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Homebrew Castable 21" Kent (UK)

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  • Homebrew Castable 21" Kent (UK)

    Hello all - I am a first time poster, medium term lurker planning a 21" homebrew castable oven for my son in Kent (UK). Space and budget are tight so a 21" along the lines of David S's ovens looks like a good fit for us. I think we are broadly following convention (concrete slab, brick base, 100 mm Thermalite blocks resting on lintels, homebrew dome cast over sand followed by second cast for gallery, 50 mm ceramic blanket, 15mm render and probably waterproofed with white tanking slurry). The two areas where we might diverge a little is that I am planning to use AR glass fibres in the homebrew rather than stainless needles (but will include the PP fibres) and we are also planning to use a cast 50 mm base (probably cast as two halves) as our cooking floor rather than firebricks. Are there any reasons why a cast cooking floor is a bad idea beyond aesthetics and the possibility to replace firebricks? Also, any thoughts on AR glass fibres?

    Regards

    Peter

  • #2
    You will need a fire brick floor. Cast floors just do not work due to expansion and contraction of the material. AR glass fibres are not a substitute for melt extract needles.
    Russell
    Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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    • #3
      I was also wondering if ar glass fibre would do, as presumably they never get close to melting point.... I bottled out and used stainless but would be interested in knowing if they work from others experience.... I wonder whether the heat might cause them to shatter.

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      • #4
        Hi Nick, I read this paper and my main takeaway was that glass fibres were good to at least 400C so worth a try. They also appear to have a similar stress-deflection curve to stainless needles up to 8Mpa (1160psi) and according to the study delay macrocrack propagation and reduce brittleness. Now I'm not a mechanical engineer so there's every chance I'm misinterpreting the data and it's only one study but bottom line for me given our constraints it's either glass and PP fibres or just PP fibres.

        Glass fibres in refractory.pdf

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        • #5
          I suggest you reach out to David S, he is our resident cast oven expert.
          Russell
          Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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          • #6
            Thanks Bamforp, that's an excellent paper. Actually I've been using both stainless and AR fibres in my product for a few years now, so the paper confirms what I thought. Both fibres are quite expensive and difficult to purchase in small quantities. I really like the AR fibres to work with because they are much softer. I also now use nano fibres with refractory, but have yet to test their improvement in strength, which I have done with regular concrete which shows a 50% increase in flexural strength.
            Kindled with zeal and fired with passion.

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            • #7
              So I learned something new about AR fibers.
              Russell
              Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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              • #8
                That is good to know. I have a phobia to ss needles. Both to being injected and ingested
                Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build

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                • #9
                  That is reassuring Dave. There was a supplier on eBay in the UK selling small quantities of the Eccotexx glass fibres so I grabbed a 100g bag for 3.50 which is enough for around 100 litres of the homebrew. That should be enough for the dome and cooking floor, assuming I go ahead with a cast floor rather than firebricks. Do you have any thoughts on the idea of using a homebrew cast floor instead of firebricks? Is it a really bad idea?

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                  • #10
                    There are 3 things holding the homebrew together, cement, lime and clay. The cement begins to fail north of 300C, the lime north of 500C while the clay does not become permanent until 573C. In addition the 500-600C range is quite unstable with materials experiencing different expansion rates. While the brew proves adequate for the dome the fire sitting directly on top of it leads to fluctuating temperature and uneven expansion. In addition the larger any casting the greater the tendency to cracking due to uneven heat. For this reason floors are better with smaller units and laid loose to allow for individual expansion and contraction.
                    So no the homebrew cast floor is not a good choice, a firebrick floor is better. A number of UK builders have found storage heater bricks seem to work ok, although as they’ve been designed for a different purpose some may be ok while others not, no guarantees.


                    Kindled with zeal and fired with passion.

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                    • #11
                      Thanks Dave, that makes sense. I haven't managed to source fire bricks locally yet but will keep looking otherwise I will just have to have them shipped but that gets expensive for a small quantity.

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