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Post by gwmckay on Jul 14, 2020 16:18:29 GMT
Hi folks,
I've been thinking about building a wood oven for months now and I keep going off in a tangent with different ideas every time I watch a new Youtube video of one being built. Originally I had intended to build a movable one, but having just built a new patio area I have left an area perfect for a permanent structure. Ideally Id like to build the dome from a castable concrete (something like the vermiculite gym ball ones you see on Youtube, but out of a stronger and more dense refractory concrete material) Does anyone have a recipe for such a material??
I like the idea of the 'home-brew' sand/cement/lime/clay mixture, but presumably this gives a material more like mortar with which to bond bricks together etc? Is there a home-brew recipe for concrete which could be used to mould the dome structure itself??
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Post by downunderdave on Jul 15, 2020 4:25:33 GMT
You can use the homebrew as a castable, just make it a bit stiffer than mortar to get a workable consistency. I’d strongly recommend that you add some burnout fibres to the mix to give it some fireproof protection from steam spalling. Proprietary refractory castables have these already added to the mix. If you access the plastic fibres used in concrete reinforcing , they usually come as a two part pack. Save the longer/thicker fibres to reinforce the outer shell and use the thinner/shorter fibres to add to the homebrew castable. They are polypropylene fibres thinner than human hair and will melt at 160C leaving a network on mini pipes out of which steam can escape. Addition is around half a handful for every 10litres of castable. They take quite a bit of mixing to disperse evenly into the mix.
Throw away the gym ball, build a sandcastle and cast in situ over it. This means you won’t have to move the casting and also does not require formwork around the base of the ball.
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Post by gwmckay on Jul 15, 2020 11:10:09 GMT
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Post by downunderdave on Jul 15, 2020 21:04:57 GMT
Yes they’re the type you need.
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Post by gwmckay on Jul 16, 2020 8:08:38 GMT
Good job, have you any recommendations for the insulation layer and top layer. I had thought Vermiculite/cement for the insulation layer but concerned about getting it to dry out properly.. safer to go with fire insulation blanket?
Top weatherproof layer more home brew?
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Post by hughjamton on Jul 16, 2020 8:17:48 GMT
They're a bit cheaper on ebay |
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Post by downunderdave on Jul 16, 2020 19:24:50 GMT
They're a bit cheaper on ebay Ceramic fibre blanket is easy to apply and it is dry. But it’s also expensive. Vermiculite/cement is cheaper but harder to apply and the more cement you add to it the less it will insulate. I use both but apply a 19:1 vermicrete brew over the blanket no thicker than one and a half inches with a week to dry out before proceeding with drying fires. |
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Post by mikeb21 on Jul 16, 2020 21:08:49 GMT
Ceramic fibre blanket is easy to apply and it is dry. But it’s also expensive. Vermiculite/cement is cheaper but harder to apply and the more cement you add to it the less it will insulate. I use both but apply a 19:1 vermicrete brew over the blanket no thicker than one and a half inches with a week to dry out before proceeding with drying fires. Yes the fire blanket is silly prices here in the UK. Cheapest I could find was £19.99 per metre with free delivery off Amazon. Luckily found people selling offcuts online for cheap and have about 2.5m which cost me less than £25. As I'm planning a vaulted oven, my plan is to use calcium silicate board underneath and around the sides, then the blanket over the top, chicken wire to hold it down then some form of render over the top, not sure precisely what yet. |
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Post by gwmckay on Jul 17, 2020 9:08:14 GMT
I’d love to get a load of small sandstone pieces and cement them on as the outer layer, that’s why I had thought that vermicrete would be the better option
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Post by downunderdave on Jul 18, 2020 20:02:46 GMT
Ceramic fibre blanket is easy to apply and it is dry. But it’s also expensive. Vermiculite/cement is cheaper but harder to apply and the more cement you add to it the less it will insulate. I use both but apply a 19:1 vermicrete brew over the blanket no thicker than one and a half inches with a week to dry out before proceeding with drying fires. Yes the fire blanket is silly prices here in the UK. Cheapest I could find was £19.99 per metre with free delivery off Amazon. Luckily found people selling offcuts online for cheap and have about 2.5m which cost me less than £25. As I'm planning a vaulted oven, my plan is to use calcium silicate board underneath and around the sides, then the blanket over the top, chicken wire to hold it down then some form of render over the top, not sure precisely what yet. The usual procedure is to make a layer of quite lean vermicrete around and inch and a half thick, over the blanket which enables you to restore the hemispherical form, covering the lumps and bumps of the blanket, adding additional insulation and providing a firm substrate to render against. I guess you could render directly onto the blanket but it will be pretty springy. Most folk cover the blanket in chicken wire, I don't because it's too laborious and the vermicrete holds it in place ok.  |
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Post by mikeb21 on Jul 20, 2020 19:00:19 GMT
Yes the fire blanket is silly prices here in the UK. Cheapest I could find was £19.99 per metre with free delivery off Amazon. Luckily found people selling offcuts online for cheap and have about 2.5m which cost me less than £25. As I'm planning a vaulted oven, my plan is to use calcium silicate board underneath and around the sides, then the blanket over the top, chicken wire to hold it down then some form of render over the top, not sure precisely what yet. The usual procedure is to make a layer of quite lean vermicrete around and inch and a half thick, over the blanket which enables you to restore the hemispherical form, covering the lumps and bumps of the blanket, adding additional insulation and providing a firm substrate to render against. I guess you could render directly onto the blanket but it will be pretty springy. Most folk cover the blanket in chicken wire, I don't because it's too laborious and the vermicrete holds it in place ok. Interesting, I've been debating best way to finish the top, don't really want to spent more money on vermiculite when I've got a lot of blanket for insulation. Was thinking of using any leftovers of the lime/clay from homebrew to make some form of concrete for the outside/roof. |
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Post by gwmckay on Jul 22, 2020 14:29:52 GMT
Any advice on the type of clay that would be suitable, Ive read about fire clay but having some trouble sourcing it. There’s a pottery supplier local to me sells powdered ball and china clays which are relatively cheap (https://www.ulsterceramicspotterysupplies.co.uk/product-category/rawmaterials/ball-and-china-clays/?v=ac46d1e7cde3)
Then they have powdered stoneware and earthenware clays which are more expensive.
(https://www.ulsterceramicspotterysupplies.co.uk/shop/claysandslips/powder-clays/white-stoneware-powdered-clay/?v=ac46d1e7cde3)
They also have a selection of wet clays
I’m just confused which would be best to go for.. obviously cheaper is good but not if it won’t do the job!
Also roughly talking about quantity, assuming a 30” internal diameter dome made from castable home brew, a vermicrete insulation layer and then an outer weatherproof layer of more home brew?.. approx weight of clay required?
Sorry for the questions, I just don’t want to go off on a tangent at this early stage!
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Post by downunderdave on Jul 22, 2020 18:55:48 GMT
Any powdered clay should be suitable apart from Bentonite which has extremely small particles that lead to exssive shrinkage that you don’t want. Get the cheapest, Ball clay will be fine. To a potter fire clay is a different material than to a builder. If sourcing the clay through a building supplier try to look for “bricklayers clay”.
Get the potters Ball clay. One bag should be sufficient, they usually come in 25kg bags.
The homebrew recipe is made up by volume not weight.
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Post by gwmckay on Jul 22, 2020 19:14:40 GMT
Any powdered clay should be suitable apart from Bentonite which has extremely small particles that lead to exssive shrinkage that you don’t want. Get the cheapest, Ball clay will be fine. To a potter fire clay is a different material than to a builder. If sourcing the clay through a building supplier try to look for “bricklayers clay”. Get the potters Ball clay. One bag should be sufficient, they usually come in 25kg bags. The homebrew recipe is made up by volume not weight. Once again, thank you for your help! |
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