Relatively Lightweight Refractory Cement oven build

[Jumbalaya]

Hello,

I plan to build an oven using a 30” exercise ball as a mold poking through a piece of supported plywood.

Inner Dome Layer –
I plan on the inner dome layer of approx 2” thick of homebrew refractory (3:1:1:1 Sand, lime, portland, clay) + SS needles(if I can source them)+sisal rope fibers.

Outer dome Layer-
Approx 2” layer of perlicrete with some type of stucco sealing layer over top.

I have a few initial questions:

1. Do you think if I make the mix fairly dry I will I be able to cover the whole ball in one go or will this mix slump down the sides as I apply it necessitating it to be done in stages with drying time in between? If done in stages, will this cause weakness where the layers meet?
2. Are the stainless needles a must for this build?
3. Once the inner layer is completed, can I simply apply the insulating layer directly over the refractory layer or do I need foil, chicken wire etc in between the layers to avoid cracking?

I am at the stage of planning/collecting raw materials now although the mold is basically ready. Any advice would be appreciated. Pics to follow.

Thanks,

Jumbalaya

[david s]

I advised some folk to use the homebrew recipe as a castable for an oven around five years ago and they both report their ovens are still working ok, although they only get infrequent and occasional use. Can't say how they'll hold up long term.

1. You need to make the mix wet enough to be workable. Too dry wil leave you with voids and a weak casting. Too wet will slump. It can be done in one go but if you do it in two or more goes, yes it will leave a weakness where the layers meet.

2. No the needles are not essential. They need to be added a min of 2% by weight of the dry mix to be effective and as you've no doubt found are expensive.they also make placement more difficult, they're not called needles for nothing.

[david s]

3. Once you've done the inner dome allow it to dry for a couple of weeks before doing the insulating layer. A foil layer will impair drying and because the insulating layer is not particularly strong adding chicken wire to it is a waste of time.

Adding sisal rope to your mix will do little. Burn out fibres are usually contained in a proprietary castable mix to assist safer water elimination. They will melt at 160 C sisal rope will not melt or burn away at these low temperatures so pretty useless to assist drying. If you want to add some burn out fibres get polypropelene fibres used for concrete reinforcing. They are finer than human hair and need around double the normal mixing time for proper dispersal.

[Jumbalaya]

Thanks David. Is it worth making ridges on the refractory layer so the insulating layer sticks better?

About the needles, I'm assuming if you work slowly the needles will line up and smooth out and not jut out of the finished product? Is it easy to stab yourself if using rubber gloves?

I guess an inflated ball is not the best thing to attempt working with the needles the first time...Could anyone point to a video on how to work with the stainless needles? I haven't been able to locate any.

[Jumbalaya]

Hi,

I've begun my build. Here are a few pics of the wooden base.

J

[Jumbalaya]

Here are the next steps a mold for the vermicrete layer(s).

[david s]

Add some diagonals to your wooden stand. You may think it's strong enough, but over time your joints may loosen with all the weight on top.
The vermicrete takes longer to dry than you would think, see attached experiment.

Vermicrete insulating slab copy.doc.zip

[Jumbalaya]

Hi David. Thanks for the advice. I plan to either brace it with diagonals before moving it or enclosing it on 3 sides somehow with doors in front but only if weight allows. Here are some more pics.

[Jumbalaya]

Hi,

Here is where I am currently at. I'd like some advice on my execution of the mold. Note: I'm going to duct tape and grease up the wooden parts before the pour.

The plan is to make a sand hemisphere to the dimension of the inner dome, cover with wet newspaper and butt the wooden mold up to it and then pour the whole igloo in one go.

- Is there is a compelling reason to pour the doorway and dome separately as I've seen others do? I will be using stainless steel needles with the homebrew refractory recipe.

I will place wooden strips under the mold so I can pull them out and drop the mold down. As well, the curved block under the chimney is not attached to the arch and should drop down after the arch is pulled out as it's tapered on 4 sides.

- The door rebate will be at the front not at the back of the arch. Given the small diameter of the oven (24"/61cm) I figure if I want to use it to cook bread etc I can remove the 6" chimney, plug the vent hole with an insulated stopper and seal with an insulated door at the front. Any issue with that design?

Any comments would be appreciated.

J

[david s]

Removing a hot flue pipe is not too easy and you'll get some abrasion wear from the casting that surrounds it, but if you only do it occasionally I guess it should work ok.
removing the front mould could be difficult because of the rebate, so be sure to place decent wedges around 1/2" at both the front and rear of the form. The addition of some burn out fibres in the mix will help with the water removal to reduce steam spalling. What you need is very fine polypropylene fibres well dispersed in the mix (you don't need much). How thick are you planning to make the casting?

Don't be in too much of a rush to cast the dome just yet, your vermicrete slab needs plenty more drying. See attachment on post #7. As the whole thing is on wheels, can you roll it into the sun?

[Jumbalaya]

I'm planning to make it approx 2" thick. I've yet to find Polypropylene fibres in small quantities, I have sisal fibers but you already said in another post that won't burn out easily enough. Any worries about doing the whole casting at once?

[david s]

I'm planning to make it approx 2" thick. I've yet to find Polypropylene fibres in small quantities, I have sisal fibers but you already said in another post that won't burn out easily enough. Any worries about doing the whole casting at once?

You should be able to get the fibres from a concrete engineering supplier. The ones I use are called Novomesh 950 (there would be other brands) and come in a two pack system, one the fine fibres and the other 50mm long plastic fibres. Added in the correct proportion they are the equivalent of steel mesh reinforcing, but so much faster and easier. You don't want to add these fibres though as they'd probably melt too.

You can do the casting in one go and I've done a couple that way. The advantage is that you get a little more oven area especially for a small oven. The disadvantage apart from the flue removal problem is that the flames want to jump straight to the flue hole. With a separate gallery you can make it thinner and lighter and create the inverted funnel to the flue pipe base and thus keeping the flames in the oven proper. Hope you understand this. What you have planned though, will work ok and if you plan on mainly cooking pizza it'll be fine with the occasional roasting and baking.

[Jumbalaya]

Don't be in too much of a rush to cast the dome just yet, your vermicrete slab needs plenty more drying. See attachment on post #7. As the whole thing is on wheels, can you roll it into the sun?

I won't be able to bring it outside due to the cold weather just yet but I do leave a small fan pointed at it. I will wait a while before the next steps.

[Jumbalaya]

Hi David, since I plan to remove the chimney whenever the oven is not in use, to avoid abrasion wear where the chimney connects to the arch I could embed a chimney coupling like in the picture below half way into the casting and then could simply place a cap on it when not in use?

[david s]

Yes, good thinking, I do the same if I want to build an oven with a removable flue ( have done a few this way), but I find a split in the sleeve so you can get an exact sliding fit, works pretty well. Make sure you only use stainless.Some plastic between the sleeve and pipe makes removal easier.Also make sure the sleeve is a slightly loose fit (2mm) to allow for the stainless expansion because you don't want it cracking your casting. I then surround the sleeve with 5:1 vermicrete.