Another attempt at a brickless oven build with HomeBrew

[Chicharron]

Hi all,

So after my first attempt at building something out of the box has been discarded (Thanks for your input david s ) I've decided to go the cast oven route.

As mentioned on my intro thread, the premises are to be able to build a WFO that's light and "movable" enough so I can take it with me when I move as I don't own a property. Also looking at reducing the cost as much as I can but ending up with a usable "good enough" oven for pizzas, roasts and the rare bread here and there.

So the idea is to build a cast oven using homebrew as my casting material over a Gym ball, The reason for that is that I just don't see the need to make a sand mold (as it's recommended) and then throw all that away later.

I've read a lot and gathered as much info as possible and here are the things I know and doubts I have so far:

1. The most proven formula for homebrew is 3:1:1:1 (By Volume) of Sand, Hydrated Lime, Portland Cement, Fire clay respectively + about 10% water. With the addition of SS needles (2% of dry material weight) for reinforcement and polypropylene Fibers (about 1%) - Can anyone advise on where to get this in Melbourne? otherwise is there any alternative to this? I understand a normal polypropylene rope cut to small strings won't work due to the thickness of the fiber and their melting point.

2. I understand that the best solution is to have a layer of "Castable" or heat retaining material, then an isolation layer of ceramic blanket or something similar and then have an outer layer of vermicrete/percrete with a final rendering for finishing purposes but my question is would it be good enough if I don't use the two outer isolation layers? as I mentioned before I'm not too concerned about heat retention or fuel efficiency all that much maybe making my walls a bit thicker?. Looking at this from bunnings https://www.bunnings.com.au/chapala-...-oven_p3180436 seems to be similar to what I'm aiming at. Are those ovens complete rubbish? as in are they never going to be able to produce good pizzas because of its lack of insulation?

3. Minimum wall thickness is recommended to be 50mm(2in) with 30mm(1.8in) entrance/flue gallery. If I make my walls 60-70mm (~2.5 in) would it be much better or just marginally? considering the lack of insulation layers

4. If I use only one layer of Homebrew, should/can I put a layer of render on top of that just to make it look nicer? if so what would this be ideally? I'm just a little bit concerned about the heat that's going to come out of my wall cracking the rendering, again, as I won't have an insulation layer.

5. I'm unsure about the flue. stainless steel flues seem to be the most common choice but looking at the guy on youtube that built his with the same casting material (He used vermicrete) I'm wondering if that's any good? I'm not too concerned about some smoke coming out of the oven opening as long as most of it comes out of the flue. What's the minimum height?.

6. Is there any way to "Sit" my dome on the fire bricks without "Gluing" it to them? i.e. Some kind of clay/sand or some material I can remove later, or maybe using gasket tape (like in big green eggs), some kind of fireproof sealant /adhesive? The question is again, to make it easier to move when needed so I can detach all the parts (dome, Floor and table) and move them separately.

I'm sure I'll come up with more questions when I start my build.

I'll post my design later.

Thanks all

[cobblerdave]

Gday
You might want to consider building on a brick pallet. They are designed to support a large weight of bricks and be movable by forklift.
I've seen a oven built of clay pavers on a brick pallet covered with 12 mm compressed cement sheet.
Ive long lost contact with the ovens builder. It was moved onto a box trailer
By a bunch of burley blokes and was towed to Sydney from Brisbane.
Regards dave

[david s]

As well as retaining heat an insulated oven also protects the refractory. Uninsulated means high temps inside and much lower temps on the outside surface, which leads to uneven expansion and cracking. Apart from the tiny (50mm) flue hole in the cheap Bunnings dome being next to useless, it won't last too long being uninsulated. I'd expect it to crack pretty badly just like the uninsulated ceramic chimeas do.

Regarding the rest of your plan, give it a shot and hope it works.

[AndrewT]

I have the Bunnings oven. I put some pics in my introduction post here:
https://community.fornobravo.com/for...om-new-zealand

Mine has a crack on the floor. It happened before I insulated it so maybe I should have insulated it earlier.

It still works great and I use it every 2 or three days at the moment often to cook naan which turns out great.

I think if I got another one of these I might try a layer of homebrew right over the entire outside surface including the floor to make it a bit thicker. Hopefully it would expand and contract at the same rate as the clay. (what do you think David?)

I did away with the chimney all together. If I could do it again I would extend the front doorway a bit to enable a larger diameter chimney (it would be better for the health of my eyes). I would read on here to find out how big it should be (I think oven inside floor diameter is about 600mm)

Since I insulated it, it uses a fraction of the amount of wood and also of course makes much less smoke while I get it up to temperature.

Also before I insulated it I had to move the fire left or right every single time I cooked a pizza. Now I put the fire to the left and the floor stays hot enough to cook as many pizza as I want without moving the fire again.

[AndrewT]

Couple more photos

[Chicharron]

Sorry I've been away from the forum Thanks all for your replies!!!


cobblerdave Thanks for your suggestion, I tried looking around but could find that build anywhere. david s yeah at the end I decided I was going to add insulation to it (as minimal as practically possible). Thanks for your input AndrewT As David Mentioned, leaving it without insulation doesnt seem like a good idea.

So I decided to bite the bullet and do my oven with homebrew, after all it's relatively cheap in case I mess up (And boy did I mess up!!!).

Ill include details of all my mistakes in another post. But for now Id like to know what went wrong with my concrete and if its fixable at all or if its better just to start over again.

As I mentioned before. I went with the 3:1:1:1 mix and applied SS Needles at 5% by weight. I didnt measure water as I found very different opinions about that so I was adding water little by little until it felt Workable It was hard enough so that it didnt fall from the trowel held at an angle (only until shaken).

I didnt have any means of vibrating the whole thing so what I did is I hit the board with a hammer as I was applying the layers. The concrete would slump a bit when hammering the board so I had to readjust it again with the trowel. Thats why I think it wasnt too dry too.

At the end I was pretty happy with the concrete application it was all relatively smooth and fairly easy to apply. I covered the whole thing with a wet blanket and a tarp over it for 7 days.
But it was today that I removed the ball and entry form that I notice the disaster that it was internally!

As you can see from the photos theres areas with pretty bad gaps and you can even see lots of needles everywhere.

So my question is it possible to apply an internal layer of homebrew or something else and smooth it all out or is it better to just scratch it and start all over again? Or should I just leave as is and see how it works? I really dont mind the aesthetical part as long as the whole thing doesnt collapse. The homebrew feels pretty solid and strong.

Cheers

[AndrewT]

Thanks for the pics. I'm curious to know what the others here will conclude. I haven't tried making a homebrew castable yet so very interested in following your thread. I'm guessing the mix might have been a little dry.

[Chicharron]

Yeah I thought so too. but as I said I don't even know how people get to cast in a wetter mixture. I found that it would slump a lot and I had to constantly push it up again specially when I Hammered the board where it was sitting (for vibration purposes).

[Chicharron]

I'm looking at this product from Bunnings https://www.bunnings.com.au/lanko-5k...ortar_p0760382 looks like it'd be something similar to Heat Stop from the US
It seems to be good for what I'm thinking of doing. Sort of rendering the interior of the oven with it. But I have no idea
Couldn't find much info about it but in this site in Spanish They mention it's highly adhesive and good for interior/exterior use in fireplaces, BBQs and Ovens http://www.parex.es/productes/ficha_124.pdf

Has anyone used this? would that be good for coating the interior of the oven with?

[dakzaag]

Ok, here are a few observations. First what you did was try to build a cast dome using home brew castable and no form to contain the cast. This is not how castable is designed to work. It is designed to be contained in a mold and the vibration takes out all the air pockets. You sort of combined the cob technique of layering up a dry castable around an exercise ball. This left a bunch of gaps because there was no way to force the air pockets out.

I'm not sure of an equivalent comparison, but perhaps throwing an ear of raw popcorn into an open fire and saying you are making popcorn would be close. For a proper cast, you need a form on the outside and inside to fill up and vibrate and let cure. The inside of the dome can be rendered, I don't think heat stop is the right stuff, but frankly this whole thing is backwards so maybe it would work just fine.

You could just leave it and see what happens, I suspect it would work just fine. It should be insulated, but don't spend a lot of money on the insulation cause you might be redoing the whole thing. With the needles in the mix, it is usually very strong so I would not be surprised if it lasts for 25 years of occasional use. Yes the inside gaps are an issue, but almost anything you apply will fall out over time and usually a little at a time. If you just leave it, the gaps will fill with carbon and could leave an interesting cob web pattern after a long hot burn.

As an experimental build, this is interesting and if you don't really care about the looks, you can just cook with it and see what happens. As an example for others to look at, it probably belongs in the "don't do this" category for others to see how not to cast an oven. Good luck with it whatever you do, thanks for sharing a project that doesn't exactly fit the norms of a regular build.

[david s]

Not too bad for a first attempt. Wriggling the mix against the form as you place it works well to reduce voids. You can fill those voids with the same mix provided the casting is still moist. The needles will make the process a bit difficult (and painful)

[david s]

I'm looking at this product from Bunnings https://www.bunnings.com.au/lanko-5k...ortar_p0760382 looks like it'd be something similar to Heat Stop from the US
It seems to be good for what I'm thinking of doing. Sort of rendering the interior of the oven with it. But I have no idea
Couldn't find much info about it but in this site in Spanish They mention it's highly adhesive and good for interior/exterior use in fireplaces, BBQs and Ovens http://www.parex.es/productes/ficha_124.pdf

Has anyone used this? would that be good for coating the interior of the oven with?

This stuff might work but as it will be a calcium aluminate based mortar it will go off fast so use small batches. It would be much easier to apply if you can flip the whole casting upside down and make sure that the casting is moist when you apply it. If the casting is either too wet or too dry the stuff will flake off. Try making some up pretty thin and work it in with a brush.

Don't forget that this stuff is a mortar so probably not suitable as a castable. It's also pretty expensive. If you want a proprietary castable rather than homebrew, better to buy it in 25 kg bags from a refractory supplier.

[AndrewT]

I wonder how the guy in this thread managed to get such a good result (pic attached):
https://community.fornobravo.com/for...canberra/page2

He hasn't mentioned using an outer form.

Is the workabilty of the proprietary castable that much different from the homebrew?

I have found a supplier of Shinagawa Shiracast 145 near me. Do you know if that will be a suitable product for me to use for my oven dome david s ?

What about if I was to build the dome up in say 4 layers (each one 1/2 inch thick, kind of like how people build up their outer render layers). Perhaps I could get it more filled in and more even in thickness? I will probably need to do my one without using any stainless needles because haven't got a source for those yet.

[Chicharron]

dakzaag Thanks for your input mate. I understand what you're saying the ideal method should be. And agree this is an experimental attempt as I'm a very inexperienced builder but I've seen quite a few people build ovens successfully with a similar method. Although most people do it with a sand castle but the principle is the same I believe. Re. The insulation, would a 50mm layer of Percrete work? I know it's ideal to have a layer of Ceramic blanket and a layer of percrete (or similar insulating concrete) on top of that but the oven is 700mm right now and I only have 100mm to play with so I can fit this thing through the door if I need to take it with me. So the 2 options I'm considering are applying a 50mm layer of percrete directly on top of the dome or a 25mm layer of Ceramic Blanket and 25mm Percrete on top of that. not sure what's better.

david s I did try to wiggle the thing as I was applying it and also hammered the board so it would slump down and I had to push it up again. Not sure if I had too many needles too. the casting is not moist anymore. as I mentioned before it's been over a week now. I did cover it but it doesn't feel moist (at least to touch) Can I just spray if with water and wait a little bit for it to absorb some? if I use homebrew, should I skip the needles (as I'm just filling the gaps or applying a thin layer).

[david s]

Yes, could be too much needles. I use 2% by weight of the dry material and don't have problems but I've been using the stuff for 10 years, so have had plenty of practice. If you used 5% it may hav contributed to the problem. I've not tried using the homebrew as a castable, but use a proprietary castable so it's quite possible the two behave differently on placement. As the proprietary mixes are designed to be gunned or troweled there may be all kinds of stuff they put in it to make it workable. Their recipes are secret so it's hard to know what's in it.

if your casting is dry you will have to wet it again before applying anything. Don't add more needles. Also sieving all but fine aggregate (sand) from the mix makes it easier to fill small voids.