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Homebrew cast over yoga ball

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  • strangelove
    I found this data sheet for the clay even thou its in swedish the chemical name would be the same.
    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 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

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  • 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 itís 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.
    Last edited by david s; 04-12-2018, 02:03 PM.

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  • 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 be suitable to mix in my homebrew?
    Last edited by strangelove; 04-12-2018, 03:14 AM.

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  • david s
    Your plan looks pretty sound, although Iím not sure how dense your Leca bricks are and therefore how good theíl 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 youíll 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.

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  • strangelove
    started a topic Homebrew cast over yoga ball

    Homebrew cast over yoga ball

    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. 22Ä for 20kg
    I think i found lime at a decent price, about 12Ä 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

    all hints and tips are welcome!