casting over a sand mould allows you to cast in situ and doesn't require extensive formwork. Providing you get the consistency of the castable right it will stand up vertically against the sand mould without the need for an outside mould. You can also build up the first 6" or so, allow it to harden for a couple of hours then proceed higher.

[Michaelh]

That's good to know that I won't need the outside form, thanks.
So I'll go for the sand mold. Couple of questions
The vermiculite mixes are 10:1 for the floor and 6:1 for the dome?
The door height I understand should be in the region of 62% of the dome height, but what's a good width? Inner diameter will be around 75-85cm.
And the tunnel length is that OK just be slightly bigger than the chimney basically?

[david s]

You have the ratios around the wrong way. The underfloor needs to be 5:1 (6:1 probably still ok) for sufficient strength to take the weight. Over the dome where it's not holding weight and you want its insulation value 10:1. Any leaner and it's much harder to apply.
Door height approx 63% of internal height, but a few cm either way will make no noticeable difference in performance.
The width just needs to be enough to give reasonable access, to wide contributes to greater heat loss. Around 400 mm is pretty good.
The depth of the flue gallery can be whatever works for your design. I have pushed the design a lot to make mine as shallow as possible because I like the better access it provides. Additionally you can reduce the thermal mass of it by casting. (mine: Weight around 11 kg ). A big thermal mass sitting outside your oven will act as a heat sink, with heat travelling from hot to cold, therefore a low thermal mass entry, also well insulated should be more efficient especially when the oven is used for retained heat cooking. Pic shows how I managed to keep the depth of mine to a min (125 mm). Same size as the internal diameter of my flue pipe.

[Michaelh]

Thanks again David.
Are you suggesting to cast the door/tunnel part from the Vermicrete rather than refractory cement?
If so, how do you make sure the two sections adhere properly (might be a silly question)

[UtahBeehiver]

I am sure DavidS means using dense castable refractory on door and tunnel not V or P crete which is not strong enough and used for insulation purposes only.

[david s]

Thanks again David.
Are you suggesting to cast the door/tunnel part from the Vermicrete rather than refractory cement?
If so, how do you make sure the two sections adhere properly (might be a silly question)

No, as Russell has pointed out the vermicrete should be used as insulation only, used dense castable refractory for the dome and flue gallery castings. Here's a well documented build to answer many questions.
#1

[Michaelh]

Thanks for that thread it's really thorough and answered a lot of my questions (some that I didn't even know I had yet).
So the thermal break seems like quite a bit of effort rather than a full and single cast, is it worthwhile or does it make it easier anyway to cast the dome separately to the flue gallery?
Also, I understand the chimney should not be too snug in the castable, does that mean it should be not actually fixed down, ie removable?

[david s]

You need a rebate at the oven mouth for the door to sit against. This is tricky to do if you do the mould in one piece. Far easier to cast the dome then build a mould in sand against the dome casting for the flue gallery. As the flue gallery sees less heat than the dome anyway it is a good place for a join because of thermal expansion. Regarding the fitment of the flue pope you can fix an anchor plate to the gallery rather than have the pipe fitting inside the casting. I prefer not to go the anchor plate route but seal above that join surrounding the pipe with 5:1 vermicrete which can handle some expansion. It also supports the base of the pipe somewhat. If you don’t understand this explanation I can do a sketch.

[david s]

Here are a few drawings to explain it more clearly. An anchor plate requires some method of fixing to the casting. drilling into it and using metal fixings is asking for trouble IMO, and you must make sure that the fixings are stainless or they'll corrode from the heat. You could cut the anchor plate down somewhat and embed it into the casting on placement (as shown) or do away with it altogether. The third pic shows also shows how some bricks can be used for the sides of the flue gallery. This is far easier than casting it including the sides. I'm sure there are other solutions too.

[Michaelh]

OK that makes sense about the two part mould. I'll have a think on exactly how that will work in practice.
Erm the chimney I think I get you, not sure. The chimney comes with a protruding ring near to the base as well, not sure how that should be used

[Michaelh]

Sorry I didn't see the pictures when I sent that one. OK think I get it now. So the cast should be a loose fit, then fit the Vermicrete nice and snug and sit the ring around the top outer edge, high temp siliconed into place

[david s]

No, the high temp silicon.sits in a small gap between theflue pipe and the outer shell. If you wrap some corrugated c’board around the pipe, render up to it, then remove it when the render has set hard, you can then fill the gap with high temp silicon. The anchor plate, if you are using one goes further below.

[Michaelh]

OK I understand thank you.
I've just completed the Vermicrete base. Its hot and sunny, sk I've covered it with tarpaulin, is that right? Should I keep this a little damp over the next few days?