Homebrew cast over yoga ball

[strangelove]

Hi,
Iv'e decided to give the pizza oven a go!
My plan is to cast it on a yoga ball, use vermicrete as insulation, fire bricks as floor, leca blocks as insulation under the floor place it on a casted concrete board and place it all on a wooden furniture.
I made a drawing of it with some dimenstion, calculation and materials.
Really dont know what size i should go with, i used a 750mm gymball for this drawing, but its easy to change... thinking it might be bigger then i need, thought about size?

for the homebrew i had problem finding a supplier for the clay, but i finally found a clay powder suitable for use in tiled stoves, hope that should be ok. 22€ for 20kg
I think i found lime at a decent price, about 12€ for 20 kg
Sand shouldnt be a problem at a local supplier
and portland concrete from a building material supplier (No need for any refractory mortar/concrete?)
Is 4 cm of homebrew enough?

For the insulation im thinking Vermicrete (10:1) (still just plane portland?)

1. I guess i'll start by casting the bottom plate, about 30mm thick reinforced with some rebar. (Portland concrete)
2. After that i'll start to cast the homebrew over the gymball, let that cure for a week (under damp blankets?) i can cast this in my garage to avoid rain and some cold weather, its below 0 degrees C during the nights still, but i think the temp in the garage stay above 0 degrees.
3. cut Leca blocks and fire bricks to the form of the oven. and cast with vermicrete around it, ontop of my bottom plate made of concrete.
4. place the homebrew ontop of the firebricks (some vermicrete in the shape of the oven under) and put my first insulation layer (Vermicrete 10:1) ontop of the homebrew. Let it cure (for how long?) and put another layer of vermicrete, ill aim for 60mm of insulation for a total thickness of 100mm. this step must be done outside, think the oven will be to heavy to move after this step.
5. start small fires a week after the last insulation layer is cast.
6 when its all set i'll put some plaster or something on the outside for protection and beauty.

When i move outside, should i cover the oven with plastic to keep it from rain?
i will use a 150mm chimney. how important is the hight? 0.5m enough?

Sorry if the english is bad, not my native language.
attached a drawing as PDF
Pizzaugn.pdf

all hints and tips are welcome!

[david s]

Your plan looks pretty sound, although I’m not sure how dense your Leca bricks are and therefore how good the’l be as insulators. 50 mm of 5:1 vermicrete between them and the floor bricks should be enough. There are two problems casting over an exercise ball. Firstly you have to build formwork up to the halfway mark which requires a fair amount of work and materials for a single casting, secondly that casting will need to be lifted and set into place over the oven floor. Calculate the weight and you’ll realise you have a problem unles you have lifting equipment at hand. It may help to cast in some steel loops to assist this operation and of course they may present problems during your oven firing. A far simpler, cheaper and faster method is to form the casting in situ over a sand mould built directly over the oven floor. Not only easier, faster and cheaper, it removes the need to relocate the casting.

[strangelove]

Thx for a good answer david, your the expert on this i understand from reading other topics.

1. so ill start by casting the bottom from concrete with some rebarb in it.
2. then ill build the insulation floor, maybe i should skip the leca blocks and just go with the 5:1 vermicrete instead, if i replace it, is 100mm enought or should i go with 150?
3. cut the firebricks to the shape of the innerdiameter, maybe a few mm wider to get all of the inside floor in firebricks. Then place it ontop of the insulation, a thin layer of sand to get the surface as even as possible
4. another cast of vermicrete outside of the firebricks to get it up to even level with the bricks so i i can start builing my dome and arch
5. build up a mold from sand, ill try to fill up with some wooden parts or empty bottles, thing i have around to make it faster.
6. cast the homebrew layer, since i dont have fire clay, should i mix in the leftovers from my firebricks here if i crush them? what should i remove if i add the firebricks? if i add 0.5 unit firebrick, do i remove 0.5 unit clay?
7. let it dry for a week and then cast the first insulation layer of 10:1 mix.
8. let that dry for a week and cast the last layer of insulation and after a week start some small fires in it.

Since ill be outside and the swedish spring can be cold and rainy, should i wait untill its not very likely with freezing degrees during the night or can it stand one or two cold nights?
should i build a temporary roof as protection from the rain? guess it will be dry faster if i dont cover it with plastic so the wind and sun can help me.
will the floor insulation need a week before i can start putting some loads on it? put firebricks and more casting

Edit: Would this https://www.wsochcompany.se/produktf..._18_micron.pdf be suitable to mix in my homebrew?

[david s]

Very good. You have researched this well. You will need to cover the damp sand mould with strips of wet newspaper about 50 mm wide. This acts as a slip layer and will give you a nice smooth surface on the inside of the casting.
The clay content in the homebrew mix is pretty important as it imparts a stickiness to the mix which helps in placement. Crushed firebrick will not give you theextremely small particle size of that of unfired clay, so you should try a bit harder to source it. Try pottery suppliers, but make sure you get powdered clay. Alternatively you can dig down in your garden until you get to the layer of clay. You then need to
1. dry it
2. break it up to nothing larger than your fist
3. soak it in water for 24 hrs
4. Sieve out coarse material
5. Dry again
6. Pulverise to a powder
This is a bit of work, but it’s free and far easier than pulverising dense firebrick.

Those fibres are correct. You notice their low melting point (160 C) you have to mix them in about double the time that you would normally use, so they disperse into the mix properly.

[strangelove]

Thx
I found this data sheet for the clay even thou its in swedish the chemical name would be the same.https://josefdavidssons.se/wp-conten...elugnslera.pdf
I'll add the fibres then, and follow the instruction on them. I will mix by hand in bucket, maybe showel in wheel barrow. I guess i could use it in the bottom plate(between oven and wooden surface) instead of iron bars too.

i've read thru this topic a couple of times https://community.fornobravo.com/for...ent-oven-build and will try to copy that design more or less. But use sand for the opening too, not build a wooden arch.

Will start my build as soon as i get all the material i need and have some spare time.
You think one bag of Vermiculite (100 litres) will be enough?

Will post some pics when i get any progress

[david s]

Here is a build that would be worth you reading.

#1

[strangelove]

Here is a build that would be worth you reading.

#1

Thx for that "guide" read it and hope i am wiser now.

Have ordered vermiculite, the polypropylene fibres and clay (things i had to order on internet), and made up a list of things that i need.
Think ill need about:
10kg of lime
80kg of portland cement
100kg of sand, should i buy any special sand, right now ive put sandbox-sand in my shoppinglist (0-2mm) what fraction is prefered for the sand?
22 fire bricks. 230x114x50 mm.

A chimney, but not sure what to buy and how i should place it. i dont want it to crack up my dome so it will be a loose fit, and i think i wanna use a connector, but i guess its the same cracking problem with that.
Should i place some metal bars and let the chimney connector rest on that or should i leave a rim for the connectior to rest on?
do i want the chimney close to the door or close to the dome?

i must use a steel pipe and not something enamelled?
im thinking of 150mm diameter and the length of it is just to keep the smoke out of my face?

[david s]

The diameter of the flue pipe depends on the internal diameter of your oven. A 750 mm dial oven requires a 150 mm diam pipe.thin galvanised steel will rust out better to use thin (0.55mm) stainless steel. A clay pipe is an option but needs to be insulated on the outside or it will crack.Using an anchor plate is problematic because of the high heat the fixings must be stainless or they'll rust away. Drilling into the refractory is asking for cracks to develop as it weakens it and any solid metal fixing will expand before the surrounding refractory causing stress. The same would apply if you embed some steel bars in the refractory for support. My solution, attached, is to cut three tags in the thin pipe, bend them out 90 degrees and insert the pipe as a slightly loose fit to allow for expansion. The joint can then be packed around and up about 500 mm with a 5:1 vermicrete mix which is strong enough and will allow some expansion against it.There are of course other ways to do it, you're the builder.

[strangelove]

I will start my casting this weekend i hope.
I will do the bottomplate from concrete and the bottom insulation like jumbalaya in his 6th post https://community.fornobravo.com/for...ent-oven-build.
i will keep the concrete under plast and spray it for 3 days or so. some questions...
1. do i need to treat the vermicrete in that way too?
2. For the vermicrete, ive read that a 5:1 ratio is used for the bottom insulation, is it okay to use that for the whole bottom insulation part?
3. when i move the insulation to the concrete, should i "glue" it with a ring of vermicrete or how do i make it stick to the concrete?
4. do i need to wait a week for the vermicrete to cure before i place the fire bricks and do the last casting to make a nice even surface of fire bricks and vermicrete?

i will cast both parts in the garage, and move outside for the next step, building sand mold and cast the dome.

[david s]

Not a bad idea to cast around a few wooden dowels near the centre of the concrete slab. They can be knocked out easily to reveal drain holes to help the elimination of water from your vermicrete insulation. Most builders would recommend a 100 mm concrete slab containing steel reinforcing bar. At only 50mm yours looks a bit thin.

1. I don't because the vermicrete mix requires about double the amount of water to that of a standard concrete mix, because each grain is made up of tiny absorbent holes. This means that after the hydration process has used up the water from the mix there is still lots of free water present. The problem is getting the water out. see my attached experiment
2. Yes
3. usually the insulating slab is cast in place, but you could do it separately. Just let its weight sit it in place, no need to stick it down.
4. Yes at least a week, see attached experiment again.
Vermicrete insulating slab copy.doc.zip

[strangelove]

Thanks for quick answer, ill go to the store today to shop for material.
The polypropylene fibres isnt enought to strenghten the slab?

[david s]

Sorry, no they're not. They provide quite good compressive strength but negligible flexural strength. There are thicker and longer plastic fibres that would work, but the normal reinforcement for concrete is steel bar.

[strangelove]

david s
Finally ive found the time to start my build!
ive casted the base of concrete ontop of my wooden stand.
And got my first cast of bottom insulation in place.
one thing i noticed was that i used alot more vermiculite then expected, used abit more then 1 bag (100 litres) and i had to use a bit more water then i thougt, 5verm:1portland:2water, but i saw no excess of water in the bottom of the wheelbarrow.

1. Should i keep the vermicrete under a tarp during the nights and when rain is expected and let the sun and wind help with the elimination of water uncovered?
2. Should i remove my form ASAP to help it get rid of water? (I think i will use it again when i put my firebricks inplace and cast to get an even surface)
3. got a neighbour that can help me get a chimned, works with sheet metal, so i can get it in any diameter and thickness? is 150mm ID ideal for a 700mm oven?
3.a what thickness of the stainless steel?
3.b is there a best spot for the chimney? how far from the center of the oven?

Added pics in link to imgur.com due to file size

https://imgur.com/a/554pXKR

[strangelove]

So i made my sand castle and casted the homebrew over it, it was ok, maybe the first batch was a bit dry, see if i have cracks in it when i remove the sand.
It may be a bit thinner then i planned, so i may have to cast another layer. Should i do that as soon as possible or should i wait for it to get dry?
i casted sunday, and think ill remove the sand wednesday, and maybe cast another layer and patch the inside of the dome if its to much cracks.

i sprayed it with water twice a day.

Should i start some small curing fire before i insulate it?
im gonna try rockwool and cover it with 10:1 vermicrete

[david s]

Apologies, I missed your last post. 150 mm diam is the correct size for your oven. Usually 0.55 mm thick 304 stainless. The chimney location should be placed in front of the oven mouth (crossdraft), so that the heat rises, is then drawn down before exiting out the flue. Look at the many builds on this site. If the flue is placed in the actual chamber (updraft) a lot of heat is lost. Judging by your pic, you got it right.

If you plan on adding another layer over your casting, do it ASAP, while there is still moisture in the casting, to achieve a decent bond between the layers. Homebrew requires a minimum of a week of damp curing to achieveI anything like full strength. I think it is better to do the insulation before lighting any fires.The reasoning behind this is that for example, a clay chimney will almost certainly crack unless it is insulated, because there is a huge temperature difference between the inner and outer surfaces and therefore also thermal expansion. The same principle applies with an uninsulated oven. I tried rockwool for two ovens partly because it is a little cheaper than ceramic fibre blanket (for me) and very little difference in insulation value. However I wouldn’t use it again as it’s harder to cut, more compressible, much more difficult to add the vermicrete layer against and more water absorbent. I concluded the small saving not worth it. But as all these products vary they may not necessarily have the same characteristics, this was only my experience of the materials I used.