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  • #16
    "
    Rob,
    I use castable refractory which is a blend of CAC (calcium aluminate cement), high temperature aggregates and burnout fibres which assist in water elimination during initial firing. The stuff is expensive and you need to do a calculation of how much is required. As itís rated to 1450C it uses high temperature aggregates that are way in excess of what we require. The use of lime in conjunction with CAC is a no no because it acts as an accelerant reducing the already limited working time of the mix. The stuff is also very temperature dependant. There are recipes out there (including this site) that recommend CAC and lime together. Unfortunately those that attempt it will find their mix going off as soon as itís been mixed.Another alternative that works (just) for our temperature range is what we call homebrew. It is used as a mortar for brick builds and can be used as a castable. It is 3:1:1:1 sand, hydrated lime, Portland cement and powdered clay. Very cheap and good working time, but the lime is vicious on the hands, wear rubber gloves. It also requires the addition of the burnout fibres which can be the really fine polypropylene ones used in reinforcing concrete. They are finer than human hair and take extensive mixing to disperse in the mix properly. I also use them for adding to mortar if brick building as they provide some measure of reducing steam spalling wher mortar joints can be extremely thick.
    Regarding my mobile, it sits on a timber trolley that has 6 wheels that enable me to roll it on and off the trailer easily.
    On the subject of information sharing, most of us here are old farts and well beyond wanting to keep secrets in order to build empires. Rather, we are excited by the enjoyment of cooking by fire in ancient yet highly efficient designs and we want to ďspread the loveĒ.

    Dave"


    Hi Dave, I don't get to write here everyday so please excuse me if it seems long.

    I can locate CAC here (Ciment fondu) though sourcing appropriate aggregate might fare a little more tricky, though not out of the question yet as I will try a few more places this week. I may just go with ref castable with 42% aluminate aggregate already in it for convenience. I will now be opting to keep mobile oven small-ish. (I read somewhere that your oven is 21"-is that inside measurement surely) alot smaller than the one I was planning originally (approx 750mm internal) but after much consolidating of the variables. I am now aiming at an oven around 600mm internal max-smaller if I feel I can get away with it.

    I am thinking 50mm thick ref castable + adding the novomesh 950, (around 2%/100ml per 100litres of mix?) The polyprop fibres all melt out at 160C and in this application are only to aid the elimination of moisture through the curing process right. Do you think I need to add SS needles too, that is the question?

    Finishing with a 50mm layer of fibre insulation blanket, covering dome and flue gallery, wire to secure it then 3 watered down layers of flexible acrylic pointing cement painted on surface leaving an expansion joint at the brick arch junction to allow for fluctuations. Would you be able to provide an example of the one you did or similar.

    I am not entirely sure how best to figure out calculating amounts based on the sizes stated. I was told 2000kg per M2 but still a bit confused how to apply this info. Is there a formula I need to work with or a easy method with which to go forth?
    I am definitely keen to avoid the tendancy to make an oven unecessarily big, after all a pizza shouldn't take that long to cook once heated, so cooking one 12" pizza at a time is fine. I actually opt for and prefer the opportunities it brings to enable people to prep, cook & eat together. You will be glad to know I not aiming for world domination here, Though I do appreciate the purist 90 second multi pizza brick oven view/approach too, and aspire to operate at around 450-500C. I have built a cob oven and operated a pompie esque oven before. So experience helps.

    How does all that sound to you?




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    • #17
      If you use a proprietary castable refractory then the burnout fibres should already be in the mix so you wonít have to add them. If you make your own castable with either homebrew or CAC then it is wise to add them. Only a handful for every 20 litres of dry castable is required but they must be dispersed well, which requires some extended mixing. It is only the fine hairlike fibres that should be added, not the longer thicker ones. The addition of stainless needles is not mandatory. They will improve strength and crack resistance, but make handling the mix more difficult the addition is 2-4% by weight. Most manufacturers donít add them because of the added expense and handling difficulties. FB donít, or at least they donít advertise that they do, so assume they donít.
      The normal procedure for insulation is one or more layers of 25 mm blanket over which a layer of 10:1 vermicrete is applied. This produces a firm substrate over which can be applied a stucco finish that can then be waterproofed if you wish. Working straight over the blanket is difficult because itís a bit soft and the surface is lumpy.

      Formula for working out volumes is V=4/3 x Pi x r3 (volume of a sphere) calculate volume ofspher using radius to outside, then subtract volume of sphere using internal radius. Halve the result because itís a hemisphere.
      Kindled with zeal and fired with passion.

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      • #18
        Originally posted by david s View Post
        If you use a proprietary castable refractory then the burnout fibres should already be in the mix so you wonít have to add them. If you make your own castable with either homebrew or CAC then it is wise to add them. Only a handful for every 20 litres of dry castable is required but they must be dispersed well, which requires some extended mixing. It is only the fine hairlike fibres that should be added, not the longer thicker ones. The addition of stainless needles is not mandatory. They will improve strength and crack resistance, but make handling the mix more difficult the addition is 2-4% by weight. Most manufacturers donít add them because of the added expense and handling difficulties. FB donít, or at least they donít advertise that they do, so assume they donít.
        The normal procedure for insulation is one or more layers of 25 mm blanket over which a layer of 10:1 vermicrete is applied. This produces a firm substrate over which can be applied a stucco finish that can then be waterproofed if you wish. Working straight over the blanket is difficult because itís a bit soft and the surface is lumpy.

        Formula for working out volumes is V=4/3 x Pi x r3 (volume of a sphere) calculate volume ofspher using radius to outside, then subtract volume of sphere using internal radius. Halve the result because itís a hemisphere.
        Thats great thanks. Spoke to supplier earlier and the castablwe they sell didnt seem to have fibre in it or at least the pretty helpful guy had never heard of it. Awaiting a call back from Sika UK regards sourcing some but can easy gets hold of some online I think (unbranded.)

        Might need to work on that formula cos I don't gets it. 4/3? r3

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        • #19
          Pi Thereís no symbol on my key board for the Greek letter. It is 3.142
          The salesman probably has no clue. Try looking at the data sheet on the product, it may tell you if there are fibres in it, or try ringing the manufacturer. You can also sieve a sample to see if there are fibres there.
          Kindled with zeal and fired with passion.

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          • #20
            I buy all my refractory products from these guys , very good prices and very helpful https://www.castreekilns.co.uk/casta...ctory-56-c.asp

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            • #21
              Originally posted by fox View Post
              I buy all my refractory products from these guys , very good prices and very helpful https://www.castreekilns.co.uk/casta...ctory-56-c.asp
              Thanks for the tip Fox.

              Oddly enough the 'very helpful' company you mentioned was the same 'pretty helpful' guy I mentioned, James Castree. Kudos James. Good prices, agreed. though he had heard of, and sells SS needles he had never heard of said fibres but was very interested, don't be surprised if he starts selling them. Requested the data sheet from these guys to see if there are fibres in the castable. Can't ask manufacturers as they are in plain white bags. I will wait for clarifacation and buy fibres seperately if necessary.

              Getting closer

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              • #22
                Here you go, show the salesman this.
                https://www.azom.com/article.aspx?ArticleID=2034
                I did have a look at the data sheet for the product I use and there was no mention of the fibres that are in the mix, as all the manufacturers are pretty secretive about their recipes, but if you sieve a sample you can can easily find them. Note the tiny amount, on the link provided, that is required. 0.05- 0.01 % by weight. The castable may be in plain white bags which is unusual and perhaps a bit suspicious. They should have the name of the manufacturer and the date it was manufactured displayed on each bag. There must also be a data sheet for the product available somewhere, but the salesman must know who the manufacturer is and should be able to either contact them or provide it to you to follow up.
                Last edited by david s; 08-16-2019, 01:06 PM.
                Kindled with zeal and fired with passion.

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                • #23
                  The last batch I bought was around 14 months ago and I am just about ready to order another 30 bags.
                  The bags that were supplied to me most definitely do contain fibres....
                  I have used the product in several rocket stoves where you can expect prolonged temperatures in excess of 1000c and it has performed faultlessly.
                  I have also built 7 pizza ovens using the same product with no issues ... however I mould my ovens in several sections and they are made with a very dry mix and vibrated to expel as much air as possible.
                  I also add stainless needles.

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                  • #24
                    Ha Ha I got a bit paranoid after making that last post so I took a picture, I had forgotten that the product also has a certain amount of stainless needles as well as fibres.

                    Comment


                    • #25
                      Originally posted by fox View Post
                      Ha Ha I got a bit paranoid after making that last post so I took a picture, I had forgotten that the product also has a certain amount of stainless needles as well as fibres.
                      Why the paranoia? It's great to see product (Thanks Fox) doesn't seem to have many needles in it looks like its got only one, lol. I will be asking James about this and send him the link provided (thanks Dave) Seems like I may only require the dense castable bags. But I will be re-requesting/checking the data to be sure, as I will not be making it in parts but a whole dome (with 2 part cast cooking surface or Bricks).

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                      • #26
                        I will also request a sample of course. (Fox) Do you know if they have a limit spend. As other places I have tried here in the UK requires a min £200 spend....and I am aiming to keep within this figure not at the expense of performance/longevity, ambitious perhap, but still.

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                        • #27
                          Ha not really paranoid.... but I was was worried that I had dreamt about the fibres!

                          No ideas about the min cost as I also buy ceramic insulation at the same time so my orders are quite big. As I live in Guernsey, I have to pay for a pallet delivery anyway!

                          I do add additional needles to the mix.

                          To be honest I donít imagine it will be very easy to use high temp castable without retaining it in a mould but it seems others have done so with success.
                          Home brew is a much more pleasant material to use and seems quite popular for home casting Ďone offí ovens.

                          I can only say that... the end results for me worked as well as I had hoped for, but ... the product is a gritty, fast setting and a dry mixture that is not particularly pleasant to work with.
                          I make my mix in an electric mixer and fill my mould sections in just a few minutes, so I donít have to worry about the working time of the mix but I then have to completely clean the mixer before the next section can be filled.
                          (I have a video if you are interested)
                          Last edited by fox; 08-17-2019, 09:55 AM.

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                          • #28
                            Originally posted by fox View Post
                            Ha not really paranoid.... but I was was worried that I had dreamt about the fibres!

                            No ideas about the min cost as I also buy ceramic insulation at the same time so my orders are quite big. As I live in Guernsey, I have to pay for a pallet delivery anyway!

                            I do add additional needles to the mix.

                            To be honest I donít imagine it will be very easy to use high temp castable without retaining it in a mould but it seems others have done so with success.
                            Home brew is a much more pleasant material to use and seems quite popular for home casting Ďone offí ovens.

                            I can only say that... the end results for me worked as well as I had hoped for, but ... the product is a gritty, fast setting and a dry mixture that is not particularly pleasant to work with.
                            I make my mix in an electric mixer and fill my mould sections in just a few minutes, so I donít have to worry about the working time of the mix but I then have to completely clean the mixer before the next section can be filled.
                            (I have a video if you are interested)
                            I would very interested to see any video relating to this.
                            Fast setting....I have a variety of skills but not used this castable before, so any aids will be very welcomed. Is there a way of extending the working time? I don't have a cement mixer, though I do have a drill mixer attachment, Though have read somewhere not to bother with this when mixing outer vermicrete mix. I was planning on small batches by hand (with gloves) and possibly using an electric sander to vibrate mix (saw this method in a video)

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                            • #29
                              OK this vid is a couple of years old, I have since made another mould for a slightly bigger 32Ē dome.
                              https://m.youtube.com/watch?v=pRnlKzJSpfo

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                              • #30
                                Originally posted by fox View Post
                                OK this vid is a couple of years old, I have since made another mould for a slightly bigger 32Ē dome.
                                https://m.youtube.com/watch?v=pRnlKzJSpfo
                                Hello Fox, I see what you mean now by work quickly! I had actually seen this video before. How easy is it to make a mold, did you design it. I presume its fibre glass.

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